Protioacyl Dications:  Hydrogen/Deuterium Exchange, Rearrangements, and Theoretical Studies¹

Hydrogen/deuterium exchange was observed by 2 H-NMR spectroscopy at the CH 3 groups of the long-lived alkanoyl cations CH 3 CH 2 CO + ( 4 ), (CH 3 ) 2 CHCO + ( 8 ), and (CH 3 ) 3 CCO + ( 13 ) when treated with excess DF·SbF 5 superacid. The intermediacy of the corresponding protio(deuterio)acylium dications is suggested to account for the exchanges. Under similar conditions, no exchange was observed in the acetyl ion CH 3 CO + ( 1 ) in DF·SbF 5, 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 acetaldehyde. Density functional theory calculations at the B3LYP/6-31G** level were carried out to support the suggested exchange mechanism as well as lack of exchange in the acetyl cation. On the basis of the calculated results, two alternative mechanisms are also suggested for the Nenitzescu rearrangement of pivaloyl chloride in isobutane with excess AlCl 3 .