Bromination of cyclic olefins. Part I. Critical dependence of carbonium ion stability on coplanarity

Abstract

The kinetics of bromination of 3-phenylindenes (1), 1,2-dihydro-4-phenylnaphthalenes (2), and 6,7-dihydro-9-phenyl-5H-benzocycloheptenes (3) in methanol at 25° were studied. The rate constants for compounds with various substituents in the phenyl group were correlated in terms of the Hammett equation by use of σ for compounds (2) and σ⁺ for compounds (3). This difference in the transmission of resonance effects has been interpreted in terms of the geometries of the systems: in compounds (3) the substituted phenyl ring is coplanar with the carbonium ion intermediate, but in compounds (2) the phenyl ring rotates out of this plane. Carbonium ion stability (and therefore the σ value appropriate to a given substituent) is dependent on the amount of p-π overlap between the carbonium ion centre and the aryl ring.