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.

The phosphenium ion CH3OPOCH3+ readily attacks hydroxyl groups of neutral substrates in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. The electrophilic character of CH3OPOCH3+ is in agreement with molecular orbital calculations (Becke3LYP/6-31G(d) + ZPE) that predict a singlet electronic ground state for this species. The observed reactions provide a convenient synthetic route to various larger phosphenium ions in the gas phase. Most importantly, however, CH3OPOCH3+ was found to be extremely sensitive to the stereochemical structure of the neutral substrate. The dramatically different reaction product distributions obtained for diastereomeric cyclic vicinal diols suggest that CH3OPOCH3+ provides a powerful chemical ionization reagent for the mass spectrometric determination of the stereochemistry of diols.

Gas-phase reactions of stereoisomeric cyclic diols with CH3OB+OCH3 were examined in a small Fourier transform ion cyclotron resonance mass spectrometer. CH3OB+OCH3 is a strong electrophile and rapidly abstracts an OH group from the diols studied. This very exothermic reaction is followed by spontaneous fragmentation of the resulting ion. In addition to this reaction, cis-diols also react with CH3OB+OCH3 by an intramolecular displacement of CH3OH in the initially formed, short-lived adduct ion. The product distributions allow distinction between the cis- and trans-isomers of 1,2-cyclopentanediol, and between the cis-(diendo- and diexo-) and trans-isomers of 2,3-trinorbornanediol.

The intrinsic gas-phase reactivity of simple dicoordinated boron cations toward alcohols was studied using a dual-cell Fourier-transform ion cyclotron resonance device. Reactions of the ions CH3BCH3+, CH3OBOCH3+ and CH3(CH2)(2)OBOH+ occur at or near collision rate with simple alcohols and are dominated by abstraction oh water or a hydroxyl group by the ion. These two reactions likely occur via the same proton-bound intermediate, analogous to the dissociation of protonated ethanol to ethylene and H3O+. The branching ratio depends primarily on the stability of the alkyl cation formed in the hydroxyl abstraction reaction. Hence, hydroxyl abstraction dominates for longer chain alcohols. The proton affinity of (CH3)(2)BOH was determined to be 179 kcal/mol through proton affinity bracketing experiments. Using this value, the heat of formation of (CH3)(2)BOH2+ is estimated to be 86 kcal/mol. Hence, dehydration of ethanol by CH3BCH3+ is concluded to be exothermic by 21 kcal/mol. The same reaction for CH3OBOCH3+ to yield (CH3O)(2)BOH2+ is exothermic by about 33 kcal/mol; a reaction yielding (CH3O)(HO)B(HOCH3)(+) as the final ionic product is significantly more exothermic, by about 54 kcal/mol. Relative to water abstraction, hydroxyl abstraction is thermodynamically more favorable for the ion CH3BCH3+ than for CH3OBOCH3+. Analogous to these water and hydroxyl abstraction reactions of alcohols, competitive ethanol and ethoxy abstractions were observed when the boron cations were allowed to react with ethyl acetate.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Energy-resolved collisionally-activated dissociation (CAD) was carried out in a dual-cell FT-ICR for the molecular ions of n-propanol, hexafluoropropene, three ionized organophosphorus esters, and four C3H7O+ isomers. The results are compared to data measured earlier in triple quadrupole, BQQ, BEQQ, and quadrupole ion trap mass spectrometers. In general, low-energy CAD spectra generated in a dual-cell FT-ICR are comparable to those measured using other types of tandem mass spectrometers when roughly similar activation conditions are used (number of activating collisions, laboratory ion kinetic energy), and this agreement is best at low collision energies. However, ions which undergo slow isomerization prior to collisional activation are an exception. These ions can yield CAD product distributions in an FT-ICR and other ion traps that are significantly different from those obtained in mass spectrometers which sample ions with shorter lifetimes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The proton affinity of trimethyl borate was determined to be 197 ± 3 kcal mol−1 by bracketing in both directions in a Fourier-transform ion cyclotron resonance mass spectrometer. This value was used to calculate a heat of formation of −46.3 ± 3.0 kcal mol−1 for CH3O(H)B(OCH3)+2. Collision-induced dissociation threshold energy measurements carried out in a flowing afterglow-triple quadrupole instrument yield an activation energy of 39.5 ± 3.5 kcal mol−1 for the dissociation of CH3O(H)B(OCH3)+2 to CH3OH and CH3O)2B+. This bond dissociation energy, together with the heat of formation of CH3O(H)B(OCH3)+2, was used to calculate a heat of formation of 41.4±4.6 kcal mol−1 for (CH3O)2B+.

Condensed-phase studies of two-coordinated boron cations are limited to ions with conjugatively stabilizing substituents; the reactions of these ions are dominated by simple addition. We have investigated the gas-phase reactions of two simple, dicoordinated boron cations, CH3OBOCH3+ and CH3BCH3+, in a dual-cell Fourier-transform ion cyclotron resonance device. These ions are found to undergo a bimolecular reaction with no well-known precedent in the gas phase or in solution: facile abstraction of a water molecule from organic ethers (the efficiency, or k(obsd)/k(ADO), is measured to be 0.5-0.8). The mechanism of the reaction was investigated by using different neutral ethers, deuterium labeling, collision-activated dissociation of the ionic reaction products and the proposed ionic intermediates, bimolecular reactions of the proposed intermediates, and generating the intermediates using independent routes. On the basis of these data, dehydration of organic ethers by the boron cations is proposed to occur via consecutive 1,2-elimination of two alkene molecules from the ether. The overall reaction is estimated to be highly exothermic.