Hypochlorite oxidation of morpholine-borane

Article abstract of DOI:10.1021/ic00134a069

Morpholine-borane reacts with sodium hypochlorite in a 1:4 mole ratio wherein three OCl^- species are utilized for hydride oxidation and a fourth is consumed in the chlorination of morpholine. The determination of kinetic parameters, based upon the stopped-flow spectrophotometric measurement of the rate of disappearance of OCl^- at 290 nm (pH 9-11), is complicated by these consecutive competitive reactions of hypochlorite. At a given pH, the second-order rate constant for the reaction of OCl^- with morpholine is about 10³ times greater than that for attack of hypochlorite on amine-borane; thus, a reliable determination of the latter constant was based upon initial rate studies under pseudo-first-order conditions involving a large stoichiometric excess of morpholine-borane. The rate of reaction of hypochlorite with amine-borane is also first order in hydrogen ion and is subject to a normal substrate isotope effect with O(CH₂)₄NH·BH₃ reacting about 1.6 times more rapidly than O(CH₂)₄NH·BD₃. At a given lyonium ion concentration, the reaction is enhanced by a factor of about 3.5 in D₂O. It is proposed that the rate-limiting step involves oxidative attack of hypochlorous acid at a boron-hydrogen bond in the amine-borane and that subsequent oxidation of the two remaining hydridic hydrogen atoms is rapid relative to the chlorination of morpholine. The inverse solvent isotope effect is attributed to a higher concentration of DOCl in D₂O relative to that of HOCl in normal water at a given pD (pH), but is is likely that this influence is partially offset by a normal secondary isotope effect associated with attack of HOCl (DOCl) at the B-H bond. A four-center activated complex involving the elements O, Cl, B, and H that is formally similar to other transition-state configurations proposed for selected reactions of amine-boranes is considered.