DISUBSTITUTED BORON CATIONS CLEAVE CARBONYL BONDS

The disubstituted boron cations CH3OBOCH3+ and CH3BCH3+ readily cleave C=O and C-C bonds in gaseous long-chain aldehydes and ketones in a du al-cell Fourier transform ion cyclotron resonance mass spectrometer. A bstraction of OH by the borocations yields a hydrocarbon product ion t hat contains the entire carbon skeleton of the aldehyde or ketone. A c ompeting abstraction of part of the carbonyl compound as a small aldeh yde results in a borocation product that is indicative of the location of the carbonyl group in the neutral substrate. The mechanisms of the se two reactions likely involve common intermediates formed via 1,2-hy dride shifts in an initially formed B-O=C adduct. Both reactions are h ighly exothermic. The OH abstraction reaction is the thermodynamically favored pathway while aldehyde abstraction is kinetically favored by the smaller carbonyl compounds. The overall enthalpy change associated with the latter reaction is likely to be relatively insensitive to th e size of the carbonyl compound. In contrast, the OH abstraction react ion becomes more exothermic as the size of the substrate increases. Th is results in a predominant hydrocarbon ion product for the larger ald ehydes and ketones.