Conformational studies by dynamic NMR. 74. Stereomutations of the conformational enantiomers in peri-substituted 1-acylnaphthalenes

Naphthalenes bearing an acyl and a phenyl group in a peri relationship give rise to a pair of enantiomers in the temperature range where the rotations of the acyl group are slow. Such enantiomers were observed by means of low temperature NMR spectra in chiral environments. The barrier to rotation fo r the acyl substituents, that causes the interconversion of the enantiomers , was demonstrated to be lower than that for the phenyl group. In an approp riately synthesized derivative it was possible to measure the two barriers that were found equal to 10.4 and 15.9 kcal mol(-1), respectively. The barr iers for the acyl group rotation increase regularly (from 9.5 to 13.2 kcal mol(-1)) with the increasing dimension of the RCO groups (R = Me, Et, Pr-i, Bu-t). When a bromine atom replaces the phenyl group, the enantiomerizatio n barrier for the corresponding acyl derivatives increases significantly.