Isomerization Rates of Iminocarbonates

Marullo, N. P.; Wagener, E. H.

doi:10.1021/ja00973a054

Cited by 40 publications

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“…57•61 Attempts to elucidate the mechanisms (lateral shift, rotational, or intermediate possibilities) of individual thermal imine isomerizations continue to challenge the ingenuity of various research groups and provide fuel for controversy. [64][65][66][67][68][69][70] As part of an ongoing program concerned with the photochemistry of the C-N double bond, we have investigated the solution phase syn-anti photoisomerization of an oxime ether in order to clarify the mechanistic details of the C-N double bond photoisomerization. The great configurational stability of oxime ethers at room temperature stands in striking contrast to the behavior of vV-aryl-and alkylimines.57 These oxime ethers are attractive candidates for mechanistic photostudies since the presence of the methoxyl group drastically reduces the rate of thermal interconversion (k < 10-13 at 60°).…”

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confidence: 99%

Photochemical syn-anti isomerization about the carbon-nitrogen double bond

Padwa¹,

Albrecht²

1974

J. Am. Chem. Soc.

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Upon irradiation with ultraviolet light, the O-methyl oxime ethers of acetophenone undergo facile syn-anti photoisomerization. The photostationary state ratio obtained by direct irradiation of the syn and anti oxime ethers was 2.20 ± 0.03. Photoisomerization about the C-N double bond could also be induced by triplet excitation. A photostationary state vs. triplet energy of the sensitizer plot shows (1) a high-energy region in which the stationary state ratio is ca. 1.5, (2) a gradual increase in the syn/anti ratio from 72 down to 59 kcal of triplet energy, and (3) a sharp decrease from 59 down to 54 kcal. The latter two observations were explained by nonvertical excitation of the acceptor. Analysis of the sensitized quantum yields for the syn-anti isomerization reaction shows that energy transfer to the oxime ethers does not proceed with identical rates. The inefficiency encountered in these sensitized experiments suggests that not all triplet sensitizers are effective in generating a twisted imine triplet. A double sensitization experiment using the oxime ethers and r/ww-stilbene indicates that triplet energy transfer can occur from the oxime ethers to trans-stilbene. Since the direct irradiation could not be quenched with high concentrations of piperylene, it would seem as though an electronically excited singlet is the reactive state in the direct isomerization. ne of the most active areas of organic photochemistry has been the study of systems which possess a carbonyl group.2-5 As a result of these studies the photochemical transformations of organic molecules containing this functional group have been categorized into a number of primary photochemical processes.6 This state of affairs contrasts sharply with the present status of the structurally related imine system, the photochemistry of which is mainly qualitative with relatively little available in the way of quantum yield data and kinetic studies. While photochemical reactions have been observed with a number of systems possessing a C-N double bond,7-31 a more (1) NDEA Title IV Fellow, 1967 (2) N.

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confidence: 99%

Photochemical syn-anti isomerization about the carbon-nitrogen double bond

Padwa¹,

Albrecht²

1974

J. Am. Chem. Soc.

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“…In addition, the rates for this kind of reactions vary on an extremely wide range, at least 16 powers of 10 1, 9. To explain these interconversions about the CN bond two limiting mechanisms have been proposed early in the literature 2, 6, 7, 10. Thus, if α G and α T are the CNX bond angles in the ground and transition states respectively (see Fig.…”

Section: Introductionmentioning

confidence: 99%

“…There has been considerable discussion concerning the mechanism that operates in imines, although the pathway which has been supported by most workers has been the inversion mechanism 1, 3, 11–13. However, the interpretation of the experimental data have in some cases given conflicting results and so, Marullo and Wagener10 interpreted the isomerization of iminocarbonates on the basis of rotation, whereas Leibfritz and Kessler14 and Vögtle et al15 favored an inversion mechanism for this process. Also, Hall et al7 in their study of the isomerization of hexafluoroacetone N ‐phenylimine and its p ‐chloro, p ‐fluoro, p ‐methoxi, p ‐methyl, and p ‐nitro derivatives, concluded that “whether our series offers examples of isomerization by both rotation and lateral shifts is a difficult question.” Clearly, these difficulties are related to the fact that transition states are not amenable to experimental study and, therefore, mechanisms must be proposed on the basis of the interpretation of the substituent effects, e.g., dependence of rates or thermodynamic activation parameters on these substituents.…”

Section: Introductionmentioning

confidence: 99%

A theoretical study of topomerization of imine systems: Inversion, rotation or mixed mechanisms?

Gálvez

Guirado

2009

J Comput Chem

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The different mechanisms, rotation, inversion, or intermediate mechanism, by which occur the topomerization of imine systems R(2) C=N-X have been studied by applying ab initio, B3LYP, and MP2 methods. The effect of a wide variety of substituents R and X on the isomerization pathway have been examined by computing fully optimized structures of the ground and transition states (136 isomers belonging to different imine families were studied and more than 300 transition structures were determined at various levels of theory). Energy barriers have been also obtained and it was found that the groups R and X have a strong influence on the type of mechanism involved and the activation energies. Thus, and depending on the type of substituents, transition state structures related to the following kinds of processes were found: pure inversion, intermediate mechanisms, rotation, and enhanced rotation (hyper-rotation). In turn, the corresponding activation energies range between very low (<10 kcal/mol) and extremely high (> 70 kcal/mol) values. A simple index that allows us to quantify the percentage of inversion or rotation mechanism is proposed.

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The Study of Intramolecular Rate Processes by Dynamic Nuclear Magnetic Resonance

Binsch

1968

Topics in Stereochemistry

327

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Isomerization Rates of Iminocarbonates

Cited by 40 publications

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Photochemical syn-anti isomerization about the carbon-nitrogen double bond

Photochemical syn-anti isomerization about the carbon-nitrogen double bond

A theoretical study of topomerization of imine systems: Inversion, rotation or mixed mechanisms?

The Study of Intramolecular Rate Processes by Dynamic Nuclear Magnetic Resonance

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