Pregledni rad

A Kinetic Approach to the Mechanism of Cationic Polyolefinic Cyclization. Simple and Extended π-Participation

Stanko Borčić ; Faculty of Pharmacy and Biochemistry, University of Zagreb, A. Kovačića 1, P. O. Box 156, 41000 Zagreb, Croatia
Olga Kronja ; Faculty of Pharmacy and Biochemistry, University of Zagreb, A. Kovačića 1, P. O. Box 156, 41000 Zagreb, Croatia
Krešimir Humski ; Faculty of Pharmacy and Biochemistry, University of Zagreb, A. Kovačića 1, P. O. Box 156, 41000 Zagreb, Croatia

Sažetak

This review deals with the mechanism of the biomimetic olefinic cationic polycyclizations considering relative rate effects, activation parameters, substituent rate effects, a, p 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 substrates with one double bond at position 5 (models for mono- cyclization) which are structurally related to structures 16U and 22U suggests that the formation of the first cyclohexane ring is a concerted process with some exceptions (16U with p-OCH3). In solvolysis of chlorides with double bonds at positions 5 and 9 (28 and 29U) extended it-parcipation occurs, i.e. both double bonds of the aliphatic chain are involved in the rate determining step. Substrastes with more then two double bonds (30U and 31U) solvolyze by way of extended 7i-participation. It remains unclear if two or three double bonds are involved. The paper also shows clear evidence that the magnitude of 3-deuterium secondary KIE are the most sensitive probe for neighboring group participation.

Ključne riječi

Hrčak ID:

136883

URI

https://hrcak.srce.hr/136883

Datum izdavanja:

1.8.1994.

Posjeta: 984 *