Boron-11 nuclear magnetic resonance study of the reactions of 2-functionalized pyridines with borane–tetrahydrofuran and –dimethyl sulphide. Formation of borinic estes and N → B bond energy differences in five- and six-membered ring borates

Abstract

A ¹¹B n.m.r. study of the reactivity of 2-substituted pyridines containing carboxy, acyl, and hydroxy groups with borane–tetrahydrofuran (BH₃·THF) and BH₃·Me₂S showed that pyridines behave differently from their aliphatic analogues. The formation of stable borinic esters was observed. Variable-temperature n.m.r. data allowed us to calculate that five-membered Bâ†�N co-ordinated heterocycles are 28 kJ mol^–1 more stable than six-membered ones.