Hindered rotation in a 1-alkylnapthalene. A dynamic nuclear magnetic resonance, difference nuclear overhauser enhancement, and molecular mechanics investigation of 1-neopentylnaphthalene. The t-butyl group as a probe of chirality
Abstract
A consideration of earlier results suggests that in contrast to other simple 1-alkylnapthalenes, the barrier to rotation of the neopentyl group in 1-neopentylnaphthalene may be high enough to be measured by dynamic n.m.r. spectroscopy. A synthesis is reported, and changes in the n.m.r. spectrum at low temperature suggest a barrier to rotation of 5.15 kcal mol–1. Molecular mechanics calculations and difference NOE n.m.r. experiments suggest the structure of the ground state and transition state conformations, and the latter is not, apparently, the one with the t-butyl group in the plane of the naphthalene ring. The barrier to rotation of the naphthyl group in the much more highly substituted 2-t-butyl-2-(1-naphthyl)-1,3-dioxolane is only 5.15 kcal mol–1 as shown by two methyls of the slowly rotating t-butyl group becoming diastereotopic as rotation of the napthyl group also becomes slow.