Boroxines are the six-membered cyclotrimeric
dehydration products of organoboronic
acids, 3R–B(
OH)2 → R3B3O3 + 3H2O, and in recent years have emerged as a useful class of organoboron molecules with applications in organic synthesis both as
reagents and catalysts, as structural components in
boronic-acid-derived
pharmaceutical agents, and as
anion acceptors and
electrolyte additives for battery materials [Korich, A. L.; Iovine, P. M. Dalton Trans. 2010, 39, 1423−1431]. Second-order Møller–Plesset perturbation theory, in conjunction with the Dunning–Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets, was used to investigate the structures and relative energies of the endo–exo, anti, and syn conformers of the aliphatic
boronic acids R–B(
OH)2 (R = H, H3C, H2N, HO, and F), as well as the thermodynamics of their boroxine formation; single-point calculations at the MP2/aug-cc-pVQZ, MP2/aug-cc-pV5Z, and CCSD(T)/aug-cc-pVTZ levels using the MP2/aug-cc-pVTZ optimized geometries were also performed in selected cases. The endo–exo conformer was generally lowest in energy in vacuo, as well as in PCM and CPCM models of aqueous and
carbon tetrachloride media. The values of ΔH(298)(0) for boroxine formation via
dehydration from the endo–exo conformers of these aliphatic
boronic acids ranged from −2.9 for (H2N)3B3O3 to +12.2 kcal/mol for H3B3O3 at the MP2/aug-cc-pVTZ level in vacuo; for H3B3O3, the corresponding values in PCM/UFF implicit
carbon tetrachloride and aqueous media were +11.2 and +9.8 kcal/mol, respectively. On the basis of our calculations, we recommend that ΔHf(298K) for boroxine listed in the JANAF compilation needs to be revised from −290.0 to approximately −277.0 kcal/mol.