Dehydrocoupling of amine boranes via tin(IV) and tin(II) catalysts

• New tin catalysts for amine borane dehydrocoupling.
• Moderate activity for alkyl and parent amine boranes.
• Apparent dependence on substrate for mechanism.
• Loss of organic substrate suggests protonation by amine functionality.
• No difference in activity for tin pre-catalyst oxidation state.

Catalytic dehydrocoupling of primary and secondary amine boranes and ammonia borane using tin(IV) and tin(II) catalysts is presented. These tin compounds have been demonstrated to dehydrocouple phosphines where catalytic activity was dependent on metal oxidation state. In contrast, the mechanism of amine borane dehydrocoupling appears to be substrate dependent. The most sterically encumbered substrate, tBuNH2BH3, appears to engage in β-hydrogen elimination based on the observation of tBuN = BH2, whereas Me2NHBH3 appears to be involved in a chain-growth process as evidenced by linear diborazane and the absence of amino borane. Though these tin catalysts are moderately active in the spectrum of amine borane dehydrocoupling catalysts, the dependence of mechanism on substrate appears to be unique.

Tin(II) and tin(IV) complexes are competent catalysts for the dehydrocoupling of amine boranes under modest conditions. There is a strong substrate effect in the dehydrocoupling whereby tBuNH2BH3 appeared to be dehydrocoupled by β-hydrogen elimination where Me2NHBH3 and NH3BH3 appear to be dehydrocoupled by a chain-growth process.Figure optionsDownload as PowerPoint slide