Determination of the nucleophilicities of silyl and alkyl enol ethers

The kinetics of the reactions of benzhydryl cations with 19 silyl enol ethers, four silyl ketene acetals, and two alkyl enol ethers have been determined photometrically in dichloromethane solution. All reactions reported in this investigation follow second-order rate laws, and the rates are independent of the nature of the complex counterion (BF4-, F3CSO3-, or ZnCl3-) in accord with rate-determining C-C bond formation. The nucleophilic reactivities span over a range of 10(8) from the vinyl ethers 1a,x as the least reactive compounds (comparable to allylsilanes) to the highly nucleophilic silyl ketene acetal 1u (comparable to enamines). Linear free enthalpy relationships are used to compare the reactivities of these compounds with those of other aliphatic and aromatic pi-nucleophiles.