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
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.