•Novel iminium ions [R1CH+NCHR2 ↔ R1CHN+CHR2] have been prepared.•Gas-phase reactions of these ions were studied by mass spectrometry and density functional theory.•Two main reaction pathways were assumed: one is simple cleavage, the other is intramolecular cyclization.The preparation of the novel iminium ions [R1CH+NCHR2 ↔ R1CHN+CHR2] and mechanistic study of their gas-phase reactions have been carried out by mass spectrometry and density functional theory. DFT calculations show that this kind of iminium ion is similar to the allene in structure. Two main reaction pathways have been assumed: one is the simple cleavage, including homolytic and heterolytic cleavages of the CC bond; the other might be the intramolecular cyclization, which is supported by the loss of HCN.Graphical abstract

•Regioselectivity of intramolecular nucleophilic attack in [RCH−NCHC6H4Y] was studied.•NO2 favors C3-attack to form spiro-σ-adduct and it also contributes to the loss of HCN.•Other substituents (H, OMe, NMe2 and Cl) favor C4-attack to yield bicyclo-σ-adduct.•It is difficult for bicyclo-σ-adduct to lose HCN.•DFT calculations are well consistent with the tandem mass spectrometry experiments.The investigation into the intramolecular nucleophilic attack in carbanion [RCH −NCHC6H4Y] in the gas phase has been carried out by mass spectrometry and density functional theory (DFT). The main results of the present work are as follows: the regioselectivity of the intramolecular attack depends on the substituent on the para-position of the benzene ring. NO2 favors C3-attack to form spiro-σ-adduct and it also contributes to the loss of HCN, while other substituents (H, OMe, NMe2 and Cl) favor C4-attack to yield bicyclo-σ-adduct and in this condition, HCN cannot be lost. In the meantime, DFT calculations also demonstrate that the loss of HCN from the spiro-σ-adduct is both thermodynamically and kinetically beneficial, while that from the bicyclo-σ-adduct is disadvantageous either in kinetics or thermodynamics.Download full-size image

The interception of fluorine radical produced in the electrophilic fluorination with a series of radical traps (5,5-dimethyl-l-pyrroline N-oxide (DMPO), 2,2,6,6-tetramethyl piperidine N-oxide (TEMPO), 2-methyl-2-nitropropane (MNP), phenyl-tert-butynitrone (PBN)) has been investigated by electrospray ionization-mass spectrometry (ESI-MS) and density functional theory (DFT). The interception experiments by ESI-MS indicate that only DMPO possesses the capacity of trapping fluorine radical, while others (TEMPO, PBN and MNP) do not. Meanwhile, ESI-MS and tandem mass spectrum (MS/MS) experiments manifest that DMPO can trap fluorine radical to produce two kinds of adducts, namely ion B at m/z 132 containing one fluorine atom and ion C at m/z 152 containing two fluorine atoms, which process has been reasonably explained by DFT calculations. Ionization principle in ESI-MS suggests that ions B and C originate from the precursor compounds IM1 and IM2 respectively. The formation mechanisms of IM1 and IM2 are described as follows: first, the addition of fluorine radical to DMPO yields the pivotal intermediate M1, whose hydrogen is then abstracted by fluorine radical to form IM1. Furthermore, the addition of fluorine radical to IM1 can produce the intermediate M17, which subsequently abstracts hydrogen from other compounds to form IM2. The current work is to prove that ESI-MS and DFT provide a suitable tool for studying the interception of the radical.Highlights► DMPO can succeed in trapping fluorine radical. ► Two kinds of adducts of DMPO and fluorine radical have been observed by ESI-MS. ► The structures of adducts are explored by MS/MS technique and DFT calculations.

On the basis of density functional theory (DFT), an investigation has been conducted about the dehydration reaction mechanism of the allicin radical cation in the gas phase as well as that of the protonated allicin. According to DFT calculations, the former is easier to take place than the latter, which agrees with the experiments.In the allicin radical cation, the allylic hydrogen atom on C1′ (adjacent to the divalent sulfur atom) attacks oxygen to yield the intermediate M1, in which the radical addition then take place to form six-member-ring intermediates (M5 or M5′). Finally, the hydrogen atom on C1 (adjacent to the S–O bond) attacks the hydroxyl group together with the dissociation of S–O bond to produce the dehydration product (M7 or M7′).On the other hand, in the protonated allicin, although the allylic hydrogen atom on C1′ is prior to that on C1 to attack the hydroxyl group, the former reaction will not yield the intact dehydration product but several fragments.

The dissociation of the NO bond and NC bond in the protonated trimethylamine oxide (Me3NOH+) and in adducts of alkali metal cations (Li+, Na+ and K+) and the trimethylamine oxide is investigated through the density functional theory (DFT) and tandem mass spectrometry. DFT calculations show that due to the introduction of the cations (H+, Li+, Na+, K+), the NC bond is strengthened and the corresponding bond dissociation energy follows the order of Me3NO < Me3NOK+ < Me3NONa+ < Me3NOLi+ < Me3NOH+, while the NO bond is weakened and the corresponding bond dissociation energy follows the order of Me3NO > Me3NOK+ > Me3NONa+ > Me3NOLi+ > Me3NOH+. In addition, in Me3NO, Me3NOK+, Me3NONa+ and Me3NOLi+, the dissociation of the NC bond has precedence over that of the NO bond, but in Me3NOH+, the dissociation of the NO bond is easier to take place. The MS/MS experiments are consistent with the DFT calculations. On the one hand, the fragment ion at m/z 59 in the CID spectrum of Me3NOH+ indicates the dissociation of the NO bond. On the other hand, the fragment ion at m/z 83 in that of Me3NONa+ manifests the dissociation of NC bond, while in the CID spectra of the other two adducts (Me3NOK+ and Me3NOLi+), there exists no corresponding fragment ion to indicate the dissociation of NC bond. The reason might be as follows: according to the calculations, in Me3NOK+ the OK bond is weaker than the NC bond and thus it’s easier to break, and in Me3NOLi+, as all chemical bonds are very strong, its dissociation might be difficult to take place in MS/MS experiments.

•Ag(I)-catalyzed amination of CH bonds in alkyltriflamides has been studied with DFT method.•A non-radical mechanism is suggested.•Ag(III) intermediate is probably involved in the catalytic cycle.•Rate-determining step is the concerted metallation/deprotonation (CMD) process.A systematic investigation has been conducted into the mechanism for the silver-catalyzed direct amination of unactivated CH bonds in triflamides by density functional theory (DFT). The present work is in favor of the non-radical mechanism, which consists of four parts: formation of NAg(I) intermediate, oxidation of the NAg(I) to NAg(III) intermediate by PhI(OTFA)2, concerted metallation/deprotonation (CMD) process and reductive elimination. Thereinto, it is the CMD process that determines the reaction rate as well as the selectivity of the reaction.Download high-res image (135KB)Download full-size image