Borane transfer reactions
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Lewis base borane adducts (LB·BH3) constitute a well-known class of molecules with a number of diverse applications, including use as protected phosphines. The kinetics and thermodynamics of borane transfer reactions from a wide range of Lewis base borane complexes have been studied. The data generated has been used both as a quantitative tool to describe the nucleofugality (leaving group ability) of Lewis bases and as a means to improve the efficiency of phosphine borane deprotection reactions. The kinetics of borane transfer from a range of tertiary phosphine borane complexes to a wide range of amines have been determined. All kinetic data obtained, in addition to computational evidence, are consistent with a direct (SN2-like) mechanism, rather than a dissociative (SN1-like) process. The identities of the amine, phosphine and to a lesser extent solvent, impact substantially on the rate and equilibrium of the transfer, which can span several orders of magnitude. In depth structure activity relationships have been explored both for a wide variety of amine nucleophiles and phosphine nucleofuges. Taken as a whole, the data allow informed optimisation of the “deprotection” of a phosphine borane complex from the standpoint of rate or synthetic convenience. Additionally, the kinetics of ethanolysis of tri(o-tolyl)phosphine borane complex have also been studied. Using bridgehead amine quinuclidine as a benchmark, the kinetics of borane transfer from a wide range amine borane adducts have also been determined. Parameterisation of these data, in addition to that obtained for the analogous phosphine borane complexes, has allowed development of a novel nucleofugality scale (NFB) that quantifies the leaving group ability of a wide range of Lewis bases. Additivity in the kinetics across a series R3−nR’nX·BH3 (X = P, N; R/R’ = aryl, alkyl) has led to the formulation of related substituent parameters (nf PB, nfAB) that quantify the nucleofugal influence of a substituent. Using the substituent nucleofugality parameter in concert with additivity provides a mean of calculating ligand nucleofugality (NFB) values for a wide range of Lewis bases that extends far beyond those experimentally derived. Good agreement was found between predicted (using NF B and nFB values) and experimental rates of borane transfer for ligands outside the training set, thus providing a means to predict the relative rate of phosphine borane deprotections. The utility of both parameters was demonstrated through correlations to rates of redox transformations at iridium (bearing phosphine spectator ligands) and MIDA boronate hydrolysis, (MIDA = N-methyliminodiacetic acid). Through these correlations, interesting subtleties in the mechanism of MIDA boronate hydrolysis have been identified.