SECONDARY DEUTERIUM-ISOTOPE EFFECTS IN SOLVOLYSIS OF 1-ARYL-5-METHYL-5-HEPTEN-1-YL CHLORIDES - COMPARISON OF PI-PARTICIPATION AND NORMAL-PARTICIPATION MECHANISMS

Three series of deuterated compounds related to 1 were prepared and their solvolysis rates measured. Since the relative contribution of kc and kΔ to the observed solvolysis rates of 1 are known and since the isotope effects on the kc route can be estimated, it is possible to extract from the data secondary deuterium isotope effects associated with the kΔ process. In all cases, the α-deuterium isotope effects on kΔ are reduced in magnitude as compared to those in the kc process, which is consistent with bridging. β-Deuterium isotope effects on kΔ are essentially nil which can be explained as being due to both conformational restrictions to hyperconjugative electron release from the C-D bonding orbitals caused by bridging and to charge delocalization away from the reaction center. Deuteration of the aliphatic double bond produces inverse isotope effects in all cases where anchimeric assistance is operative. The latter observation clearly demonstrates the direct involvement of the aliphatic double bond in the rate-determining step of the kΔ process. A direct comparison of rates show that 1-aryl-4-methoxy-1-butyl chlorides 3 solvolyze slightly faster than the corresponding chlorides of the series 1. The rate factors are, however, too small to confirm the possibility that n and π participations proceed by different mechanisms. If the aliphatic methoxyl group is directly involved in the rate-determining step of the kΔ process throughout the series 3, then measured isotope effects reflect transition-state structures with a variable amount of bridging, depending upon the solvent and/or the nature of the aromatic substituent. © 1979 American Chemical Society.