SET-induced photorearrangement of 2-phenylallyl phosphites. Stereochemistry at phosphorus. Application to cyclic nucleotide derivatives

The stereochemistry at phosphorus of the SET-induced photorearrangement of diastereomeric 4-tert-butyl-2-phenylallyl-1,3,2-dioxaphosphorinanes (8) to the corresponding 2-phenylallylphosphonates (9), which involves exicted singlet 1,4-dicyanonaphthalene (1DCN*) as one-electron oxidant, was investigated. The rearrangement occurs with close to complete retention of configuration at phosphorus. The previously postulated mechanism for this photorearrangement is shown to be consistent with the stereochemical finding. Thus, one-electron reduction by DCN.- of the presumably stereospecifically formed distonic cyclic 1,3-cation radical intermediate 15, generated from cis-8 (Scheme 2), yields the thermodynamically stable diradical 16. beta scission of 16 forms phosphonate cis-9. An alternative mechanism involving beta scission of 15 to a styryl cation radical, prior to one-electron reduction to 15, is discounted on the basis of unpublished trapping studies using MeOH. The direct, kinetically controlled formation of diradical 16 rather than the thermodynamically less stable 21 with CH2 bonded apically to phosphorus is argued to be consistent with the essentially equal values of the quantum yield for phosphonate formation (phi P) on SET-induced rearrangement of the acyclic 2-phenylallyl phosphite 1 and phosphite 7 with phosphorus incorporated in a six-membered (1,3,2-dioxaphosphorinane) ring. This mechanism is contrasted to that for the previously reported triplet-sensitized photorearrangements of phosphites 1 and 7, which have greatly different phi P values. For these reactions, kinetic formation of the triplet analogue of 21, but without the tert-butyl substituent, requires a permutation of substituents for conversion to diradical 16 prior to intersystem crossing and beta scission to form the phosphonate corresponding to 7. The preparative-scale SET-induced photorearrangement of the thymidine-based 2-phenylallyl 3',5'-phosphite 10 gave both diastereomers of phosphonate 11 that were separated by HPLC. The 2-phenylallyl functionality provides an opportunity for further functionalization. As reported elsewhere, 11 was not formed in useful amounts via triplet-sensitized reaction of 10.