Intrinsic alkyl radical properties inferred from the study of unimolecular dissociations of gaseous carboxylic acid cation radicals

Competing Simple Cleavage Reactions: The Elimination of Alkyl Radicals from Amine Radical Cations

Two different alkyl radicals can be expelled when unsymmetrical aliphatic amine radical cations undergo C-C bond cleavage. The branching ratio is strongly dependent on the internal energy of the reactant, even when the competition involves loss of closely related alkyl radicals. In mass spectrometers, the rate of loss of the smaller radical (excepting methyl) always exceeds the rate of loss of the larger close to threshold. The preference is reversed for the more highly energized ions that react in the ion source, demonstrating that the rate of loss of the larger radical rises much more rapidly with increasing internal energy than does the rate of loss of the smaller radical. This result is not easily reconciled with a simple RRKM model, given the expected strong resemblance between the transition states involved, whereas it agrees well with a description based on variational transition state theory. The heats of formation of the products determined with the G3 composite ab initio method show that loss of the smaller radical is without exception the more favorable reaction. The relative rates of the competing C-C bond cleavage reactions of the metastable ions vary with the number of degrees of freedom of the reactant, with the critical energy of the reaction, and with the difference between the heats of formation of the products. The presence of intermediate energy barriers when cleavage occurs at a branching point can give rise to variations in relative rates that are not easily interpreted.

Diastereoselectivity in the McLafferty Rearrangement of Photoionized 3-Methyl Valeramide

The McLafferty rearrangement of photoionized 3-methyl valeramide proceeds quasi-barrierless and with high regioselectivity. The mass spectra of the stereospecifically labeled syn- and anti-[4-D(1)]-diastereomers reveal a strong preference for activation of the gamma-hydrogen/deuterium in anti-position relative to the methyl group at C(3), which serves as a steric marker. Quantitative analysis of the fragmentation patterns of other photoionized isotopomers permits the determination of primary and secondary kinetic isotope effects (KIEs), the branching ratios of competing McLafferty reactions, and the steric effect (SE) associated with transfer of the diastereotopic H(D) atoms at C(4). While the associated KIEs of the title reaction are negligible, the steric effect (SE = 2.9) is remarkably large for the otherwise flexible, monofunctional compound. The findings can be explained by a preferentially chairlike transition structure for the initial gamma-H atom transfer.

Evidence for a 1,2 Shift of a Hydrogen Atom in a Radical Intermediate of the Methylmalonyl-CoA Mutase Reaction

An excellent substrate of methylmalonyl-CoA mutase, methylmalonyl-carba-(dethia) coenzyme A (methylmalonyl-CH(2)-CoA), was synthesized by a chemoenzymatic method and its alpha-proton was exchanged with deuterium by long-term incubation in deuterium oxide at pH 6.9. After addition of highly purified epimerase-free methylmalonyl-CoA mutase the enzymatic rearrangement was monitored by 1H NMR spectroscopy. Already in the initial phases of the reaction only 72% of the produced succinyl-CH(2)-CoA was monodeuterated, while unlabeled and geminally dideuterated species, 14% of each, were also formed. After the addition of more enzyme the equilibrium (methylmalonyl-CoA:succinyl-CoA = 1:20) was quickly established, while the proportion of unlabeled succinyl-CH(2)-CoA rose to 30% and the geminally dideuterated species were slowly transformed to vicinally dideuterated ones. After 19 h of incubation the ratio of the unlabeled, monodeuterated, and dideuterated species was roughly 1:1:1 while no appreciable deuterium incorporation from the solvent occurred. The unexpected disproportionation of deuterium can be best explained by a 1,2 shift of a hydrogen atom in the succinyl-CH(2)-CoA radical intermediate competing with the hydrogen transfer from 5'-deoxyadenosine. A precedence for such a hydrogen shift in a radical was previously observed only in the mass spectrometer and was supported by ab initio calculations. Copyright 2000 Academic Press.

The structure and mechanism of formation of C5H 9O (+) from ionized phytyl methyl ether

The recent proposal that ionized phytyl methyl ether [C16H33(CH3)C=CHCH2OCH 3 (+·) ] undergoes an allylic rearrangement to ionized isophytyl methyl ether [CH2=CHC(C16H33)(CH3)OCH 3 (+·) ] before elimination of an alkyl radical is discussed. Both literature precedent and new results in which the structure of the [M-C16H 33 (·) ](+) fragment ion is established by comparison of its collision-induced dissociation mass spectrum with the spectra of isomeric C5H9O(+) ions of known structure are inconsistent with this proposal. The forma Hon of CH3CH=CHCH=O(+)CH3 by loss of a γ-alkyl substituent without skeletal isomerization rather than CH2=CHC(CH3)=O(+)CH3 after allylic rearrangement is explained in terms of a mechanism that involves two 1,2-H shifts, followed by σ-cleavage of the resultant ionized enol ether, C16H33(CH3)CH-CH=CHOCH 3 (+·) .