Isolation and Characterization of Borane Complexes of Dimethylsulfoxonium Methylide

Trialkylboranes (R 3 B) catalyze the repetitive insertion of methylene from dimethylsulfoxonium methylide ( 1 ) to form polymethylene. A proposed intermediate in this reaction is a 1:1 complex between R 3 B and ylide 1 . Following complexation, an alkyl group (boron-substituted) undergoes a 1,2-migration to the methylide carbon with displacement of a molecule of DMSO. A series of complexes of dimethylsulfoxonium methylide ( 1 ) and various organoboranes, X 3 B (X = H, Ph, F, C 6 F 5 ), have been prepared and isolated. Molecular structures, obtained by single-crystal X-ray diffraction, for ylide·BF 3 ( 3 ) and ylide·B(C 6 F 5 ) 3 ( 4 ) were found to contain geometries with potential migrating groups anti-periplanar to the carbon−sulfur bond. The stability of solutions of these complexes ranges considerably. For example, ylide·BPh 3 ( 6 ) undergoes reaction at room temperature, while ylide·B(C 6 F 5 ) 3 ( 3 ) is stable to temperatures > 100 °C. All complexes can be prepared as solids stable at room temperature. The solid-state stability of ylide·BR 3 complexes was evaluated by differential scanning calorimetry. The decomposition temperature increases across the series for R = H, Ph, C 6 F 5, F. The heats of reaction of ylide·BR 3 are R = H (−54.7), Ph (−15.7), C 6 F 5 (−21.6), and F (−17.1) kcal mol - 1, respectively.