Kinetics of the Reactions of Electron Rich CC-Double Bonded Systems with Carbenlum Ions

In spite of the enormous importance, reactions of cationic electrophiles with silylated enol ethers have adopted in organic synthesis,* kinetic data on such reactions are rare.u Previously we have reported rate constants for the reactions of alkenes, allylelement compounds, and enol ethers toward chloro-, methyl-, and alkoxy-substituted benxhydryl cation& Because of the high nucleophilicity of alkyl and silyl enol ethers, the rate constants of their reactions with these carbocstions are very high, often approaching the diffusion limit. In order to obtain insight into the relationship between structure and nucleophilicity of silyl enol ethers, we have now investigated their reactivities toward the considerably less electrophilic bis(p dimethylaminophenyl)carbenium ion 1. In addition, we report some rate constants for the reactions of this carbenium ion with allylsilanes and allylstannanes, which provide a connection to the reactivity scale of xnucleophiles, previously reported.5 The kinetic experiments have generally been carried out with the his@-dimethylaminophenyl)carbenium triflate(l-UTF), prepared by reaction of the corresponding alcohol with 1 equivalent of trifluoromethanesulfonic acid in dry THF at 20 “C. In accord with the reaction mechanism described in Scheme 1, the photometrically monitored reactions follow second order kinetics, as previously reported for analogous reactions of other benxhydryl cations.43