Experimental Investigation of the Primary and Secondary Deuterium Kinetic Isotope Effects for Epoxidation of Alkenes and Ethylene with <i>m</i>-Chloroperoxybenzoic Acid

Koerner, Terry; Slebocka‐Tilk, H.; Brown, R. S.

doi:10.1021/jo981652x

Cited by 44 publications

(20 citation statements)

References 33 publications

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“…For the S E Ar reaction involving 1Br 2 at 298 K, inverse values of 0.75 and 0.84 for the closed shell and biradical procedures, respectively, were obtained and 0.83 for the biradical Ad E addition. The latter in particular is typical of addition to an sp 2 center . We have already established, however, that the 1Br 2 reaction is not a good model on the basis of the activation barrier alone and the predicted isotope effect confirms this.…”

Section: Kinetic Isotope Effectsmentioning

confidence: 78%

Noncatalytic bromination of benzene: A combined computational and experimental study

et al. 2015

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The non-catalytic bromination of benzene is shown experimentally to require high 5-14M concentrations of bromine in order to proceed at ambient temperatures to form predominantly bromobenzene, along with detectable (<2%) amounts of addition products such as tetra and hexabromocyclohexanes. The kinetic order in bromine at these high concentrations is 4.8 ± 0.06 at 298K and 5.6 ± 0.11 at 273K with a small measured inverse deuterium isotope effect using D 6 -benzene of 0.97 ± 0.03 at 298K. These results are rationalized using computed transition states models at the B3LYP+D3/6-311++G(2d,2p) level with an essential continuum solvent field for benzene applied. The model with the lowest predicted activation free energies agrees with the high experimental kinetic order in bromine and involves formation of an ionic, concerted and asynchronous transition state involving a Br 7 -cluster resembling the structure of the known Br 9 -. This cluster plays three roles;; as a Br + donor, as a proton base and as a stabilizing arm forming weak interactions with two adjacent benzene C-H hydrogens, these aspects together combining to overcome the lack of reactivity of benzene induced by its aromaticity. The computed inverse kinetic isotope effect of 0.95 agrees with experiment, and arises because CBr bond formation is essentially complete whereas C-H cleavage has not yet commenced. The computed free energy barriers for the reaction with 4Br 2 and 5Br 2 for a standard state of 14.3M in bromine are reasonable for an ambient temperature reaction, unlike previously reported theoretical models involving only one or two bromines.

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Section: Kinetic Isotope Effectsmentioning

confidence: 78%

Noncatalytic bromination of benzene: A combined computational and experimental study

et al. 2015

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“…As in the DMD case, the olefinic carbons have undergone only a small degree of rehybridization with little change in the bonding of the peroxy acid oxidant, as displayed in the C Re ϶ and C Si ϶ transitions structures. [29] The opposite sense in the observed stereoselectivity for m-CPBA vs. DMD implies that the transition structure C Si ϶ is favored over the C Re ϶ ( Figure 3); consequently, factors other than those for DMD must be operative in this case. Indeed, it has previously been established that in peracid epoxidations of substrates containing carbonyl groups (carbamates, [30a-30d] amides, [30c,30e] esters, [30d-30e] ketones [30f] ), the effective hydrogen bonding between the peracid proton and the carbonyl group specifies syn stereoselectivity.…”

Section: Discussionmentioning

confidence: 99%

Diastereoselective Reactions of the Tiglic Acid Functionality Mediated by Oxazolidine Chiral Auxiliaries: A Mechanistic Comparison of DMD andm‐CPBA Epoxidations versus Singlet Oxygen and PTAD Ene Reactions

et al. 2005

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Keywords: Ene reaction / Epoxidation / Hydrogen bonding / Singlet oxygen / Stereochemistry 2,2-Dimethyloxazolidines have been utilized as chiral auxiliaries for the diastereoselective functionalization of the optically active tiglic acid derivatives (S)-1 by means of epoxidation with DMD or m-CPBA and ene reactions with 1 O 2 or PTAD. In the DMD and m-CPBA epoxidations, high diastereoselectivities but opposite senses of diastereomer selection were observed. In contrast, the stereochemistry of the 1 O 2 and PTAD ene reactions depended on the size of the attacking enophile: whereas essentially perfect diastereoselectivity was obtained with PTAD, much lower stereoselection

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“…Epoxidation of linoleate will mean that the sp 2 -hybridized carbons C12 and C13 are converted to sp 3 -hybridized carbons in vernoleate. Inverse secondary kinetic isotope effects have been reported for this change in hybridization both for chemical epoxidation (38) and for dehalogenation carried out by thymidylate synthetase (39,40). Thus, the isotopic profile observed is compatible with a mechanism in which epoxidation of linoleic acid occurs.…”

Section: Resultsmentioning

confidence: 65%

Quantitative Deuterium Isotopic Profiling at Natural Abundance Indicates Mechanistic Differences for Δ12-Epoxidase and Δ12-Desaturase in Vernonia galamensis

Billault

Duan

Guiet

et al. 2005

Journal of Biological Chemistry

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Quantitative2 H NMR spectroscopy can determine the natural abundance ( 2 H/ 1 H) ratio at each site of a molecule. In natural products, variation in these values is related to the reaction mechanisms in the pertinent biosynthetic pathway. For the first time, this novel approach has been exploited to probe for mechanistic differences in the introduction of different functionalities into a long-chain fatty acid. Vernolic acid, a major component of the seed oil of Vernonia galamensis, contains both an epoxide and a desaturation. The site-specific isotopic distribution ( 2 H/ 1 H) i has been determined for both vernolic acid and linoleic acid isolated from the same V. galamensis oil. It is found that the ( 2 H/ 1 H) ratio of vernolic acid shows a pattern along the entire length of the chain, consistent with linoleic acid being its immediate precursor. Notably, the C13 relates to the C13 of linoleic acid but not to the C13 of oleic acid. Furthermore, the C12 and C13 positions in vernolic acid are less depleted, consistent with a change in hybridization state from sp 2 to sp 3 . However, the C11 position shows a marked relative enrichment in the vernolic acid, implying that it plays a role in the epoxidase but not the desaturase mechanism. Thus, although it can be concluded that the catalytic mechanisms for the epoxidase and desaturase activities are similar, marked differences in the residual ( 2 H/ 1 H) patterns indicate that the reaction mechanisms are not identical.

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Experimental Investigation of the Primary and Secondary Deuterium Kinetic Isotope Effects for Epoxidation of Alkenes and Ethylene with m-Chloroperoxybenzoic Acid

Cited by 44 publications

References 33 publications

Noncatalytic bromination of benzene: A combined computational and experimental study

Noncatalytic bromination of benzene: A combined computational and experimental study

Diastereoselective Reactions of the Tiglic Acid Functionality Mediated by Oxazolidine Chiral Auxiliaries: A Mechanistic Comparison of DMD andm‐CPBA Epoxidations versus Singlet Oxygen and PTAD Ene Reactions

Quantitative Deuterium Isotopic Profiling at Natural Abundance Indicates Mechanistic Differences for Δ12-Epoxidase and Δ12-Desaturase in Vernonia galamensis

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