Structure and properties of 3-(Diethylboryl)pyridines

3-(Diethylboryl)pyridine (1a), a versatile starting material for the preparation of arylpyridines, is notable for its stability under ambient conditions, in spite of little steric hindrance on the boron atom. ¹H and ¹³C spectra of 1a indicated that the boryl group does not act as a mere π-acceptor and that the boron atom is shielded by ca. 50 ppm even when compared with trivalent boron atoms conjugated with the π-donor. A single-crystal X-ray crystallographic study for 1a revealed formation of a cyclic-tetramer with a void via the intermolecular boron-nitrogen coordination bond. Vapor pressure osmometry in various solvents suggested that 1a comprises the tetramer in these solutions. In order to know the actual structure, synthesis of 3-(2-methoxyethoxy)-5-(diethylboryl)pyridine (1b) and its scrambling experiment with 1a were carried out. Heating at 100 °C for 24 h was required to attain the equilibrium of the scrambling of the component molecules in the tetramers. This means that 3-(diethylboryl)pyridines 1a and 1b comprise the rigid cyclic-tetramer in solution at ambient temperature. Compound 1b is stable in aerated tetrahydrofuran containing up to 33 % water.