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

The first typical examples are described on the solvolysis rate enhanc ements ascribed to the relief of F-strain between an alkyl group and t he leaving group atom directly attached to the reaction center. The ra tes and products of solvolyses in ethanol were studied for 2-methylene - and (Z)- and (E)-2-ethylidenebicyclo[2.2.2]oct-1-yl triflates. Solvo lyses were also conducted in ethanol and 2,2,2-trifluoroethanol (TFE) on 2-methylene- and (Z)- and (E)-2-ethylidene-1-adamantyl compounds ha ving OMs, F, Cl, Br, or I asa leaving group. Ah the substrates gave th e corresponding bridgehead substitution products as kinetic control pr oducts. The Z:E rate ratios at 25 degrees C were 217 +/- 6 for 2-ethyl idenebicyclo[2.2.2]oct-1-yl triflates (ethanol) and 109 +/- Il(ethanol ) and 117 fl(TFE)for 2-ethylidene-1-adamantyl mesylates. O-18 scrambli ng studies on the ethanolyses of(Z)- and (E)2-ethylidene-1-adamantyl m esylates showed that the titrimetrically determined Z:E rate ratios ca n 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 ha lides varied in the sequence F (ca. 70), Cl(1020 +/- 160), Br (2230 +/ - 90), and I (9500 +/- 280). The significant increases in the rate rat io with the increase in the atomic size of halogen were explained in t erms 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 bet ween the Z and E isomers (slope 1.0) and against Hansch's Es, demonstr ating the significance of F-strain effect in the enhanced rates of the (Z)-2-ethylidene-1-adamantyl system. These correlations showed an int ercept of 0.8 kcal mol(-1), which suggested the greater stability of t he (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 tha t the large Z:E rate ratios are in part ascribed to the difference in the cation stability.