Desulfination of Allylic Sulfinic Acids: Characterisation of a Retro-Ene Transistion State

Abstract

Allylic sulfinic acids have been prepared by cleavage of trialkyltin allylic sulfinates with trifluoroacetic acid (TEA). Allylsulfinic acid prepared in this way could be trapped as the sodium salt or observed by NMR and UV spectroscopy to undergo spontaneous decomposition to propene and sulfur dioxide at ambient temperatures. The corresponding reaction of l-methylprop-2-enylsulfinic acid was regiospecific, yielding (E)- and (Z)-2-butene (E:Z = 82:18). Acidolysis of E/Z mixtures of trimethyltin 5-methylcyclohex-2-enylsulfinate with [2H]-TFA followed by desulfmation indicated that the latter process proceeded with -syn deuterium migration, consistent with a concerted, retro-ene mechanism. A detailed kinetic investigation of allylsulfinic acid desulfmation supported this conclusion. This reaction obeyed a first order rate law (&297k = (5.5 ± 0.1) x 10-4 s-1 in toluene) with a large, negative activation entropy (AS* = -146 ± 17 J K-1). The rate of desulfmation was essentially independent of solvent. The volume profile (AV* = —5.5 ± 1.0 cm3 mol-1, AV = 15 ± 5 cm3 mol-1) is comparable to that of a retro Diels—Alder reaction and taken together with the above evidence and a relatively small deuterium kinetic isotope effect (&h/&d = 2.5 ± 0.1) strongly supports a concerted desulfmation, proceeding via a relatively compact, early transition state.