Boryl Substitution of Acetaldehyde Makes It an Enol: Inconsistency between Gn/CBS and Ab Initio/DFT Data

Tautomerism, a particular case of isomerism, plays ail important role in modern organic chemistry, biochemistry, medicinal chemistry, pharmocology, and molecular biology. Inconsistency between results of complex energy computation methods Gn/CBS (G2, G3, CBS-4M, and CBS-QB3) and high-level ab initio/DFF ones (CCSD(T)/CBS, MP2/CBS, and B3LYP/aug-cc-pVTZ) is found. Gn/CBS methods provide a qualitatively different description Of tautomeric (keto-enol) equilibrium in 2-substitUted acetaldehydes. According to valence focal point analysis (FPA) based oil CCSD(T)/aug-cc-pVTZ, MP3/aug-cc-pVQZ, and MP2/aug-cc-pV5Z energies, boryl substitution of acetaldehyde makes it an enol. Ill other words, enol was found to be the global minimum on the potential energy surface (PES) of C2H5BO. Gn/CBS methods predict the keto form to be the minimum. The relative energy of alkenol, CH(BH2)=CH(OH), is Calculated to be -1.67 +/- 0.82 kcal mol(-1) at CCSD(T)/CBS level of theory. Hydrogen shift effects are also calculated in two other 2-substituted acetaldehydes, namely, 3-oxopropanenitrile (C3H3NO) and ethanal (C2H4O), With a general formula of XH2C-CHO (X = BH2, CN, and H). Electron density (charge) transfer between the C=C double bond and the free p orbital of the boron atom (B) in a boryl group (BH2) greatly stabilizes enol with respect to ketone, CH2(BH2)-CHO. The first known stabilization of enol in ail acetaldehyde derivative, without ail intramolecular hydrogen bond (H-bond), questions the accuracy of complex energy Computation methods for boron-containing molecules. The possible reasons and consequences of this finding are discussed.