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

Allylic sulfinic acids have been prepared by cleavage of trialkyltin a llylic sulfinates with trifluoroacetic acid (TFA). Allylsulfinic acid prepared in this way could be trapped as the sodium salt or observed b y NMR and UV spectroscopy to undergo spontaneous decomposition to prop ene and sulfur dioxide at ambient temperatures. The corresponding reac tion of 1-methylprop-2-enylsulfinic acid was regiospecific, yielding ( E)- and (Z)-2-butene (E:Z = 82:18). Acidolysis of E/Z mixtures of trim ethyltin 5-methylcyclohex-2-enylsulfinate with [H-2]-TFA followed by d esulfination indicated that the latter process proceeded with gamma-sy n deuterium migration, consistent with a concerted, retro-ene mechanis m. A detailed kinetic investigation of allylsulfinic acid desulfinatio n supported this conclusion. This reaction obeyed a first order rate l aw (k(297K) = (5.5 +/- 0.1) x 10(-4) s(-1) in toluene) with a large, n egative activation entropy (Delta S double dagger = -146 +/- 17 J K-1) . The rate of desulfination was essentially independent of solvent. Th e 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 Die ls-Alder reaction and taken together with the above evidence and a rel atively small deuterium kinetic isotope effect (k(H)/k(D) = 2.5 +/- 0. 1) strongly supports a concerted desulfination, proceeding via a relat ively compact, early transition state.