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.