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

The direct UV irradiation of the 1-arylethyl phosphites 7, 8, and 9 was car ried out in acetonitrile, benzene, and cyclohexane, as was the triphenylene -sensitized reaction of 9. Dimethyl I-phenylethyl phosphite, 7, gives the p hoto-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 o ptically active, predominantly (R)-1-phenylethyl phosphite 7 (R/S = 97/3; 9 4% eel, at 35-40 degreesC proceeds with a high degree of stereospecificity at the stereogenic migratory carbon to give predominantly (R)-10 (R/S = 86/ 14, 72 +/- 2% eel. Use of the nitroxide radical trap TEMPO affords phosphon ate 10, presumably all cage product, from predominantly (R)-7 (RIS = 97/3; 94% eel in 64% yield (80% ee, R/S = 90/10). By contrast, the 1-(4-acetylphe nyl)-ethyl phosphite, predominantly (S)-8 (SIR = 98/2, 96% eel, on direct i rradiation gives the corresponding phosphonate (12) in only 20% yield along with dimer 11b in 40% accountability yield. Phosphonate 12 is nearly racem ic (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 s imilar 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 eel. By contrast, the triplet triphenylene-sensitized photorearrange ment of largely (R)-9 (R/S = 98/2, 96% eel leads to product distributions s imilar to those from direct irradiation of predominantly (S)-8 and is accom panied 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 ibr radical pair formation. Furthermore, thes e stereochemical results are diagnostic of the multiplicity of the initial radical pair formed. Values for k(comb)/k(rot) for the proximate free radic al 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 degreesC. The possibility that k(comb) is increased for the pairs from 7 an d 9 is proposed.