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

George A Olah; Thomas Mathew; Alain Goeppert; Golam Rasul; G K Surya Prakash; Pierre Mothé Esteves

doi:10.1016/j.jasms.2004.05.002

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

J Am Soc Mass Spectrom. 2004 Jul;15(7):959-65. doi: 10.1016/j.jasms.2004.05.002.

Authors

George A Olah¹, Thomas Mathew, Alain Goeppert, Golam Rasul, G K Surya Prakash, Pierre Mothé Esteves

Affiliation

¹ Department of Chemistry, Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, 90089-1661, USA. [email protected]

PMID: 15234354
DOI: 10.1016/j.jasms.2004.05.002

Abstract

Both pivaloyl cation in the presence of hydride donors and protonated pivalaldehyde in superacid media (both aprotic and protic) rearrange to protonated methyl isopropyl ketone involving gitionic dicationic intermediates. In our earlier studies we have found that the rearrangement of pivaladehyde to methyl isopropyl ketone occurs quantitatively 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. Our present study with environmentally more benign and stable amine:HF complexes, namely pyridinium poly(hydrogen fluoride) (PPHF) (5), poly(4-vinylpyridinium) poly(hydrogen fluoride) (6), and poly(ethyleniminium) poly(hydrogen fluoride) (PEIHF) (7) shows that these modified HF equivalents can carry sufficient amount of immobilized HF and provide ample acidity for complete isomerization of pivalaldehyde to methyl isopropyl ketone. Calculations on protioformyl, acetyl and pivaloyl dications at the B3LYP/6-311 ++ G(d,p) and CCSD(T)/6-311 ++ G(d,p)//B3LYP/6-311 ++ G(d,p) levels have been performed to compare the nature of protosolvation of formyl, acetyl, pivaloyl cations and protonated pivaladehyde in superacid media. These studies further suggest protosolvation of protonated pivalaldehyde leading to gitionic dications at high acidities resulting in the carbocatioinic rearrangement. The reported carbocationic rearrangement under superacidic activation represents a novel solution chemistry equivalent of the well known gas-phase McLafferty rearrangement.