HYDROLYSIS OF STRAINED BRIDGEHEAD BICYCLIC VINYL ETHERS AND SULFIDES

Rates of hydrolysis of the bridgehead bicyclic vinyl ether 9-oxabicyclo[3.3.1 ]non-l-ene (6) and its vinyl sulfide counterpart 9-thiabicyclo[3.3.1]non-l-ene (7), catalyzed by the hydronium ion, were measured in H2O and in D2O solution. These data give isotope effects, kH/kD = 2.4 and 1.9 respectively, which show that these reactions occur by the normal, rate-determining carbon protonation, mechanism. The vinyl ether 6 is less reactive than its olefin analogue, bicyclo[3.3.1]non-l-ene (relative rates 1:1/]400), as may have been expected for a constrained bicyclic system such as this, where stabilization of the bridgehead carbocation intermediate by conjugation with oxygen is severely impaired. The vinyl sulfide 7, however, is even less reactive than the vinyl ether (relative rates 1:1/]400); this is a remarkable result in view of the fact that conjugation between the sulfur atom and the cationic center is presumably also strongly inhibited. © 1979, American Chemical Society. All rights reserved.