Resonance Effects in the Solvolysis of α -t-Butyl- α-neopentylbenzyl and α-t-Butyl-α-isopropylbenzyl p-Nitrobenzoates

Article abstract of DOI:10.1246/bcsj.70.929

Substituent effects on the solvolysis rates of α-t-butyl-α-neopentylbenz.yl and α-t-butyl-α-isopropylbenzyl p-nitroben/oates in 50% (v/v) aqueous EtOH at 75 °C were correlated with the Yukawa-Tsuno equation to give p = -3.37 and r = 0.78. and p= -3.09 and r = 0.68, respectively. The reduction in r values from r= 1.00 characteristic of full conjugation is ascribed to deviation from coplanarity of the carbocationic center and the benzylic π-system in the transition state. The torsion angles for twisting out of coplanarity were estimated from the relationship r/r_max = cos² θ for a series of sterically hindered tertiary dialkylbenzylic solvolyses. The torsion angles assigned for these twisted benzylic systems are in good agreement with theoretically calculated dihedral angles of the corresponding 6-3IG ^* optimized carbenium ions. This identity provides convincing evidence for occurrence of steric inhibition of resonance through loss of coplanarity, and, in turn, provides strong support for our characterization of the r value as a resonance demand parameter reflecting the degree of resonance (π-delocalization) interaction between an aryl group and the reaction site.