S. Borcic et al., A KINETIC APPROACH TO THE MECHANISM OF CATIONIC POLYOLEFINIC CYCLIZATION - SIMPLE AND EXTENDED PI-PARTICIPATION, Croatica chemica acta, 67(2), 1994, pp. 171-188
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.