Comparative basicity of 1,3,2-oxazaborinanes and tetrahydro-1,3-oxazines

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Substituted 1,3,2-oxazaborinanes and their nonboron analogues, tetrahydro-1,3-oxazines, are of interest for stereochemical studies and are used in organic synthesis [1–7]. The first stage of heterolytic reactions of the discussed compounds is known to be the formation of oxonium and ammonium ions [5]. However, the conjugation of n-electron pairs of heteroatoms with the vacant orbital of boron atoms in the cyclic molecules of the boron esters decreases their basicity, which was confirmed experimentally by the example of substituted 1,3,2-dioxaborinanes compared with 1,3-dioxanes [8]. If the heteroatoms are different, the redistribution of the electron density in the X–B–Y heteroatomic fragment should increase the basicity of one center at the expense of another. However, such systems remain unexplored so far. The purpose of this paper is to assess the relative basicity of 2-methyland 2,3-dimethyl-1,3,2-oxazaborinanes and tetrahydro-1,3oxazine and its 3-methyl analog by calculating the heat of protonation using ab initio HF/6-31G(d) approach within a HyperChem software [9] and PBE/3z DFTmethod (PRIRODA package [10]) for the most stable conformers of the studied compounds. The most stable conformers of cyclic boronic esters are the halfchair form [2] and in the case of tetrahydro-1,3-oxazines molecules, the chair conformers [11].