Dioxane-monochloroborane: A new and highly reactive hydroborating reagent with exceptional properties

[GRAPHICS] Dioxane-monochloroborane was prepared by the reaction of dioxane and dioxan e-BCl3 with diborane or NaBH4 in 98% purity. The adduct thus obtained is a liquid, 6.2 M in BH2Cl, stable indefinitely at either 0 or 25 degrees C, Th e adduct hydroborates simple unhindered olefins to the corresponding dialky lchloroboranes within 0.5 h, while moderately hindered olefins take 1-4 h a t room temperature. Hindered tetrasubstituted olefins rapidly hydroborate t o the monoalkylchloroborane stage with further hydroboration slow, Regiosel ectivity studies of representative olefins reveal that this new reagent pos sesses selectivities similar to those from the monochloroborane-diethyl eth er adduct, Consequently, dioxane-monochloroborane can readily substitute fo r the older hydroborating agents, BH3:THF and BH3:DMS.