Sir: Monochloroborane-methyl sulfide, H2BCl.SMe2, and dichloroborane-methyl sulfide, HBC12.SMe2, are readily synthesized by redistribution of HsB.SMe2 with C13B.SMe2 in the appropriate ratios (eq 1, 2). The products are stable indefinitely at room temperature. Yet they hydroborate ole- fins readily with high regiospecificity and provide a valuable advantageous route to the corresponding dialkylboron chlo- rides, R2BC1, and monoalkylboron ...