Olefination with sulfonyl halides and esters: Mechanistic DFT and experimental studies, and comparison with reactivity of phosphonates

Olefination of carbonyl compounds with carbanions of sulfonyl halides and activated sulfonates runs <i>via</i> aldol-type addition, cyclization to four-membered ring intermediates, and fragmentation to alkenes. The sequence differs from mechanism of sulfur-based Julia and (one-pot) Julia-Kocienski reactions, and mimics behavior of phosphorus-based reagents. In our report, we present energetic profile of the process based on DFT calculations, and discuss factors, which control <i>E</i>/<i>Z</i>-selectivity of produced alkenes. Importantly, the calculations reveal that the fragmentation step displays the highest barrier and involves anti-apicophilic oxathietanes, in which the OCH₂CF₃ group is located on the opposite side to the carbon atom. Aldehyde-exchange experiments, designed to keep steric constrains virtually unchanged, reveal effect of carbanion nucleophilicity on equilibration of the aldol addition step. We also demonstrate synthesis of non-stabilized and semi-stabilized carbanion precursors: octane- and phenylmethane-<i>phosphonates</i> of fluorinated alcohols (TFE and HFIP), and probe their reactions with benzaldehyde.