Solvolyses of Bicyclo[2.2.2]oct-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
作者:Yasushi Ohga、Motohiro Munakata、Toshikazu Kitagawa、Tomomi Kinoshita、Ken'ichi Takeuchi、Yasuo Oishi、Hiroshi Fujimoto
DOI:10.1021/jo00094a012
日期:1994.7
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 asa leaving group. Ah the substrates gave the corresponding bridgehead substitution products as kinetic control products. The Z:E rate ratios at 25 degrees C were 217 +/- 6 for 2-ethylidenebicyclo[2.2.2]oct-1-yl triflates (ethanol) and 109 +/- Il(ethanol) and 117 fl(TFE)for 2-ethylidene-1-adamantyl mesylates. O-18 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 degrees 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 x log[k(Z)/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/G-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.