Croat. Chem. Acta 67 (1994) 171-188.
A Kinetic Approach to the Mechanism of Cationic Polyolefinic Cyclization. Simple and Extended pi-Participation
Stanko Borcic, Olga Kronja and Kresimir Humski
Faculty of Pharmacy and Biochemistry, University of Zagreb, A. Kovacica 1, P. O. Box 156, 10000 Zagreb, Croatia
This review deals with the mechanism of the biomimetic olefinic cationic polycyclizations considering relative rate effects, activation parameters, substituent rate effects, sigma, ro correlation and secondary deuterium kinetic isotope effects (KIE) in solvolysis, obtained with tertiary and benzylic substrates comprising one, two or more double bonds, respectively, sited at the same positions as in the natural precursor (C-5, C-9, C-13). Kinetic behaviour of substrstes with one double bond at position 5 (models for monocyclization) which are structurally related to the structures 16U and 22U suggests that the formation of the first cyclohexane ring is a concerted process with some exeptions (16U with p-OCH3). In solvolysis of chlorides with double bonds at positions 5 and 9 (28 and 29U) extended pi-participation occurs, i. e. both double bonds of the aliphatic chain are involved in the rate determining step. Substrates with more than two double bonds (30U and 31U) solvolyze 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 sensitive probe for neighboring group participation.