Reversible generation of metastable enols in the 1,4-addition of thioacetic acid to α,β-unsaturated carbonyl compounds.

Addition of thioacetic acid to reactive α,β-unsaturated carbonyl compounds like acrolein or crotonaldehyde in acetone-d(6) generates metastable (E)- and (Z)-1-alkenols, which tautomerize slowly at ambient temperature. The 1,4-addition of thioacetic acid and crotonaldehyde to (Z)-3-(acetylsulfanyl)-1-propen-1-ol is reversible with K(eq) = 5.5 ± 0.5 L/mol. A concerted, cyclic 1,4-addition mode is proposed to explain the preferred (Z)-stereoselectivity in lower polarity, nonprotic solvents.