Stilbene amine adduct

The proposition that locally excited triplet states can be formed from back electron transfer within a doublet-doublet radical ion pair has firm theoretical (88) and experimental support. For example, with time-resolved Resonance Raman spectroscopy, one can directly monitor the chemical fate of the exciplex, solvent separated ion pair, and doublet free radical ion pairs formed between stilbene and amines. As might be expected from the above discussion, adduct formation is observed from the exciplex or contact ion pairs, whereas enhanced intersystem crossing ensues from the solvent separated ion pairs, producing spectroscopically observable stilbene triplets. This back electron transfer process, eq. 30 (89),... 

Photoinduced intramolecular interaction of t-S and tertiary amine moieties linked with a polymethylene chain has also been studied24. The photoexcitation of fraws-stilbene in which a tertiary amine is attached to the ortho position with a (CH2)i-3 linker leads to fluorescent exciplexes by intramolecular electron transfer, and results in no more than trans-cis isomerization. The failure to give adducts from the intramolecular exciplexes could arise from the unfavourable exciplex geometry to undergo the necessary bond formation. 

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

TABLE 13. trans-Stilbene-Amine Adduct Yields and Ratios3... 

The results described in this article establish that the stilbenes are among the most versatile of organic reactants in bimolecular photochemical reactions. Only triplet cyclo-alkenones can rival the ability of It to dimerize, form [2+2] adducts with both electron-rich and electron-poor alkenes, and form acyclic adducts with amines, heterocycles, and noncon-jugated dienes. All of the known bimolecular photochemical reactions of excited stilbenes involve It as the reactive excited state. The failure of - -c, and 3C to undergo bi-... 

The addition reactions of It with amines are also presumed to occur via exciplex or radical-ion pair intermediates however, exciplex fluorescence is observed only under conditions where chemical reactions do not occur. Transfer of hydrogen from the amine a-C-H (tertiary amine) or N-H (secondary amine) bond results in the formation of a radical pair which ultimately gives rise to stilbene amine adducts and other free-radical derived products. The radical-ion pairs can also be intercepted by external electrophiles and nucleophiles, leading to formation of radical-ion-derived products. 

These products can both be explained as arising from radical coupling derived from proton transfer within the initially formed radical ion pair, N-H deprotonation giving adduct and C-H deprotonation giving reduction product. With stilbene and tertiary alkyl amines (dimethylaminoalkanes), two products characteristic of radical coupling are observed, eq. 54 (162) ... 

As discussed earlier, deprotonation of a-carbon forms a major reaction pathway for the disappearance of the amine radical cation. Studies of photoinduced electron-transfer reactions of tertiary amines by Lewis [7, 11] and by Mariano [5, 10] have contributed significantly towards our understanding of the factors that control this process. Lewis and coworkers used product-distribution ratios of stilbene-amine adducts to elucidate the stereoelectronic effects involved in the deprotonation process [5, 10, 121, 122]. In non-polar solvents, the singlet excited state of tran -stilbene forms non-reactive but fluorescent exciplexes with simple trialkylamines. Increasing solvent polarity brings about a decrease in the fluorescence intensity and an increase in adduct formation. For non-symmetrically substituted tertiary amines two types of stilbene-amine adduct can be formed, as is shown in Scheme 9, depending on whether the aminoalkyl radical adding to the stilbene radical is formed by de-... 

Table 6. fra -Stilbene-amine adduct yields and ratios. ... 

The cycloaddition of A -sulfonyl amines (Af-sulfonyl imides) (e.g., 431 ) to alkenes gives 1,2-thiazetidine 1,1-dioxides. " The stereochemistry about the double bond of the alkene is preserved in the adduct, and a tight zwitterionic intermediate is favored. Considerable amounts of six-membered cyclic adducts also are formed in these additions. Yields vary from good to poor. Thermolysis of 432 yields sulfonyl amine 433 the higher reaction temperatures involving 433 enable less reactive alkenes such as cis-stilbene to be used successfully. The success of cycloadditions to enamines depends on the latter having no protons on the -alkene carbon or on an sp hybridized carbon attached to the a-position. ... 

Reactions of stilbene with nonsymmetrical tertiary amines also yields mixtures of adducts (Scheme 3). - The product ratios... 

Whereas most reactions of singlet arenes and aryl olefins with secondary amines result in N-H addition, there are exceptions to this generalization. Cookson et al. reported that irradiation of 1-phenylcyclohexene in isopropylamine yielded a mixture of N-H and a-C-H adducts. Gilbert and co-workers ° found that the reaction of benzene with dimethylamine yields N-H adducts, whereas pyridine reacts with diethylamine to yield the substitution product 11, which presumably is formed by aromatization of the a-C-H adduct. Irradiation of stilbene and indole in mixed crystals yields both N-H and C-H adducts, product ratios being dependent upon the reactant ratio. Investigation of the reactions of 9-cyanophenanthrene with primary and secondary... 

The ET-sensitized photoamination of 1,1-diarylethylenes with ammonia and most primary amines yields the anti-Markovnikov adducts. Photoamination of unsymmetrically substituted stil-benes yields mixtures of regioisomers 15 and 16. Modest re-gioselectivity is observed for p-methyl or p-chloro substituents however, highly selective formation of adduct 15 is observed for the p-methoxy substituent (Table 5). Selective formation of 15 was attributed to the effect of the methoxy substituent on the charge distribution in the stilbene cation radical. This re-gioselectivity has been exploited in the synthesis of intermediates in the preparation of isoquinolines and other alkaloids." Photoamination of 1-phenyl-3,4-dihydronaphthalene yields a mixture of syn and anti adducts 17 and 18 (Scheme 5)." Use of bulky primary amines favors formation of the syn adduct (Table 5), presumably as a consequence of selective anti protonation of the intermediate carbanion. 

The photochemical behavior of the orr/to-(aminoalkyl)slil-benes 92—94 is also dependent upon the polymethylene linker (Scheme II). Irradiation of 93 and 94 results in formation of the benzazepines 96 and 97. These adducts are presumably formed by regioselective N—H transfer to the proximal end of the stilbene double bond in 93 and the distal end in 94, in both cases resulting in the formation of a 1,7-biradical intermediate. Since intramolecular stilbene-amine addition reactions are non-regioselective, the regioselectivily of N-H transfer must be subject to exciplex conformational control. The biradical inter-... 

The regioselective ET-sensitized addition of primary amines has been cleverly employed by Yasuda and co-woikers in the synthesis of isoquinolines and related alkaloids. For example, the DCNB-sensitized photoamination of stilbene with 2-hy-doxypropylamine in an acetonitrile-water solution yields the adduct 98, which is converted to the benzylisoquinoline 99 upon treatment with CFjSOjH, The P-hydroxyl group of ethano-lamine can be replaced with acetal or vinyl groups which also can serve as precursors of the carbocation required for the acid-catalyzed ring-forming reaction. Analogous reaction sequences have been used to convert phenanthrene to aporphines. ... 

The addition reactions of singlet stilbene with nonsymmetrical tertiary amines yield addition products resulting from proton transfer from each of the nonequivalent a-carbons (Scheme 2). The adduct resulting from proton transfer from the less substituted carbon of simple trialkylamines, such as N,N-dimethylisopropylamine, is formed selectively. In contrast, the adduct resulting from proton transfer from the more substituted carbon is formed selectively in the case of N,N-dimethylallylarnine and other amines possessing a radical-stabilizing a-substituent. The former result was attributed to a stereoelec-tronic effect on the proton transfer process and the latter result to the effects of substituents upon the kinetic acidity of the a-protons. Similar trends have been reported for other amine photooxidation reactions. The selectivity pattern is dependent upon the identity of the proton acceptor. 

Direct irradiation of [60]fuUerene 47 in the presence of trimethylamine is reported to yield the adducts 47a and 47b as primary and secondary products (Scheme 29). Only small amounts of these adducts are isolated from complex product mixtures. Formation of 47a is proposed to occur via an electron transfer, proton transfer, radical coupling mechanism similar to that for addition of stilbene and trialky-lamines. Formation of 47b is found to require oxidation of 47a by singlet fuUerene. Direct irradiation of 47 with dimethylamine in the presence of air is reported to result in the formation of the tetra(amino)fullerene epoxide 47c (Scheme 29). Isolated yields as high as 98% are reported for the reaction of N-methylpiperazine. More hindered secondary amines such as EtjNH and primary amines fail to undergo this extraordinary reaction. 

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