Substituent Effects on the Solvolysis Rates and Gas Phase Stabilities of 1,2,2-Trimethyl-1-phenylpropyl and 1,2,2-Trimethyl-1-(2-methylphenyl)propyl Systems

摘要

Substituent effects on the solvolysis rates of 1,2,2-trimethyl-1-phenylpropyl chlorides in 80% (v/v) aq acetone at 45 deg C and 1,2,2-trimethyl-1-(2-methylphenyl)propyl p-nitrobenzoates in 50% (v./v) aq ethanol at 75 deg C were correlated with the Yukawa-Tsuno equation to give #rho#=-4.28 and r=0.91, and #rho#=-2.78 and r=j 0.70, respectively. The reduction in r values from r=1.00 for full conjugation is ascribed to deviation from coplanarity of the carbocationic center and the benzylic #pi#-system in the transition state. Substituent effects on the gas phase stabilities of 1,2,2-trimethyl-1-phenyl propyl actions and 1,2,2-trimethyl-1-(2-methylphenyl)propyl cations were correlated to give #rho#=-9.1 and r=0.89 and #rho#=-6.6 and r=0.70, respectively. The identical r values were obtained for the transition states and for the intermediates. The experimentally obtained torsion angles for twisting estimated from the relationship r/r_(max)=cos~2 #theta# are in good agreement with theoretically calculated dihedral angles of the corresponding Becke3LYP/6-31G~* optimized carbenium ions. This identity provides convincing evidence for occurrence of stearic inhibition of resonanace through loss of coplanarity and provides strong support for our characterization of the r value as a resonance demand parameter.