Desulfinylation of Prop-2-enesulfinic Acid: Experimental Results and Mechanistic Theoretical Analysis

The potential energy surfaces of the desulfinylation of prop-2-enesulfinic acid (13) in CH2Cl2 solution at -15 degrees C have been explored by quantum calculations and analyzed with kinetic data obtained for the reaction in absence or presence of additives. Monomeric 13 adopts a preferred conformation with gauche S=O/sigma(C(1)-C(2) bond pairs and the O-H bond pointing toward C(3). It equilibrates with the more stable dimer (13)(2) (at - 15 degrees C) formed by two O-H center dot center dot center dot O=S hydrogen bonds and in which the S=O/sigma C(1)-C(2) are gauche also, but the SOH moieties are antiperiplanar with respect to sigma(C(1)-C(2)). Dimer (13)(2) undergoes desulfinylation into propene + SO2 + 13 following a one-step, concerted mechanism. The preferred transition state is a six-membered, chairlike transition structure (C center dot center dot center dot S elongation and S-O center dot center dot center dot H center dot center dot center dot C(3) hydrogen transfer occur in concert) in which the S=O/sigma(C(1)-C(2)) bonds are gauche (S=O adopt pseudoaxial positions). There are at least 48 transition states, each one defining a different pathway, all with similar calculated free energies (Delta G(double dagger) = 25.3-28.6 kcal/mol), which makes the bimolecular (autocatalyzed) retro-ene elimination Of SO2 competing (entropy factor) with a monomolecular process for which the transition state (calculated Delta G(double dagger) = 24.3 kcal/mol) implies only one molecule of sulfinic acid. This agrees with the experimental rate law of the reaction which is first order in the concentration of dimer (13)(2). SO2, CF3COOH, and BF3 center dot Me2O do not catalyze the reaction. In the presence of an excess of BF3 center dot Me2O the desulfinylation is completely inhibited due to the formation of a stable tetramolecular complex of type (CH2=CHCH2SO2H center dot BF3)(2) (18), for which quantum calculations show that the S=O/sigma(C(1)-C(2)) bonds are antiperiplanar whereas the S-OH/sigma(C(1)-C(2)) bonds are gauche. Independently of the additive, the retro-ene eliminations of SO2 are calculated to be concerted and have transition states adopting six-membered cyclic structures in which S=O and sigma(C(1)-C(2)) are gauche, the S=O interacting with the additive. Preliminary experiments suggested that the thermodynamically unfavored ene reaction Of SO2 with propene can occur at low temperature using 1 equiv of BF3.