Computational study of the substituent effects on the gas-phase stabilities of phenylboranylmethyl anions

Abstract

Abstract The relative gas-phase stabilities of ring-substituted phenylboranylmethyl anions were computationally determined using isodesmic reactions. The energies of species included in the reactions were calculated at the B3LYP/6-311+G(2d,p) level of theory. The obtained substituent effects were analyzed by the extended Yukawa-Tsuno equation, and unexpectedly substantial r − (0.59) and s (0.65) values were found for the fully-optimized planar anion. The substantial through-resonance effect quantified by the r − value was observed, although it is not possible to draw a canonical form in which the negative charge is delocalized on the benzene ring. Substituent effects were also analyzed for the anions in which the dihedral angle (φ) between the side chain plane and the benzene ring was fixed. The r − value decreased significantly by changing the φ from 0° to 90°, while the s value changed little. NBO analyses revealed that the r − value is proportional to the sum of the π–π* and σ–π* orbital interactions between the side chain and the benzene ring. This fact shows that the through-resonance effect quantified by the r − value is present at all φ, and therefore, the anion cannot become an ideal σ 0-reference system. The constant saturation effect quantified by the s value can be explained by the constant charge distributed to the benzene ring. The combination of substituent-effect analysis and NBO analysis successfully revealed the nature of the anion.