A KINETIC APPROACH TO THE MECHANISM OF CATIONIC POLYOLEFINIC CYCLIZATION - SIMPLE AND EXTENDED PI-PARTICIPATION

This review deals with the mechanism of the biomimetic olefinic cation ic polycyclizations considering relative rate effects, activation para meters, substituent rate effects, sigma, rho correlation and secondary deuterium kinetic isotope effects (KIE) in solvolysis, obtained with tertiary and benzylic substrates comprising one, two or more double bo nds, respectively, sited at the same positions as in the natural precu rsor (C-5, C-9, C-13). Kinetic behaviour of substrates with one double bond at position 5 (models for monocyclization) which are structurall y related to structures 16U and 22U suggests that the formation of the first cyclohexane ring is a concerted process with some exceptions (1 6U with p-OCH3). In solvolysis of chlorides with double bonds at posit ions 5 and 9 (28 and 29U) extended pi-parcipation occurs, i.e. both do uble bonds of the aliphatic chain are involved in the rate determining step. Substrastes with more then two double bonds (30U and 31U) solvo lyze by way of extended pi-participation. It remains unclear if two or three double bonds are involved. The paper also shows clear evidence that the magnitude of beta-deuterium secondary KIE are the most sensit ive probe for neighboring group participation.