Conformational analysis of α-alkyl-ββ-di-isopropylstyrenes. A dynamic 1H nuclear magnetic resonance spectroscopic and molecular mechanics investigation

Abstract

The conformational equilibria and barriers to rotation in α-R-ββ-di-isopropylstyrenes with R = H, Me, Et, Prⁱ, CH₂Bu^t and Bu^t, and in tri-isopropylethylene and 1,1,2-tri-isopropylpropene investigated by dynamic ¹H n.m.r. spectroscopy and molecular mechanics (MM2, MMP2 force fields) are reported. The following conclusions are drawn: (i) the phenyl group is twisted ca. 70–90° out of the ethylene plane, (ii) the conformational populations of the geminal isopropyl groups indicate that the effective steric size of the phenyl group is larger than that of methyl, ethyl, isopropyl, and neopentyl groups, but smaller than that of the t-butyl group, (iii) barriers (ΔG^‡) to interchange of the isopropyl groups fall in the range 10.5–12.2 kcal mol^–1, (iv) molecular mechanics calculations fail to reproduce the experimentally determined conformational populations but satisfactorily reproduce the barriers to rotation of the isopropyl and phenyl groups. According to these calculations the isopropyl group rotations are not correlated.