Sd. Hiscock et al., DESULFINATION OF ALLYLIC SULFINIC ACIDS - CHARACTERIZATION OF A RETRO-ENE TRANSITION-STATE, Journal of organic chemistry, 60(22), 1995, pp. 7166-7169
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.