CuI‑Catalyzed Conjugate Addition of Silyl Boronic Esters: RetracingCatalytic Cycles Using Isolated Copper and Boron EnolateIntermediatesJacqueline Plotzitzka and Christian Kleeberg*

Institut fur Anorganische und Analytische Chemie, Technische Universitat Carolo-Wilhelmina zu Braunschweig, Hagenring 30, 38106Braunschweig, Germany

*S Supporting Information

ABSTRACT: Copper(I)-catalyzed conjugate additions of silylboronic esters to α,β-unsaturated aldehydes, ketones, and estersare synthetically well-established reactions. For the first timecentral reactive intermediates as well as the boron enolates asthe primary reaction products are isolated and employed inorder to deduce catalytic cycles on an experimental basis.Employing an NHC CuI complex as a model catalyst, it ispossible to perform efficient catalytic transformations as well asto isolate and characterize the formed copper enolatecomplexes as the key intermediates. It is shown that for thiscatalytic system the nature of this enolateO- or C-enolateiscrucial for the catalytic process. For α,β-unsaturated aldehydesand ketones the O-enolate is formed predominantly, while forα,β-unsaturated esters the C-enolate is the major product.Catalytic turnover is only facile for copper O-enolates, as they react efficiently with the silyl boronic ester under (re)formation ofthe catalytically active Cu−Si species and a thermodynamically favored boric acid ester. Thus, the formation of copper C-enolatesis inhibiting the catalytic process, and effective turnover is possible only after solvolysis by an alcohol additive. The individualcatalytic processes were retraced by performing stepwise stoichiometric reactions monitored by in situ NMR spectroscopy.

■ INTRODUCTIONSilyl boronic esters, especially pinB−SiMe2Ph (1) (pin =OCMe2CMe2O), are well-established reagents in transition-metal-catalyzed silylation reactions (e.g., Pt, Pd, Cu, Ni, Rh,CuII). More recently, transition-metal-free Lewis-base-pro-moted silylation and borylation reactions employing 1 havealso emerged.1−4 Furthermore, copper(I)-catalyzed silylationreactions employing 1 with various organic substrates such asα,β-unsaturated carbonyl and carboxyl compounds, aldehydes,imines, amides, but also, for example, allyl/propargyl chlorideshave been reported in the past few years.1a,2 Using α,β-unsaturated carbonyl and carboxyl compounds as substrates,the products obtained, possibly after hydrolytic workup, are thecorresponding β-silyl carbonyl/carboxyl compounds, respec-tively (Scheme 1).A generalized catalytic cycle was proposed in agreement with

the experimental data, well-established stoichiometric cupratechemistry, and the apparently closely related CuI-catalyzeddiboration reactions (Scheme 2).1a,2,4−8,9a,b While diversecatalyst systems and reaction conditions are employed inthese transformations, two general points regarding the(pre)catalyst are to be noted: The CuI source may be apreformed, isolated copper complex. More often, a CuI salt anda ligand or ligand precursor, possibly under addition of anauxiliary reagent (e.g., a base in the case of imidazolium salt in

order to furnish a (NHC)Cu species), is employed. In a simpleand yet versatile catalytic system CuCN is used in the absenceof any additional ligand.1a,2,4 The second general observation isthat catalytic amounts of an alkali metal alkoxide are required. Ithas been conclusively argued that the alkoxide is necessary(besides the possible generation of an NHC ligand, vide supra)to form a copper alkoxido species that is crucial for an initialB−Si bond activation step in order to generate the catalytically

Received: September 29, 2014

Scheme 1. General Scheme of Copper(I)-CatalyzedSilylation Reactions of α,β-Unsaturated Carbonyl (R = H,Alkyl; R′ = H, Alkyl, Aryl) and Carboxyl (R = Alkoxido, R′ =H, Alkyl, Aryl) Compounds Employing the Silyl BoronicEster 1

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crucial Cu−Si species (Scheme 2). The effectiveness of ligandcontrol especially by NHC ligands has been demonstrated byHoveyda et al.: Using a chiral NHC-Cu complex as catalyst, theconjugate silyl addition to α,β-unsaturated ketones and estersresults in high yields and enantioselectivity.4a

The commonly accepted catalytic process consists of threedistinct steps (Scheme 2): (1) formation of the centralcopper(I) silyl complex (via σ-bond metathesis) from thesilyl boronic ester and a copper alkoxido species, initiallyformed from the primary copper source and the alkali metalalkoxide. (2) This copper(I) silyl complex reacts in an additionreaction with the α,β-unsaturated substrate to give a β-silylatedcopper enolate. (3) The reaction of this enolate with the silylboronic esters regenerates the copper(I) silyl complex (and,hence, completes the catalytic cycle), and the primary productof the catalytic process, a β-silylated boron O-enolate, isreleased. Finally, hydrolytic workup of the latter results in theformation of the finally isolated β-silylated carbonyl/carboxylcompounds. The individual steps within the catalytic cycle arealso, as far as plausible, in agreement with computational dataon a CuI/amine-cocatalyzed enantioselective conjugate silyla-tion of α,β-unsaturated carbonyl compounds and withexperimental studies on the 1,2-addition of 1 to aldehydesand CO2.

4g,5,10

For a further rational development of CuI-catalyzedconjugate addition of silyl boronic esters with α,β-unsaturatedcarbonyl and carboxyl compounds a more detailed under-standing of the species present during the individual reactionsteps and of their reactivity is highly desirable. Hence, we setout to study the CuI-catalyzed reaction of 1 with exemplary α,β-unsaturated aldehydes, ketones, and esters employing a(IDipp)CuI complex (IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene) as model catalyst. In particular we aimed toisolate and thoroughly characterize the central intermediates

and finally to study their reactivity and retrace the catalyticcycle stepwise by stoichiometric reactions.We reasoned that using a model catalyst based on the

(IDipp)CuI fragment is advantageous in order to have well-defined, mononuclear species and to prevent the formation ofaggregates and/or polynuclear complexes as suggested by thelong-discussed aggregation behavior known from, for example,alkyl and aryl cuprates.6 Furthermore, the (IDipp)CuI systemhas already been successfully employed as a model catalyststudying 1,2-addition reactions of 1 to aldehydes and CO2 aswell as in a number of related studies.5,9

■ RESULTS AND DISCUSSIONCatalytic Experiments. The exemplary α,β-unsaturated

carbonyl and carboxyl compounds 3a−f as substrates werereacted with 1 in the presence of catalytic amounts of(IDipp)CuOtBu (2). The substrates were selected in order tocover carbonyl and carboxyl compounds (ketones, aldehydes,esters) and different substitution patterns at the β-carbon atom(Table 1).

For the ketones 3a−c and the mono-β-substituted aldehyde3d the expected β-silylated boron enolates (4a,b,d) and afterhydrolytic workup the corresponding β-silylated carbonylcompounds (5a−d) were isolated in good yields. Thisdemonstrates that 2 is indeed an efficient precatalyst and thatfurther studies may be fruitful to gain more mechanistic insight.Moreover, to the best of our knowledge, the β-silyl boronenolates as the primary reaction products of a CuI-catalyzedsilyl conjugate addition reaction of a silyl boronic ester have sofar not been obtained in substance and characterized in anydetail.11

The conjugate addition reactions of 1 to 3a, 3b, and 3d werestudied by in situ 1H NMR spectroscopy under conditionsclosely resembling the conditions used above but employing 10mol % of (IDipp)Cu−SiMe2Ph (6) or (IDipp)Cu−OtBu (2) asprecatalysts and C6D6 as solvent. As, according to the proposedcatalytic cycle (Scheme 2) and in agreement with our studieson related 1,2-addition chemistry, the alkoxide complex 2 israpidly converted to 6 upon reaction with 1, both precatalystseventually lead to the same catalytically active species.5

For all substrates the formation of the boron enolate as thevirtually exclusive reaction product is observed (Figure 1).

Scheme 2. Proposed Catalytic Cycle for the CuI-CatalyzedConjugate Silylation of α,β-Unsaturated Carbonyl andCarboxyl Compounds with 1a

aAdopted from ref 1a.

Table 1. Conjugate Addition of 1 to α,β-UnsaturatedSubstrates in the Presence of 2

substrate t/h 4 (E/Z)a 5a

R = Et, R′ = H 3a 3 87% (1:7) 77%R = Me, R′ = Me 3b 6−8 67% 87%cyclohexenone 3c 6 not isolated 82%R = H, R′ = H 3d 2 93% (2:5) 68%R = H, R′ = Me 3e 16 0%b 0%b

R = OMe, R′ = H 3f 4 79%c

aIsolated yields. E/Z ratio determined by 1H NMR spectroscopy. b1,2-Addition product in 92% (7)/83% (8) yield (vide infra, Scheme 3). cInthe presence of iPrOH.

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However, the reaction times to reach complete conversion aresignificantly different and generally in accordance with thepreparative study (vide supra). The different rates may berationalized with the steric influence of the β-substituents andthe electrophilicity of the carbonyl group. Thus, for the mono-β-substituted aldehyde 3d complete conversion is achievedwithin 2 h, while for the sterically comparable ketone 3a 6 h isrequired. The β-dimethyl-substituted, sterically more encum-bered ketone 3b does not reach completion within 20 h (62%1H NMR conversion). In comparison with other Cu-basedcatalytic systems the catalyst employed here is of only moderateactivity; yet, this may render it especially suitable for theisolation and study of catalytically relevant intermediates.1a,2

The boron O-enolates 4a and 4d are obtained as E/Zmixtures according to 1H NMR data with ratios of 1:8 and 2:5,respectively, resembling the ratios observed in the isolatedproducts (Table 1). For the enolate 4b only one isomer wasdetected by 1H NMR spectroscopy. The isomers were studiedby 1H−1H NOESY NMR experiments on isolated samples of4a,b,d. For the major isomer of 4a,d no (or only very weak)NOESY contacts were observed between the hydrogen atomsin the positions 1 and 3, while strong contacts were observedbetween the hydrogen atoms in the positions 1 and 2,suggesting mutual Z orientation of the latter. In agreement withthese findings the opposite is observed for the minor isomer:strong 1′−3′ (and 1′−4′) and only comparably weak 1′−2′contacts were found, corroborating the assignment of thepredominant isomer as Z configured (Figure 1). In the case of4b a final assignment is not possible on the basis of NOESYNMR data, as comparable contacts 1−2 and 1−4 are observed.Furthermore, for 4d the assignment is confirmed by theobserved mutual coupling constants for the olefinic hydrogenatoms of 3JHH = 6.0 Hz for the major Z isomer and of 3JHH =11.9 Hz for the minor E isomer. These data are in agreementwith the general trend observed for vicinal coupling constants

and in particular with reported values for related silyl enolethers.12

In addition to the signals of the silylation products 4a,b,dsignals indicative for complex 6 as well as for residual 1 areobserved in all reactions. Moreover, unreacted starting materialis observed in the reaction of 3b as well as pinB−OtBu in thereaction of 3d. The latter results from the initial conversion ofthe precatalyst 2 to the silyl complex 6 in order to enter thecatalytic cycle (vide supra).5 In the case of the reaction of 3a asmall amount (<5% relative to 4a) of 5a was identified due tohydrolysis by adventitious moisture. The magnitude of all otherremaining signals is negligible, and their nature was not furtherevaluated.Employing the β-dimethyl-substituted aldehyde 3e under the

established conditions, the products of the 1,2-addition of 1, thesilyl alcohol derivatives 7 and 8, are obtained as major productsin excellent isolated yields (Scheme 3). The existence of a 1,2-addition pathway is not surprising considering that the catalyticsystem used is also effective for the 1,2-silylation of aryl andalkyl aldehydes.5

An in situ 1H NMR experiment reveals that the 1,2-additionproduct 7 is indeed the major product (Figure 2). However,additional signals in the in situ 1H NMR spectrum aresuggestive of the competitive formation of the 1,4-additionproduct 4e (5% relative to 7).13,14 All other signals are inagreement with the findings discussed above.In light of the observation of the 1,2-addition reaction for 3e

it may be speculated that the additional, very weak signals in therange 5−6 ppm observed during the silylation reaction of 3dmay suggest the formation of traces of the 1,2-addition productalso in this case. However, finally conclusive data could not beobtained (Figure 1, bottom).For the α,β-unsaturated ester 3f no conjugate addition

product 4f was obtained under the conditions employed for thecarbonyl substrates, while in the presence of a stoichiometric

Figure 1. Details of in situ 1H NMR spectra of conjugate addition reactions of 1 (1.2 equiv, ■) to hex-4-en-3-one (3a), mesityloxide (3b, □), andcrotonaldehyde (3d) (1.0 equiv) (from top) in the presence of 10 mol % precatalyst (6 (●) for 3a,b and 2 for 3d) at room temperature (300 MHz,rt, C6D6). Signals of the minor isomers are denoted by a prime; only selected NOESY contacts are shown (○: pinB−OtBu; ×: 5a); † denotes thatonly a part of the signal is visible due to overlapping.

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amount of iPrOH the β-silylated ester 5f was obtained (Table1). This is unexpected insofar as the conjugate addition to 3fwas reported to proceed in the absence of alcohol, although asimilar catalytic system with a different NHC ligand wasused.4a,b The in situ 1H NMR spectrum of the silylationreaction of 3f corroborates this and shows essentially a mixtureof the starting materials 1 and 3f after 8 h (Figure 3, bottom).Nevertheless, the precatalyst 2 has been converted to a singlenew (IDipp)Cu species along with the formation of pinB−OtBu, indicating the initial conversion of 2 to 6. The novel(IDipp)Cu species was later identified as the copper C-enolate9f, the insertion product of 3f into the Cu−Si bond in 6 (videinfra).In contrast, the catalytic silylation reaction does perform very

well in the presence of a stoichiometric amount of iPrOH, ascomplete conversion to 5f is observed within 30 min at roomtemperature (Figure 3, top). Additionally, the in situ 1H NMRspectrum reveals the formation of pinB−OtBu, pinB−OiPr, andthe (IDipp)Cu complexes 6 and 9f. This observation may berationalized assuming the alcoholysis of the insertion product 9fresulting in the direct formation of 5f, without the intermediateformation of a boron enolate, along with the formation of thealkoxido complex (IDipp)Cu−OiPr. The latter then reacts with1, yielding pinB−OiPr and regenerating 6 to complete thecatalytic cycle. The observation of both copper complexes 6

and 9f may be rationalized by a slightly less (approximately 5%)than equimolar amount of iPrOH, leading to incompletealcoholysis and as a result incomplete conversion of 9f to 6(Figure 3).In summary, it is shown that, by preparative as well as in situ

NMR studies, the CuI-catalyzed conjugate addition of the silylboronic ester 1 employing a (IDipp)Cu complex as catalyst iseffective for α,β-unsaturated ketones (3a−c) and certainaldehydes (3d). This is in general agreement with previousreports on related systems.4 Beyond that, it is demonstratedthat the bis-β-substituted aldehyde 3e gives the 1,2-additionproduct and that the ester 3f is unreactive in the absence of analcohol as additive.

Stoichiometric Experiments. To gain further insight intothe individual reaction steps, we aimed to retrace the catalyticcycle by stoichiometric experiments. The first step to be studiedis the insertion of the α,β-unsaturated substrates into the Cu−Si bond of 6 (Scheme 4).

An in situ NMR study of the stoichiometric reaction of3a,b,d,f with 6 revealed that single products are formed, whicheventually were assigned to the corresponding copper enolates(Figure 4). In agreement with the results of the catalyticreactions the rate of the reactions depends greatly on thecarbonyl compound. The conversions of aldehyde 3d and ester3f are fast (complete conversion within 30 min), while theketones 3a,b react slower (complete conversion within 1.5 h inthe case of 3a, while for the β-bis-substituted ketone 3bcompletion is not reached within 16 h).The reaction products were isolated (62−87% yield) and

identified as the copper enolates 9a,b,d,f (and also 9c) by 1Dand 2D NMR techniques as well as single-crystal X-raydeterminations for 9a−d. The complexes 9a,b,d exhibit 1HNMR signals just below 4 ppm (Figure 4). These signals

Scheme 3. Reaction of 1 with 3e in the Presence of 2 Leadsto the Predominant Formation of the 1,2-Addition Product7 (and 8)

Figure 2. Details of in situ 1H NMR spectrum of the conjugateaddition reaction of 1 (1.2 equiv, ■) to 3e (1.0 equiv) in the presenceof 10 mol % 2 as precatalyst after 6 h at room temperature giving 7and 4e (300 MHz, rt, C6D6, ○: pinB−OtBu; ●: 6).13

Figure 3. Details of in situ 1H NMR spectrum of the conjugateaddition reaction of 1 (■) to 3f (□). Bottom: 1 (1.2 equiv), 3f (1.0equiv), 10 mol % 2 after 8 h at room temperature. Top: 1 (1.2 equiv),3f (1.0 equiv), iPrOH (1.0 equiv), 10 mol % 2 after 0.5 h at roomtemperature (300 MHz, rt, C6D6, ○: pinB−OtBu; ●: 6; ×: pinB−OiPr; +: 9f); † denotes that only a part of the signal is visible due tooverlapping.

Scheme 4. Insertion of 3a,b,d,f into the Cu−Si Bond in 6

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correspond to carbon atoms with chemical shifts of 96−103ppm and 1J(C,H) coupling constants of 150−155 Hz(determined from 1H−13C HMBC spectra), indicating an sp2-character of the carbon atom. Therefore, these signals wereassigned to the 2-CH groups of the O-enolate isomer of 9a,b,d(Figure 4). In contrast, for 9f no 1H/13C NMR signals in therespective ranges are observed and the signal for the 2-CHgroup is detected at 2.35 ppm (13C: 37.6 ppm). Thus, a C-enolate structure was assigned to 9f (Figure 4). In accordancewith these assignments a 13C NMR signal at 179.4 ppmindicative for a carboxyl group is observed for 9f, while for9a,b,d no signals indicative for carbonyl groups are observed.Here, the signals of the O-bound carbon atoms are detected at151−162 ppm.A computational study on the closely related CuI-catalyzed

diborylation reaction generally agrees with this observation: foraldehydes (and presumably ketones) the copper O- and C-

enolates differ only slightly in energy, while for esters thecopper C-enolate is significantly thermodynamically favored.Moreover, in this study it is also shown that for copper O-enolates the metathesis reaction with a diboron compound isassociated only with a small energy barrier, while for copper C-enolates the energy barrier is substantial and, hence, thereaction pathway is ineffective. Efficient catalytic conversion ispredicted to be possible only after alcoholysis and formation ofa Cu alkoxide intermediate.7

Only single isomers of the complexes 9a,b are observedduring the in situ NMR study and in the isolated compounds;unfortunately, the nature of the isomers could not beestablished unambiguously by means of NMR spectroscopy.For 9d, however, a mixture of the E and Z isomers in a 1:2 ratiois observed on the basis of 1H NMR data (Figure 4). This is inagreement with a 2:5 E/Z ratio found for the boron enolate 4d.The E and Z isomers are assigned on the basis of the coupling

Figure 4. Details of in situ 1H NMR spectra of the reaction of 6 (●) with hex-4-en-3-one (3a), mesityloxide (3b), crotonaldehyde (3d), andmethylcrotonate (3f) (□) at room temperature (300 MHz, rt, C6D6). Signals of the E isomer are denoted by a prime (*: (IDipp)CuPh15); †denotesthat only a part of the signal is visible due to overlapping.

Table 2. Selected Crystallographic and Geometrical Parameters for 9a, 9b, 9c(C5H12)1/2, and 9d(10)(PhMe)16

9aa 9ba 9c(C5H12)1/2 9d(10)(PhMe)a

formula C41H57CuN2OSi C41H57CuN2OSi C43.5H61CuN2OSi C79H109BCu2N4O4Sicryst syst monoclinic monoclinic monoclinic triclinicspace group, Z C2/c, 8 P21/n, 4 C2/c, 8 P1, 2wR2 (all data), Rint 0.1145, 0.0489 0.1468, 0.0436 0.1564, 0.0779 0.1462, 0.0513temp (K) 100(2) 150(2) 100(2) 101(3)CCDC no. 1019948 1019947 1019946 1019950CNHC−Cu1 (Å) 1.861(2) 1.855(2) 1.865(3) 1.849(2)Cu1−O1 (Å) 1.819(1) 1.799(2) 1.823(2) 1.815(2)C1−Cu1−O1 (deg) 174.35(7) 172.48(8) 179.4(1) 174.78(9)O1−C1 (Å) 1.342(2) 1.298(3) 1.345(3) 1.293(4)C1−C2 (Å) 1.350(3) 1.347(4) 1.323(4) 1.331(6)C2−C3 (Å) 1.503(3) 1.508(3) 1.512(4) 1.529(6)C1−C7 (Å) 1.513(3) 1.507(4) 1.509(6)C1−C2−C3 (deg) 125.4(2) 128.9(2) 124.8(3) 124.6(4)τ (deg)b −5.5(3) −8.9(5) 179.0(3) −173.0(4)

aValues for the major of disordered parts only. bTorsion angle along O1,C1,C2,C3.

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constants between the olefinic protons (5.6 Hz (Z) and 11.1Hz (E), respectively); this is in accordance with findings for theboron O-enolate 4d.12 Intriguingly the E and Z isomers of 9dare in a slow equilibrium, as concluded from the observation ofsharp 1H NMR signals as well as signals indicative for chemicalexchange in a 1H−1H NOESY spectrum of pure 9d.15,16

Single crystals suitable for X-ray structure determination ofthe enolates 9a−d were obtained by crystallization fromsolutions in PhMe/n-pentane at low temperatures (Table 2),although for 9d the crystals obtained were repeatedly of lowquality.16 A single crystal of higher quality containing 9d wasobtained by slow evaporation of a solution in PhMe/n-pentane,but it contains, apart from one molecule of 9d, one molecule ofPhMe and one molecule of (IDipp)Cu−OBpin (10) in theasymmetric unit (Table 2).15 While the geometrical data ofboth molecular structures of 9d are comparable, the latterstructure determination is of significantly better quality and isused exclusively for further discussion.16 The molecularstructures of 9a,d show both enantiomers of the enolatemoieties mutually disordered; for 9b a structurally relateddisorder of two conformers is observed (Figure 5).17

For 9a−d the distances O1−C1 are in between a single and adouble bond, the distances C1−C2 indicate clearly a CCdouble bond, and the C1−C2−C3 angles are above 120° andsuggestive for an sp2-hybridized C2 carbon atom.18 Thedistances C1−C7 and C2−C3 are in the range expected for aC(sp2)−C(sp3) single bond.18 Moreover, the geometry of theCNHC−Cu1−O1 moieties is similar to those found in alkoxidocomplexes of the type (NHC)Cu−OR (Cu1−O1: 1.80−1.84Å, CNHC−C1: 1.86−1.87 Å, CNHC−Cu1−O1: 173−179°).9d,e,19a All four crystal structures consist of a single Z/Eisomer. For 9a,b the solid-state structures represent the Zisomer, while for 9c,d the E isomer is found as indicated by theO1−C1−C2−C3 torsion angles. However, asat least for 9d

(vide supra)the Z/E isomers equilibrate in solution, noconclusions on the isomer present in solution can be drawnfrom the solid-state structures.Having established the copper enolates as central inter-

mediates (Scheme 2), a last step is required in order to closethe catalytic cycle and to retrace it by stoichiometric reactions.In this last step a copper enolate reacts with 1 to form therespective boron enolate, the primary product of the catalyticprocess (vide supra), and to regenerate 6 (Scheme 5).

The reaction of 9a,b,d with an excess of 1 in C6D6 wasstudied by in situ 1H NMR spectroscopy (Scheme 5, Figure 6).For all three copper enolates virtually clean and quantitativeconversions to the expected products 4a,b,d and 6 wereobserved within 2 h at rt. Again, for 4a and 4d a mixture of theE and Z isomers is observed with ratios of 1:8 (4a) and 1:3(4d), in reasonable agreement with the values obtained fromthe catalytic reactions (vide supra). It is worth mentioning thatthose values are not necessarily directly correlated to the E/Zratio observed for the enolates 9a,d asat least for 9dthe E/Z isomers of the copper enolates are interconvertible.On performing the reaction of the copper C-enolate 9f with

an equimolar amount of 1, virtually no formation of 5f isobserved within 2 h. Nevertheless, clean and completeconversion to 5f, 6, and pinB−OiPr is observed upon additionof iPrOH within a few minutes (Figure 7). This demonstrates

Figure 5. Molecular structures of 9a−d. For clarity only selected hydrogen atoms and only the ipso-carbon atoms of the arene groups are shown;disorder present in the structures of 9a,b,d is omitted.16 Thermal ellipsoids are drawn at the 50% probability level; an adopted numbering schemewas used.

Scheme 5. Reaction of the Copper Enolates 9a,b,d with 1 tothe Boron O-Enolates 4a,b,d and 6

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that the copper C-enolate 9f does not efficiently react with 1(assuming that the traces of 5f observed are due to adventitiousmoisture). Hence, under the catalytic conditions employed hereand in agreement with a computational study on the relateddiboration reaction, efficient conversion is not possible in theabsence of an alcohol, as the catalytic cycle rests at the stage ofthe copper C-enolate.7 In accordance with this and excludingpossible side reactions, further NMR experiments show that theCu−Si complex 6 does not efficiently react with iPrOH and arein agreement with the formation of 5f and (IDipp)CuOiPrupon reaction of 9f with iPrOH.16

■ CONCLUSION

Six exemplary α,β-unsaturated carbonyl/carboxyl compounds(3a−f) are efficiently β-silylated by the silyl boronic esterpinB−SiMe2Ph (1) in the presence of catalytic amounts of(IDipp)Cu−SiMe2Ph (6). Depending on the steric andelectronic properties of the substrates, the reaction gives

Figure 6. Details of in situ 1H NMR spectra of the reaction of 9a,b,d with excess 1 (■) giving 6 (●) and 4a,b,d at room temperature after 2 h (300MHz, rt, C6D6). Signals of the E isomers of 4a,d are denoted by a prime (×: 5a); † denotes that only a part of the signal is visible due to overlapping.

Figure 7. Details of the in situ 1H NMR spectrum of the reaction of 9fwith 1 (■) in the absence and presence of iPrOH (300 MHz, rt, C6D6,× : pinB−OiPr; □: iPrOH; ●: 6).

Scheme 6. Generalized Catalytic Cycles for the CuI-Catalyzed Conjugate Silyl Addition to Carbonyl/Carboxyl Compoundsa

aIsolated species are denoted by their respective identifiers (species in parentheses suggested by NMR data only).

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different products and/or pursues different reaction pathways.Using the (IDipp)Cu-based catalyst as a model system, thedifferent pathways were studied in detail, isolating centralintermediates, retracing the different catalytic cycles stepwise bystoichiometric experiments, and monitoring the individual stepsby in situ NMR spectroscopy. In conclusion three different, yetclosely related, catalytic cycles for the Cu-catalyzed silylation ofcarbonyl/carboxyl compounds can be deduced (Scheme 6).In agreement with previously proposed catalytic cycles and

our findings on the silylation of alkyl/aryl aldehydes and CO2,the Cu-silyl complex (IDipp)Cu−SiMe2Ph (6) is initiallyformed as a catalytically active species.5 This complex reactsefficiently with the α,β-unsaturated substrates by insertion intothe Cu−Si bond (9a−d,f). The following insertion products areobserved (Scheme 6): (a) for α,β-unsaturated ketones the 1,4-addition product, a copper O-enolate, is formed; (b) for α,β-unsaturated aldehydes, depending on the β-substitution pattern,either the 1,4-addition product as for ketones (one β-substituent) or the 1,2-addition product (two β-substituents)is predominantly formed; (c) for α,β-unsaturated esters the 3,4-addition product, a copper C-enolate, is formed. While theisolated 1,4- and 3,4-insertion products alone allow noconclusions on the mechanism of the insertion reaction, thefindings agree with a computational study on the closely relatedborylation of α,β-unsaturated substrates. Here generally a 3,4-addition of a Cu−boryl species onto the α,β-unsaturatedcompound is suggested. This primary 3,4-insertion productthen rearranges for aldehydes (and presumably ketones) in asubsequent reaction step via a keto-to-enol isomerization to thethermodynamically favored 1,4-addition product (copper O-enolate).7

For the conditions and the model catalyst employedconversion of the insertion product to the primary catalyticreaction products 4a−d accompanied by regeneration of theCu−Si species 6 and hence completion of the catalytic cycleupon reaction with the silyl boronic ester 1 is effective only forthe copper O-enolates 9a−d. It is ineffective for the copper C-enolate 9f, and, hence, the ester 3f is not reactive under thecatalytic conditions described. However, in the presence of astoichiometric amount of iPrOH alcoholysis of the C-enolate 9fto the β-silylated ester 5f and a Cu-alkoxido complex is facile,and the latter regenerates efficiently 6 upon reaction with 1.Hence in the latter reaction the β-silylated ester (5f) is obtainedas the primary reaction product, while for aldehydes andketones the β-silylated carbonyls (5a−d) are obtained onlyafter hydrolytic workup.An exception to this general scheme is the bis-β-substituted

aldehyde 3e. Here, presumably due to steric reasons, the 1,2-addition product is predominantly formed, and only a minoramount of the 1,4-addition product is observed. Still, catalyticconversion is effective but leading to α-silyl alcohols, asreported for alkyl and aryl aldehydes.5a

While experimental investigations are inevitably exemplaryand strictly valid only for the particular system studied, thefindings discussed may be carefully generalized, as they agreewell with a computational study on the related conjugateaddition reactions of tetraalkoxidodiboron reagents.7 So ourfindings may contribute to the understanding and furtherdevelopment of CuI-catalyzed conjugate silyl additions andother related transformations.

■ EXPERIMENTAL SECTIONGeneral Considerations. pinB−SiMe2Ph (1), (IDipp)CuOtBu

(2), and (IDipp)CuSiMe2Ph (6) were prepared according to literatureprocedures.5b,19 All other compounds were commercially available andwere used as received; their purity and identity was checked byappropriate methods. All α,β-unsaturated compounds 3a−f werecommercially available andif applicablepredominantly (>95%)the E isomer. All solvents were dried using a solvent purificationsystems, deoxygenated using the freeze−pump−thaw method, andstored under purified nitrogen. All manipulations were performedusing standard Schlenk techniques under an atmosphere of purifiednitrogen or in a nitrogen-filled glovebox. Air-sensitive samples weremeasured in NMR tubes equipped with screw caps. Chemical shifts(δ) are given in ppm, using the residual resonance signal of thesolvents (C6D6: 99.5% deuteration, 1H NMR: 7.16 ppm, 13C NMR:128.06 ppm).20 11B chemical shifts are reported relative to externalBF3·Et2O.

11B NMR spectra were processed applying a back linearprediction in order to suppress the broad background signal due to theborosilicate glass of the NMR tube and a Lorentz-type windowfunction (LB = 10 Hz); the spectra were carefully evaluated to ensurethat no genuinely broad signals of the sample were suppressed. Ifnecessary 2D NMR techniques were employed to establishconnectivity and to assign the individual signals (1H−1H NOESY (1s mixing time), 1H−1H COSY, 1H−13C HSQC, and 1H−13C HMBC).Complicated coupling patterns were analyzed with the aid ofsimulations. The same numbering scheme as in Figures 1−4 and 6/7 is used in the experimental part. Melting points were determined inflame-sealed capillaries under nitrogen and are not corrected.Elemental analyses were performed at the Institut fur Anorganischeand Analytische Chemie of the Technische Universitat Carolo-Wilhelmina zu Braunschweig. GC/MS measurements were performedin positive EI mode (70 eV, 60−700 m/z) with the followingconditions: injection temperature 250 °C; interface temperature 280°C; temperature program: start temperature 50 °C for 3 min, heatingrate 12 °C min−1, end temperature 300 °C for 8 min; column type: 5%phenyl-arylene/95% dimethylpolysiloxane, 30 m × 0.25 mm, 0.25 μmfilm thickness; He carrier gas (1.5 mL min−1). For HRMSmeasurements a time-of-flight mass spectrometer operating in EImode (70 eV) coupled to a gas chromatograph was used. Infrared (IR)spectra were recorded using an ATR unit. Analytical thin-layerchromatography (TLC) was performed on precoated silica gelpolyester sheets; flash column chromatography (⦶ 3 cm × 20 cm)on silica gel 60.

Preparation of 4a,b,d and 7 (General Procedure). In anitrogen-filled glovebox 2 (18 mg, 34 μmol, 3 mol %) and 1 (300 mg,1.14 mmol, 1.0 equiv) were mixed in dry toluene (5 mL). The α,β-unsaturated substrate (1.14 mmol, 1.0 equiv) was added, and themixture stirred at room temperature. The progress of the reaction wasmonitored by GC/MS analysis of an aliquot of the reaction mixture(0.05 mL) after dilution with Et2O (2 mL) and filtration over Celite.After complete consumption of 1 the solvent and excess α,β-unsaturated carbonyl substrate were evaporated at room temperatureunder vacuum (10−3 mbar), and the residue was extracted with n-pentane (2 × 2 mL) and filtered over Celite, in order to removeinsoluble NHC-Cu complexes, to give a clear solution. Evaporation ofthe solvent and possibly residual α,β-unsaturated carbonyl substrate atroom temperature under vacuum (10−3 mbar) yielded the reactionproducts.

pinB-O-C6H10-SiMe2Ph (4a): hex-4-en-3-one (3a) (112 mg, 1.14mmol); 3 h; pale yellow, sticky oil (357 mg, 0.99 mmol, 87%). Despiteall efforts the contamination of 4a with a small amount (<5 mol %) ofthe hydrolysis product 5a was unavoidable according to 1H NMRspectroscopy, likely due to adventitious moisture. 1H NMR (600 MHz,C6D6, rt): δ (Z isomer) 7.55−7.53 (m, 2 H, o-CHPh), 7.22−7.19 (m, 3H, m,p-CHPh), 4.49 (dt, 1 H, 3JHH = 10.9 Hz, 4JHH = 1.0 Hz, 2-CH),2.38 (dq, 3JHH = 10.9 Hz, 3JHH = 7.3 Hz, 1 H, 3-CH), 2.28 (qt, 3JHH =7.5 Hz, 4JHH = 1.0 Hz, 2 H, 1-CH2), 1.10 (d, 3JHH = 7.3 Hz, 3 H, 4-CH3), 1.06 (bs, 12 H, 6-CH3), 1.03 (t,

3JHH = 7.5 Hz, 3 H, 1a-CH3), 2× 0.33 (s, 3 H, 5-CH3); δ (E isomer) 7.58−7.56 (m, 2 H, o-CHPh),

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7.25−7.21 (m, 3 H, m,p-CHPh), 4.98 (d, 1 H,3JHH = 11.6 Hz, 2′-CH),

2.19−2.13 (m, 1 H, 1′-CH2), 2.10−2.05 (m, 1 H, 1′-CH2), 1.71 (dq,3JHH = 11.6 Hz, 3JHH = 7.3 Hz, 1 H, 3′-CH), 1.01 (d, 3JHH = 7.3 Hz, 3H, 4′-CH3), 1.06 (s, 12 H, 6′-CH3), 1.03−1.00 (m, 3 H, 1a′-CH3),0.17 (s, 3 H, 5-CH3), 0.16 (s, 3 H, 5-CH3). The signals of 1, 1a, 1′, 1a′,and m,p-CHPh overlap with signals of 4a or 5a. 11B{1H} NMR (96MHz, C6D6, rt): δ 22.0 (pinB-O).

13C{1H} NMR (150 MHz, C6D6,rt):δ (Z isomer) 149.5 (C-O), 138.3 (ipso-CPh), 134.5 (o-CHPh), 129.1(m/p-CHPh), 128.8 (m/p-CHPh), 112.2 (2-CH), 82.9 (C(CH3)2), 28.4(1-CH2), 19.2 (3-CH), 24.7 (C(CH3)2), 24.6 (C(CH3)2), 15.5 (4-CH3), 11.1 (1a-CH3), −3.8 (5-CH3), −5.2 (5-CH3); δ (E isomer)149.3 (C-O), 138.1 (ipso-CPh), 134.7 (o-CHPh), 113.6 (2′-CH), 23.8(1′-CH2), 20.7 (3′-CH), 16.2 (4′-CH3), −4.6 (5′-CH3), −5.1 (5′-CH3). The m/p-CHPh, 1a′-CH3, C(CH3)2, and C(CH3)2 signals couldnot be assigned unambiguously and are not given. MS (GC/MS, EI):360 [M]+, 331 [M − C2H5]

+, 135 [SiMe2Ph]+. Anal. Calcd for

C20H33BO3Si: C, 66.66; H, 9.23. Found: C, 66.54; H, 8.98.pinB-O-C6H10-SiMe2Ph (4b): mesityl oxide (3b) (112 mg, 1.14

mmol); 6 h; pale yellow oil (275 mg, 0.76 mmol, 67%). 1H NMR (600MHz, C6D6, rt): δ 7.57−7.54 (m, 2 H, CHPh), 7.25−7.20 (m, 3 H,CHPh), 4.37 (q, JHH = 1.0 Hz, 1 H, 2-CH), 1.94 (d, JHH = 1.0 Hz, 3 H,1-CH3), 1.29 (s, 6 H, 4-CH3), 1.05 (s, 12 H, 6-CH3), 0.42 (s, 6 H, 5−CH3).

11B{1H} NMR (96 MHz, C6D6, rt): δ 21.8 (pinB-O). 13C{1H}NMR (150 MHz, C6D6, rt): δ 144.6 (C-O), 138.1 (ipso-CPh), 135.4(o/m-CHPh), 129.1 (p-CHPh), 128.1 (o/m-CHPh), 117.3 (2-CH), 82.9(C(CH3)2), 25.0 (4-CH3), 24.6 (C(CH3)2), 24.4 (3-C), 22.6 (1-CH3),−5.2 (5-CH3). MS (GC/MS, EI): 360 [M]+, 345 [M − CH3]

+, 135[SiMe2Ph]

+. Anal. Calcd for C20H33BO3Si: C, 66.66; H, 9.23. Found:C, 66.53; H, 9.07.pinB-O-C4H6-SiMe2Ph (4d): crotonaldehyde (3d) (80 mg, 1.14

mmol); 2 h; colorless, highly viscous oil (350 mg, 1.06 mmol, 93%).1H NMR (600 MHz, C6D6, rt): δ (Z isomer) 7.51−7.49 (m, 2 H, o-CHPh), 7.24−7.18 (m, 3 H, m,p-CHPh (overlapping with signal of Eisomer)), 6.74 (dd, 3JHH = 6.0 Hz, 4JHH = 1.0 Hz, 1 H, 1-CH), 4.44(dd, 3JHH = 10.8 Hz, 3JHH = 6.0 Hz, 1 H, 2-CH), 2.66 (dqd, 3JHH = 10.8Hz, 3JHH = 7.3 Hz, 4JHH = 1.0 Hz, 1H, 3-CH), 1.02 (d, JHH = 7.4 Hz, 3H, 4-CH3), 1.00 (s, 12 H, 6-CH3), 0.31 (s, 3 H, 5-CH3), 0.30 (s, 3 H,5-CH3); δ (E isomer) 7.44−7.42 (m, 2 H, o-CHPh), 7.24−7.18 (m, 3H, m,p-CHPh (overlapping with signal of Z isomer)), 6.72 (dd, 3JHH =11.9 Hz, 4JHH = 1.3 Hz, 1 H, 1′-CH), 5.51 (dd, 3JHH = 11.9 Hz, 3JHH =8.9 Hz, 1 H, 2′-CH), 1.57 (dqd, 3JHH = 8.9 Hz, 3JHH = 7.2 Hz, 4JHH =1.3 Hz, 1 H, 3′-CH), 1.00 (s, 12 H, 6-CH3), 0.97 (d, JHH = 7.2 Hz, 3H, 4′-CH3), 0.19 (s, 3 H, 5′-CH3), 0.18 (s, 3 H, 5′-CH3).

11B{1H}NMR (96 MHz, C6D6, rt): δ 22.0 (pinB-O). 13C{1H} NMR (150MHz, C6D6,rt): δ (Z isomer) 138.1 (ipso-C), 135.8(1-CH), 134.5 (o-CHPh), 129.1 (m/p-CHPh), 128.0 (m/p-CHPh), 115.6 (2-CH), 83.1(C(CH3)2), 24.7 (C(CH3)2), 18.4 (3-CH), 15.3 (4-CH), −4.2 (5-CH3), −5.4 (5-CH3); δ (E isomer) 137.8 (ipso-C), 137.6 (1′-CH),134.4 (o-CHPh), 129.2 (m/p-CHPh), 128.0 (m/p-CHPh), 116.0 (2′-CH), 83.1 (C(CH3)2), 24.6 (C(CH3)2), 20.8 (3′-CH), 14.6 (4′-CH),−4.6 (5-CH3), −5.5 (5-CH3). MS (GC/MS, EI): 332 [M]+, 317 [M −CH3]

+, 135 [SiMe2Ph]+. Anal. Calcd for C18H29BO3Si: C, 65.06; H,

8.80. Found: C, 65.16; H, 8.51.pinB-O-C5H8-SiMe2Ph (7): 3-methylcrotonaldehyde (3e) (96 mg,

1.14 mmol); 16 h; from the n-pentane extract after filtration overCelite colorless crystals separated that were dried in vacuo (365 mg,1.05 mmol, 92%). 1H NMR (300 MHz, C6D6, rt): δ 7.65−7.61 (m, 2H, o−CHPh), 7.22−7.17 (m, 3 H, m,p-CHPh), 5.52 (d of v sept.,

3JHH =10.1 Hz, J = 1.3 Hz, 1 H, 2-CH), 5.13 (d, 3JHH = 10.1 Hz, 1 H 1-CH),1.55−1.54 (m, 3 H, 3/4-CH3), 1.44 (d,

3JHH = 1.3 Hz, 3 H, 3/4-CH3),1.02 (s, 6 H, 6-CH3), 1.01 (s, 6 H, 6-CH3), 0.40 (s, 3 H, 5-CH3), 0.35(s, 3 H, 5-CH3).

11B{1H} NMR (96 MHz, C6D6, rt): δ 22.7 (pinB-O).13C{1H} NMR (75 MHz, C6D6, rt): δ 137.0 (ipso-C), 134.8 (CHPh),131.6 (C(CH3)2), 129.4 (CHPh), 127.9 (CHPh), 125.2 (2-CH), 82.3(C(CH3)2), 67.7 (1-CH), 25.9 (3/4-CH3), 24.8 (C(CH3)2), 24.7(C(CH3)2), 18.5 (3/4-CH3), −5.3 (5-CH3), −5.6 (5-CH3). Mp: 57−60 °C. MS (GC/MS, EI): 346 [M]+, 331 [M − CH3]

+, 135[SiMe2Ph]

+. Anal. Calcd for C19H31BO3Si: C, 65.89; H, 9.02. Found:C, 65.57; H, 9.04.

Preparation of 5a−d,f and 8 (General Procedure). In anitrogen-filled glovebox 2 (12 mg, 23 μmol, 6 mol %) and 1 (100 mg,0.38 mmol, 1.0 equiv) were mixed in dry toluene (2 mL). The α,β-unsaturated substrate (0.38 mmol, 1.0 equiv) was added, and themixture stirred at room temperature. The progress of the reaction wasmonitored by GC/MS analysis of an aliquot of the reaction mixture(0.05 mL) after dilution with Et2O (2 mL) and filtration over Celite.After complete conversion the solvent was evaporated and the residuepurified on air by flash column chromatography using the indicatedeluents. Compounds 5a−d,f were reported earlier, and thespectroscopic data match the literature.

(C2H5)C(O)CH2CH(SiMe2Ph)CH3 (5a): hex-4-en-3-one (3a) (37mg, 0.38 mmol); 3 h; n-pentane/EtOAc (8:1); colorless oil (69 mg,0.29 mmol, 77%). 1H NMR (300 MHz, CDCl3, rt): δ 7.52−7.46 (m, 2H, CHPh), 7.39−7.32 (m, 3 H, CHPh), 2.45−2.23 (m, 3 H, OC-CH2), 2.17 (dd, 3JHH = 10.8 Hz, 3JHH = 16.2 Hz, 1 H, OC-CH2),1.60 (bs, 1 H, OH), 1.58−1.46 (m, 1 H, Si-CH), 1.00 (t, 3JHH = 7.3Hz, 3 H, CH2-CH3), 0.91 (d,

3JHH = 7.3 Hz, 3 H, CH-CH3), 0.27 (s, 6H, Si(CH3)2).

4d 1H NMR (300 MHz, C6D6, rt): δ 7.45−7.39 (m, 2 H,CHPh), 7.24−7.19 (m, 3 H, CHPh), 2.1 (dd,

3JHH = 4.0 Hz, 3JHH = 16.4Hz, 1 H, OC-CH2), 1.98−1.72 (m, 3 H, OC-CH2), 1.68−1.54(m, 1 H, Si-CH), 0.91 (d, 3JHH = 7.3 Hz, 3 H, CH-CH3), 0.89 (t, 3JHH= 7.3 Hz, 3 H, CH2-CH3), 2 × 0.17 (s, 6 H, Si(CH3)2). MS (GC/MS,EI): 234 [M]+, 219 [M − CH3]

+, 205 [M − C2H5]+, 135 [SiMe2Ph]

+.H3CC(O)CH2CH(SiMe2Ph)(CH3)2 (5b): mesityl oxide (3b) (37.4

mg, 0.38 mmol); 8 h; n-pentane/Et2O (5:1); colorless oil (77 mg, 0.33μmol, 87%). 1H NMR (300 MHz, CDCl3, rt): δ 7.52−7.47 (m, 2 H,CHPh), 7.38−7.32 (m, 3 H, CHPh), 2.29 (s, 2 H, CH2), 2.04 (s, 3 H,CH3), 1.04 (s, 6 H, CH3), 0.32 (s, 6 H, Si(CH3)2).

21a MS (GC/MS,EI): 234 [M]+, 219 [M − CH3]

+, 135 [SiMe2Ph]+.

OC6H9SiMe2Ph (5c): cyclohexenone (3c) (37 mg, 0.38 mmol); 6h; n-pentane/Et2O (8:1); colorless oil (73 mg, 0.31 μmol, 82%). 1HNMR (300 MHz, CDCl3, rt): δ 7.50−7.47 (m, 2 H, CHPh), 7.40−7.34(m, 3 H, CHPh), 2.42−2.04 (m, 5 H, CH/CH2), 1.87−1.60 (m, 2 H,CH/CH2), 1.50−1.18 (m, 2 H, CH/CH2), 2 × 0.31 (s, 6 H,Si(CH3)2).

21b MS (GC/MS, EI): 232 [M]+, 217 [M − CH3]+, 156,

135 [SiMe2Ph]+.21b

HC(O)CH2CH(SiMe2Ph)CH3 (5d): crotonaldehyde (3d) (27 mg,0.38 mmol); 2 h; n-hexane/EtOAc (5:1); yellow oil (53 mg, 0.26mmol, 68%). 1H NMR (300 MHz, CDCl3, rt): δ 9.67 (dd, 4JHH = 3.3Hz, 4JHH = 1.2 Hz, 1 H, OCH), 7.52−7.46 (m, 2 H, CHPh), 7.40−7.33 (m, 3 H, CHPh), 2.43 (ddd,

2JHH = 16.2 Hz, 3JHH = 3.6 Hz, 4JHH =1.2 Hz, 1 H, OC-CH2), 2.15 (ddd, 2JHH = 16.2 Hz, 3JHH = 10.9 Hz,3JHH = 3.3 Hz, 1 H, OC-CH2), 1.51 (dqd, 3JHH = 10.9 Hz 3JHH = 7.3Hz, 3JHH = 3.6 Hz, 1 H, Si-CH), 0.98 (d, 3JHH = 7.3 Hz, 3 H, CH3), 2× 0.30 (d, 6 H, Si(CH3)2).

4g,21a MS (GC/MS, EI): 191 [M − CH3]+,

135 [SiMe2Ph]+.

HO−HC(SiMe2Ph)CHC(CH3)2 (8): 3-methylcrotonaldehyde (3e) (32mg, 0.38 mmol); 16 h; n-hexane/Et2O (5:1); colorless oil (70 mg, 0.32mmol, 84%). 1H NMR (300 MHz, C6D6, rt): δ 7.63−7.56 (m, 2 H,CHPh), 7.25−7.18 (m, 3 H, CHPh), 5.29 (dsept., 3JHH = 10.1 Hz, 4JHH= 1.4 Hz, 1 H, OC(H)-C(H)), 4.25 (d, 3JHH = 10.1 Hz, 1 H,OC(H)-C(H)), 1.57−1.55 (bs, 3 H, CH3), 1.38 (bs, 1H, OH), 1.29(bd, 4JHH = 1.4 Hz, 3 H, CH3), 0.35 (s, 3 H, Si(CH3)2), 0.32 (s, 3 H,Si(CH3)2).

13C{1H} NMR (75 MHz, C6D6, rt): δ 137.3 (ipso-CPh),134.7 (CHPh), 131.2 (C(CH3)2), 129.4 (CHPh), 128.0 (CHPh),126.8 (OC(H)-C(H)), 64.1 (OC(H)-C(H)), 25.9 (C(CH3)2),18.3 (C(CH3)2), −5.3 (Si(CH3)2), −5.5 (Si(CH3)2). IR (liquid): ν3382, 3069, 2963, 2911, 2856, 1662, 1427, 1375, 1247, 1113, 971, 831,812, 776, 734, 698, 642. MS (GC/MS, EI): 220 [M]+, 205 [M −CH3]

+, 135 [SiMe2Ph]+. HRMS (GC/MS, EI): calcd for C13H20OSi

220.1283, found 220.1267.OC(OMe)CH2CH(SiMe2Ph)CH3 (5f). The reaction was conducted

as described for 5a−d and 8, but additionally iPrOH (29 μL, 23 mg,0.38 mmol) was added after combining 3f, 1, and 2: methylcrotonate(3f) (38 mg, 0.38 mmol); 4 h; n-hexane/Et2O (8:1); colorless oil (71mg, 0.30 mmol, 79%). 1H NMR (300 MHz, CDCl3, rt): δ 7.52−7.46(m, 2 H, CHPh), 7.39−7.33 (m, 3 H, CHPh), 3.61 (s, 3 H, OCH3), 2.39(dd, 3JHH = 15.2 Hz, 3JHH = 4.2 Hz, 1 H, CH2), 2.06 (dd, 3JHH = 15.2Hz, 3JHH = 11.2 Hz, 1 H, CH2), 1.44 (dqd, 3JHH = 11.2 Hz 3JHH = 7.6

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Hz, 3JHH = 4.2 Hz, 1 H, CH), 0.97 (d, 3JHH = 7.6 Hz, 3 H, CH3), 0.28(s, 6 H, Si(CH3)2).

21c 1H NMR (300 MHz, C6D6, rt): δ 7.41−7.35 (m,2 H, CHPh), 7.20−7.15 (m, 3 H, CHPh), 3.32 (s, 3 H, OCH3), 2.33(dd, 3JHH = 15.3 Hz, 3JHH = 4.2 Hz, 1 H, CH2), 2.02 (dd, 3JHH = 15.3Hz, 3JHH = 10.8 Hz, 1 H, CH2), 1.53 (dqd, 3JHH = 10.8 Hz, 3JHH = 7.3Hz, 3JHH = 4.2 Hz, 1 H, CH), 0.97 (d, 3JHH = 7.3 Hz, 3 H, CH3), 0.13(s, 3 H, Si(CH3)2), 0.12 (s, 3 H, Si(CH3)2). MS (GC/MS, EI): 236[M]+, 221 [M − CH3]

+, 205 [M − OCH3]+, 159 [M − Ph]+, 135

[SiMe2Ph]+.21c

Preparation of 9a-d,f (General Procedure). In a nitrogen-filledglovebox 6 (50 mg, 85 μmol, 1 equiv) was dissolved in toluene (2mL), and a small excess of the α,β-unsaturated carbonyl compound3a−d,f was added. After the given reaction time at room temperaturethe mixture was layered with n-pentane (approximately 2 mL) andstored at −40 °C. After separation of a (crystalline) solid thesupernatant solution was decanted and the residue washed with n-pentane (approximately 2 × 1 mL) and dried in vacuo.(IDipp)Cu-O-C6H10-SiMe2Ph (9a): hex-4-en-3-one (3a) (9 mg, 92

μmol, 1.1 equiv); 3 h; light brown microcrystals (64 mg, 74 μmol,87%). 1H NMR (600 MHz, C6D6, rt): δ 7.74−7.70 (m, 2 H, o-CHPh),7.28−7.22 (m, 3 H, m,p-CHPh), 7.19 (t, JHH = 7.8 Hz, 2 H, p-CHDipp),7.04 (d, JHH = 7.8 Hz, 4 H, m-CHDipp), 6.25 (s, 2 H, NCH), 3.97 (d,JHH = 9.5 Hz, 1 H, 2-CH), 2.72 (dq, 3JHH = 9.5 Hz, 3JHH = 7.5 Hz, 2 H,3-CH), 2.54 (sept., 3JHH = 6.9 Hz, 4 H, CH(CH3)2), 1.96 (q, 3JHH =15.0 Hz, 3JHH = 7.5 Hz, 1 H, 1-CH2), 1.93 (dq,

3JHH = 16.0 Hz, 1 H, 1-CH2), 2 × 1.36 (d, JHH = 6.9 Hz, 6 H, CH(CH3)2), 1.14 (d, JHH = 7.6Hz, 3 H, 4-CH3), 1.06 (bd, 3JHH = 6.9 Hz, 12 H, CH(CH3)2), 1.02 (t,3JHH = 7.5 Hz, 3 H, 1a-CH3), 0.41 (s, 3 H, Si(CH3)2), 0.39 (s, 3 H,Si(CH3)2).

13C{1H} NMR (150 MHz, C6D6, rt): δ 182.8 (CCarbene),162.1 (O-CCH), 145.7 (ipso-CDipp), 142.1 (ipso-CPh), 135.2 (ipso-NCDipp), 134.8 (o-CHPh), 130.7 (p-CHDipp), 128.2 (m/p-CHPh), 127.6(m/p-CHPh), 124.4 (m-CHDipp) 122.6 (NCH), 95.5 (2-CH), 35.5 (1-CH2), 29.0 (CH(CH3)2), 24.9 (CH(CH3)2), 23.9 (CH(CH3)2), 17.8(3-CH), 16.8 (4-CH3), 13.6 (1a-CH3), −2.7 (Si(CH3)2), −4.8(Si(CH3)2). Mp: 110−114 °C. Anal. Calcd for C41H57CuN2OSi: C,71.83; H, 8.38; N, 4.09. Found: C, 69.56; H, 8.23; N, 4.17. Repeatedelemental analysis for different, independently prepared samples failedto give more satisfactory results; all samples gave consistent values butwere low in carbon. It may be speculated that this is caused by SiCand/or carbide/carbonate formation.22

(IDipp)Cu-O-C6H10-SiMe2Ph (9b): mesityl oxide (3b) (9 mg, 92μmol, 1.1 equiv); 20 h; off-white microcrystals (39 mg, 57 μmol, 67%).1H NMR (600 MHz, C6D6, rt): δ 7.78−7.74 (m, 2 H, o-CHPh), 7.26−7.18 (m, 3 H, m,p-CHPh), 7.20 (t, 3JHH = 7.8 Hz, 2 H, p-CHDipp), 7.05(d, JHH = 7.8 Hz, 4 H, m-CHDipp), 6.26 (s, 2 H, NCH), 3.87 (bs, 1 H,2-CH), 2.53 (sept., 3JHH = 6.9 Hz, 4 H, CH(CH3)2), 1.59 (s, 3 H, 1-CH), 1.36 (d, 3JHH = 6.9 Hz, 12 H, CH(CH3)2), 1.36 (s, 6 H, 4-CH3),1.06 (d, 3JHH = 6.9 Hz, 12 H, CH(CH3)2), 0.52 (s, 3 H, Si(CH3)2).13C{1H} NMR (150 MHz, C6D6, rt): δ 182.7 (CCarbene), 157.0 (O-CCH), 145.7 (ipso-CDipp), 142.6 (ipso-CPh), 135.2 (o-CHPh), 135.1 (ipso-NCDipp), 130.7 (p-CHDipp), 127.9 (m/p-CHPh), 127.3 (m/p-CHPh),124.3 (m-CHDipp), 122.6 (NCH), 102.6 (2-CH), 30.4 (1-CH2), 29.0(CH(CH3)2), 26.8 (4-CH3), 24.9 (CH(CH3)2), 23.9 (CH(CH3)2),22.6 (3-CH), −3.0 (Si(CH3)2). Mp: 128−134 °C. Anal. Calcd forC41H57CuN2OSi: C, 71.83; H, 8.38; N, 4.09. Found: C, 70.70; H, 8.29;N, 4.22. Repeated elemental analysis for different, independentlyprepared samples failed to give more satisfactory results; all samplesgave consistent values but were low in carbon. It may be speculatedthat this is caused by SiC and/or carbide/carbonate formation.22

(IDipp)Cu-O-C6H8-SiMe2Ph (9c): cyclohexenone (3c) (9 mg, 94μmol, 1.1 equiv); 1 h; crystallization at −78 °C, colorless microcrystals(36 mg, 53 μmol, 62%). 1H NMR (600 MHz, C6D6, rt): δ 7.64−7.60(m, 2 H, o-CHPh), 7.25−7.21 (m, 3 H, m,p-CHPh), 7.20 (t, 3JHH = 7.7Hz, 2 H, p-CHDipp), 7.04 (d, JHH = 7.7 Hz, 4 H, m-CHDipp), 6.23 (s, 2H, NCH), 4.56 (bs, 1 H, O−CCH), 2 × 2.53 (sept., 3JHH = 6.9 Hz,2 H, CH(CH3)2), 2.05−1.97 (m, 2 H, CH-Si and CH2), 1.94−1.89(m, 1 H, CH2), 1.79−1.69 (m, 2 H, CH2), 1.66−1.56 (m, 1 H, CH2),1.49−1.43 (m, 1 H, CH2), 1.34 (d, 3JHH = 6.9 Hz, 6 H, CH(CH3)2),1.33 (d, 3JHH = 7.0 Hz, 6 H, CH(CH3)2), 2 × 1.06 (d, 3JHH = 6.9 Hz, 6H, CH(CH3)2), 0.32 (s, 3 H, Si(CH3)2), 0.31 (s, 3 H, Si(CH3)2).

13C{1H} NMR (150 MHz, C6D6, rt): δ 182.8 (CCarbene), 161.0 (O-CCH), 2 × 145.7 (ipso-CDipp), 140.7 (ipso-CPh), 135.1 (ipso-NCDipp),134.6 (o-CHPh), 130.7 (p-CHDipp), 128.5 (m/p-CHPh), 127.8 (m/p-CHPh), 124.3 (m-CHDipp), 122.6 (NCH), 90.4 (O−CCH), 35.1(CH-Si), 29.0 (CH(CH3)2), 25.7 (CH2), 25.5 (CH2), 25.1 (CH-(CH3)2), 24.5 (CH2), 2 × 23.8 (CH(CH3)2), −3.7 (Si(CH3)2), −4.3(Si(CH3)2). Mp: 86−92 °C (dec). Anal. Calcd for C41H55CuN2OSi:C, 72.04; H, 8.11; N, 4.10. Found: C, 72.17; H, 8.36; N, 4.02.

(IDipp)Cu-O-C4H6-SiMe2Ph (9d): crotonaldehyde (3d) (7 mg, 100μmol, 1.2 equiv); 2 h; colorless microcrystals (41 mg, 62 μmol, 73%).1H NMR (600 MHz, C6D6, rt): (Z isomer) δ 7.72−7.69 (m, 2 H, o-CHPh), 7.26−7.21 (m, 3 H, m,p-CHPh), 7.19 (dd,

3JHH = 5.6 Hz, 4JHH =0.9 Hz, 1 H, 1-CH), 7.19 (t, 3JHH = 7.8 Hz, 2 H, p-CHDipp), 7.04 (d,3JHH = 7.8 Hz, 4 H, m-CHDipp), 6.23 (s, 2 H, NCH), 3.91 (dd, 3JHH =10.0 Hz, 3JHH = 5.6 Hz, 1 H, 2-CH), 2.74 (dqd, 3JHH = 10.0 Hz, 3JHH =7.5 Hz, 4JHH = 0.9 Hz, 1 H, 3-CH), 2 × 2.53 (sept, 3JHH = 6.8 Hz, 2 H,CH(CH3)2), 1.35 (d,

3JHH = 6.8 Hz, 6 H, CH(CH3)2), 1.34 (d,3JHH =

6.8 Hz, 6 H, CH(CH3)2), 1.09 (d,3JHH = 7.5 Hz, 3 H, 4-CH), 1.05 (d,

3JHH = 7.8 Hz, 12 H, CH(CH3)2), 0.40 (s, 6 H, Si(CH3)2); (E isomer)δ 7.63−7.60 (m, 2 H, o-CHPh), 7.26−7.21 (m, 3 H, m,p-CHPh), 7.17 (1H, 1′-CH, overlapping with signals of Z isomer), 6.23 (s, 2 H, NCH,overlapping with signal of Z isomer), 3.98 (dd, 3JHH = 11.1 Hz, 3JHH =9.4 Hz, 1 H, 2′-CH), 1.74 (dqd, 3JHH = 9.4 Hz, 3JHH = 7.6 Hz,4JHH =0.6 Hz, 1 H, 3′-CH), 1.13 (d, 3JHH = 7.6 Hz, 3 H, 4′-CH), 0.33 (s, 3 H,Si(CH3)2), 0.31 (s, 3 H, Si(CH3)2), the signals of the Dipp moietywere not independently assigned due to overlapping with the signals ofthe Z isomer. 13C{1H} NMR (150 MHz, C6D6, rt): (Z isomer) δ 182.6(CCarbene), 151.4(O-CCH), 145.7 (ipso-CDipp), 142.0 (ipso-CPh),135.0 (ipso-NCDipp), 134.9 (o-CHPh), 130.8 (p-CHDipp), 128.3 (m/p-CHPh), 127.6 (m/p-CHPh), 124.4 (br, m-CHDipp), 122.6 (NCH), 101.4(2-CH), 29.0 (CH(CH3)2), 25.0 (CH(CH3)2), 24.9 (CH(CH3)2), 23.8(CH(CH3)2), 23.7 (CH(CH3)2), 16.8 (4-CH3), 16.5 (3-CH), −3.1(Si(CH3)2), −4.5 (Si(CH3)2); (E isomer) δ 182.7 (CCarbene), 154.7 (1′-CH), 141.8 (ipso-CPh), 134.7 (o-CHPh), 128.5 (m/p-CHPh), 127.8 (m/p-CHPh), 100.1 (2′-CH), 20.1 (3′-CH), 18.0 (4′-CH3), −2.9(Si(CH3)2), −5.2 (Si(CH3)2), the signals of the Dipp moiety werenot independently assigned due to overlapping with the signals of theZ isomer. Mp: 94−99 °C. Anal. Calcd for C39H53CuN2OSi: C, 71.24;H, 8.13; N, 4.26. Found: C, 70.35; H, 8.07; N, 4.21. Repeatedelemental analysis for different, independently prepared samples failedto give more satisfactory results; all samples gave consistent values butwere low in carbon. It may be speculated that this is caused by SiCand/or carbide/carbonate formation.22

(MeO)OC-((IDipp)Cu)-C4H5-SiMe2Ph (9f): methyl crotonate (3f)(10 mg, 100 μmol, 1.2 equiv); 48 h; colorless solid (46 mg, 67 μmol,79%). 1H NMR (600 MHz, C6D6, rt): δ 7.59−7.55 (m, 2 H, o-CHPh),7.17−7.12 (m, 3 H, m,p-CHPh), 7.19 (t,

3JHH = 7.7 Hz, 2 H, p-CHDipp),7.07 (dd, 3JHH = 7.7 Hz, 3JHH = 1.3 Hz, 2 H, m-CHDipp), 7.06 (dd,

3JHH= 7.7 Hz, 3JHH = 1.3 Hz, 2 H, m-CHDipp), 6.30 (s, 2 H, NCH), 3.24 (s,3 H, 1-CH3), 2.55 (m, 3JHH = 7.0 Hz, 2 H, CH(CH3)2), 2.55 (m, 3JHH= 6.9 Hz, 2 H, CH(CH3)2), 2.35 (bd,

3JHH = 7.9 Hz, 1 H, 2-CH), 2.05(dq, 1 H, 3JHH = 7.9 Hz, 3JHH = 6.8 Hz, 1 H, 3-CH), 1.38 (d, 3JHH = 6.9Hz, 6 H, CH(CH3)2), 1.38 (d, 3JHH = 6.9 Hz, 6 H, CH(CH3)2), 1.07(d, 3JHH = 7.0 Hz, 12 H, CH(CH3)2), 0.88 (bd,

3JHH = 6.8 Hz, 1 H, 4-CH), 0.28 (s, 3 H, Si(CH3)2), 0.25 (bs, 3 H, Si(CH3)2).

13C{1H}NMR (150 MHz, C6D6, rt): δ 183.8 (CCarbene), 179.4 (O-CO),145.8 (ipso-CDipp), 141.4 (ipso-CPh), 135.4 (ipso-NCDipp), 134.5 (o-CHPh), 130.5 (p-CHDipp), 128.2 (m/p-CHPh), 127.6 (m/p-CHPh), 2 ×124.2 (br, m-CHDipp), 122.5 (NCH), 49.2 (1-CH), 37.6 (br, 2-CH),29.0 (CH(CH3)2), 24.9 (CH(CH3)2), 24.8 (CH(CH3)2), 24.0(CH(CH3)2), 23.9 (CH(CH3)2), 21.1 (4-CH), 19.8 (br, 3-CH3),−2.7 (br, Si(CH3)2), −5.2 (br, Si(CH3)2). Mp: dec >100 °C. Anal.Calcd for C40H55CuN2O2Si: C, 69.88; H, 8.06; N, 4.07. Found: C,69.72; H, 8.02; N, 4.21.

In Situ NMR Experiments: Catalytic Conjugate Silyl Additionto 3a,b,d,e,f (Figure 1−3). In a nitrogen-filled glovebox a screw capNMR tube was charged with 3a,b,d,e or f (57 μmol, 1.0 equiv), 1 (18mg, 68 μmol, 1.2 equiv), and 2 (3.3 mg, 5.7 μmol, 10 mol %) or 6 (3.0mg, 5.7 μmol, 10 mol %) and C6D6 (0.7 mL). 1H NMR spectra wererecorded after the given time intervals at rt: 3a: 5.6 mg, 6 h. 3b: 5.6, 20

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h. 3d: 4.0 mg, 2 h. 3e: 4.8 mg, 6 h. 3f: 5.7 mg, 8 h; a separate, identicalexperiment was performed under addition of iPrOH (approximately3.4 mg, 57 μmol, 1.0 equiv, 0.5 h).In Situ NMR Experiments: Formation of 9a,b,d,f (Figure 4).

In a nitrogen-filled glovebox a screw cap NMR tube was charged with6 (15 mg, 26 μmol, 1.0 equiv), 3a,b,d or f (26 μmol, 1.0 equiv), andC6D6 (0.7 mL). 1H NMR spectra were recorded after the given timeintervals at rt. 3a: 2.8 mg, 1.5 h. 3b: 2.8 mg, 16 h. 3d: 2.0 mg, 0.5 h. 3f:2.8 mg, 0.5 h.In Situ NMR Experiments: Formation of 4a,b,d/5f and 6 from

9a,b,d,f and 1 (Figure 6,7). In a nitrogen-filled glovebox a screw capNMR tube was charged with 9a,b,d or f (15 mg, 1 equiv), 1 (7 mg, 27μmol, 1.2 equiv), and C6D6 (0.7 mL).

1H NMR spectra were recordedafter 2 h at rt. 9a: 22 μmol. 9b: 22 μmol. 9d: 23 μmol. 9f: 22 μmol;after 2 h a 1H NMR spectrum was recorded, iPrOH (approximately 2mg, 3 μmol, 1.5 equiv) was added, and another 1H NMR spectrumwas recorded.

■ ASSOCIATED CONTENT*S Supporting InformationThe SI available contains additional NMR spectroscopic andcrystallographic data. This material is available free of charge viathe Internet at http://pubs.acs.org. Crystallographic data(excluding structure factors) for the structures reported inthis paper have been deposited with the Cambridge Crystallo-graphic Data Centre as supplementary publication nos. CCDC1019946−1019951. Copies of the data can be obtained free ofcharge on application to CCDC, 12 Union Road, CambridgeCB2 1EZ, UK [fax: +44(1223) 336-033; e-mail: deposit@ccdc.cam.ac.uk].

■ AUTHOR INFORMATIONCorresponding Author*E-mail: ch.kleeberg@tu-braunscheweig.de.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge financial support by the DeutscheForschungsgemeinschaft (DFG). C.K. thanks the Fonds derChemischen Industrie for a Liebig-Stipendium. The authorswish to thank Oliver Haslett (RISE program, DAAD) for helpwith the laboratory work.

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(14) The molecular structure of 7 has also been established by an X-ray structure analysis; see SI for details.(15) Crystallization of 9d proved especially challenging as repeatedlycrystals of (IDipp)CuPh were obtained from samples containing 9dand that were originally free of (IDipp)CuPh. Moreover, formation of(IDipp)CuPh was also observed in C6D6 solution after prolongedperiods of time. The intriguing observations regarding 9d, itsdecomposition to (IDipp)CuPh, the formation of 10, and theequilibrium of the E/Z isomersmay it be related or notaresubjects of ongoing studies.16 For the spectroscopic data of (IDipp)CuPh see ref 9d.(16) See SI for details.(17) In all cases but 9d the refinement of suitable split atom modelswas successful. Similarity restraints had to be employed and a commonADP was refined for each disordered atom pair. The refinedoccupancies of the main component were 9a: 0.950(1), 9b:0.811(1), and 9d(10)PhMe: 0.656(3). Only the geometrical data ofthe major component are discussed.16

(18) Wiberg, N. Holleman-Wiberg, Lehrbuch der AnorganischenChemie; Walter de Gruyter: Berlin/New York, 2007; p 2006.(19) (a) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics2004, 23, 3369−3371. (b) Hintermann, L. Beilstein J. Org. Chem. 2007,3, No. 22. (c) Jarkauskas, V.; Sadighi, J. P.; Buchwald, S. L. Org. Lett.2003, 5, 2417−2420. (d) Andersen, M. W.; Hildebrandt, B.; Koster,G.; Hoffmann, R. W. Chem. Ber. 1989, 122, 1777−1782. (e) Suginome,M.; Matsuda, T.; Ito, Y. Organometallics 2000, 19, 4647−4649.(20) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I.Organometallics 2010, 29, 2176−2179.(21) (a) Lipshutz, B. H.; Sclafani, J. A.; Takanami, T. J. Am. Chem.Soc. 1998, 120, 4021−4022. (b) Ito, H.; Ishizuka, T.; Tateiwa, J.;Sonoda, M.; Hosomi, A. J. Am. Chem. Soc. 1998, 120, 11196−11197.(c) Lipshutz, B. H.; Tanaka, N.; Taft, B. R.; Lee, C.-T. Org. Lett. 2006,8, 1963−1966.(22) (a) Culmo, R. F. Application Note Elemental Analysis; PerkinElmer, Inc.: Shelton, CT, 2013. (b) Gawargious, Y. A.; MacDonald, A.M. G. Anal. Chim. Acta 1962, 27, 300−302.

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