Electron transfer photocyclization

Xu, W., Jeon, Y. T., Hasegawa, E., Yoon, U. C., Mariano, P. S., Novel Electron transfer Photocyclization Reactions of a Silyl Amine a,P Unsaturated Ketone and Ester Systems,... 

Electron transfer initiated photocyclization of a methanolic solution of 90 followed by catalytic hydrogenation gave a mixture of benzoindolizines 91 and tetrahydroquinoline. Hydrogenation is necessary to stabilize one of the proposed products (82TL919, 83JA1204) (Scheme 17). 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

The direct photocyclization of another interesting acceptor-donor pair, the amine-enone system, has been reported by Mariano [224-226]. Direct irradiation of -(aminoethyl)cyclohexenones 281 leads to the excitation of the conjugated cyclohexenone chromophore. Intramolecular single-electron transfer from the amine donor to the cyclohexenone excited state results in the formation... 

Further research on intramolecular photocyclization of amino enones and amino ketones based on electron transfer has been made by Kraus and Chen [228]. In analogy to the earlier results of Roth and El Raie [229], Kraus and Chen obtain the cyclopropanol derivative 286 as single stereoisomer by direct irradiation of 285. Photolysis of the amino enone 287 does not lead to a three-membered ring product, but only to pyrrolidine 288. The irradiation of the unsaturated keto ester 289 results in the even more unexpected formation of a nine-membered ring product 290. Such remote photocyclizations have rarely been described so far. 

The photochemistry of imides, especially of the N-substituted phthalimides, has been studied intensively by several research groups during the last two decades [233-235]. It has been shown that the determining step in inter- and intramolecular photoreactions of phthalimides with various electron donors is the electron transfer process. In terms of a rapid proton transfer from the intermediate radical cation to the phthalimide moieties the photocyclization can also be rationalized via a charge transfer complex in the excited state. 

From the foregoing discussion, it is clear that DPM rearrangements are very general for a variety of 1,4-unsaturated systems, such as, 1,4-dienes, (3,7-unsaturated aldehydes and ketones, and different 1-aza-1,4-diene derivatives. Surprisingly, the literature was devoid of studies describing the photoreactivity of the closely related 2-aza-1,4-diene derivatives. For many years, the only studies in this area were carried out by Mariano and his co-workers [60] on the photochemistry of iminium salts derived from 2-aza-1,4-dienes. The results obtained demonstrated the synthetic utility of the photocyclizations of iminium salts to different heterocycles, in reactions that are initiated by intramolecular single electron transfer [60]. 

The intramolecular photocyclization of l-dimethylamino-3-phenylpropane to give a me la cycloadduct via photoinduced electron transfer was first reorted by Bryce-Smith et al. in 1973 [377], Yoon and Mariano reported the intramolecular photocyclization of 416 in the presence of DCA to give 417 and 418 via the radical cation of amido moiety followed by desilylation (Scheme 124). 

The intramolecular photoinduced electron transfer reaction of N-(o-chlorobenzyl)aniline 440 in the presence of sodium hydroxide in aqueous acetonitrile afforded, 9,10-dihydrophenanthridine and its dimer, which is reasonably explained by dechlorination from the radical anion of chlorobenzene chro-mophore followed by the cyclization (Scheme 130) [481], Similar photocyclization 9-(io-anilinoalkyl)-10-bromophenantherens 441 takes place to give spiro compounds, cyclized products, and reduction products dependent on the methylene chain length. The efficient intramolecular photocyclization occurs when the methylene tether is n = 3 [476] (Scheme 131). 

One strategy to overcome the regioselectivity problem and to enhance the efficiency for the photocyclization is the use of a suitable leaving group in a-position to the donor (Sch. 31). The intermediate donor radical-cation formed after electron transfer subsequently undergoes... 

Regioselective photocyclization of ethylthiopropyl phenylglyoxalate 793, by an electron-transfer process, produced the 1,5-oxathiocin 773 in 96% yield. The mechanism proposed for such a photocyclization involved excitation of the keto ester 793 to its triplet state (T) upon irradiation (350 nm), and electron transfer between the donor and acceptor occurred in T producing the ketyl radical anion (A). This was deactivated to ground state 793 by back electron transfer or was converted to biradical (B) by proton transfer process. Cyclization occurred between the excited carbonyl group and the carbon a to the sulfur on the remote side (Scheme 156) <1997T7165>. 

Intramolecular photocyclization between the vinyl moieties of (189) occurs using 9,10-dicyanoanthracene as an electron transfer sensitizer. In general, the route leads to the moderately efficient synthesis of the macrocyclic ring system (190, 191). The stereoselectivity shown by the reaction is, in some instances, dependent on the solvent used. Thus with (189a, b, and c), the cis-isomers (190) were the major products with acetonitrile as solvent, but with benzene the frans-isomer (191) predominated. In the case of (189d, e, and f), the trans-isomers predominate with both solvents. 

The electron-transfer initiated photoaddition of allylsilanes to iminium salts has been examined in detail. Irradiation of l-methyl-2-phenylpyrrolinium perchlorate (177) and the allylsilanes (178) in methanol, for example, gave the 2-allylpyrrolidines (179) " the proposed pathway is outlined in Scheme 11. Photoaddition is initiated by electron transfer and driven to completion by desilylation of the allylsilane-derived cation radical. Analogous conversions have been reported in the allyl iminium perchlorates (180), " and cyclization to the spiro-compounds (181) occurs on irradiation of the /3-enaminone-derived allyliminium salts (182). " A photocyclization of this type has been... 

Photocyclization of methoxynaphthyl analogs of chalcone is reported to proceed via (unusual) electron transfer from excited vinyl arenes. Copper(II) gives an organocopper intermediate which evolves via a radical cation to a cyclized radical and the final naphthofuran 12 (eq. (6)) [71]. 

Direct photolysis of 50-53 in 02-saturated acetonitrile solution also leads to the corresponding carbazoles with a quantum yield of ca 0.64 in all cases (equation 13)161. Apparently, substituents have only a little effect on the chemical yield of carbazole produced by steady-state irradiation in aerated acetonitrile. However, an attempt to carry out such a photocyclization reaction by using photoinduced electron-transfer sensitization has failed, presumably due to fast back electron transfer that quenches the net reaction. It is also interesting to note that chemical oxidation and electrochemical oxidation of 50-53 does not result in carbazoles. Instead, benzidine products are formed. These results are consistent with the AMI calculations, which suggest that the cyclization reaction is both kinetically and thermodynamically more favorable from the triplet state than from the cation radical or dication. 

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