STEREOELECTRONIC EFFECTS AND GAS-PHASE AFFINITIES OF DICOORDINATED BORINIUM CATIONS

The relative affinities of the borinium cations (CH3OB+ and CH3OB+H to wards pyridines were determined by the kinetic method using a pentaqua drupole mass spectrometer and the absolute affinities of these ions to wards pyridine itself were estimated with the aid of ab initio calcula tions 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-s ubstituted pyridines, but steric effects decrease the affinities for o rtho-substituted pyridines. A set of gas-phase stereoelectronic parame ters (S-k) for these isomers was measured from the deviation of the va lue of the logarithm of the experimentally measured ion abundance rati o from the regression line established for meta- and para-substituted pyridines. The S-k value of 2,6-dimethylpyridine with (CH3OB+ is small er than that for other dimethylpyridines, owing to the stabilizing eff ects of auxiliary hydrogen bonding interactions between one hydrogen o f each of the ortho-methyl substituents and the two oxygens of the (CH 3OB+ ion. The dimethoxy boron cation(CH3OB+ is shown to have smaller s teric effects than CH3OB+H, even though the former ion has two bulky m ethoxy groups. This is ascribed to the relatively longer B-N bond in ( CH3OB+-Py as a result of reduced charge density on the boron atom due to resonance stabilization within the dimethoxy boron cation. (C) 1997 by John Wiley & Sons, Ltd.