DESULFINATION OF ALLYLIC SULFINIC ACIDS - CHARACTERIZATION OF A RETRO-ENE TRANSITION-STATE

Allylic sulfinic acids have been prepared by cleavage of trialkyltin allylic sulfinates with trifluoroacetic acid (TFA). 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 1-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 [H-2]-TFA followed by desulfination indicated that the latter process proceeded with gamma-syn deuterium migration, consistent with a concerted, retro-ene mechanism. A detailed kinetic investigation of allylsulfinic acid desulfination supported this conclusion. This reaction obeyed a first order rate law (k(297K) = (5.5 +/- 0.1) x 10(-4) s(-1) in toluene) with a large, negative activation entropy (Delta S double dagger = -146 +/- 17 J K-1). The rate of desulfination was essentially independent of solvent. The volume profile (Delta V double dagger = -5.5 +/- 1.0 cm(3) mol(-1), Delta V = 15 +/- 5 cm(3) 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 (k(H)/k(D) = 2.5 +/- 0.1) strongly supports a concerted desulfination, proceeding via a relatively compact, early transition state.