Kinetics of iodination of some amine-boranes with molecular iodine in toluene

Abstract

R₃N.BH₃ (R=Me, Et, Prⁿ) at room temperature reacts with l₂ in toluene quantitatively to produce R₃N.BHI₂ when concentrations of the reactants are kept low. But with reactant concentrations ≥ 50 mmol dm^{- 3},R₃N<i>.</i> BH₂l and H₂ are produced as oibhserved earlier (<i>lnorg Synth, </i> 12 (1970) 120). A reaction scheme has been proposed to explain this difference. Production of R₃N.BHI₂ from R₃N.BH₃ proceeds in two consecutive steps: <img src='http://www.niscair.res.in/jinfo/equation43.gif' border=0> Both steps are first order each in [borane] and [I₂l. <i>k</i>₁ is greater than <i>k₂ </i> but both increase in the order Et &gt; Me &gt; Prⁿ. The secondary amine-borane (Et)₂NH.BH₃ reacts with l₂ much faster than any of the three tertiary amine-boranes studied. The observed order of reactivity of amine-boranes with homopolar l₂ is the same as that observed by earlier workers with heteropolar HOCI A bimolecular mechanism with a four-membered transition state and pentacoordinate B is proposed and the sequence of reactivity appears to be primarily a result of steric influence, though electronic effects might have a modifying role.