Mechanistic studies of unimolecular ionic decompositions by deuterium and heavy-atom isotope effects. Concerted elimination of acetaldehyde from the benzyl ethyl ether radical cation

Stone, David J.; Bowie, John H.; Underwood, Dennis; Donchi, Kevin F.; Allison, C. E.; Derrick, Peter J.

doi:10.1021/ja00344a064

Cited by 18 publications

(9 citation statements)

References 4 publications

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“…Unlike deprotonated pinacol itself, deprotonated benzpinacol does undergo a pinacol-type rearrangement, presumably because of (i) the formation of the strong intramolecular hydrogen bond shown in Eqn (15) and (ii) the ease of phenyl rnigrati~n.~' R R [3,3] Rearrangements like the oxy Cope [Eqn (16)I4l and Claisen rearrangement42 are facile reactions, and occur in both the condensed and gas phases. [1,3] Rearrangements [Eqn (17)] do occur in the condensed phase, but are not observed in the gas phase when R is alkyl or aryl. In contrast, if the vinylidene alkoxide system contains a small ring, the gas-phase [1,3] rearrangement does occur.…”

Section: Intramolecular Rearrangement Reactions Of Even-electron Anionsmentioning

confidence: 99%

“…[1,3] Rearrangements [Eqn (17)] do occur in the condensed phase, but are not observed in the gas phase when R is alkyl or aryl. In contrast, if the vinylidene alkoxide system contains a small ring, the gas-phase [1,3] rearrangement does occur. For example, the collisioninduced spectra of deprotonated 1-vinylcyclobutanol and cyclohexanone are identical [see Eqn (18)];43 the fragmentation is exclusively that of deprotonated cyclohexanone.…”

Section: Intramolecular Rearrangement Reactions Of Even-electron Anionsmentioning

confidence: 99%

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Twenty‐five years of negative‐ion studies at adelaide

Bowie

1993

Org. Mass Spectrom.

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PREAMBLEIn common with many Australian organic chemists of my generation, as a graduate student I was trained in the general area of natural products chemistry. In the mid-1960s I continued this work at Cambridge (UK), working with A. R. Todd on aphid pigments. Dudley Williams arrived from Stanford during this period, and my collaboration with him in those early days introduced me to the mysteries and delights of a technique and a topic completely new to me: applied mass spectrometry and the fundamental reactivity of positive ions in the gas phase. Dudley's drive, enthusiasm and scientific curiousity have always been an inspiration to his students and colleagues, and it is therefore of no surprise that after my appointment to Adelaide my students and I continued work in the area of positive-ion mass spectrometry with particular emphasis on rearrangement reactions. Apart from a joint project (with Peter Derrick) on the application of isotope effects on the McLafferty and allied rearrangements,'*' and my recent and arguably belated return to natural product chemistry with our recent structure determination of a variety of amphibian peptides using fast atom bombardment mass spectrometry (FABMS) and other techn i q u e~,~ our work on positive ions terminated in the late 1960s. But this was the catalyst to all of our subsequent work in gas-phase ion chemistry during the next 25 years. FRAGMENTATION PROCESSES OF MOLECULAR ANIONSTowards the end of the 1960s I decided to commence work on gas-phase negative ions for two major reasons. First, because negative ions are so very important in many condensed-phase reactions of organic and organometallic systems, and I believed that the corresponding gas-phase chemistry would certainly be equally fascinating. The second reason was much more pragmatic: our Hitachi RMU 7D instrument, presumably because of its source design, produced an excellent yield of negative ions following electron impact ionization of many organic substrates. Although the resolution of this instrument left much to be desired, few instruments either then, or indeed subsequently, have bettered its sensitivity for negative-ion formation following electron impact.The prognosis at that time for the useful application of negative ions for analytical purposes in organic chemistry was certainly not promising. Apart from the pioneering work of von Ardenne and colleagues on large organic molecule^,^ early reviews' and an important study by Carl Djerassi and colleagues6 were not particularly favourable in this regard. Major difficulties appeared to be the paucity of formation of molecular anions by the then existing ionization techniques, together with small ion currents then reported to be produced by organic negative ions. Even so, we were not deterred: our long-term aim was to study the chemistry of organic and organometallic negative ions, and any possible analytical application would simply be a bonus.In essence, our initial idea was simple. We wished to study the fragmentations of all 'organic' functional groups ...

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Section: Intramolecular Rearrangement Reactions Of Even-electron Anionsmentioning

confidence: 99%

Section: Intramolecular Rearrangement Reactions Of Even-electron Anionsmentioning

confidence: 99%

Twenty‐five years of negative‐ion studies at adelaide

Bowie

1993

Org. Mass Spectrom.

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“…In particular, an extensive experimental investigation, utilizing both 13 C and 2 H kinetic isotope effects, clearly indicated that the McLafferty rearrangement of ketone radical cations is stepwise. 9 However, Stone et al 7 proposed a concerted mechanism for the loss of acetaldehyde from the benzyl ethyl ether radical cation, which is also a McLafferty type reaction, in collisional activation spectra measurements. A few years later, the same group of authors 8 confirmed this observation by using Rice-Ramsperger-Kassel-Marcus (RRKM) theory on the same reaction.…”

Section: Introductionmentioning

confidence: 99%

McLafferty rearrangement of the radical cations of butanal and 3‐fluorobutanal: A theoretical investigation of the concerted and stepwise mechanisms

Norberg

Salhi-Benachenhou

2007

J Comput Chem

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The stepwise and concerted pathways for the McLafferty rearrangement of the radical cations of butanal (Bu(+)) and 3-fluorobutanal (3F-Bu(+)) are investigated with density functional theory (DFT) and ab initio methods in conjunction with the 6-311+G(d,p) basis set. A concerted transition structure (TS) for Bu(+), (H), is located with a Gibbs barrier height of 37.7 kcal/mol as computed with CCSD(T)//BHandHLYP. Three pathways for the stepwise rearrangement of Bu(+) have been located, which are all found to involve different complexes. The barrier height for the H(gamma) transfer is found to be 2.2 kcal/mol, while the two most favorable TSs for the C(alpha)-C(beta) cleavage are located 8.9 and 9.2 kcal/mol higher. The energies of the 3F-Bu(+) system have been calculated with the promising hybrid meta-GGA MPWKCIS1K functional of DFT. Interestingly, the fluorine substitution yields a barrier height of only 20.5 kcal/mol for the concerted TS, (3F-H). A smaller computed dipole moment, 12.1 D, for (3F-H) compared with 103.2 D for (H) might explain the stabilization of the substituted TS. The H(gamma) transfer, with a barrier height of 4.9 kcal/mol, is found to be rate-determining for the stepwise McLafferty rearrangement of 3F-Bu(+), in contrast to the unsubstituted case. By inspection of the spin and charge distributions of the stationary points, it is noted that the bond cleavages in the concerted rearrangements are mainly of heterolytic nature, while those in the stepwise channels are found to be homolytic.

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“…More evidence for this rearrangement reaction was found with the decomposition of ionized aliphatic aldehydes, ketones, esters, amides and other derivatives with observation of elimination of vinyl hydrocarbons . The origin of γ‐hydrogen transfer has been established by extensive experiments involving deuterium and heavy‐atom labeling . McLafferty proposed the electronic mechanism for this type of rearrangement reaction, involving γ‐hydrogen transfer.…”

Section: Introductionmentioning

confidence: 99%

“…[3][4][5][6][7] The origin of g-hydrogen transfer has been established by extensive experiments involving deuterium and heavy-atoml abeling. [8][9][10] McLafferty proposed the electronic mechanism for this type of rearrangement reaction, [11,12] involv-ing g-hydrogent ransfer.T his type of reactionw as nameda fter him and hasb een reported generally for other chemical systems. [13][14][15][16][17][18][19][20] Aromatic ethylamines are fundamentalc hemical materials and include phenethylamine-derivedn eurotransmitters.…”

Section: Introductionmentioning

confidence: 99%

Roaming-Mediated CH₂NH Elimination from the Ionization of Aromatic Ethylamines

2017

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The ionization of aromatic ethylamines by photons or electrons leads to elimination of CH2NH fragments, supposedly deriving from the McLafferty rearrangement involving intramolecular γ‐hydrogen transfer. Using tryptamine and phenethylamine as examples, the results reported here suggest that the McLafferty mechanism is inadequate for interpreting the observations of CH2NH elimination due to much higher calculated appearance energy than experimentally measured values. Furthermore, by considering the roaming‐mediated effect, the calculated appearance energy for the elimination of CH2NH fragments is reduced and matches well with the experimental results and verifies the existence of roaming‐mediated effect. This effect could potentially be extended to explain the general CH2NH elimination of aromatic ethylamines. Due to the similar hydrogen transfer to that of the McLafferty mechanism, the roaming‐mediated effect was taken into account to suggest a novel mechanism, termed the “roaming‐modified McLafferty rearrangement”, that explains the observations of CH2NH elimination in the ionization of aromatic ethylamines. This is a reasonable modification of the McLafferty rearrangement mechanism.

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Mechanistic studies of unimolecular ionic decompositions by deuterium and heavy-atom isotope effects. Concerted elimination of acetaldehyde from the benzyl ethyl ether radical cation

Abstract: Partial support for the purchase of a Bruker WP-200 NMR spectrometer from the National Science Foundation is also acknowledged.

Cited by 18 publications

References 4 publications

Twenty‐five years of negative‐ion studies at adelaide

Twenty‐five years of negative‐ion studies at adelaide

McLafferty rearrangement of the radical cations of butanal and 3‐fluorobutanal: A theoretical investigation of the concerted and stepwise mechanisms

Roaming-Mediated CH₂NH Elimination from the Ionization of Aromatic Ethylamines

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Mechanistic studies of unimolecular ionic decompositions by deuterium and heavy-atom isotope effects. Concerted elimination of acetaldehyde from the benzyl ethyl ether radical cation

Abstract: Partial support for the purchase of a Bruker WP-200 NMR spectrometer from the National Science Foundation is also acknowledged.

Cited by 18 publications

References 4 publications

Twenty‐five years of negative‐ion studies at adelaide

Twenty‐five years of negative‐ion studies at adelaide

McLafferty rearrangement of the radical cations of butanal and 3‐fluorobutanal: A theoretical investigation of the concerted and stepwise mechanisms

Roaming-Mediated CH2NH Elimination from the Ionization of Aromatic Ethylamines

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Roaming-Mediated CH₂NH Elimination from the Ionization of Aromatic Ethylamines