Thermodynamics of Boroxine Formation from the Aliphatic Boronic Acid Monomers R–B(OH)₂ (R = H, H₃C, H₂N, HO, and F): A Computational Investigation

Abstract

Boroxines are the 6-membered cyclotrimeric dehydration products of organoboronic acids: 3 R– B(OH)2 → R3B3O3 + 3 H2O, 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, as anion acceptors and electrolyte additives for battery materials [AL Korich and PM Iovine, Dalton Trans. 39 (2010) 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 level 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 ΔnormalH2980 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. Based on our calculations, we recommend that ΔHf(298 K) for boroxine listed in the JANAF compilation needs to be revised from −290.0 kcal/mol to approximately −277.0 kcal/mol.

N-Heterocyclic Carbene-facilated Condensation of 3-Methylphenylboronic Acid to the Boroxine

“…The B-C (carbene) bond length of 1 is 1.651(3)Å which is shorter when compared with that of NHC·BF 3 [26]. In general, formation of (RBO) 3 from organic boronic acids RB(OH) 2 requires phosphorus pentoxide or sulfuric acid for the dehydration or prolonged heating in toluene [27]. In contrast to these methods, the condensation of RB(OH) 2 reported here is more facile and gives higher yields without heating or other auxiliaries.…”

Heats of formation for the boronic acids R–B(OH)2 and boroxines R3B3O3 (R=H, Li, HBe, H2B, H3C, H2N, HO, F, and Cl) calculated at the G2, G3, and G4 levels of theory

“…Indeed, our recent MP2/aug-cc-pVTZ calculations suggest the reaction is significantly endothermic, with a $\mathrm{normal\Delta }{H}_{298}^0$ of +12.2 kcal/mol. (12) Thus, there appears to be a problem with the heats of formation currently in the literature for H–B(OH) 2 and/or for H 3 B 3 O 3 .…”

A comparison of the structure and bonding in the aliphatic boronic R–B(OH)2 and borinic R–BH(OH) acids (R=H; NH2, OH, and F): a computational investigation

“…The optimized structures of the lowest-energy endo - exo conformers of R–B(OH) 2 (R = H; NH 2 , OH, and F) are all planar at the MP2(FC)/aug-cc-pVTZ level [46–48, 64–69]. Selected structural parameters are shown in Figure 1 and additional values are listed in Table 1A; the corresponding parameters from MP2(FC)/aug-cc-pVQZ and MP2(FC)/aug-cc-pV5Z optimizations are also given in Table 1A.…”

“…Lone-pair orbitals available from simple aliphatic substituents provide competitive donor-acceptor interactions that also impact the boron-oxygen bonding. Boronic acids differ from borinic acids by the presence of two boron–oxygen bonds and an intramolecular B–O–H⋯O hydrogen-bonding interactions inherent in the lower-energy endo-exo conformers of boronic acids[46–48]. These differences are expected to result in distinct steric and electronic properties of corresponding boronic and borinic acids that will influence their effectiveness as diol receptors to specific substrates.…”