Acid-Catalyzed Isomerization of Pivalaldehyde to Methyl Isopropyl Ketone via a Reactive Protosolvated Carboxonium Ion Intermediate

Abstract

Quantitative rearrangement of pivalaldehyde to methyl isopropyl ketone is observed in acids such as trifluoromethanesulfonic acid, anhydrous HF, and trifluoroethyl alcohol-BF3 but not in trifluoroacetic acid. Studies in a mixture of trifluoroacetic acid and trifluoromethanesulfonic acid show that acids with H(o) < or = -11 are able to carry out complete isomerization. These results and density functional theory calculations at the B3LYP/6-31G level suggest that protonated pivalaldehyde undergoes further protosolvation at higher acidities to a reactive superelectrophilic species resulting in rearrangement. A mechanism for the pivalaldehyde rearrangement to methyl isopropyl ketone in strong protic acids involving a reactive protosolvated superelectrophilic intermediate is suggested. Aspects of the related mechanism of the reaction with isobutane with CO in HF/BF3 medium leading to methyl isopropyl ketone are also discussed.

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.…”

“…We suggested that the reaction proceeds through the intermediacy of gitionic diprotonated pivalaldehyde, which undergoes subsequent rearrangement to methyl isopropyl ketone. We explained the effect of acidity on pivalaldehyde rearrangement to methyl isopropyl ketone by monitoring the rearrangement under varying acidities provided by mixtures of trifluoroacetic acid and trifluoromethanesulfonic acid [4].…”

“…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.…”

“…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].…”

“…We have studied the rearrangement of pivaladehyde to methyl isopropyl ketone in the presence of various superacidic media such as anhydrous HF, triflic acid, boron trifluoride-2,2,2-trifluoroethanol complex (BF 3 .2CF 3 CH 2 OH) etc. [4]. In this paper we report the rearrangement of pivalaldehyde to methyl isopropyl ketone with stable amine:HF complexes.…”