Photoinduced H-Abstraction in Homo- and Protoadamantylphthalimide Derivatives in Solution and in Organized and Constrained Media

FULL PAPER

DOI: 10.1002/ejoc.201201332

Photoinduced H-Abstraction in Homo- and ProtoadamantylphthalimideDerivatives in Solution and in Organized and Constrained Media

Nikola Cindro,[a] Ivan Halasz,[b] Kata Mlinaric-Majerski,[a] and Nikola Basaric*[a]

Keywords: Photochemistry / Rearrangement / Polycycles / Cyclodextrins / Adamantanes

The photochemical reactivity of homoadamantylphthalimide5 and protoadamantylphthalimides 9 and 10 was investi-gated in solution, in the β-cyclodextrin (CD) complex, and inthe solid state. The triplet excited states of 5, 9 and 10 werecharacterized by laser flash photolysis. Sensitized irradiationin solution was found to give rise to polycyclic productsthrough a transiently generated higher excited triplet state,not detectable by LFP. Direct excitation of 5, 9 and 10 gives,in addition to the products arising from the triplet state,

Introduction

Regio- and stereocontrol of chemical reactions is at theforefront of research in organic synthesis. There is an in-creasing demand for the ability to carry out organic reac-tions with regio- and stereocontrol that minimize the timeand cost for subsequent purification steps. Supramolecularchemistry provides a very useful approach to stereocontrolof chemical reactions.[1] Formation of supramolecular as-semblies with the appropriate spatial orientation of reactingmolecules immobilizes reactants and ensures the correct re-action trajectory leading to specific transition states and en-suing product selectivity. In recent decades supramolecularchemistry has also gained importance in photochemistry.[2]

It is now well established that formation of supramolecularassemblies with chiral auxiliaries enables diastereoselectiveand enantioselective reactions.[3] Such systems have beenused as key steps in the synthesis of natural products.[4]

Furthermore, stereoselectivity can be attained by using cir-cularly polarized light[5] or by carrying out photochemicalreactions in the solid state[6] or in confined space. Morerecent examples of supramolecular assemblies applied tophotochemical transformations include the use of zeolites,[7]

proteins,[8] cyclodextrins,[9] curcubiturils,[10] molecular cap-sules[11] and porous self-assembled organic frameworks.[12]

[a] Department of Organic Chemistry and Biochemistry,Rueer Boskovic Institute,Bijenicka cesta 54, 10000 Zagreb, CroatiaFax: +385-1-4680195E-mail: [email protected]: www.irb.hr

[b] Division of Material Chemistry, Rueer Boskovic Institute,Bijenicka cesta 54, 10 000 Zagreb, CroatiaSupporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejoc.201201332.

Eur. J. Org. Chem. 2013, 929–938 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 929

cleavage products probably generated through a singletstate. Complexation with β-cyclodextrin changes the regi-ochemistry of H-abstraction. In the solid state, 5 and 9 areboth photoreactive though state-dependent product mixturesare noted. Irradiation of 5 affords only one product whereasirradiation of 9 affords the same product mixture regardlessof whether irradiation is performed in the solid state or inCH3CN solution.

However, to date there are not many reports in which com-plexation in a confined space altered the regiochemical out-come of photochemical transformations.[13]

N-alkyl-substituted phthalimides, similar to simple carb-onyl derivatives (cf., aldehydes and ketones), undergo pho-toinduced H-abstraction[14] and afford reduction,[15] or cy-clization products in the event of intramolecular reac-tions.[16] We recently became interested in intramolecularphotoinduced H-abstraction reactions using a series of ada-mantylphthalimides.[17] The interest in these compoundswas inspired by their interesting biological activity.[18] Speci-fically, designed N-(adamantylalkyl)phthalimides undergophotoinduced domino reactions giving complex polycyclicstructures, probably arising from higher excited triplet orthe singlet excited states.[17] Intramolecular photoinducedH-abstraction in the series of adamantylphthalimides wasalso investigated in the solid state.[19] Irradiation of 1 in thesolid state selectively affords 2, whereas photolysis of 1 insolution yields products 2–4; see Equation (1). Investi-gations were expanded to include photochemical reactionsof homoadamantylphthalimide 5 which, upon irradiation insolution, gave three photoproducts 6–8; see Equation (2).[20]

Herein we report photoinduced H-abstraction reactionswith three polycyclic phthalimides 5, 9 and 10. The com-pounds investigated are characterized by the presence ofmore than one γ-H atom that may, in principle, be availablefor photoinduced abstraction by the phthalimide carbonylmoiety. In addition to preparative photolyses, we carriedout laser flash photolysis (LFP) to characterize the tripletexcited state thus gaining insight into the photochemicalreaction mechanism. Irradiations were carried out in solu-tion as well as in the solid state and in complex with β-cyclodextrin (CD). These differing reaction conditions al-

N. Cindro, I. Halasz, K. Mlinaric-Majerski, N. BasaricFULL PAPER

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lowed us to determine if restricted geometry in the solidstate, or within the cavity of CD (constrained media) canalter the selectivity of the H-abstraction process. Indeed, wefound that irradiation of the compounds in the solid stateproduced different product ratios, whereas irradiation ofCD complexes gave rise to new products that cannot beattained by irradiation in solution.

Results and Discussion

Photochemistry in Solution

Phthalimide derivatives 5, 9 and 10 were synthesized ac-cording to a previously published procedure.[18] To isolatethe photoproducts of protoadamantylphthalimides we firstcarried out preparative irradiations in acetone and CH3CN.Irradiation of exo-protoadamantane isomer 9 in acetonegave two major products, 11 and 12, formed and isolated ina ratio 1:1; see Equation (3) and Table 1. Contrary to thephotochemical reaction of the exo-isomer, irradiation of

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Table 1. Photochemistry of 5, 9 and 10 under different conditions.

Irradiation Conv. Photoproductconditions [%] (isolated yield; %) [b]

5 acetone/H2O (3:1), 4 h[a] 58 6 (33) 7 (10), 8 (10)5 acetone, 2 h 100 6 (78)5 CH3CN, 4 h 49 6 (35), 16 (trace)[b]

5 CD, 45 min 100 6 (24) 7 (5), 8 (5)5 solid state, 48 h 10 6 (5)9 acetone, 2 h 95 11 (39), 12 (38), 14 (trace)[b]

9 CH3CN, 4 h 83 11 (2), 12 (9), 15 (12)9 CD, 30 min 86 11 (30)9 solid state, 48 h 45 11 (7), 12 (2), 15 (6)10 acetone, 2 h 100 13 (80), 14 (trace)[b]

10 CH3CN, 4 h 61 13 (39), 15 (6)10 CD, 40 min 80 13 (21), 17 (20)10 Solid state, 48 h 0

[a] Irradiation performed at a larger scale with 4 lamps, taken fromref.[20] [b] Not isolated, detected by GC.

endo-protoadamantane 10 in acetone gave only one mainproduct, 13, that was isolated in 80% yield; see Equation(4)]. GC analyses of photolysis mixtures obtained usingboth 9 and 10 in acetone revealed traces of cleavage prod-uct, protoadamantene (14) in both reactions. Photolytic for-mation of 14 was confirmed using GC data generated withauthentic 14.[21] Products 11–13 were fully characterized by1D and 2D NMR, whereas the structures of 11 and 12 wereadditionally validated by X-ray crystallography (see Sup-porting Information). Photolysis of both diastereomers 9

Photoinduced H-Abstraction in Adamantylphthalimides

and 10 in CH3CN gave, in addition to polycyclic derivatives11–13, a significant amount of cleavage product, phthal-imide (15) – see Equation (5) – which was isolated and char-acterized by NMR spectroscopy. In addition, formation of15 was confirmed by HPLC analysis of the crude photolysismixture. Attempts to isolate 14 after photolysis failed dueto the high volatility of the compound. Conversion to pho-toproducts was found to take place faster in acetone thanin CH3CN (Table 1), in accordance with sensitization byacetone and formation of products from the triplet excitedstate. Parallel analytical irradiations of 9 leading to lowconversions in CH3CN, CH3CN/H2O, acetone, and ace-tone/H2O followed by the analysis of the photolysis mixtureby HPLC revealed that addition of H2O to both CH3CNand acetone resulted in a 2–3-fold rate enhancement forphotoproduct generation. Water probably enhances the rateof photochemical reactions, or leads to switching of the rel-ative order of the excited state levels, as seen previously inthe series of adamantylphthalimides.[17]

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After detection of cleavage products 14 and 15 in photo-chemical reactions of protoadamantane derivatives, we re-visited the photochemistry of homoadamantylphthalimide5. Irradiation of 5 under the same conditions identified for9 and 10 in neat acetone gave only alcohol 6 (Table 1) andno traces of amides 7 and 8, or cleavage products. Uponprolonged irradiation of 5 (to achieve complete conversion),amides 7 and 8 underwent secondary photochemical reac-tions and decomposed giving unidentified high weight ma-terial, as we have previously reported.[20] Alternatively, fol-lowing irradiation in CH3CN, in addition to 6 which wasisolated in 35 % yield, traces of homoadamantene 16 weredetected by GC; see Equation (6). Formation of 16 wasconfirmed by comparing the retention time with that of au-thentic sample.[22] This finding suggests that irradiation ofphthalimides in solvent incapable of acting as a triplet sen-sitizer enhances formation of the cleavage photoproducts.

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Quantum efficiencies of the photochemical reactions of5, 9 and 10 were determined using a valerophenone acti-nometer. Since analytical irradiations indicated faster con-version to the photoproducts upon addition of H2O, irradi-ations were performed in a mixture of CH3CN/H2O. Pho-tolyses were performed at 254 nm, absorbances of the solu-tions were measured prior to irradiation, and solution com-positions were analyzed by HPLC. The measured efficienc-ies are low, on the order of 10–3 (Table 2). This is consistentwith previous reports involving intramolecular H-abstrac-tions for adamantylphthalimide scaffolds.[17] Interestingly,diastereomer 9 undergoes photochemical reaction aboutfive times more efficiently than 10. Acetone acts as an ef-ficient sensitizer and, accordingly, significantly enhancesphotoproduct generation. This finding, in tandem with thelow quantum efficiencies for reactions involving direct exci-tation, suggests that direct excitation of phthalimides leadsto an inefficient population of the reactive triplet excitedstate from which products are generated.

Table 2. Quantum yields for reactions of phthalimides 5, 9 and10.[a]

Φ

5 (3�1)�10–3 [b]

9 (5�1)�10–3 [c]

10 (1.5�0.2) �10–3

[a] Irradiation performed in CH3CN/H2O (7:3), determined by useof valerophenone actinometer. [b] Efficiency for the formation of6–8. [c] Efficiency for the formation of 11 and 12.

Laser Flash Photolysis (LFP)

It is known that N-alkyl-substituted phthalimides gen-erally have very weak fluorescence due to short singlet ex-cited state lifetimes (� 0.2 ns). Consequently these sub-strates undergo photochemical reactions from the triplet ex-cited state; the triplet is relatively efficiently populated.[23]

To get more information on the reaction mechanisms at

N. Cindro, I. Halasz, K. Mlinaric-Majerski, N. BasaricFULL PAPERplay here and to characterize the triplet state presumed tobe at play in these systems, we carried out LFP experiments.Transient absorption spectra were recorded for 5, 9 and 10in N2- and O2-purged CH3CN solutions. All derivatives ex-hibited characteristic transient absorptions with maxima at340 nm. The transient decayed in N2-purged solution withthe rate constant in the range k = (1 to 20)� 105 s–1 andwas quenched by O2. On the basis of comparisons with pre-viously established spectra[17b,23] we assigned the transientabsorption to the triplet excited state of phthalimide. Thequantum efficiency for intersystem crossing (ISC) was de-termined by comparing the intensities of the transient ab-sorption signal with the N2-purged optically matched solu-tion of N-methylphthalimide in CH3CN (ΦISC = 0.8).[23h]

As can be seen from the data in Table 3, all phthalimidederivatives display similar quantum yields of ISC (10%)which cannot be correlated with the measured quantumefficiency of the photochemical reactions. Measured life-times of the triplet excited state also do not correlate withthe quantum efficiency of the photoreactions. This findingstrongly suggests that the spectroscopically observed tripletexcited state is not the reactive state responsible for productgeneration. Consequently, we postulate that the photo-chemical reaction probably takes place from an upper ex-cited triplet state or singlet excited state, as seen previouslyfor assorted phthalimide derivatives.[17b,23h,23i]

Table 3. Photophysical properties of phthalimides 5, 9 and 10 inCH3CN.

τ [μs][a] ΦISC[b] kq [dm3 mol–1 s–1] [c]

5 4.4�0.2 0.13 �0.03 1.4�109

5·CD 28�4 0.13�0.03 –9 1.29�0.04 0.16�0.03 1.5 �109

10 0.89�0.02 0.13�0.03 1.3�109

[a] Lifetime of the triplet in N2-purged CH3CN. [b] Determined bycomparing intensity of the signal with the optically matched solu-tion of N-methylphthalimide in CH3CN (ΦISC = 0.8).[23h] [c] Rateconstant for the quenching of the triplet excited state with O2.

LFP measurements were also conducted for the CDcomplex of 5 in CH3CN/H2O (95:5). The transient absorp-tion spectrum was identical to the one measured in CH3CNearlier assigned to the triplet excited state of phthalimide.However, the lifetime of the triplet was significantly pro-longed to 28 μs (in N2-purged solution k = 3.5�104 s–1).In principle, this longer lifetime could be due to an increasein solvent polarity as previously reported.[17,23] Alterna-tively, complexation of 5 within the CD cavity might ac-count for the longer triplet lifetime. Since phthalimides arenot soluble in aqueous solvent without CD, we were notable to measure the lifetime of the 5-derived triplet in isola-tion in CH3CN/H2O (95:5). An explanation for the veryslow decay of the 5·CD triplet therefore remains elusive.

Irradiations in the Presence of CD

There are several examples of stereoselective intramolec-ular photochemical reactions that take place in the cavity


of CD[24] or structurally-modified CD.[25] Therefore, we car-ried out irradiations in the presence of CD. The use of CDhas several advantages. Primarily, the inclusion of CD inreactions is expected to enhance or alter stereoselectivityrelative to irradiations in solution. Substrate complexationin CD is also expected to alter photophysical properties ofthe substrate. Furthermore, the inclusion of CD enables ir-radiations to be performed in H2O, which has importantecological ramifications.

Compounds 5, 9 and 10 form complexes with CD. Sincethese compounds are very lipophilic, without CD they arecompletely insoluble in H2O. On complexation with CD,solutions of 5, 9 and 10 were prepared containing only 5%of CH3CN in H2O [c(CD) = 3.5� 10–3 m, and c(phthalim-ide) = 7 �10–4 m]. Due to the insolubility of compounds inH2O we could not perform titration experiments with CDto determine the association constants of the correspondingcomplexes. However, it is known that adamantane deriva-tives form stable 1:1 complexes with β-CD in H2O charac-terized by association constants in the range 103–105 m–1.[9b]

The phthalimide moiety can also complex with CD al-though the ΔG for phthalimide complexation is lower thanthat for the adamantane moiety.[9b] Therefore, it is plausiblethat 5, 9 and 10 may also form complexes wherein eachsubstrate would be placed in a capsule formed by two CDs.To get information on the molecular structure of these com-plexes we acquired NOESY spectra of DMSO/H2O (1:1)solutions of 5, 9 and 10 in complex with CD [c(CD) =0.02 m, and c(phthalimide) = 0.01 m]. Since no NOE inter-actions were observed between CD and the phthalimide H-atoms, it is suggested that 1:1 complexes form wherein onlythe adamantane moiety of each compound enters the CDcavity; the phthalimide moiety remains solvent exposed andfree of CD associations.

Irradiation of the aqueous solutions of the CD complexwith 5 gave the same products as irradiation in CH3CN oracetone, with alcohol 6 being the main product (Table 1).On the other hand, upon complexation of protoadaman-tanes 9 and 10 in CD, a different selectivity for the photore-action was observed. Irradiation of 9 gave azepine 11 as themajor product (isolated in 30 % yield), along with numerouscompounds that were present in very small amounts(� 5 %) and which were not isolated. The absence of 12among the products of 9 photolysis indicates that complex-ation with CD induces a change in the geometry of 9 ren-dering different H-atoms of the protoadamantane skeletonavailable for abstraction. Irradiation of 10 in the presenceof CD gave two products, 13 and 17; both were isolated in20% yield; see Equation (7). Similar to the photochemistryof 9, complexation of 10 with CD probably induces a geo-metric change enabling abstraction of the H-atom at the γ-position leading to new product 17, not seen in the photo-chemistry in CH3CN or acetone. All products isolated inthe photochemical reactions with CD were purified, andcharacterized by NMR spectroscopy. Additionally, to checkfor product formation with enantiomeric excess, optical ro-tations of samples were measured. Notably, no enantio-meric excess was observed since all products were obtained


as racemates. Nevertheless, to the best of our knowledge,these are the first examples of photochemical H-abstrac-tions within a CD cavity involving phthalimide chromo-phores. Although H-abstractions were found not to bestereoselective, reactions in the presence of CD are impor-tant since they reveal that CD complexation enables dif-ferent regiochemical outcomes for photolytic chemistry.

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In addition to the change of regiochemistry on complex-ation of protoadamntanes 9 and 10 with CD, complexationalso induced faster conversion to photoproducts. For exam-ple, irradiation of 9 at the same concentrations in acetone,CH3CN, and H2O containing CD gave rise to conversionsof 73, 7 and 76%, respectively. The finding suggests fasterphotochemical reactions in the presence of CD. As statedabove, the adamantane moiety likely forms a complex withCD, whereas the phthalimide remains solvent exposed. Inthis way the photophysics of the phthalimide chromophoreis altered. The change in the photophysical properties ofadamantylphthalimides on complexation with CD is alsosuggested by LFP measurements. It was demonstrated thatthe triplet excited state of 5 has a lifetime that is 5–6 timeslonger in the CD complex than in neat CH3CN. Further-more, it has been suggested by Kubo that the change ofpolarity and H-bonding capability of the solvent may resultin switching of the character of the S1 and T1 state for thephthalimide, being either n,π* or π,π*.[26] In turn, this maychange the quantum efficiency of ISC, and lead to an en-hanced population of the more reactive triplet excited state.Our LFP measurements did not suggest a change of theISC quantum yield. However, from our measurements wecannot say anything about the population of the higher ex-cited triplet states that are probably the reactive states re-sponsible for product generation.

Irradiation in the Solid State

Scheffer et al. have developed a structure–reactivity rela-tionship for solid state intramolecular photochemical γ-Habstractions by carbonyl chromophores.[27] Four param-eters (d, Δ, θ, and ω) direct the reactivity of the carbonylgroup in the excited state, as shown in the figure withinTable 4. The optimal value for d is 2.72 Å, an optimal valuefor Δ is between 90–120°, θ is expected to have an optimalvalue at 180°, whereas ω should be around 0°. We haveshown that the same parameters can be extended to exam-ine the photochemical reactivity of adamantylphthalimidederivatives in the solid state.[19] In addition, we have demon-strated that restriction of the molecular motion within the


crystal lattice leads to different regio- and stereochemistryof the intramolecular H-abstraction.[19]

Table 4. Relevant geometric parameters d, ω, Δ, and θ in the crystalof phthalimide 9 for photochemistry in the solid state.

d [Å] [a] ω [°][b] Δ [°] θ [°]

O2A–H3A 2.647 21.5 90.6 100.2O2A–H8A 2.515 16.0 119.4 111.6O2A–H7A2 3.042 55.5 80.5 113.2O1A–H5A2 2.651 46.3 78.5 105.5O1B–H3B 2.686 23.0 90.2 98.1O1B–H8B 2.495 21.5 121.0 111.1O1B–H7B2 2.963 52.6 81.5 114.4O2B–H5B1 2.716 51.2 77.5 103.6O1B–H3B 2.686 23.0 90.2 98.1O1B–H8B 2.495 21.5 121.0 111.1

[a] The atom notations correspond to the ORTEP drawings givenin the Supporting Information. [b] ω is approximated with torsionangle comprising one atom neighbouring the carbonyl group andthe hydrogen atom of interest.

Irradiations of 5, 9 and 10 in the solid state gave productsonly for 5 and 9. However, both 5 and 9 appear to undergoa very slow photochemical reactions in the solid state. Irra-diation of 5 afforded only alcohol 6 whereas photochemicalreaction of 9 was not selective and gave three products 11,12 and 14. Irradiation of 9 in the solid state afforded thesame products as found after irradiation in CH3CN solu-tion.

We obtained single crystals suitable for X-ray analysisonly for protoadamantylphthalimide 9. Investigation of thecrystal structure revealed that some H-atoms are appropri-ately positioned for intramolecular H-abstraction by thecarbonyl group (Table 4, and Figure 1). It is interesting tonote that geometric parameters suggest the highest prob-ability for H-abstraction to be at position 8 of the protoada-mantane skeleton. However, no product was detected re-sulting from the corresponding 1,5-biradical formed by H-

Figure 1. Crystal packing of 9 with marked short distances betweenthe hydrogen atoms and the carbonyl groups of the phthalimidemoieties.

N. Cindro, I. Halasz, K. Mlinaric-Majerski, N. BasaricFULL PAPER8 abstraction. Besides intramolecular short contacts be-tween the phthalimide carbonyls and the H-atoms of theprotoadamantane skeleton, some intermolecular short con-tacts were found which may enable H-abstraction. In par-ticular, the distance between the H-atom at position 2 ofthe protoadamantane and the carbonyl of the neighbouringmolecule is shorter than the sum of the van der Waals radii(2.70 Å). Additional short contacts (2.82 Å) were found be-tween H-atoms at positions 2 and 9 of the protoadaman-tane and neighbouring carbonyl moieties. These intermo-lecular short contacts may lead to formation of diradicalpairs that could give rise to high-molecular weight material,or back H-transfer to afford starting material. Inefficientphotolysis may be explained by these intermolecular H-ab-stractions that do not give products, or by formation ofcleavage products 14 and 15 which destroy the crystal latticethus blocking further reaction.

Photochemical Reaction Mechanism

On the basis of isolated products, LFP data and thenoted influence of solvent on reactivity, reaction mecha-nisms for the photoinduced rearrangement of 9 and 10 canbe proposed. Direct excitation of 9 leads to population ofthe singlet state, which with the efficiency of only 10 %, un-dergoes ISC to the triplet state. Cleavage products 14 and15 are formed in significantly higher yields in CH3CN thanin acetone. Moreover, 16 was only detected in the photo-chemical reaction of 5 when carried out in CH3CN; theuse of acetone prevented generation of 16. These findingsstrongly suggest that the cleavage products arise from thesinglet state. They are probably formed in a β-cleavage reac-tion that involves mixing of the LUMO orbital of thephthalimide with the σ* orbital of the C–N bond that isbeing cleaved. Such cleavage events are ubiquitous in thephotochemistry of carbonyl compounds.[28] Such reactionstake place if the excited state has higher energy than is re-

Scheme 1.


quired for the cleavage of σ-bond; the S1 state of phthal-imide has a very high energy (Es = 384 kJ mol–1).[23g] Theresulting radical pair 20 (Scheme 1) undergoes dispropor-tionation to give alkene 14 and phthalimide 15. The samecleavage event also appears to take place in the solid state.However, we cannot rule out the possibility that 14 and 15may also be formed from the triplet state in a typical Nor-rish-II cleavage from biradical 18.

Irradiation of 9 in the presence of acetone leads directlyto population of the triplet excited state. However, the low-est excited triplet state of phthalimide in CH3CN is not re-active. Population of higher excited triplet states is requiredto induce H-abstraction. Two H-atoms can be abstracted,at positions 5 and 7, leading to 1,4-biradical 18 or 1,6-bi-radical 19, respectively. Combination of the biradicals fur-nishes products 21 and 12. Azetidinol 21 is not stable andundergoes ring-opening to afford stable benzazepindione11. Formation of trace quantities of 14 and 15 can be ex-plained by Norrish II cleavage from biradical 18.

Complexation of 9 in CD changes the geometry of themolecule rendering only H-5 available for abstraction. Ab-straction of H-7 is probably restricted due to deep position-ing within the CD cavity. In addition, exposure of thephthalimide to H2O on complexation with CD changes thephotophysics available to 9 making it more reactive in H-abstraction reactions.

Direct excitation of 10 in CH3CN also leads to the for-mation of 14 and 15 in a process that probably involves theS1 state and β-cleavage. Sensitization with acetone or ISCpopulates the triplet excited state, but the H-abstractionprobably takes place from a higher triplet excited state. Inthe case of 10, only H-2 is abstracted giving selectively 1,5-biradical 22 that recombines to generate isolated product13 (Scheme 2). However, on complexation with β-CD geo-metries change rendering the γ-H atom available for reac-tion. Subsequent irradiation thus gives 1,4-biradical 23 as areaction precursor to azepindione 17, the result of cycliza-tion and ring enlargement.


Scheme 2.

Conclusions

Photochemical reactivity of phthalimides 5, 9 and 10 wasinvestigated in the solid state, in solution, and in complexwith β-CD. In solution all compounds undergo photosenzi-tized H-abstraction through a higher excited triplet stateand give rise to polycyclic products. Upon direct irradiationof phthalimide derivatives 5, 9 and 10, in addition to theformation of polycyclic molecules, cleavage of the phthal-imide takes place, presumably through a singlet excitedstate. In the CD complex, homoadamantyl derivative 5gives the same photoproducts as noncomplexed molecules,whereas the regioselectivity of the H-abstraction in the pro-toadamantylphthalimides 9 and 10 changes, due to CDcomplexation-induced conformational changes. In the so-lid-state only 5 and 9 were photoreactive. Whereas solid-state photoreaction for 5 is selective, irradiation of 9 gavethe same products as irradiation in the CH3CN solution.These new examples of photochemical reactions provideclear demonstration of the influence of organized and con-strained media on reactivity and selectivity, which may haveapplications in the synthesis of new complex polycyclic mo-lecules.

Experimental SectionGeneral: 1H and 13C NMR spectra were recorded with a BrukerAV-300 or AV-600 spectrometer at 300 or 600 MHz, respectively.All NMR spectra were measured in CDCl3 using tetramethylsilaneas a reference. Melting points were obtained with a Kofler Mikro-heiztisch apparatus (Reichert, Wien). IR spectra were recordedwith a FT-IR ABB Bomem MB 102 spectrophotometer. The sam-ples were analyzed using a Shimadzu HPLC equipped with a diodearray detector on a Phenomenex Luna 3u C18(2) column usingCH3OH/H2O (20%) as a solvent, or on a GC using DB-1701 capil-lary column. Temperatures on the GC were set as follows: injector230 °C, column oven 80 °C and FID detector 300 °C. Retention


times for 14 and 16 were 4.04 min, and 7.65 min, respectively. Silicagel (Merck 0.05–0.2 mm) was used for chromatographic purifica-tions. Solvents were purified by distillation. Phthalimides 5, 9 and10 were prepared according to the published procedure.[18] An au-thentic sample of 14 was prepared from 4-protoadamantanone[29]

by reduction of the corresponding tosylhydrazone,[21] whereas 16[22]

was obtained from the corresponding 4-homoadamanol.[30] Irradi-ations were performed with a Rayonet reactor equipped with 12lamps with 300 nm output, or in a Luzchem reactor equipped with8 lamps. During irradiations in Rayonet, the solutions were contin-uously purged with a stream of argon and cooled with tap waterusing a finger-condensor.

Irradiation Experiments in Solution. General Procedure: In a quartzvessel, phthalimide 5, 9 or 10 (100 mg, 0.36 mmol) was dissolvedin acetone or CH3CN (150 mL). The solution was purged with Arand irradiated in a Rayonet reactor at 300 nm 15 min. After irradi-ation, an extraction with pentane was carried out (in case of ace-tone, 10 mL of H2O was added prior to extraction), and the pent-ane extracts were analyzed by GC. The solvent (CH3CN or ace-tone) was removed on a rotary evaporator and the residue purifiedby crystallization or chromatography.

Irradiation of 9 in Acetone: After irradiation and removal of thesolvent the residue was dissolved in hot ethyl acetate. Crystalli-zation afforded pure product 11 (39 mg, 39%) as colorless crystals.The mother liquor was evaporated and the residue chromato-graphed on a TLC (SiO2) using CH2Cl2/ethyl acetate (4:1) as eluentto afford pure product 12 (38 mg, 38%) as colorless crystals.

rel-(1R,11S)-2-Aza-3,10-dioxopentacyclo[9.7.0.112,16.04,9.014,18]-nonadeca-4,6,8-triene (11): Colorless crystals, m.p. 269–270 °C. 1HNMR (600 MHz, CDCl3, 20 °C): δ = 7.91–7.87 (m, 1 H), 7.62–7.55(m, 2 H), 7.38–7.32 (m, 1 H), 5.95 (s, 1 H), 4.35–4.29 (m, 1 H),3.05 (br. s, 1 H), 2.99 [d, 3JH,H = 7.1 Hz, 1 H], 2.69–2.58 (m, 1 H),2.32 (dd, 3JH,H = 6.0, 2JH,H = 12.1 Hz, 1 H), 2.23 (m, 1 H), 1.98–1.89 (m, 3 H), 1.77–1.72 (m, 1 H), 1.64–1.60 (dd, 3JH,H = 2.9, 2JH,H

= 11.2 Hz, 1 H), 1.52 (dd, 3JH,H = 3.8, 2JH,H = 13.6 Hz, 1 H), 1.44(d, 2JH,H = 12.3 Hz, 1 H), 1.35 (d, 2JH,H = 13.0 Hz, 1 H) ppm. 13CNMR (150 MHz, CDCl3, 20 °C): δ = 203.44 (s), 177.22 (s), 139.91(s), 132.05 (d), 130.75 (d), 130.43 (s), 128.96 (d), 126.01 (d), 60.82

N. Cindro, I. Halasz, K. Mlinaric-Majerski, N. BasaricFULL PAPER(d), 50.59 (d), 41.99 (t), 39.16 (d), 38.44 (t), 36.10 (t), 35.02 (d),30.96 (d), 30.94 (t), 26.55 (d) ppm. IR (KBr): ν = 3182, 3066, 2929,2866, 1697, 1660, 1477, 1388, 1396, 975, 817, 750, 617 cm–1. HRMS(MALDI), calcd. for C18H20NO2 282.1449, found 282.1488.

rel-(1S,11S)-2-Aza-10-hydroxy-3-oxohexacyclo[9.5.2.114,17.02,10.04,9.012,16]nonadeca-4,6,8-triene (12): Colorless crystals, m.p. 208–210 °C. 1H NMR (300 MHz, CDCl3, 20 °C): δ = 7.72 (d, 3JH,H =7.5 Hz, 1 H), 7.59 [dd(t, 3JH,H = 7.5 Hz, 1 H, )], 7.53 (d, 3JH,H =7.4 Hz, 1 H), 7.47 [ddd(dt), 3JH,H = 7.5, 4JH,H = 1.2 Hz, 1 H, )],4.24 [dd(t), 3JH,H = 5.0 Hz, 1 H, )], 2.68 (s, 1 H), 2.61 (dd, 3JH,H =7.8, 15.6 Hz, 1 H), 2.54–2.45 (m, 1 H), 2.41 (d, 2JH,H = 13.6 Hz, 1H), 2.37–2.31 (m, 1 H), 2.27–2.20 (br. s, 1 H), 1.89 (dd, 3JH,H =4.9, 2JH,H = 13.7 Hz, 1 H), 1.85–1.77 (m, 1 H), 1.68–1.54 (m, 2 H),1.49–1.40 (m, 1 H), 1.35–1.22 (m, 3 H) ppm. 13C NMR (75 MHz,CDCl3, 20 °C): δ = 172.02 (s), 148.10 (s), 132,87 (d), 131.97 (s),129.58 (d), 123.42 (d), 122.26 (d), 90.30 (s), 49.30 (d), 42.71 (d),38.11 (t), 38.03 (t), 37.87 (d), 35.28 (d), 34.89 (d), 32.83 (t), 29.65(t), 26.30 (d) ppm. IR (KBr): ν = 3281, 2930, 1661, 1358, 1090,1048, 983, 775, 715 cm–1. HRMS (MALDI), calcd. for C18H20NO2

282.1449, found 282.1488.

Irradiation of 10 in Acetone: After irradiation and removal of sol-vent the residue was dissolved in hot CCl4. Crystallization affordedpure product 13 (80 mg, 80%) as colorless crystals.

rel-(1R,10S)-2-Aza-10-hydroxy-3-oxohexacyclo[9.6.1.112,16.02,10.04,9.014,18]nonadeca-4,6,8-triene (13): Colorless crystals, m.p. 193–194 °C. 1H NMR (600 MHz, CDCl3, 20 °C): δ = 7.70 (d, 3JH,H =7.6 Hz, 1 H), 7.64–7.56 (m, 2 H), 7.50–7.43 (m, 1 H), 4.35 (ddd,3JH,H = 7.5, 7.1, 2JH,H = 2.5 Hz, 1 H), 2.93 (dd, 3JH,H = 9.7, 4.5 Hz,1 H), 2.66 (dd, 3JH,H = 7.9, 16.0 Hz, 1 H), 2.54 (s, 1 H), 2.51 (d,3JH,H = 13.5 Hz, 1 H), 2.42 (br. s, 1 H), 2.32–2.26 (m, 1 H), 2.24–2.15 (m, 2 H), 2.01 (d, 2JH,H = 14.0 Hz, 1 H), 1.92–1.78 (m, 2 H),1.67–1.62 (m, 1 H), 1.57 (d, 3JH,H = 11.2 Hz, 1 H), 1.37 (d, 2JH,H

= 13.0 Hz, 1 H) ppm. 13C NMR (150 MHz, CDCl3, 20 °C): δ =171.33 (s), 150.45 (s), 133.12 (d), 130.85 (s), 129.22 (d), 123.39 (d),122.26 (d), 96.58 (s), 56.68 (d), 51.07 (d), 50.30 (d), 46.18 (t), 36.31(t), 36.24 (d), 34.85 (t), 34.10 (t), 31.22 (d), 29.61 (d) ppm. IR(KBr): ν = 3364, 2935, 1673, 1388, 1328, 1066, 1018, 888, 750,703 cm–1. HRMS (MALDI), calcd. for C18H20NO2 282.1449,found 282.1488.

Irradiation in the Presence of CD: Phthalimide derivative 5, 9 or 10(20 mg) was dissolved in CH3CN (5 mL). The solution was addedto 80 mL of H2O giving a milky coloidal mixture, to which anaqueous solution of CD (20 mg in 1 mL) was added. The resultingclear solution of phthalimide-CD complex was purged with Ar andirradiated in a Rayonet reactor for 40 min. After irradiation thesolution was extracted with a 1:1 mixture of ethyl acetate andCH2Cl2 (5 � 20 mL). The extracts were dried with anhydrousMgSO4, filtered and the solvent was removed on a rotary evapora-tor. The remaining residue was chromatographed on a TLC to af-ford pure products.

rel-(11R)-10-Aza-2,9-dioxopentacyclo[9.7.0.113,17.03,8.01,15]nona-deca-3,5,7-triene (16): Colorless crystals, m.p. 184–185 °C. 1HNMR (600 MHz CDCl3, 20 °C): δ = 7.91–7.86 (m, 1 H), 7.63–7.58(m, 2 H), 7.52–7.47 (m, 1 H), 5.97 (br. s, 1 H), 4.16–4.04 (m, 1 H),2.52–2.29 (m, 3 H), 2.21–2.10 (m, 2 H), 2.06–1.97 (m, 1 H), 1.88(dd, 3JH,H = 3.2, 2JH,H = 12.5 Hz, 1 H), 1.84–1.79 (m, 1 H), 1.52–1.36 (m, 3 H), 1.25 (br. s, 2 H) ppm. 13C NMR (150 MHz, CDCl3,20 °C): δ = 209.26 (s), 169.66 (s), 137.81 (s), 131.93 (d), 131.54 (d),129.02 (d), 128.14 (d), 64.07 (s), 49.37 (d), 42.19 (d), 39.61 (t), 39.11(t), 36.00 (d), 34.82 (t), 34.67 (t), 31.67 (t), 27.95 (d) ppm. IR (KBr):ν = 3198, 3060, 2930, 1656, 1381, 1275, 941, 727 cm–1. HRMS(MALDI), calcd. for C18H20NO2 282.1488, found 282.1480.


Irradiation in the Solid State: Phthalimide derivatives 5, 9 or 10(100 mg) were dissolved in CH2Cl2 (10 mL). The solutions werespread on the walls of quartz cuvettes (200 mL) to form thin filmsupon evaporation of the solvent. The quartz cuvettes with the filmof phthalimides were filled with N2 and irradiated in a Luzchemreactor over the course of 2 d. After irradiation, the samples wereanalyzed by HPLC and then subjected to column chromatographyusing hexane/EtOAc with a gradual increase of the EtOAc concen-tration.

Quantum Yields of the Photochemical Reaction: Solutions of 5, 9or 10 and valerophenone in CH3CN/H2O (7:3), were freshly pre-pared and their concentrations adjusted to have absorbances 0.4–0.8 at 254 nm. After adjustment of the concentration and measure-ment of the corresponding UV/Vis spectra the solutions wereplaced into quartz cuvettes (15 mL), purged with a stream of N2

(20 min each), and sealed with a septum. The cuvettes were irradi-ated at the same time in a Luzchem reactor equipped with a merry-go-round and 2 lamps with the output at 254 nm for 0.5, 1, 2, 5,10 and 15 min. After each irradiation, samples were taken from thecuvettes by use of a syringe and analyzed by HPLC. Quantumyields for the reaction were calculated using a valerophenone acti-nometer (formation of acetophenone in aqueous media, Φ =0.65�0.03).[31]

Laser Flash Photolysis (LFP): All LFP studies were conducted atthe University of Victoria LFP facility employing a YAG laser, witha pulse width of 10 ns and excitation wavelength 266 nm. Staticcells (0.7 cm) were used and solutions were purged with nitrogenor oxygen for 20 min prior to measurements. Absorbances at266 nm were about 0.4. LFP measurements in the presence of CD[c(CD) = 0.01 m] were carried out in CH3CN/H2O (5:95), and theabsorbance at 266 nm was 0.2.

Single Crystal X-ray Measurements and Structure Determinations:Single crystal diffraction data were collected from the crystal gluedon a glass fibre tip. Diffraction intensity data were collected byω-scans using an Oxford Diffraction Xcalibur 3 using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) and reducedusing the CrysAlis program package.[32] The structures were solvedby direct methods using SHELXS.[33] The refinement procedure byfull-matrix least-squares methods based on F2 values against allreflections included anisotropic displacement parameters for allnon-H atoms. The positions of H-atoms each riding on carbonand nitrogen atoms were determined on stereochemical grounds.Refinements were performed using SHELXL-97.[33] The SHELXprograms operated within the WinGX[34] suite. A summary of gene-ral and crystal data, intensity data collection and structure refine-ment are presented in Table S1 in the Supporting Information.Geometric calculations and molecular graphics calculations wereperformed with PLATON,[35] MERCURY,[36] andORTEP.[37]

CCDC-896925 (for 9), -896926 (for 11), and -896927 (for 12) con-tain the supplementary crystallographic data for this paper. Thesedata can be obtained free of charge from The Cam-bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Supporting Information (see footnote on the first page of this arti-cle): Laser flash photolysis of 5, 9, and 10, crystallographic data,1H and 13C NMR spectra of new compounds 11–13 and 17.

Acknowledgments

This work was financed by the Croatian Foundation for Science,Higher Education and Technological Development of the Republic


of Croatia (HRZZ grant number 02.05/25), the Ministry of ScienceEducation and Sports of the Republic of Croatia (grant number098-0982933-2911). The authors thank Professors Peter Wan andCornelia Bohne, and the University of Victoria (Victoria, Canada,BC) for allowing access to nanosecond LFP, and Professor A. G.Griesbeck for critical reading of the manuscript.

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Published Online: January 7, 2013

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Photoinduced H-Abstraction in Homo- and Protoadamantylphthalimide Derivatives in Solution and in Organized and Constrained Media