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

Abstract

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.

Unprecedented Metal-Free C(sp3)−C(sp3) Bond Cleavage: Switching fromN-Alkyl- toN-Methyl-1,3,5-triaza-7-phosphaadamantane

“…In 1 − 3 , the N -alkyl PTA ligands are more stable toward fragmentation but show a more intricate behavior than that of the corresponding free species. In general, the observed fragmentation patterns display features established under mass spectrometry conditions for various alkylamines, namely, revealing the possibility of (i) α-cleavage of the C−C bond with elimination of alkyl radicals or alkenes and (ii) ring openings with formation of various iminium ions…”

“…The search for mild and selective methods of C−C bond activation in various substrates has been of long-standing interest in view of both fundamental and practical importance in, for example, organic synthesis, organometallic and environmental chemistry, catalysis, and biochemistry. Most of the reported examples of C−C bond activation concern the dissociation of strained, cyclic, and/or multiple C−C bonds in the presence of metal centers. − In contrast, the cleavage of unstrained and inert C(sp 3 )−C(sp 3 ) bonds remains little explored and typically limited to high-temperature hydrocarbon cracking, photoinduced electron transfer, mass spectrometry ionization, or enzymatic systems . In particular, efficient and mild metal-free C−C bond dissociation reactions on the secondary and/or primary C(sp 3 ) atoms within unbranched alkyl moieties are virtually unknown.…”

“…C−C bond dissociation reactions are also known to occur under mass spectrometry conditions . In view of the yet unknown mass spectroscopic behavior of alkylated PTA derivatives, we investigated [PTA-R]I (R = n Bu, n Pr, Et, Me) and 1 − 3 by FAB + -MS spectrometry (see Supporting Information).…”

Determination of Tuaminoheptane in Doping Control Urine Samples

“…[30][31][32] The results demonstrated that a complex cascade of rearrangements precedes the dissociation of amines that competes with the formation of the commonly observed and characteristic α-cleavage products. 33 Moreover, they allowed for the explanation of frequently detected additional fragment ions in EI mass spectra of aliphatic amines. Most importantly, rearrangements of the carbon skeleton forming cyclic intermediates were reported that initiated the shift of the amino function to adjacent carbons.…”

Possible Intermediacy of Cyclopropane Complexes in the Isomerization of Aliphatic Amine Radical Cations

“…The two characteristic unimolecular reactions of aliphatic amine radical cations and alkoxy radicals are intramolecular hydrogen atom abstraction and cleavage of the CC-bond next to the heteroatom (Scheme ). − The CC-cleavage results directly in loss of alkyl radicals, whereas hydrogen atom abstraction can initiate a variety of reactions, including for amine radical cations skeletal rearrangement processes. − …”