Role of Equilibrium Associations on the Hydrogen Atom Transfer from the Triethylborane-Methanol Complex

The triethylborane-methanol system used in radical deoxygenation and dehalogenation processes has been investigated. Unambiguous evidence for the formation of a complex between triethylborane and methanol is provided. It was shown that the complexation process is exothermic (Delta H-o approximate to -7.6 kcal mol(-1)) while being entropically disfavored (Delta S-o approximate to -24 cal mol(-1) K-1). This study demonstrates that only very small quantities of complex (1-2%) are present in most of the reported conditions used in dehalogenation and deoxygenation processes. Recalculating the rate constant for the hydrogen transfer to a secondary alkyl radical with this concentration suggests a value in the 10(6) M-1 s(-1) range for the complex itself, indicating a much more important activation of the O-H bond than previously thought. The importance of solvent effects is also highlighted. The formation of a larger amount of complex by the addition of methanol is accompanied by its deactivation via hydrogen bonding. These observations open new opportunitites for the future preparation of more effective hydrogen atom donors involving borane complexes.