Picosecond Optical Grating Calorimetry of Singlet Methylene in Benzene

Khan, M. Iltaf; Goodman, Joshua L.

doi:10.1021/ja00129a045

Cited by 38 publications

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“…Eqn ). Transient 8 , probably a weak π‐complex of CH 2 and benzene, was proposed as the intermediate, with a computed dissociation energy of 7.2 kcal mol −1 (RHFMP2/6‐31 G*), similar to the observed dissociation energy of 8.7 ± 3.1 kcal mol −1 …”

Section: Carbene–solvent Complexessupporting

confidence: 56%

“…An important step toward the direct detection of carbene‐aromatic complexes was taken by Kahn and Goodman, who employed picosecond optical grating calorimetry to follow the reaction of methylene with benzene . Two heat depositions were monitored, with the faster event ascribed to the formation of a transient intermediate, whereas the slower event ( k 2 ~ 2 × 10 9 s −1 ) was assigned to its decay to products (cf.…”

Section: Carbene–solvent Complexesmentioning

confidence: 99%

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Solvation and complexation of carbenes – a perspective

Moss

2011

J of Physical Organic Chem

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Complexes of several carbenes with aryl ethers are documented by laser flash photolysis with UV‐visible spectroscopic detection. The carbenes include methylchlorocarbene, benzylchlorocarbene, phenylchlorocarbene, p‐nitrophenylchlorocarbene, and dichlorocarbene, whereas the complexing ligands are anisole, various methylanisoles, dibenzo‐18‐crown‐6, dibenzofuran, 1,3‐dimethoxybenzene, and 1,3,5‐trimethoxybenzene. The carbene complexes are characterized both by spectroscopy and computation. Complexation retards the additions of the carbenes to olefins. The complexation of phenylchlorocarbene by 1,3,5‐trimethoxybenzene is shown to be an equilibrium process, and the pertinent thermodynamic parameters are obtained by experiment and computed from theory. Copyright © 2011 John Wiley & Sons, Ltd.

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Section: Carbene–solvent Complexessupporting

confidence: 56%

Section: Carbene–solvent Complexesmentioning

confidence: 99%

Solvation and complexation of carbenes – a perspective

Moss

2011

J of Physical Organic Chem

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“…For the generation of 4 , the lowering of both the activation barrier and the energy of the intermediate (route III ) or transition state (route II ), may arise from a stabilising secondary orbital interaction involving the vacant p orbital at C(2) and the ipso ‐carbon atom on toluene. Analogous interactions of aryl π systems with carbenes have been suggested on the basis of computational studies to account for rate accelerations of carbene reactions in aromatic solvents 33. Such a process might be expected to perturb the electronic effects of the para ‐tolyl substituents and thus lead to a nonlinear Hammett correlation.…”

Section: Discussionmentioning

confidence: 99%

Intermolecular Insertion of an N,N‐Heterocyclic Carbene into a Nonacidic CH Bond: Kinetics, Mechanism and Catalysis by (K‐HMDS)₂ (HMDS=Hexamethyldisilazide)

Lloyd‐Jones¹,

Alder²,

Owen‐Smith³

2006

Chemistry A European J

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The reaction of 2-[13C]-1-ethyl-3-isopropyl-3,4,5,6-tetrahydropyrimidin-1-ium hexafluorophosphate ([13C1]-1-PF6) with a slight excess (1.03 equiv) of dimeric potassium hexamethyldisilazide ("(K-HMDS)2") in toluene generates 2-[13C]-3-ethyl-1-isopropyl-3,4,5,6-tetrahydropyrimid-2-ylidene ([13C1]-2). The hindered meta-stable N,N-heterocyclic carbene [13C1]-2 thus generated undergoes a slow but quantitative reaction with toluene (the solvent) to generate the aminal 2-[13C]-2-benzyl-3-ethyl-1-isopropylhexahydropyrimidine ([13C1]-14) through formal C-H insertion of C2 (the "carbene carbon") at the toluene methyl group. Despite a significant pKa mismatch (Delta pKa 1+ and toluene estimated to be ca. 16 in DMSO) the reaction shows all the characteristics of a deprotonation mechanism, the reaction rate being strongly dependent on the toluene para substituent (rho = 4.8(+/-0.3)), and displaying substantial and rate-limiting primary (k(H)/k(D) = 4.2(+/-0.6)) and secondary (k(H)/k(D) = 1.18(+/-0.08)) kinetic isotope effects on the deuteration of the toluene methyl group. The reaction is catalysed by K-HMDS, but proceeds without cross over between toluene methyl protons and does not involve an HMDS anion acting as base to generate a benzyl anion. Detailed analysis of the reaction kinetics/kinetic isotope effects demonstrates that a pseudo-first-order decay in 2 arises from a first-order dependence on 2, a first-order dependence on toluene (in large excess) and, in the catalytic manifold, a complex noninteger dependence on the K-HMDS dimer. The rate is not satisfactorily predicted by equations based on the Brønsted salt-effect catalysis law. However, the rate can be satisfactorily predicted by a mole-fraction-weighted net rate constant: -d[2]/dt = ({x2 k(uncat)} + {(1-x2) k(cat)})[2]1[toluene]1, in which x2 is determined by a standard bimolecular complexation equilibrium term. The association constant (Ka) for rapid equilibrium-complexation of 2 with (K-HMDS)2 to form [2(K-HMDS)2] is extracted by nonlinear regression of the 13C NMR shift of C2 in [13C1]-2 versus [(K-HMDS)2] yielding: Ka = 62(+/-7) M(-1); delta(C(2)) in 2=237.0 ppm; delta(C(2)) in [2(K-HMDS)2] = 226.8 ppm. It is thus concluded that there is discrete, albeit inefficient, molecular catalysis through the 1:1 carbene/(K-HMDS)2 complex [2(K-HMDS)2], which is found to react with toluene more rapidly than free 2 by a factor of 3.4 (=k(cat)/k(uncat)). The greater reactivity of the complex [2(K-HMDS)2] over the free carbene (2) may arise from local Brønsted salt-effect catalysis by the (K-HMDS)2 liberated in the solvent cage upon reaction with toluene.

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“…[106] LFP of diazomethane in benzene results in two heat depositions, corresponding to a fast (Ͻ100 ps) and a ''slow'' (k ϭ 2.4 ϫ 10 9 s…”

Section: Arenesmentioning

confidence: 99%

“…Two weak complexes of methylene, 70 and 71 (R 1 ϭ R 2 ϭ H), were located, which show the carbene center interacting either with a specific benzene carbon atom (70) or with the π-cloud above a CϪC bond (71) (Scheme 18). [106] At the MP2/6-31G*//HF/6-31G* level of theory, 70 and 71 were found to be stabilized by 30 and 24 kJ/mol, respectively, relative to singlet methylene ϩ benzene (no BSSE, vibrational or thermal corrections). The minimum energy conformation 70b of the CCl 2 -benzene system presents no overall molecular symmetry.…”

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confidence: 96%

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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Picosecond Optical Grating Calorimetry of Singlet Methylene in Benzene

Cited by 38 publications

References 0 publications

Solvation and complexation of carbenes – a perspective

Solvation and complexation of carbenes – a perspective

Intermolecular Insertion of an N,N‐Heterocyclic Carbene into a Nonacidic CH Bond: Kinetics, Mechanism and Catalysis by (K‐HMDS)₂ (HMDS=Hexamethyldisilazide)

Carbene Complexes of Nonmetals

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Picosecond Optical Grating Calorimetry of Singlet Methylene in Benzene

Cited by 38 publications

References 0 publications

Solvation and complexation of carbenes – a perspective

Solvation and complexation of carbenes – a perspective

Intermolecular Insertion of an N,N‐Heterocyclic Carbene into a Nonacidic CH Bond: Kinetics, Mechanism and Catalysis by (K‐HMDS)2 (HMDS=Hexamethyldisilazide)

Carbene Complexes of Nonmetals

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Intermolecular Insertion of an N,N‐Heterocyclic Carbene into a Nonacidic CH Bond: Kinetics, Mechanism and Catalysis by (K‐HMDS)₂ (HMDS=Hexamethyldisilazide)