Solvolyses of Bicyclooct-1-yl and 1-Adamantyl Systems Containing an Ethylidene Substituent on the 2-Position: Typical Examples of Rate Enhancements Ascribed to Relief of F-Strain

Article abstract of DOI:10.1021/jo00094a012

The first typical examples are described on the solvolysis rate enhancements ascribed to the relief of F-strain between an alkyl group and the leaving group atom directly attached to the reaction center.The rates and products of solvolyses in ethanol were studied for 2-methylene- and (Z)- and (E)-2-ethylidenebicyclo<2.2.2>oct-1-yl triflates.Solvolyses were also conducted in ethanol and 2,2,2-trifluoroethanol (TFE) on 2-methylene- and (Z)- and (E)-2-ethylidene-1-adamantyl compounds having OMs, F, Cl, Br, or I as a leaving group.All the substrates gave the corresponding bridgehead substitution products as kinetic control products.The Z:E rate ratios at 25 deg C were 217 +/- 6 for 2-ethylidenebicyclo<2.2.2>oct-1-yl triflates (ethanol) and 109 +/- 11 (ethanol) and 117 +/- 1 (TFE) for 2-ethylidene-1-adamantyl mesylates. 18O scrambling studies on the ethanolyses of (Z)- and (E)-2-ethylidene-1-adamantyl mesylates showed that the titrimetrically determined Z:E rate ratios can be used as a measure of the rate ratios for the ionization step.The Z:E rate ratio in TFE at 25 deg C for 2-ethylidene-1-adamantyl halides varied in the sequence F (ca. 70), Cl (1020 +/- 160), Br (2230 +/- 90), and I (9500 +/- 280).The significant increases in the rate ratio with the increase in the atomic size of halogen were explained in terms of the presence of F-strain in the Z substrates and its essential absence in the E substrates.Linear correlations were found in a plot of 1.36*logZ/k_E> against the MM2 steric energy difference between the Z and E isomers (slope 1.0) and against Hansch's Es, demonstrating the significance of F-strain effect in the enhanced rates of the (Z)-2-ethylidene-1-adamantyl system.These correlations showed an intercept of 0.8 kcal mol^-1, which suggested the greater stability of the (Z)-2-ethylidene-1-adamantyl cation than the corresponding E cation by this amount.Ab initio calculations (RHF/6-31G**)showed that the Z cation is more stable than the E cation by 1.0 kcal mol^-1, and that the large Z:E rate ratios are in part ascribed to the difference in the cation stability.