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Chemical Communications

www.rsc.org/chemcomm Volume 49 | Number 61 | 7 August 2013 | Pages 6815–6922

COMMUNICATIONUlrich Siemeling et al.Carbonylation of the simplest persistent diaminocarbene

6834 Chem. Commun., 2013, 49, 6834--6836 This journal is c The Royal Society of Chemistry 2013

Cite this: Chem. Commun.,2013,49, 6834

Carbonylation of the simplest persistentdiaminocarbene†‡

Tim Schulz,a Christian Farber,a Michael Leibold,a Clemens Bruhn,a

Wolfgang Baumann,b Detlef Selent,b Timo Porsch,c Max C. Holthausenc andUlrich Siemeling*a

The reactions of the acyclic diaminocarbenes (Me2N)2C and (Ph2N)-

(iPr2N)C with CO proceed in a 2 : 1 stoichiometric ratio, affording

unprecedented betainic oxyallyl species of type [(R2N)2C]2CO.

Acyclic diaminocarbenes (ADACs) are currently attracting increasedattention.1,2 In comparison to the more familiar N-heterocycliccarbenes (NHCs) they exhibit increased basicity, nucleophilicityand s-donicity.1e Furthermore, their wide N–C–N angle places theirsubstituents closer to the divalent carbon atom, which may beadvantageous for applications in catalysis.1b

The iconic ADAC is (iPr2N)2C (1a), which was reported by Alderand co-workers in 1996 as the first compound of this class to beisolated and structurally characterised.3 We recently demonstratedthat the reactivity and synthetic potential of ADACs have been muchunderestimated.4 For example, 1a reacts with carbon monoxideunder mild conditions, cleanly affording a b-lactam derivative.4b Thecorresponding reaction sequence shown in Scheme 1 is supportedby current high-level DFT calculations (B2GP-PLYP-D/def2-QZVP//B97-D/SVP, see ESI‡ for details).§ The primary carbonylation pro-duct is the transient diaminoketene 2a, which undergoes a remark-able follow-up reaction. An intramolecular nucleophilic attack of anamino group on the ketene unit leads to the (amino)(carboxamido)-carbene 3a, which subsequently affords the final product 4a byan intramolecular C–H activation process. The calculated freeactivation enthalpy DG‡ is moderately high for the first two steps(1a - 2a: 22.9 kcal mol�1; 2a - 3a: 21.7 kcal mol�1; 3a - 4a:17.6 kcal mol�1), which is nicely compatible with the fact that thereaction proceeds noticeably only at temperatures above ca.�20 1C.

The step 2a - 3a is a retro-Wolff rearrangement. Bona fide examplesof this reaction type are rare. Previously studied cases originate fromflash vacuum pyrolysis experiments and exhibit much higheractivation barriers.5

The surprising results obtained with 1a have prompted us to starta systematic investigation of the reaction of diaminocarbenes withCO. We have already addressed the addition of CO to carbenes in atheoretical study, which, however, did not cover any follow-upreactions of the primary ketene species.6 We here report on thereaction of CO with the simplest persistent diaminocarbene, viz.(Me2N)2C (1b). In contrast to the much bulkier 1a, this carbenecannot be isolated. Alder7 and Bertrand8 have described differentmethods for its generation, namely (i) the deprotonation of theformamidinium salt [1bH]Cl with a fairly large excess (2.6 equiv.) oflithium tetramethylpiperidide (LTMP) and (ii) the reaction of thechloroamidinium chloride [1bCl]Cl with Hg(SiMe3)2. In view of thehigh toxicity of mercury and its compounds, we chose the ‘‘Aldermethod’’ for our study, using 1.25 equivalents of LDA. A small excessof amide base proved useful for getting rid of protic impurities in thereaction system. Our protocol afforded 1b as a ca. 0.1 M solution inTHF-d8, showing slow dimerisation to (Me2N)2CQC(NMe2)2 with ahalf-life t1/2 of ca. 16 h at room temperature (t1/2 E 80 h at �40 1C).Lithium ions present in the solution have the dual effect of

Scheme 1 Carbonylation of 1a, leading to b-lactam 4a as the final product.

a Institute of Chemistry, University of Kassel, Heinrich-Plett-Str. 40, D-34132 Kassel,

Germany. E-mail: siemeling@uni-kassel.deb Leibniz Institute for Catalysis e. V. at the University of Rostock,

Albert-Einstein-Str. 29a, 18059 Rostock, Germanyc Institut fur Anorganische und Analytische Chemie, Johann Wolfgang Goethe-Universitat

Frankfurt, Max-von-Laue-Str. 7, D-60438 Frankfurt am Main, Germany

† Dedicated to Prof. Werner Uhl on the occasion of his 60th birthday.‡ Electronic supplementary information (ESI) available: Experimental, crystallographicand computational details, Fig. S1–S4. CCDC 916147 and 934512. For ESI andcrystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc42888e

Received 18th April 2013,Accepted 11th May 2013

DOI: 10.1039/c3cc42888e

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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 6834--6836 6835

coordinatively stabilising the carbene and catalysing its dimerisationin a way analogous to protons.9 Metal-free 1b generated by the‘‘Bertrand method’’ undergoes much faster, and unspecific, decom-position. The 13C NMR signal due to its divalent C atom is located atd = 259.7 ppm.8 Lithium coordination is reflected by a high-fieldshift of this signal, which we have observed at d = 253.1 ppm. Thevalue of 244.4 ppm reported by Alder7a reflects the comparativelyhigher Li+/1b ratio present in his system due to the much largerexcess of amide base. Two 13C NMR signals are observed for themethyl groups of 1b (d = 49.1 and 40.2 ppm) at �30 1C. At roomtemperature, however, the rotation of the Me2N units around therespective N–Ccarbene is rapid on the NMR time scale, leading to anaveraged signal at d = 44.5 ppm.

We have followed the carbonylation of 1b by in situ NMRspectroscopy. A stream of CO was bubbled through the carbenesolution kept in an NMR tube. In contrast to the bulkier 1a, which isessentially inert below ca. �20 1C,4b 1b turned out to react with COalready at �40 1C, affording an orange-red solution. The productgave rise to a single 1H NMR signal. Two signals were observed inthe 13C NMR spectrum. These features are not compatible with theb-lactam 4b, which was expected in analogy to the formation of 4afrom 1a. Soon after completion of the reaction, orange-red crystalsformed, which were suitable for a single-crystal X-ray diffractionstudy. The result (Fig. 1) shows that the reaction of 1b with CO leadsto the unusual betain [(Me2N)2C]2CO (5, Scheme 2), which crystal-lised as the trimeric lithium chloride adduct [{Li(m2-5)Cl}3].

The trimer exhibits molecular C2 symmetry. Its inorganic coreis a regular [Li–O]3 hexagon, with the twofold axis of rotationrunning through the diametrically opposed atoms O1 and Li2.Each Li atom is surrounded by two O atoms and one Cl atom in atrigonal-planar three-coordinate environment. This unusually lowcoordination number leads to extraordinarily short bondlengths.10 The average Li–O and Li–Cl distances are only 1.86 Åand 2.28 Å, respectively. The structural motif of an essentially

undistorted [Li–O]3 hexagon which shows no further aggregationis extremely rare. We are aware of only a single previous examplewith exclusively trigonal-planar ring atoms, viz. the lithium aryl-oxide [Li(OC6H2-2,6-iPr2)(THF)]3, whose average Li–O distance of1.83 Å is even marginally shorter than that of [{Li(m2-5)Cl}3].11

Betain 5 is an unprecedented oxyallyl cation stabilised by fouramino groups. Note that oxyallyl systems are of great currentinterest, with heteroatom-stabilised ones being of particularimportance for organic synthesis.12 The bond lengths of the betainframework (N2C)2CO have values in between those typical ofcorresponding single and double bonds (average values: C–O1.36 Å, C–N 1.37 Å, C–C 1.41 Å)13 and all C and N atoms aretrigonal-planar (sum of angles 358.11–360.01). These featuresindicate a highly efficient p-delocalisation of the cationic charge,which becomes even more evident by a comparison of the bondlengths of the two N2C units of 5 with those of tetramethyl-formamidinium salts like [(Me2N)2CH][TeCl6].14 The bonds in thebetain framework of 5 are ca. 7 pm longer than those of the [N2CH]+

unit, where the cationic charge is delocalised over only three atoms.In fact, the N2C units of 5 exhibit bond lengths essentially identicalto those reported for carbene 1a.3 Not unexpectedly, the C–Odistance of 5 is very similar to that of lithium enolates15 and ofthe structurally related aryloxide [Li(OC6H2-2,6-iPr2)(THF)]3 men-tioned above. In contrast, the di(imidazolium)ketone [Im2CQO]Cl2(Im = 1,3-diisopropyl-4,5-dimethylimidazolium) exhibits a muchshorter C–O distance of only 1.22 Å, typical of a double bond,together with a C–Ccarbonyl distance of 1.47 Å, as is typical of singlebonds between sp2-hybridised C atoms.16

Why does the diaminoketene 2b add the carbene 1b, instead ofundergoing the initially expected intramolecular process leading tothe b-lactam 4b? An obvious hypothesis is that this is due to stericreasons. This is indeed strongly supported by results of DFT calcula-tions shown in Scheme 2. The nucleophilic addition of the carbene1b to the diaminoketene 2b has only a low activation barrier (DG‡ =10.7 kcal mol�1). The analogous reaction of the bulkier partners 1aand 2a has a much higher barrier (DG‡ = 31.0 kcal mol�1) and is evenweakly endoergic (see ESI‡). The intramolecular nucleophilic attack,however, which affords the transient carbene 3a, has a barrier of only21.7 kcal mol�1 (Scheme 1). Consequently, apart from being thermo-dynamically more meaningful here than the intermolecular reaction,this intramolecular process is also kinetically strongly favoured for 2a(DDG‡ = 9.3 kcal mol�1). In the case of 2b, the intramolecular reactionhas a similar barrier (DG‡ = 24.4 kcal mol�1), but is not observed due

Fig. 1 Molecular structure of [{Li(m2-5)Cl}3] in the crystal (ellipsoids drawn at the30% probability level). Selected bond lengths (Å): Li1–O1 1.863(3), Li1–O2 1.857(3),Li1–Cl1 2.284(3), Li2–O2 1.859(2), Li2–Cl2 2.273(4), C1–N1 1.370(2), C1–N2 1.362(2),C1–C2 1.409(2), C2–O1 1.357(3), C7–N3 1.372(2), C7–N4 1.355(2), C7–C9 1.421(2),C8–N5 1.377(2), C8–N6 1.365(2), C8–C9 1.399(3), C9–O2 1.362(2).

Scheme 2 Carbonylation of 1b, leading to betain 5. The intramolecular pathleading to b-lactam 4b (in analogy to the formation of 4a from 1a and CO shownin Scheme 1) is kinetically strongly disfavoured.

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6836 Chem. Commun., 2013, 49, 6834--6836 This journal is c The Royal Society of Chemistry 2013

to the much lower barrier of only 10.7 kcal mol�1 for the inter-molecular path (DDG‡ = 13.7 kcal mol�1).

In order to probe the limits of steric congestion preventing theintermolecular path, we finally studied the carbonylation of theunsymmetrical ADAC Ph2N–C–NiPr2 (1c),17 whose steric demand ismuch higher than that of 1b, resembling that of 1a. To our surprise,the reaction of this bulky ADAC with CO did not afford a b-lactam,but rather the blue betain [(Ph2N)(iPr2N)C]2CO (6), which wasisolated as the red hydrochloride [6H]Cl (Fig. 2) after acidicwork-up. The HCl adduct was characterised by a single-crystalX-ray diffraction study (Fig. 3), which revealed that the gross featuresof the molecular structure of [6H]+ are very similar to those alreadydiscussed for 5 (vide supra). Our DFT results for the reaction of 1c(see ESI,‡ Fig. S4) show that the intramolecular pathways of theprimary carbonylation product (Ph2N)(iPr2N)CQCQO (2c), whichcan lead to two different carbenes of type 3 due to the unsymme-trical nature of 2c, exhibit higher activation barriers (23.5 and27.1 kcal mol�1, respectively) than the nucleophilic addition pathleading to betain 6 (20.2 kcal mol�1) in an exoergic reaction.

In summary, the reaction of the prototype carbene (Me2N)2C(1b) with CO proceeds in a 2 : 1 stoichiometry, affording a betainicoxyallyl species by nucleophilic addition of the carbene to theprimary carbonylation product (Me2N)2CQCQO (2b). In view ofthe analogous reaction of the bulky (Ph2N)(iPr2N)C (1c), we con-clude that this behaviour is representative of ADACs which undergocarbonylation. A reaction with CO in a 1 : 1 stoichiometry to afford ab-lactam derivative can be expected only with exceptionally bulky

carbenes like (iPr2N)2C (1a), where the nucleophilic addition of thecarbene to the transient diaminoketene is sterically prohibited sothat an intramolecular follow-up reaction ensues. We will continueour studies systematically in this direction.

We thank the Deutsche Forschungsgemeinschaft for generousfunding (grant SI 429/19-1). T.S. is grateful to the Studienstiftung desdeutschen Volkes for a doctoral fellowship. Quantum-chemical calcu-lations were performed at the Center for Scientific Computing (CSC)Frankfurt on the LOEWE-CSC high-performance computing cluster.

Notes and references§ Our present double-hybrid DFT results differ considerably from thosegiven in ref. 4b, which were calculated at a less sophisticated level ofDFT (BP86/def2-SVP).

1 For reviews, see: (a) H. G. Raubenheimer, Angew. Chem., Int. Ed., 2012,51, 5042; (b) V. P. Boyarskiy, K. V. Luzyanin and V. Yu. Kukushkin, Coord.Chem. Rev., 2012, 256, 2029; (c) L. M. Slaughter, ACS Catal., 2012, 2, 1802;(d) M. Barbazanges and L. Fensterbank, ChemCatChem, 2012, 4, 1065;(e) J. Vignolle, X. Cattoen and D. Bourissou, Chem. Rev., 2009, 109, 3333.

2 Selected recent references: (a) S. Miltsov, V. Karavan, V. Boyarsky,S. Gomez-de Pedro, J. Alonso-Chamarro and M. Puyol, Tetrahedron Lett.,2013, 54, 1202; (b) A. J. Martınez-Martınez, M.-T. Chicote, D. Bautista andJ. Vicente, Organometallics, 2012, 31, 3711; (c) S. Handa andL. M. Slaughter, Angew. Chem., Int. Ed., 2012, 51, 2912; (d) Y.-M. Wang,C. N. Kuzniewski, V. Rauniyar, C. Hoong and F. D. Toste, J. Am. Chem. Soc.,2011, 133, 12972; (e) J. Ruiz, L. Garcıa, B. F. Perandonez and M. Vivanco,Angew. Chem., Int. Ed., 2011, 50, 3010; ( f ) A. S. K. Hashmi, C. Lothschutz,C. Bohling and F. Rominger, Organometallics, 2011, 30, 2411; (g) H. Seo,B. P. Roberts, K. A. Abboud, K. M. Merz, Jr. and S. Hong, Org. Lett., 2010,12, 4860; (h) D. R. Snead, S. Inagaki, K. A. Abboud and S. Hong,Organometallics, 2010, 29, 1729; (i) E. L. Rosen, D. H. Sung, Z. Chen,V. M. Lynch and C. W. Bielawski, Organometallics, 2010, 29, 250.

3 R. W. Alder, P. R. Allen, M. Murray and A. G. Orpen, Angew. Chem.,Int. Ed. Engl., 1996, 35, 1121.

4 (a) T. Schulz, M. Leibold, C. Farber, M. Maurer, T. Porsch,M. C. Holthausen and U. Siemeling, Chem. Commun., 2012, 48, 9123;(b) U. Siemeling, C. Farber, C. Bruhn, M. Leibold, D. Selent, W. Baumann,M. von Hopffgarten, C. Goedecke and G. Frenking, Chem. Sci., 2010, 1, 697.

5 (a) G. G. Qiao, W. Meutermans, M. W. Wong, M. Traubel and C. Wentrup,J. Am. Chem. Soc., 1996, 118, 3852; (b) M. T. Nguyen, M. R. Hajnal andL. G. Vanquickenborne, J. Chem. Soc., Perkin Trans. 2, 1994, 169; (c) M. T.Nguyen, M. R. Hajnal, T.-K. Ha, L. G. Vanquickenborne and C. Wentrup,J. Am. Chem. Soc., 1992, 114, 4387.

6 C. Goedecke, M. Leibold, U. Siemeling and G. Frenking, J. Am. Chem.Soc., 2011, 133, 3557.

7 (a) R. W. Alder, M. E. Blake and J. M. Oliva, J. Phys. Chem. A, 1999,103, 11200; (b) R. W. Alder, M. E. Blake, C. Bortolotti, S. Bufali,C. P. Butts, E. Linehan, J. M. Oliva, A. G. Orpen and M. J. Quayle,Chem. Commun., 1999, 241.

8 M. Otto, S. Conejero, Y. Canac, D. Romanenko, V. Rudzevitch andG. Bertrand, J. Am. Chem. Soc., 2004, 126, 1016.

9 For a review, see: R. W. Alder, M. E. Blake, L. Chaker, J. N. Harvey,F. Paolini and J. Schutz, Angew. Chem., Int. Ed., 2004, 43, 5896.

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11 T. J. Boyle, D. M. Pedrotty, T. M. Alam, S. C. Vick andM. A. Rodriguez, Inorg. Chem., 2000, 39, 5133.

12 For recent reviews, see: (a) M. Harmata, Chem. Commun., 2010, 46, 8904;(b) M. Harmata, Chem. Commun., 2010, 46, 8886; (c) A. G. Lohse andR. P. Hsung, Chem.–Eur. J., 2011, 17, 3812; (d) H. F. Bettinger, Angew.Chem., Int. Ed., 2010, 49, 670.

13 E. V. Anslyn and D. A. Dougherty, Modern Physical Organic Chemistry,University Science Books, Sausalito, 2006, p. 22.

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Sci., 2001, 56, 129.17 S. Conejero, Y. Canac, F. S. Tham and G. Bertrand, Angew. Chem.,

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Fig. 2 The product of the carbonylation of Ph2N–C–NiPr2 (1c) and subsequentworkup with HCl in Et2O.

Fig. 3 Molecular structure of [6H]+ in the crystal (ellipsoids drawn at the 30% prob-ability level). The hydrogen dichloride anion (not shown) is engaged in a hydrogenbond with the OH group. Selected bond lengths (Å): C1–N1 1.353(3), C1–N2 1.403(3),C1–C2 1.411(4), C2–O1 1.410(3), C2–C3 1.409(4), C3–N3 1.357(3), C3–N4 1.408(3).

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