THE INTERACTION OF PI-ORBITALS WITH A CARBOCATION OVER 3 SIGMA-BONDS

The semi-pi analogue of double hyperconjugation (''hyperconjugation/co njugation'') has been examined in 4-isopropylidenecyclohexyl mesylate (4-OMs) by comparison with the saturated analogue, trans-4-isopropylcy clohexyl mesylate(5-OMs). The unsaturated substrate reacts in 97% trif luoroethanol only four times faster than the saturated substrate. Rabe r-Harris plots indicate that both substrates react by k(s) mechanisms; i.e., solvolysis occurs with solvent assistance rather than carbocati on formation. These results are consistent with the absence of a direc t, through-bond interaction of the double bond with the reactive cente r. The absence is caused at least in part by less than ideal overlap o f the gamma, delta pi orbitals with the alpha,beta sigma orbitals. In contrast, an electron-rich tin atom attached to the 4-position provide s a large rate enhancement and changes the mechanism to carbocation fo rmation through double hyperconjugation.