Solvolysis of Highly Congested Tertiary Benzylic p-Nitrobenzoates. Kinetic Probe for Detecting Resonance Contribution of Aryl Moiety to the Cationic Transition State

摘要

The solvolysis of a series of 1-aryl-2,2-dimethyl-1-phenyl-1-propyl p-nitrobenzoates (6a-6g) in a variety of solvents were studied. Both the single-parameter Hammett-Brown equation (Eqn. 1) and the dual-parameter Yukawa-Tsuno equation (Eqn. 2) showed excellent linear correlations, although inverse order of k(p-CF_3)/k(m-CF_3) was realized in five different solvent systems. A similar result was also found in 3-aryl-2,2,4,4-tetramethyl-3-pentyl p-nitrobenzoates (5) in 80% acetone. The result of Grunwald-Winstein type correlation analysis against Y_(BnOPNB) or Y_(xBnOPNB) showed ambiguity for interpretation. It provides more evidence for the lack of coplanarity between the aryl ring and the sp~2 plane at the cationic transition state, and thus an insignificant resonance contribution from the aromatic moiety. A comprehensive collection of published data is tabulated.