Novel Diborane-Analogue Transition Structures for Borane Reactions with Alkyl Halides^†

Ab initio and DFT calculations were performed to examine the mechanisms of reduction of alkyl halides and formaldehyde by borane. With alkyl halides, the optimized transition structure geometry resembled diborane, with a pair of hydrogen atoms bridging the boron and carbon atoms by three-center−two-electron bonds. A similar transition structure was found for the reduction of formaldehyde, although it was not the lowest-energy transition structure. Solvation by dimethyl ether or dimethyl sulfide disrupted this bridging with chloromethane, while both ligands dissociated from borane during the reduction of formaldehyde. The high calculated activation free energies of alkyl halide reduction are consistent with their observed lack of reactivity with borane.