Activation parameters for a 1,2 carbon shift in a carbene rearrangement

Moss, Robert A.; Ho, Guo Jie; Shen, S.; Krogh‐Jespersen, Karsten

doi:10.1021/ja00160a059

Cited by 34 publications

(30 citation statements)

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“…[97] A barrier (∆G ϶ ) of 16 kJ/mol protects 61 against conversion into the planar structure 61Ј, which is computed to be more stable than 61 by 19 kJ/mol. A 1,2-H shift of 61Ј eventually affords cyclopentadiene (62). Qualitatively similar conclusions had been drawn earlier from the fact that labeled 61 was scavenged stereoselectively (Ն 95%) with methanol (60a Ǟ 64a, 60b Ǟ 64b).…”

Section: Alkenessupporting

confidence: 82%

“…The enhanced formation of alkene 26 was attributed to a concerted migrationϪelimination of the ylide 28, although there is little precedent for such a mechanism (as a rule, methyl groups migrate to the vacant carbene p-orbital). [60] However, all carbene-stabilizing substituents R (Ph, [61] Cl, [62] CO 2 Et, [63] COCMe 3 [64] ) lower the cyclopro-Scheme 8. Solvent effects on the reactivity of tert-butylcarbene pane to alkene ratio obtained from 27 relative to that of the parent compound (R ϭ H).…”

Section: Scheme 7 Modification Of Arylcarbene Reactivities By Ethersmentioning

confidence: 99%

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Carbene Complexes of Nonmetals

Kirmse

2005

Eur J Org Chem

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Electrophilic carbenes accept a variety of n‐ and π‐donors. The products range from persistent ylides to matrix‐isolated xenon complexes. Ylides can be viewed as carbene complexes if they regenerate carbenes thermally or undergo concerted methylene transfer reactions. According to these definitions, the present review focuses on oxonium and iodonium ylides. Transient π‐complexes appear to be involved in addition reactions of carbenes with arenes (but not with alkenes). Nucleophilic carbenes form adducts with Lewis acids which are, for the most part, stable and have been well characterized structurally. Only recently attention has been directed to reversible systems. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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Section: Alkenessupporting

confidence: 82%

Section: Scheme 7 Modification Of Arylcarbene Reactivities By Ethersmentioning

confidence: 99%

Carbene Complexes of Nonmetals

Kirmse

2005

Eur J Org Chem

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“…Linear Arrhenius behavior occurs for BCC, together with simplification of its chemistry. Parallel experiments indicate that analogous results can be obtained in other, nonhalogenated polar solvents, such as methyl benzoate (ε = 6.59) 25 or ethyl acetate (ε = 6.11) …”

Section: Resultssupporting

confidence: 59%

“…In both TCE (ε = 8.20) and CHCl 3 (ε = 4.81), the intramolecular hydride shift is potentiated, effectively shutting off alternative intermolecular chemistry (e.g., azine formation, solvent insertion). Linear Arrhenius behavior occurs for BCC, together with simplification of its chemistry.…”

Section: Resultsmentioning

confidence: 99%

Benzylchlorocarbene: Origins of Arrhenius Curvature in the Kinetics of the 1,2-H Shift Rearrangement

et al. 1998

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Benzylchlorocarbene (1, BCC) was generated photochemically from benzylchlorodiazirine (2) in isooctane, methylcyclohexane (MCH), and tetrachloroethane (TCE) at temperatures from ∼30 to −75 °C. At −70 °C in isooctane, the identified products included Z/E-β-chlorostyrenes 4 (46.6%), α-chlorostyrene 5 (2.4%), 1,1-dichloro-2-phenylethane 6 (1.9%), a BCC-isooctane insertion product 8 (5.5%), carbene dimers 9 (3.8%), and azine 3 (30%). The significant incursion of intermolecular products 3, 8, and 9 implies that laser flash photolytic (LFP) kinetic data for the decay of BCC obtained at low temperature is biased and should not be employed in Arrhenius analyses. Accordingly, previously obtained curved Arrhenius correlations for BCC do not necessarily implicate quantum mechanical tunneling (QMT) in the 1,2-H shift rearrangement of BCC to 4. Similarly in MCH, where BCC affords a solvent insertion product in ∼44−53% yield, the curved Arrhenius correlation (Figure ) cannot be readily interpreted. In polar solvents such as TCE, clean H-shift reactions of BCC are obtained even at −71 °C; an Arrhenius correlation of LFP kinetic data is linear from 3 to −71 °C (Figure ), affording E a = 3.2 kcal mol-1 and log A = 10.0 s-1. Therefore, QMT does not appear to play a major role in the 1,2-H shift rearrangement of BCC at ambient or near ambient temperature in solution.

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“…The parent carbene, CH 2 , has a triplet ground state lying 9 kcal/mol below the lowest-energy singlet. , Experiment and well converged quantum mechanical studies indicate that alkyl and aromatic substitution preferentially stabilize the singlet state, with a stabilization of roughly 5 kcal/mol per methyl or phenyl substituent. − In the area of carbene reactivity, unimolecular rearrangements, particularly of singlet state carbenes, have been a focus of attention, in part because low-temperature matrix isolation techniques have permitted the chemistry of isolated carbenes to be followed in great detail − ,,,− 1,2-Alkyl shifts are also well known, as are carbene insertions into more distantly located bonds, bimolecular carbene insertions, and carbene additions to double bonds. − ,,− …”

Section: Introductionmentioning

confidence: 99%

Singlet−Triplet Splittings and 1,2-Hydrogen Shift Barriers for Methylphenylborenide, Methylphenylcarbene, and Methylphenylnitrenium in the Gas Phase and Solution. What a Difference a Charge Makes

1997

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In the isoelectronic series methylphenylborenide, methylphenylcarbene, and methylphenylnitrenium, fundamental differences are predicted for singlet state geometries, singlet-triplet state splittings, barriers to singlet 1,2-hydrogen migration, and sensitivity of 1,2-hydrogen migration to solvent effects in n-heptane and acetonitrile. We conclude that isoelectronic analogies are dangerous for systems having different formal charges, and that the interaction of the divalent center with a conjugating substituent is very sensitive to the electron donating or withdrawing nature (and power) of the hypovalent atom. Solvent effects on the singlet-triplet splitting result from static polarity differences whereas the solvent effects on 1,2-hydrogen migration result primarily from polarizability differences. For the experimentally characterized carbene case, extensive comparison of calculated and measured results is provided.

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Activation parameters for a 1,2 carbon shift in a carbene rearrangement

Cited by 34 publications

References 0 publications

Carbene Complexes of Nonmetals

Carbene Complexes of Nonmetals

Benzylchlorocarbene: Origins of Arrhenius Curvature in the Kinetics of the 1,2-H Shift Rearrangement

Singlet−Triplet Splittings and 1,2-Hydrogen Shift Barriers for Methylphenylborenide, Methylphenylcarbene, and Methylphenylnitrenium in the Gas Phase and Solution. What a Difference a Charge Makes

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