Isolation and Characterization of Borane Complexes of Dimethylsulfoxonium Methylide

Abstract

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 °C. 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.