Synthesis and comparative reactivity of thiomorpholine–borane: aqueous hydrolysis and oxidation by hypochlorite

Abstract

Thiomorpholine- N-borane was synthesized via amine displacement of BH 3 from tetrahydrofuran–borane which had been prepared from NaBH 4 and BF 3·Et 2O in tetrahydrofuran. Acid-catalyzed hydrolysis occurs only slightly faster than for morpholine–borane, the difference being attributed to a small difference in the electronic inductive effects of sulfur and oxygen in the 4-position of the respective adducts. Reaction with NaOCl exhibits a stoichiometric [OCl −]:[S(CH 2) 4NHBH 3] ratio of 5:1. This is attributed to consumption of 3 mol of hypochlorite for the oxidation of hydridic hydrogen in BH 3, one for the chlorination of nitrogen and another in attack at sulfur presumably resulting in sulfoxide formation. At pH 9.1–10.4, the initial reaction of hypochlorite with thiomorpholine–borane is several times faster than with morpholine–borane and, unlike the reaction with morpholine–borane, relatively insensitive to pH. Whereas hypochlorite oxidation of morpholine–borane has been proposed to occur primarily through reaction with HOCl, it is speculated that thiomorpholine–borane is also susceptible to attack by hypochlorite ion.