Photo-Arbuzov rearrangements of benzylic phosphites. Stereochemistry at migratory carbon

The stereochemistry of the photo-Arbuzov rearrangement of the benzylic phosphite trans-(R,R')-10 to the corresponding phosphonate, 11, has been determined by P-31 NMR spectroscopy and X-ray crystallography. The reaction is shown to occur with predominant retention of configuration at the stereogenic migratory carbon center of configuration R' in starting trans-(R,R')-10 and the predominant product cis-(R,R')-11. Thus, reaction of optically active phosphoramidite 13 (R/S ratio 98/2, 96% ee) with 1-phenylethanol of high optical purity (R'/S' ratio, 97/3, 94% ee) gives phosphite trans-10 (cis/trans ratio, 97/3) almost entirely as the single enantiomer, trans-(R,R')-10. Irradiation of trans-(R,R')-10 in acetonitrile with 254 nm ultraviolet light converted it cleanly to two diastereomers of phosphonate cis-11 in 80/20 ratio (P-31 NMR). The major (80%) isomer was isolated, recrystallized, and shown by X-ray crystallography to be cis-(R,R')-11. The lesser (20%) product is identified, on the basis of its 31P NMR chemical shift, as the diastereomer cis-(R,S')-11. (Assignments derived from photorearrangement of totally racemic cis-10 prepared from reaction of racemic 1-phenylethanol with racemic 13.) The generation of trans-(R,S')-11 is attributed to the formation (Scheme 1) from trans-(R,R')-10 of short-lived, predominantly singlet, free radical pairs (12a) that largely (similar or equal to 80%) undergo combination to form cis-(R,R')-11. To a lesser extent (similar or equal to 20%), the 1-phenylethyl radicals (C) of the pair 12a are converted by rotation to C' to generate the stereochemically distinct radical pair 12b that then combines to form cis-(R,S')-11. To a first approximation, combination (k(comb)) is four times as fast as rotation (k(rot)). During the photorearrangement the trans/cis ratios of starting phosphite 10 and product phosphonate 11 are unchanged as is consistent with the generation of phosphinoyl radical E that is configurationally stable at phosphorus.