Trans photodimerization

Figure 10.1 Photodimerization of anthracene (a) and trans-cinnamic acid (b).

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

Cowan and Drisko have studied the photodimerization of acenaphthy-lene(130,l36) in detail and have concluded that the cis dimer is derived primarily from a singlet excimer state of acenaphthylene while the trans dimer is derived entirely from the acenaphthylene triplet state ... 

Cycloadditions are in general an effective way of constructing cyclobutane rings. A wide variety of heterocyclic systems dimerize in this way. 1,3-Diacetylindole, for example, affords the head-to-tail dimer 242 on irradiation in ethanol.185 Ethyl 2-ethoxy-l,2-dihydroquinoline-l-carboxy-late is similarly converted in diethyl ether into the trans head-to-head dimer.186 Notable among many analogous photodimerizations are those reported in 1,4-dihydropyridines,187 in furo[3,2-b]pyridin-2(4//)-ones,188 in 8-methyl-s-triazolo[4,3-a]pyridine,189 and in 2H-2-benzazepine-1,3-diones.190 The [ 2 + 2] dimerization of amidopyrine is the first reported example of a photocycloaddition in a 4-pyrazolin-3-one.191... 

Cycloadditions, although relatively rare, are not unknown in heterocyclic systems. The first documented example was the photodimerization of pyrid-2-one a reinvestigation of this reaction has established that three other isomeric [ 4 + 4] dimers are formed in low yield in addition to the originally reported trans-anti dimer (264).215 Attempts to effect analogous cycloadditions in a series of l,T-polymethylenedipyrid-2-ones were unsuccessful, [ 2 + 2] and [ 4 + 2] additions being preferred.216 Thus,... 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

Feldman and Campbell, on the other hand, used hydrogen-bonding interactions to enforce a particular stereo- and regiochemical outcome of the sohd-state photocycloaddition of a naphthoic acid-derived cinnamic acid [45]. In a conceptually similar approach, Scheffer demonstrated that diamines can form double salts with a variety of trans-cinnamic acid derivatives. The locking in place of the double bonds steers the sohd-state [2+2] photodimerization [46]. 

Figure 1 summarizes the chemical structures of the topochemically polymerizable 1,3-diene monomers providing stereoregular 1,4-trans polymer (Scheme 6) [ 16]. Most of the polymerizable monomers contain benzyl, naphthylmethyl, and long alkyl-chain substituents in their chemical structures. The (ZyZ)-, (E,Z)-, and ( , )-muconic and sorbic acids as well as the other diene carboxylic acids are used as the ester, amide, and ammonium derivatives. In contrast to this, the carboxylic acids themselves have crystal structures unfavorable for polymerization while they undergo [2-1-2] photodimerization, as has already been described in the preceding sections.

Ultraviolet irradiation of the methiodide or hydrochloride of 2-styryl-pyridine (XCIII) in the solid state results in transformation of the trans-isomers to the corresponding dimers (XCVa, b) on the other hand irradiation in benzene solution gives both isomerization and dimerization.309 Dimer (XCVa) was produced in low yield on irradiation of XCIII in the powdered form in the presence of air.308 This is in contrast to the reported stability of XCIII toward photodimerization.109 Similar dimerizations have been reported in the case of 2,4-dichloro-3-cyano-6-styrylpyridine,164 2-styrylquinaldine (XCV) and frans-4,4 -diguanyl-stilbene bis(2-hydroxyethane sulfonate) (stilbamidine) (XCIV).82,83... 

Another example of a carbonyl compound whose excited singlet reacts before crossing over to a triplet is provided by coumarin. Direct irradiation in ethanol gives very low yields of a cw-fused dimer.432 However, inclusion of benzophenone results in only trans-fused dimer, even if all the light is initially absorbed by the coumarin.433 Apparently, excited singlet coumarin transfers its singlet excitation to benzophenone, which then crosses over to its triplet and transfers its triplet excitation to coumarin. Triplet coumarin then dimerizes fairly efficiently ( = 0.25) to a tranj-fused system. Morrison has demonstrated sizeable solvent effects on this photodimerization.434 Yields of the cis-dimer are increased by polar solvents and probably arise from excited singlet excimers.119... 

The photodimerization of coumarin has been studied in several solvents and their nature has an effect on this rather complex reaction. In a polar medium such as methanol, the only product is the cis head-to-head isomer (244) but in acetonitrile, this is accompanied by the trans head-to-head dimer (245), which becomes the main product of the reaction in non-polar solvents like benzene or dioxan. Small amounts of the head-to-tail isomers (246) are also formed in non-polar solvents (66JA5415). In some solvents, the presence of benzophenone as sensitizer is essential and it also alters the relative proportion of dimeric isomers (64JA3103, 66CB625). Photolysis of osthole (247) in the presence of acetophenone gives a mixture of two quite different dimers (248) and (249) (80MI22302), representing addition at the exocyclic and endocyclic double bond respectively. 

The j5-chlorobenzenesulfonate salt of N-octadecyl-trans-4-stil-bazole (27) fails to dimerize in solution or when incorporated into CTAB micelles (66a,b). However, irradiation in the solid state or in monolayer assemblies yields the syn head-to-head dimer 23 accompanied by long-wavelength fluorescence attributed to the excimer. Evidently, aggregation of the hydrophobic hydrocarbon tails is more important than like-charge repulsion in determining the stereochemistry of photodimerization in the case of 27. Whitten and co-workers (66c) recently reported the observation of strong red-shifted fluorescence attributed to... 

Keywords trans-stilbene, cinnamic acid, [2+2]photodimerization, photopolymerization, cyclobutane... 

Tomotaka and coworkers [202] studied the photodimerization of cinnamic acids incorporated in vesicles (Fig. 39). They mixed equimolarly the cinnamic acid with alkyldimethylamine N-oxide (C DAO) to produce the ion pair 72 (Fig. 39). These ion pairs form stable vesicular aggregates in water. Whereas photoirradiation of the cinnamic acids in methanol resulted in only the cis-trans isomerization to form 71, in these vesicle medium, three dimers, 68-70, were obtained. 

Rao et al. observed that photoirradiation of tran.v-stilbene in crystalline y-cyclodextrin inclusion complexes yields a single isomer of. svn-tctraphenylcy-clobutane (119) [109] stereoselectively in high yield (70%). In contrast, the photodimerization of stilbene in solution is very inefficient, and no photodimer was observed even after prolonged irradiation of pure stilbene crystals. 

Photodimerization of 2-pyridone (46) in the presence of the 2,2/-biphenyldi-carboxylic acid host (45) also proceeded via a catalytic process. First, irradiation of the 1 2 inclusion complex of 46 and 45 in the solid state gave the trans-anti dimer (47) in 92 % yield [27], The mechanism of this stereoselective photoreaction was investigated through X-ray analysis of this complex. In the complex, two 46 molecules are arranged in ideal positions for yielding 47 by dimerization [27], Secondly, a catalytic dimerization reaction of 46 was carried out. Photoirradiation for 20 h of a 1 4 mixture of powdered 45 and 46 under occasional mixing in the solid state gave 47 in 81 % yield. These data clearly show that molecules of... 

Ab initio calculations have also been carried out <2003JFC(124)99> for the photodimerization reaction of trifluoro-thioacetyl fluoride 13 which affords approximately equal amounts of trans- and >-2,4-difluoro-2,4-bis(trifluoromethyl-1,3-dithietane) 14 (Scheme 1) <1965JOC1375>. The calculations indicate that both dimers are thermodynamically stable relative to the monomer, and of almost equal stability. The Ec and G2gs.is values obtained in the G3(MP2) calculations of the dimers relative to monomers gave 115 and 45 kj mol-1 for cis-14, while the corresponding values for trans-14 were 112 and 44 kj mol-1. For both isomers, the 1,3-dithietane ring was found to be planar <2003JFC(124)99>. 

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