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| - Disubstituted Boron Cations Cleave Carbonyl Bonds
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| Abstract
| - The disubstituted boron cationsCH3OBOCH3+ andCH3BCH3+ readily cleaveCO and C−C bonds ingaseous long-chain aldehydes and ketones in a dual-cell Fouriertransform ion cyclotron resonance mass spectrometer.Abstraction of OH by the borocations yields a hydrocarbon production that contains the entire carbon skeleton ofthe aldehyde or ketone. A competing abstraction of part of thecarbonyl compound as a small aldehyde results ina borocation product that is indicative of the location of the carbonylgroup in the neutral substrate. The mechanismsof these two reactions likely involve common intermediates formed via1,2-hydride shifts in an initially formedB−OC adduct. Both reactions are highly exothermic. TheOH abstraction reaction is the thermodynamicallyfavored pathway while aldehyde abstraction is kinetically favored bythe smaller carbonyl compounds. The overallenthalpy change associated with the latter reaction is likely to berelatively insensitive to the size of the carbonylcompound. In contrast, the OH abstraction reaction becomes moreexothermic as the size of the substrate increases.This results in a predominant hydrocarbon ion product for thelarger aldehydes and ketones.
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