Diastereoselective Protonation on Radical Anions of Electron-Deficient Alkenes via Photoinduced Electron Transfer

Diastereoselective protonations in the photoinduced electron transfer (PET) reactions of 1,1-dicyanoethene derivatives (1a−e) by use of organosilicon compounds such as allyltrimethylsilane
(2) and benzyltrimethylsilane (5) are described. Irradiation of an acetonitrile−acetic acid solution
containing 4-tert-butylcyclohexylidenepropanedinitrile (1a) and an excess of 2 in the presence of
phenanthrene (Phen) as a sensitizer afforded reduction and allylated products (3a, 4a) in 63:37
and 57:43 ratios in a less regioselective manner. Photoreactions of 3,3,5-trimethylcyclohexylidenepropanedinitrile (1b), 2-methylcyclohexylidenepropanedinitrile (1c), bicyclo[2.2.1]-2-heptylidenepropanedinitrile (1d), and 1,7,7-trimethylbicyclo[2.2.1]-2-heptylidenepropanedinitrile (1e) with
2 showed higher diastereoselectivity via the PET process. Similar diastereoselectivities were
obtained in the Phen-sensitized photoreaction of 1a−e with 5. When 2,4,6-trimethylpyridinium
tosylate (7) was used as a proton source in place of acetic acid, the product ratios were substantially
changed in several cases. From these results, steric and torsional effects have been postulated as
important factors for the control of the diastereoselectivity in these PET reactions.