Photoinduced electron transfer homoquinones

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Irradiation of dimethyl-substituted homobenzoquinone 6a in the presence of EtjN or diethylamine gave dihydrogenated 41a without cyclopropane ring cleavage (Scheme 22). In the case of DMA, only the aniline adduct 40a was produced as well as in the reaction of 6c. The stereoselective formation of 40 and 41 is rationalized by the stability of their half-chair conformations. In conclusion, irradiation of homo-quinones in the presence of donors resulted in a photoinduced electron transfer that is responsible for the subsequent dimerization, donor amine addition, and hydrogenation, depending on the nature of the homoquinone and the donor. 

Homoquinones also undergo intramolecular photoinduced electron transfer reactions when the electron-donating substituent is attached to the geminal diaryl group. Semiempirical calculations revealed that... 

A survey of the literature reveals some characteristic photochemical features of homoquinones (1) [2+2]-photocycloaddition with alkenes and alkynes, (2) hydrogen abstraction and/or ring opening of incorporated cyclopropane, and (3) photoinduced electron transfer reactions, as described below. The photocycloaddition with alkenes and alkynes took place exclusively at the ethylene linkage of homobenzoquinones to afford cyclobutanes (or cyclobutene derivatives) regio- and exo/endo stereoselectively, depending on the identity of both the homobenzoquinones as well as the alkenes. 

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