PROTIOACYL DICATIONS - HYDROGEN DEUTERIUM EXCHANGE, REARRANGEMENTS, AND THEORETICAL-STUDIES/

Hydrogen/deuterium exchange was observed by H-2-NMR spectroscopy at th e CH3 groups of the long-lived alkanoyl cations CH3CH2CO+ (4), (CH3)(2 )CHCO+ (8), and (CH3)(2)CCO+ (13) when treated with excess DF .-SbF5 s uperacid. The intermediacy of the corresponding protio(deuterio)acyliu m dications is suggested to account for the exchanges. Under similar c onditions, no exchange was observed in the acetyl ion CH3CO+ (1) in DF . SbF5, but at the same time its electrophilic reactivity is greatly enhanced in superacids. The acetyl cation in a superacidic medium also abstracts tertiary hydrogens of isoalkanes to give protonated acetald ehyde. Density functional theory calculations at the B3LYP/6-31G* lev el were carried out to support the suggested exchange mechanism as wel l as lack of exchange in the acetyl cation. On the basis of the calcul ated results, two alternative mechanisms are also suggested for the Ne nitzescu rearrangement of pivaloyl chloride in isobutane with excess A lCl3.