Disubstituted Boron Cations Cleave Carbonyl Bonds

The disubstituted boron cations CH 3 OBOCH 3 + and CH 3 BCH 3 + readily cleave C O and C−C bonds in gaseous long-chain aldehydes and ketones in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer. Abstraction of OH by the borocations yields a hydrocarbon product ion that contains the entire carbon skeleton of the aldehyde or ketone. A competing abstraction of part of the carbonyl compound as a small aldehyde results in a borocation product that is indicative of the location of the carbonyl group in the neutral substrate. The mechanisms of these two reactions likely involve common intermediates formed via 1,2-hydride shifts in an initially formed B−O C adduct. Both reactions are highly 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 the size of the carbonyl compound. In contrast, the OH abstraction reaction becomes more exothermic as the size of the substrate increases. This results in a predominant hydrocarbon ion product for the larger aldehydes and ketones.