Disubstituted Boron Cations Cleave Carbonyl Bonds

Abstract

The disubstituted boron cations CH3OBOCH3 + and CH3BCH3 + 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.

Identification of Epoxide Functionalities in Protonated Monofunctional Analytes by Using Ion/Molecule Reactions and Collision-Activated Dissociation in Different Ion Trap Tandem Mass Spectrometers

“…The solvation energy released upon formation of the reactant complex in a gas-phase collision is typically between 5 and 10 kcal/mol, thus allowing proton transfers that are endothermic by 5-10 kcal/mol. The second step, addition of the neutral epoxide to the boron atom, is supported by vast literature evidence that this commonly occurs for other oxygen-containing molecules, and is followed by elimination of methanol [17,18,22,25,26]. The observation of elimination of methanol as well as ethylene oxide from the addition/methanol elimination product of protonated phenyl oxirane indicates that not only the epoxide oxygen but also the aromatic ring can act as the nucleophile in the addition reaction.…”

“…The efficiencies of all the ion/molecule reactions studied here were measured by using a method described previously [17,18,[20][21][22][23]. They were found to be high.…”

“…This reagent is known to deprotonate protonated oxygen functionalities (but not, for example, nitrogen functionalities), followed by addition of the analyte to the boron center and elimination of methanol (Table 1), without causing rearrangement of the analyte [17,18,22,25,26]. Upon SORI-CAD, the methanol elimination products have been reported to yield structurally characteristic fragment ions for many oxygencontaining analytes (Table 1) [22,35].…”

“…Most past work has focused on gas-phase ion/molecule reactions of neutral analytes with ionic reagents [17][18][19]. Fewer studies have explored the use of neutral reagents to identify functionalities in protonated analytes [10,[20][21][22][23][24].…”

Differentiation of Protonated Aromatic Regioisomers Related to Lignin by Reactions with Trimethylborate in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

“…In contrast to gas-phase dissociation reactions, gas-phase ion-molecule reactions provide a powerful technique for obtaining structural information for isomers [4][5][6][7][8][9][10][11][12][13][14][15]. Most often these experiments involve using an ionized reagent to probe the structure of a neutral analyte.…”