Methoxide-Catalyzed Decomposition of Diarylmethyl (Arylsulfonyl)methyl Sulfoxides: A Sulfine-Forming Elimination on the (E1cB)rev/(E1cB)irrev Borderline

Article abstract of DOI:10.1021/jo00004a017

In 7:3 CH3OH-DMSO (v/v) in the presence of methoxide ion, diarylmethyl (arylsulfonyl)methyl sulfoxides (Ar2CHS(O)CH2SO2Ar') 4, undergo elimination remarkably easily to afford the diarylsulfine and the aryl methyl sulfone (eq 4).Comparison of the rate of cleavage of 4 (k_elim) and the rate of disappearance of the 1H NMR signal (k_CHSO) for the Ar2CHS(O) proton in CD3OD-DMSO shows that the mechanism for the elimination is on the (E1cB)_rev/(E1cB)_irrev borderline, (k_CHSO/k_elim) ranging from 1.2 to 5.2, depending on the nature of the Ar and Ar'groups in 4.Slight changes in structure can shift the mechanism from (E1cB)_rev to (E1cB)_irrev as a result of their effect on the partitioning of the α-sulfinyl carbanion intermediate (Ar2CS(O)CH2SO2Ar') 5, between cleavage to diarylsulfine plus ArSO2CH2^- (step k_ii, eq 8) and protonation to regenerate 4 (step k_-i).Structural changes that make Ar'SO2CH2 a better leaving group increase k_ii/k_-i and shift the mechanism toward (E1cb)_irrev as does also an increase in the percentage of DMSO in the solvent.Structural changes in Ar that enhance the stability of 5 decrease k_ii/k_-i and shift the mechanism toward (E1cb)_rev.It is also shown that for 4 in general k_ii appears larger than would be expected for a leaving group of the basicity of Ar'SO2CH2^-.Repulsion between the dipoles of the S(O) and SO2 groups in 5 is thought to be responsible.