Ring strain in boroxine rings: computational and experimental considerations

B3LYP/6-311+G(d) calculations indicate that (HBO)₃ (<b>4</b>) and (HBO)₄ (<b>5</b>) possess (zero-point energy corrected) strain enthalpies of 11.4 and 31.6 kJ mol⁻¹, respectively. The absence of eight-membered (RBO)₄ rings is attributed to a combination of ring strain and the lability of the B---O bond. The synthesis, characterization and molecular structure of (PhBO)₃·pyridine (<b>1</b>) are described and chemical phenomena related to the addition of amines to triorganoboroxine rings are rationalized in terms of relief of ring strain in <b>4</b>.<br /><br /><br />