Stereoelectronic Effects and Gas-phase Affinities of Dicoordinated Borinium Cations

The relative affinities of the borinium cations (CH3O)2B+ and CH3OB+H towards pyridines were determined by the kinetic method using a pentaquadrupole mass spectrometer and the absolute affinities of these ions towards pyridine itself were estimated with the aid of ab initio calculations at the MP2/6–31G(d,p)//6–31G(d,p) level to be 75.7 and 82.2 kcal mol-1, respectively (1 kcal=4.184 kJ). The affinities were found to correlate linearly with the proton affinities of the meta- and para-substituted pyridines, but steric effects decrease the affinities for ortho-substituted pyridines. A set of gas-phase stereoelectronic parameters (Sk) for these isomers was measured from the deviation of the value of the logarithm of the experimentally measured ion abundance ratio from the regression line established for meta- and para-substituted pyridines. The Sk value of 2,6-dimethylpyridine with (CH3O)2B+ is smaller than that for other dimethylpyridines, owing to the stabilizing effects of auxiliary hydrogen bonding interactions between one hydrogen of each of the ortho-methyl substituents and the two oxygens of the (CH3O)2B+ ion. The dimethoxy boron cation (CH3O)2B+ is shown to have smaller steric effects than CH3OB+H, even though the former ion has two bulky methoxy groups. This is ascribed to the relatively longer B—N bond in (CH3O)2B+—Py as a result of reduced charge density on the boron atom due to resonance stabilization within the dimethoxy boron cation.