Study of Reactions Leading to Sulfine Formation. 4. Evidence for a Reversible E1cB Mechanism for Base-Induced Elimination of N,N-Diisopropyl-9-fluorenesulfinamide

Article abstract of DOI:10.1021/jo00292a028

Previous examination of base-catalyzed sulfine formation from methyl diarylmethanesulfinates, Ar2CHS(O)OMe, has shown that the elimination takes place by an (E1cB)_irrev mechanism, even though MeO is not a good leaving group.It raised the question as to whether base-catalyzed sulfine formation would be possible with substrates Ar2CHS(O)Y where Y was an even poorer leaving group, and if so, at what point the mechanism would shift to (E1cB)_rev.In the present study N,N-diisopropyl-9-fluorenesulfinamide (3) has been found to undergo H/D exchange of the 9-H in CD3OD in the presence of methoxide 120 times faster than it undergoes elimination to give 9-thiofluorenone S-oxide (4).This shows that when the leaving group is R2N sulfine formation is still possible but that the mechanism is now (E1cB)_rev.The solvent isotope effect associated with the formation of 4 from 3 indicates that expulsion of R2N from the intermediate carbanion (7) must be accompanied by the transfer of a proton from the solvent to the leaving group, so that it departs as R2NH rather than R2N(-).