Isolation and characterization of borane complexes of dimethylsulfoxonium methylide

Trialkylboranes (R3B) 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 R3B 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, X3B (X = H, Ph, F, C6F5), have been prepared and isolated. Molecular structures, obtained by single-crystal X-ray diffraction, for ylide.BF3 (3) and ylide.B(C6F5)(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.BPh3 (6) undergoes reaction at room temperature, while ylide.B(C6F5)(3) (3) is stable to temperatures > 100 degreesC. All complexes can be prepared as solids stable at room temperature. The solid-state stability of ylide.BR3 complexes was evaluated by differential scanning calorimetry. The decomposition temperature increases across the series for R = H, Ph, C6F5, F. The heats of reaction of ylide.BR3 are R = H (-54.7), Ph (-15.7), C6F5 (-21.6), and F (-17.1) kcal mol(-1), respectively.