Photo-Arbuzov rearrangements of 1-arylethyl phosphites: stereochemical studies and the question of radical-pair intermediates.

Abstract

The direct UV irradiation of the 1-arylethyl phosphites 7, 8, and 9 was carried out in acetonitrile, benzene, and cyclohexane, as was the triphenylene-sensitized reaction of 9. Dimethyl 1-phenylethyl phosphite, 7, gives the photo-Arbuzov rearrangement product, dimethyl 1-phenylethylphosphonate (10), in 67% average yield and minor amounts (2%) of 2,3-diphenylbutane (11a) in quantum yields of 0.32 and 0.02, respectively. The photorearrangement of optically active, predominantly (R)-1-phenylethyl phosphite 7 (R/S = 97/3; 94% ee), at 35-40 degrees C proceeds with a high degree of stereospecificity at the stereogenic migratory carbon to give predominantly (R)-10 (R/S = 86/14, 72 +/- 2% ee). Use of the nitroxide radical trap TEMPO affords phosphonate 10, presumably all cage product, from predominantly (R)-7 (R/S = 97/3; 94% ee) in 64% yield (80% ee, R/S = 90/10). By contrast, the 1-(4-acetylphenyl)-ethyl phosphite, predominantly (S)-8 (S/R = 98/2, 96% ee), on direct irradiation gives the corresponding phosphonate (12) in only 20% yield along with dimer 11b in 40% accountability yield. Phosphonate 12 is nearly racemic (R/S = 52/48). Direct irradiation of predominantly (R)-9 (R/S = 98/2, 96% ee), a 1-(1-naphthyl)ethyl phosphite, results in a product distribution similar to that from predominantly (R)-7, but with a somewhat higher degree of retention of configuration in the product phosphonate 13 (R/S = 93/7, 86 +/- 3 ee). By contrast, the triplet triphenylene-sensitized photorearrangement of largely (R)-9 (R/S = 98/2, 96% ee) leads to product distributions similar to those from direct irradiation of predominantly (S)-8 and is accompanied by almost total loss of stereochemistry in its product phosphonate, 13 (R/S = 51/49). The partial loss of stereochemistry on direct irradiation of 7 and 9 provides evidence for radical pair formation. Furthermore, these stereochemical results are diagnostic of the multiplicity of the initial radical pair formed. Values for kcomb/krot for the proximate free radical pairs from 7 and 9, derived experimentally, are severalfold larger than those for the proximate singlet pair from Ph2C=C=N-CHPhMe, corrected to 35 degrees C. The possibility that kcomb is increased for the pairs from 7 and 9 is proposed.