Comparison between electron transfer and nucleophilic reactivities of ketene silyl acetals with cationic electrophiles

The products and kinetics for the reactions of ketone silyl acetals with a series of p-methoxy-substituted trityl cations have been examined, and they are compared with those of outer-sphere electron transfer reactions from 10,10′-dimethyl-9,9′, 10, 10′- tetrahydro-9,9′-biacridine [(AcrH)₂] to the same series of trityl cations as well as other electron acceptors. The C-C bond formation in the reaction of β,β-dimethyl-substituted ketene silyl acetal (1: (Me₂C=C(OMe)OSiMe₃) with trityl cation salt (Ph₃C+ClO₄-) takes place between 1 and the carbon of para-positon of phenyl group of Ph₃C⁺, whereas a much less sterically hindered ketene silyl acetal (3: H₂C=C(OEt)OSiEt₃) reacts with Ph₃C⁺ at the central carbon of Ph₃C⁺. The kinetic comparison indicates that the nucleophilic reactivities of ketene silyl acetals are well correlated with the electron transfer reactivities provided that the steric demand at the reaction center for the C-C bond formation remains constant.