New Kinetics Methodologies Applied to Carbene Fragmentation Reactions

Moss, Robert A.; Johnson, Lauren A.; Yan, Shunqi; Toscano, John P.; Showalter, Brett M.

doi:10.1021/ja002611x

Cited by 18 publications

(47 citation statements)

References 24 publications

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“…Benzyloxychlorocarbene ( 1 ). The fragmentation of carbene 1 in MeCN led to benzyl chloride and N -benzylacetamide, a “Ritter” product formed by the attack of the benzyl cation on MeCN, followed by hydrolysis with adventitious water. , In DCE, only benzyl chloride formed from the fragmentation of 1 . These results, together with product distributions for the fragmentations of 1 in cyclohexane, benzene, and pentane, appear in Table .…”

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confidence: 95%

“…Stereochemical retention during the fragmentation of chiral α-deuterio- 1 , as well as the persistence of 43% of benzyl chloride even when the reaction occurred in methanol, led to a mechanistic formulation of the fragmentation via ion pair 2 . , The absolute rate constant of the fragmentation, measured by laser flash photolysis (LFP) using either UV , or time-resolved infrared (TRIR) monitoring, gave k frag ∼ 4.5 × 10 5 s -1 (MeCN) and ∼ 1.8 × 10 5 s -1 (DCE) …”

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The Fragmentation of Alkoxychlorocarbenes in Hydrocarbon Solvents and in Low Temperature Argon Matrixes

Moss¹,

Ma²,

Zheng³

et al. 2002

J. Phys. Chem. A

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Alkoxychlorocarbenes (ROCCl, R = benzyl, cyclohexyl, and 1-octyl) were generated from the corresponding diazirines in acetonitrile, dichloroethane, benzene, cyclohexane, and pentane solutions at 25 °C, and the fragmentations of these carbenes were examined. Formation of RCl (and alkenes when R = cyclohexyl or 1-octyl) occurred efficiently in all of the solvents. The rate constant for the fragmentation of PhCH2OCCl (determined by laser flash photolysis) was ∼105 s-1, and relatively independent of solvent. Photolysis of PhCH2OC(N2)Cl in Ar matrixes led to the carbene, PhCH2OCCl, as well as its primary fragmentation products, the benzyl and COCl radicals. A prominent product was also phenacyl chloride, PhCH2COCl, a formal rearrangement product of the carbene. Computational studies of the rearrangements and fragmentations of MeOCCl, EtOCCl, and PhCH2OCCl at the DFT and coupled cluster levels afforded transition states and energetics. These studies allowed us to identify several mechanistic pathways, including concerted and homolytic processes that are predicted to prevail in nonpolar solvents and matrixes as opposed to heterolytic (ionic) processes in polar solvents. The existence of cis and trans forms of R−OC−Cl, and their interconversion, are complicating factors that were considered computationally.

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Section: Resultsmentioning

confidence: 95%

mentioning

confidence: 99%

The Fragmentation of Alkoxychlorocarbenes in Hydrocarbon Solvents and in Low Temperature Argon Matrixes

Moss¹,

Ma²,

Zheng³

et al. 2002

J. Phys. Chem. A

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“…include reactions of benzyloxychlorocarbene, 2a sec-butoxychlorocarbene, 2b cyclobutoxychlorocarbene and cyclopropylmethoxychlorocarbene, 2c 2-norbornyloxychlorocarbene, 2d and various bridgehead ROCCl. 2e Carbene fragmentation not only offers novel entry to several classic carbocations of organic chemistry, but laser flash photolysis (LFP) 3,4 and time-resolved infrared spectroscopy (TRIR) 4 permit measurements of rate constants and activation parameters for these very fast processes.…”

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confidence: 99%

“…5 In dichloroethane (DCE), k frag lies between 6.2 × 10 4 (LFP) and 2.9 × 10 5 s -1 (TRIR). 4 PhCH 2 Cl is the only product in DCE, whereas, in MeOH, PhCH 2 -Cl (43%) and PhCH 2 OMe (57%) are produced by ion pair collapse and cation capture, respectively. 2a The B3LYP/6-31G* computed E a for the fragmentation of (cis) 1 is 6.7 kcal/mol in the gas phase and 1.4 kcal/mol in MeOH.…”

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A Duality of Mechanisms for the Fragmentation of Substituted Benzyloxychlorocarbenes

Moss

Sauers

2002

J. Am. Chem. Soc.

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Substituted benzyloxychlorocarbenes (X-PhCH2OCCl) were generated photochemically at 25 degrees C in dichloroethane from appropriate diazirine precursors. Fragmentations of the carbenes produced were determined by laser flash photolysis. The data (X, kfrag in s-1) were: p-Me, 2.6 x 105; p-Ph, 8.3 x 104; H, 6.0 x 104; p-Cl, 5.2 x 104; m-Cl, 1.3 x 105; p-F3C, 2.1 x 106; p-O2N, 6.3 x 106. A Hammett correlation of log kfrag versus sigma+ was parabolic. The curvature was taken to imply the gradual change of the fragmentation mechanism from predominantly heterolytic for X-PhCH2CCl with electron-donating X (with developing positive charge on the benzylic carbon in the transition state) to predominantly homolytic for carbenes with electron-withdrawing X. This idea was supported by computational studies.

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Ionic and radical fragmentations of alkoxyhalocarbenes – a perspective

Moss¹

2008

J of Physical Organic Chem

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Fragmentations of secondary or tertiary alkoxyhalocarbenes in polar solvents generate carbocations as components of ion pairs. A variety of carbocations can be produced including acyclic, alicyclic, benzyl, bridgehead, cyclopropyl, cyclopropylcarbinyl, and norbornyl examples. Laser flash photolysis (LFP) studies provide kinetics and activation parameters for the carbene fragmentations, which are orders of magnitude faster, and require considerably reduced activation energies, compared to analogous solvolytic carbocation‐generative processes. In some cases, the time required for solvent and anion equilibration of the ion pairs can be estimated. In nonpolar solvents, the gas phase, or cryogenic matrices, homolytic carbene fragmentation may, in certain cases, supplant heterolytic fragmentation. Copyright © 2008 John Wiley & Sons, Ltd.

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New Kinetics Methodologies Applied to Carbene Fragmentation Reactions

Cited by 18 publications

References 24 publications

The Fragmentation of Alkoxychlorocarbenes in Hydrocarbon Solvents and in Low Temperature Argon Matrixes

The Fragmentation of Alkoxychlorocarbenes in Hydrocarbon Solvents and in Low Temperature Argon Matrixes

A Duality of Mechanisms for the Fragmentation of Substituted Benzyloxychlorocarbenes

Ionic and radical fragmentations of alkoxyhalocarbenes – a perspective

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