Elimination Reactions of Alkanesulfenyl Derivatives: Effect of Structure on Reactivity in Thioketone-Forming Eliminations of Diarylmethyl Thiosulfonates

Article abstract of DOI:10.1021/jo00201a007

The reaction of a group of diarylmethyl arenethiosulfonates, ArAr'CHSSO2Ar'' (2), with (a) two alkoxide ions (i-PrO^- and MeO^-), (b) a series of secondary and tertiary amines of differing base strength, and (c) phenoxide ion has been examined.For each system both the overall rate of disappearance of 2 and the fraction (α_elim) converted to thioketone were determined.Salient results are as follows: (1) The ρ values for thioketone-forming elimination of ArAr'CHSSO2C6H4CH3-p with either isopropoxide (+3.4) or piperidine (+3.5) are large and positive, while theρ value associated with variation of the substituent in Ar'' in the elimination of Ph2CHSSO2Ar'' with i-PrO^- is quite modest (+1.3). (2) The Broensted β for the elimination reaction of p-nitrobenzhydryl p-toluenethiosulfonate with the series of amines is close to +1.0. (3) While plots of the elimination rate constant (k_elim) vs. for any of the amine-induced elimination in amine-amineH⁺ buffers are linear, plots of k_elim vs. -> for the phenoxide-induced elimination in PhO^--PhOH buffers very pronounced downward curvature (Figure 4).These various results can be explained by assuming that the different eliminations proceed by different variants of an ElcB mechanism: for the elimination involving amines and 2 a reversible (ElcB)_{ion pr} mechanism (eq 13) is suggested; in the elimination with phenoxide ion the reaction proceeds by an ordinary (ElcB)_reversible mechanism (eq 9, k_i>k_ii); in the elimination involving isopropoxide the mechanism becomes (ElcB)_irreversible (eq 9, k_-iii).Comparison of selected data on the rates of thioketone-forming eliminations of 2 with amines with data obtained previously (ref 6) on the rates of sulfene-forming eliminations of aralkyl α-disulfones with amines indicates that an arylalkanesulfonyl compound undergoes elimination approximately 300 million times faster than the equivalently substituted arylalkanesulfenyl derivative.