Solvolysis of 1,1-dimethyl-4-alkenyl chlorides:: Evidence for π-participation

Tertiary 1,1-dimethyl-4-alkenyl chloride (1) solvolyzes with significantly reduced secondary beta-deuterium kinetic isotope effect (substrate with two trideuteromethyl groups) and has a lower entropy and enthalpy of activation than the referent saturated analogue, 4 k(H)/k(D) = 1.30 +/- 0.03 vs kH/kD = 1.79 +/- 0.01; DeltaDeltaH(not equal) = -9 kJ mol(-1), DeltaDeltaS(not equal) = -36 J mol(-1) K-1, in 80% v/v aqueous ethanol), indicating participation of the double bond in the rate-determining step. Transition structure I-TS computed at the MP2(fc)/6-31G(d) level of theory revealed that the reaction proceeds through a late transition state with considerably pronounced double bond participation and a substantially cleaved C-Cl bond. The doubly unsaturated compound 3 (1.1-dimethyl-4,8-alkadienyl chloride) solvolyzes with further reduction of the isotope effect, and a drastically lower entropy of activation (k(H)/k(D) = 1.14 +/- 0.01; DeltaS(not equal) = -152 +/- 12 J mol(-1) K-1, in 80% v/v aqueous ethanol), suggesting that the solvolysis of 3 proceeds by way of extended pi-participation, i.e., the assistance of both double bonds in the rate-determining step.