Multibond reactions cannot normally be synchronous

Dewar, Michael J. S.

doi:10.1021/ja00313a042

Cited by 387 publications

(245 citation statements)

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“…Our conclusions agree with the contention of Dewar that multibond processes are usually asynchronous [1]. However, whereas we attribute this to the avoidance of orbital symmetry constraints, Dewar concluded that multiple bonds do not break simultaneously because that would require an amount of energy roughly equal to the sum of the energies required to break each bonds individually [1]. Given that we recently found that H 2 elimination from CH 3 CHϭNH ϩ CH 3 , an orbital symmetry allowed 1,4-process with a high reverse critical energy, is highly synchronous [45], we believe that orbital symmetry restraints significantly influence the mechanisms of 1,2-eliminations by preventing synchronized trajectories from being taken.…”

Section: Discussionsupporting

confidence: 93%

“…The fourth type is asynchronous 1,2-eliminations initiated by bond extension and followed by H-transfer, e.g., dissociations of 1 and CH 3 NH ϩ CH 3 . Our conclusions agree with the contention of Dewar that multibond processes are usually asynchronous [1]. However, whereas we attribute this to the avoidance of orbital symmetry constraints, Dewar concluded that multiple bonds do not break simultaneously because that would require an amount of energy roughly equal to the sum of the energies required to break each bonds individually [1].…”

Section: Discussionsupporting

confidence: 89%

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1,2-Eliminations from (CH₃)₂NH⁺CH₂CH₃ and (CH₃)₂NH₂⁺: Guided dissociations

McAdoo

2008

J. Am. Soc. Mass Spectrom.

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1,2-Eliminations are a varied and extensive set of dissociations of ions in the gas phase. To understand better such dissociations, elimination of CH 2=CH2 and CH 3CH3 from (CH 3hNH+CH2CH3 (1) and of CH 4 from (CH 3hNH2 + are characterized by quantum chemical calculations. Stretching of the CN bond to ethyl is followed by shift of an H from methyl to the bridging position in ethyl and then to N to reach (CH 3hNH2 + + CH 2=CH2 from 1. CH 3CH3 elimination by H-transfer to C 2Hs + to form CH 3NH+=CH2 + CH 3CH3 also takes place.(CH 3hNH2 + eliminates methane by CN bond extension followed by /3-H-transfer to give CH 2=NH+ + CH 4 • Low-energy reactions resembling complex-mediated 1,2-eliminations occur and constitute a hitherto largely unrecognized type of reaction. As in many complexmediated reactions, these reactions transfer H between incipient fragments. They are distinguished from complex-mediated processes by the fragments not being able to rotate freely relative to each other near the transition state for reaction, as they do in complexes. Most 1,2-eliminations are ion-neutral complex-mediated, occur by the just described lower energy reactions, have 1,l-like transition states, or utilize highly asynchronous 1,2 transition states. All of these avoid synchronized 1,2-transition states that would violate conservation of orbital symmetry. ( . Williams and Hvistendahl concluded that some 1,2-eliminations of H 2 occur by high-energy, symmetry-forbidden 1,2-trajectories and others by allowed 1,l-transition states [2,3]. Uggerud and coworkers further characterized 1,l-like transition states for 1,2-eliminations [4-6]. Uggerud concluded that orbital symmetry [7] cannot be used to predict the reaction trajectories for the 1,2-eliminations that do occur [4]. However, we conclude that formal 1,2-eliminations generally avoid violating conservation of orbital symmetry. They do this by occurring in separate steps (ion-neutral complex-mediated or parallel lower energy processes), utilizing non-forbidden Ll-like transition states, or passing through sufficiently asynchronous 1,2-transition states that they are not symmetry forbidden [8]. To categorize 1,2-eliminations, we here use theory to characterize dissociations of (CH 3hNH+CH2CH3 (1) to (CH 3hNH2 + + CH 2=CH2 , to CH 3NH+=CH2 + CH 3CH3 , to CH 3NH+=CH2 + 2 'CH 3 , and of (CH 3hNH2 + to CH=NH 2 + + CH 4 .The most common 1,2-elimination by cations in the gas phase is probably bond cleavage to form an ionAddress reprint requests to Dr. D. J.McAdoo, Department of Neuroscience and Cell Biology, University of Texas Medical Branch, 301 University Blvd., Galveston, TX 77555-1043, USA. E-mail: djmcadoo@utmb.edu neutral complex followed by H-transfer between incipient partners, often accompanied by isomerization [9][10][11][12][13][14][15][16]. Complexes are considered to be intermediaries when a bond is extended to the point that the partners can rotate relative to each other [10,11,17,18], and/or reactions occur between incipient fragments [10,12,15]. Such reactions are initia...

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Section: Discussionsupporting

confidence: 93%

Section: Discussionsupporting

confidence: 89%

1,2-Eliminations from (CH₃)₂NH⁺CH₂CH₃ and (CH₃)₂NH₂⁺: Guided dissociations

McAdoo

2008

J. Am. Soc. Mass Spectrom.

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“…A concerted reaction is defined as one that takes place in a single kinetic step without necessarily being synchronous. 2 In contrast, a stepwise reaction involves two kinetically distinct steps via a stable intermediate. Houston and co-workers 3 used the calculated molecular rotational time scale ͑ϳps͒ of the intermediate as a dividing line between a concerted and a stepwise mechanism, defining concertedness when the intermediate lifetime becomes shorter than its rotational period.…”

Section: Introductionmentioning

confidence: 99%

Direct femtosecond observation of the transient intermediate in the α-cleavage reaction of (CH3)2CO to 2CH3+CO: Resolving the issue of concertedness

Kim

Pedersen

Zewail

1995

The Journal of Chemical Physics

130

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Articles you may be interested inObservation of charge-transfer excited states in the I−⋅CH3I, I−⋅CH3Br, and I−⋅CH2Br2SN2 reaction intermediates using photofragmentation and photoelectron spectroscopies J. Chem. Phys. 97, 5911 (1992); 10.1063/1.463752Reuse of AIP Publishing content is subject to the terms: https://publishing.aip.org/authors/rights-and-permissions. When a reaction involving two equivalent bonds has sufficient energy to break both of them, it can proceed by either a concerted or a stepwise mechanism. For Norrish type-I and other reactions, this issue has been controversial since direct time resolution of the individual C-C cleavage events was not possible. Here, for the elementary ␣-cleavage of acetone, we report on the femtosecond resolution of the intermediates using mass spectrometry. The results show the nonconcertedness of the reaction, provide the times for the primary and secondary C-C breakage, and indicate the role of electronic structure ͑ *, antibonding impulse͒ and the vibrational motions involved.

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“…These reactions are very favored thermodynamically, ¦G°2 98K = ¹7.9 kcal mol ¹1 and ¦G°2 98K = ¹10.2 kcal mol ¹1 , respectively. Alternatively, the transformations 5a ¼ 6a may occur via the six-membered pericyclic two-bond 34 synchronous 34 concerted reaction through intermolecular proton transfer with the help of one additional water molecule, denoted as a "proton shuttle." Such a possibility was verified only for the transformation 5a R ¼ 6a R , and was found slightly higher in energy, see SI for details.…”

Section: Experimental Studies Of the Reactions Between Ru Andmentioning

confidence: 99%

C–F Bond Breaking through Aromatic Nucleophilic Substitution with a Hydroxo Ligand Mediated via Water Bifunctional Activation

Dub

Wang

Gridnev

et al. 2013

Bulletin of the Chemical Society of Japan

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Transformation of CF to CO bond mediated by bifunctional ruthenium and iridium complexes is described. This reaction proceeds through water OH bond cleavage via metalligand cooperation in the newly developed 16e bifunctional ruthenium and iridium complexes bearing chiral (S,S)-C 6 F 5 SO 2 -dpen ligand. The 16e Ru amido complex, [Ru{(S,S)-Pfbsdpen}(η 6 -hmb)] (1a), readily reacted with water at room temperature producing oxometallacyclic compound, (R)- [Ru{κ 3 (N,N¤,O)-(S,S)-OC 6 F 4 SO 2 dpen}(η 6 -hmb)] (3a R ), as a result of bifunctional water activation followed by ortho-oxometallation via S N Ar. Complex 3a R can be prepared either from 1a or, more conveniently from its 18e chlorido precursor, complex (R)-[RuCl{(S,S)-Pfbsdpen}(η In either case 3 R or 4 R is obtained as a single diastereomer, the structure of which has been determined by single-crystal X-ray diffraction studies in solid state and NMR-analysis in solution. Reaction mechanism was studied by NMR spectroscopy combined with continuum solvent reaction-field density functional theory (DFT) analysis. Experimental studies showed that diastereoselective oxocyclometallation 1a ¼ 3a R proceeds at temperatures >0°C in a stepwise manner through the detectable intermediate, hydroxo complex (R)-[Ru(OH){(S,S)-Pfbsdpen}(η 6 -hmb)] (6a R ), which exists in equilibrium with less-populated diastereomer (S)-[Ru(OH){(S,S)-Pfbsdpen}(η 6 -hmb)] (6a S ) in 10:1 ratio at ¹80°C in CD 2 Cl 2 . Computational analysis essentially explains the diastereoselectivity in this reaction via a significant difference in the stabilities of the corresponding transition states: although diastereomers 6a R and 6a S are in equilibrium via complex 1a, only 6a R is transformed into 3a R via ratedetermining Meisenheimer-type transition state.In the last few decades, bifunctional molecular catalysis based on metalligand cooperation has been rapidly developed and has become a general strategy for highly efficient molecular transformations in synthetic organic chemistry. 1,2We have successfully developed a series of half-sandwich-type bifunctional Ru and Ir catalysts, [Ru(N-sulfonylated dpen)-(η 6 -arene)] and [Cp*Ir(N-sulfonylated dpen)], bearing the chelating chiral N-sulfonylated diamine ligands (dpen: 1,2-diphenylethylenediamine) for a wide range of molecular transformations. They include practical asymmetric transfer hydrogenation of ketones 3 and imines, 4 asymmetric hydrogenation of polar functionalities, 5 asymmetric Michael reaction of 1,3-dicarbonyl compounds with cyclic enones 6 and nitroalkenes, 7 stereoselective catalytic CN 8 and CC 8b,9 bondforming reactions, aerobic oxidative kinetic resolution of racemic secondary alcohols 10 and asymmetric hydration of nitriles.11 The key to success in these catalytic reactions relies on the intrinsic nature of the 16e amido complexes to activate reversibly XH ¤+ bonds (X = H ¤¹ , C, O, N) via the metalligand cooperation with no change in the formal oxidation state of the metal (bifunctional activation). For example, facile reversible...

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Multibond reactions cannot normally be synchronous

Cited by 387 publications

References 2 publications

1,2-Eliminations from (CH₃)₂NH⁺CH₂CH₃ and (CH₃)₂NH₂⁺: Guided dissociations

1,2-Eliminations from (CH₃)₂NH⁺CH₂CH₃ and (CH₃)₂NH₂⁺: Guided dissociations

Direct femtosecond observation of the transient intermediate in the α-cleavage reaction of (CH3)2CO to 2CH3+CO: Resolving the issue of concertedness

C–F Bond Breaking through Aromatic Nucleophilic Substitution with a Hydroxo Ligand Mediated via Water Bifunctional Activation

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Multibond reactions cannot normally be synchronous

Cited by 387 publications

References 2 publications

1,2-Eliminations from (CH3)2NH+CH2CH3 and (CH3)2NH2+: Guided dissociations

1,2-Eliminations from (CH3)2NH+CH2CH3 and (CH3)2NH2+: Guided dissociations

Direct femtosecond observation of the transient intermediate in the α-cleavage reaction of (CH3)2CO to 2CH3+CO: Resolving the issue of concertedness

C–F Bond Breaking through Aromatic Nucleophilic Substitution with a Hydroxo Ligand Mediated via Water Bifunctional Activation

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1,2-Eliminations from (CH₃)₂NH⁺CH₂CH₃ and (CH₃)₂NH₂⁺: Guided dissociations

1,2-Eliminations from (CH₃)₂NH⁺CH₂CH₃ and (CH₃)₂NH₂⁺: Guided dissociations