A new perspective in the Lewis acid catalyzed ring opening of epoxides. Theoretical study of some complexes of methanol, acetic acid, dimethyl ether, diethyl ether, and ethylene oxide with boron trifluoride

Several 1: 1, 1: 2, and 2: 2 complexes between BF3 and CH3OH ( Met), CH3COOH (AcA), (CH3)(2)O (DME), (CH3CH2)(2)O ( DEE), and (CH2)(2)O (EOX) have been studied using ab initio (MP2) and density functional theory (DFT) (PBE, B3LYP) methods and the 6-311++ G(3df, 2pd) basis set. Geometrical structures and vibrational frequencies are reported, in most cases, for the first time. A detailed comparison of the vibrational frequencies for the O center dot center dot center dot BF3 vibrational modes, as well as for the v( OH) band in the methanol and acetic acid complexes with BF3, is performed, and the theoretical frequency shifts are compared with the available experimental information. Thermochemical properties are calculated by employing counterpoise correction to alleviate the basis set superposition error. The DFT enthalpy of complexation of the 1: 1 complexes results in the order of stability ( AcA)(2) > AcA: BF3 > DEE: BF3 > DME: BF3 > Met: BF3 > EOX: BF3 > ( Met)(2); in contrast, MP2 shows the noticeable difference that the AcA: BF3 complex is much less stable ( similar to Met: BF3). The order of stability shows that, even though acetic acid prefers dimerization to complexation with BF3, the case is exactly the opposite for methanol. In both cases, the interaction of BF3 with the dimer gives rise to very stable trimers. However, in contrast to the interaction of BF3 with the methanol dimer being stronger than that with the monomer, the interaction of BF3 with the acetic acid dimer is weaker than that with the monomer. The relative strength of the complexes, discussed in the context of BF3-catalyzed ring opening of epoxides, suggests that the effect of the catalyst in a nonprotogenic solvent should be more properly ascribed to activation of the nucleophile instead of activation of the epoxide.

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