| 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-phenylethylphosphite, 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 yieldsof 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 °C proceeds with a high degree ofstereospecificity 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 allcage 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), ondirect irradiation gives the corresponding phosphonate (12) in only 20% yield along with dimer11b in 40% accountability yield. Phosphonate 12 is nearly racemic (R/S = 52/48). Direct irradiationof predominantly (R)-9 (R/S = 98/2, 96% ee), a 1-(1-naphthyl)ethyl phosphite, results in a productdistribution similar to that from predominantly (R)-7, but with a somewhat higher degree ofretention of configuration in the product phosphonate 13 (R/S = 93/7, 86 ± 3 ee). By contrast, thetriplet triphenylene-sensitized photorearrangement of largely (R)-9 (R/S = 98/2, 96% ee) leads toproduct distributions similar to those from direct irradiation of predominantly (S)-8 and isaccompanied 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 9provides evidence for radicalpair formation. Furthermore, these stereochemical results are diagnostic of the multiplicity of theinitial 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 Ph2CCN−CHPhMe, corrected to 35 °C. The possibility that kcomb is increased for the pairs from 7 and9 is proposed.
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