Rates of H/D Exchange of 9-Fluorenyl Sulfoxides: Evidence for an Irreversible E1cB Mechanism for Base-Induced Sulfine Formation from Methyl Diarylmethanesulfinates

Article abstract of DOI:10.1021/jo00250a033

A previous study had shown that the base-catalyzed, sulfine-forming eliminations of methyl 9-fluorenesulfinate (4) and other methyl diarylmethanesulfinates take place by either an E1cB-irreversible or an E2 mechanism, rather than the E1cB-reversible mechanism that had been expected.In the present work the rates (k_exch) of DABCO (diazacyclooctane)-catalyzed H/D exchange of the 9-H in a series of 9-fluorenyl sulfoxides (3) have been determined at 25 deg C in CD3OD.From a plot of log k_exch vs ?^* for R in 3 the anticipated rate for the DABCO-catalyzed formation ofthe 9-fluorenyl carbanion from 4 (eq 3, R = OCH3) can be estimated.Comparison of this rate with the actual rate (k_elim) of the DABCO-catalyzed, sulfine-forming elimination of 4 (eq 5) indicates that the elimination does not take place by an E2 mechanism in which there is a significant degree of cleavage of the S(O)-OCH3 bond in the rate-determining transition state.The results are, however, entirely consistent with an E1cB-irreversible mechanism for the elimination.