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

Abstract

Hydrogen/deuterium exchange was observed by 2H-NMR spectroscopy at the CH3 groups of the long-lived alkanoyl cations CH3CH2CO+ (4), (CH3)2CHCO+ (8), and (CH3)3CCO+ (13) when treated with excess DF·SbF5 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 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 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 AlCl3.

Carbocationic rearrangement of pivaloyl cation and protonated pivalaldehyde in superacid medium: A novel solution equivalent of the McLafferty rearrangement

“…As we have shown earlier [4,5], the rearrangement of pivalaldehyde to methyl isopropyl ketone is strictly dependent on the acidity of the medium. Since pivalaldehyde and pivaloyl cation differ only by a hydride ion, it was worthwhile studying by theoretical methods the nature and reactivity of the pivaloyl ion under superacidic conditions as well as in the presence of hydride donors.…”

“…Also the acylium salts in superacid media are found to be excellent acylating agents for deactivated aromatics. However, Olah et al have shown that such reactions do not take place with acylium salts themselves in the absence of superacids in aprotic solvents such as SO 2 , SO 2 ClF, AsF 3 , or CH 2 Cl 2 [3].As we have shown earlier [4,5], the rearrangement of pivalaldehyde to methyl isopropyl ketone is strictly dependent on the acidity of the medium. Since pivalaldehyde and pivaloyl cation differ only by a hydride ion, it was worthwhile studying by theoretical methods the nature and reactivity of the pivaloyl ion under superacidic conditions as well as in the presence of hydride donors.…”

“…No ketone was isolated when SnCl 4 was used instead of AlCl 3 . One of the two possible mechanisms, based on our theoretical calculations [5], shows that the reaction involves direct hydride abstraction by the O-complexed aprotic superacidic pivaloyl cation-AlCl 3 Complex (13), followed by further complexation to protosolvated pivalaldehyde-AlCl 3 Complex (15), which, in turn, undergoes rapid rearrangement to methyl isopropyl ketone (2) (Scheme 6).…”

“…Over the past decade Olah et al have explored extensively the nature and behavior of acyl ions in superacid medium [4,5,16,18]. They suggested that the de facto reactive intermediate in the reaction of acyl ions under superacidic conditions is not simple acyl cations itself, but its O-protonated (protosolvated) form R-COH 2ϩ (9a-c), which is a highly electron deficient superelectrophilic gitonic dication substantially more reactive than its parent monocation [5].…”

Attempted hydrogen–deuterium exchange of the protio -trimethyloxonium dication (CH 3 ) 3 OH 2+ , study of methylating ability of (CH 3 ) 3 O + in superacids and theoretical investigations

“…, with various counterions such as SbF 6 Ϫ , BF 4 Ϫ , SbCl 6 Ϫ , and PF 6 Ϫ are excellent alkylating agents for nucleophiles containing heteroatoms such as N, O, or S. In general they are not capable of C-alkylation of aromatic or aliphatic compounds. In conjunction with protic superacid FSO 3 H:SbF 5 (Magic Acid), however, they were found to alkylate aromatics (1).…”