Structural organic chemistry by nmr. III. Isomerization of compounds containing the carbon-nitrogen double bond

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

doi:10.1016/s0040-4039(01)88566-x

Cited by 20 publications

(9 citation statements)

References 15 publications

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“…Another issue previously discussed6, 9, 16 refers to the fact that the transition state for isomerizations that occur by rotation should involve a dipolar species with positive and negative charges on the imino carbon and imino nitrogen, respectively, i.e., R 2 C (+) − N −) X. To verify this, differences between the NBO charges of the C and N atoms in the ground and transition states [Δ q ( G ) and Δ q ( T )], as well as the values of Δ q ( T , G ) = Δ q ( T ) − Δ q ( G ) are given below for some selected imine systems (values are in electrons and were computed at the B3LYP/6‐31G(d) level):…”

Section: Resultsmentioning

confidence: 99%

“…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. In addition, these studies must be limited to those imine systems which have stable Z/E isomers, i.e., alkylimines, arylimines, oximes, N ‐haloimines, and N ‐cyanoimines, mainly 1, 4, 7, 16. Conversely, computational methods are more suitable for such studies because they give detailed information on the TS structures and the reaction pathways.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Resultsmentioning

confidence: 99%

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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“…Their highest experimentally obtained value was 19.8 kcal/mol (coalescence temperature 102 °C) 15a. Marullo and Wagener determined the free activation energies of the azomethines derived from aceton and aniline and benzylamine, respectively, to 21 kcal/mol and > 23 kcal/mol by measurement of the coalescence temperature 16c. Hofmann et al calculated on the STO‐3G level of theory values of 23−30 kcal/mol for the linear inversion transition state of three different anil types 16d…”

Section: Resultsmentioning

confidence: 99%

Metal 4‐Alkylidene‐4H‐pyridin‐1‐ides and 2H‐Imidazol‐4‐ones from Novel HighlyN‐(Pyridin‐4‐yl)methyl‐Substituted Azomethines

et al. 2004

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Novel azomethines 3 synthesized from 4‐(aminomethyl)pyridine (1) and various ketones 2 exhibit a higher acidity of the Cα‐protons compared with other 4‐alkylpyridines like 4‐benzylpyridine or 4‐(tosylmethyl)pyridine. Therefore, they can easily be metalated to give a variety of specific anions 3(−), which deserve interest for further synthetic applications. Important properties of compounds 3 and their lithiated derivatives 4 (lithium 4H‐pyridin‐1‐ides, synthesized by deprotonation of the α‐CH2 group in 3) can be controlled by the carbonyl reactant 2. These carbanions 3(−) are remarkable intermediates regarding their NMR properties. Comparing the NMR spectra of the lithium 4H‐pyridin‐1‐ides 4 with their parent azomethines 3 reveal very distinct changes in the chemical shifts from which important structural and electronic properties of the carbanions can be deduced. The observation of E/Z isomers in the case of 3c and their mutual conversion at higher temperature inspire DFT calculations on the B3LYP/6‐311++G(d,p) level of theory to get insight into the energetic demand of such isomerization processes. Moreover, calculations on the same level of theory were carried out on the model azomethine anion 5 and different monomeric lithium coordination isomers 6a−d. Experimentally, two selected examples will exemplify the reactivity of 4H‐pyridin‐1‐ide anions towards heterocumulenes. The 2‐(pyridin‐4‐yl)acetate 7 obtained from the reaction of 4e with carbon dioxide appears to be a CO2 storage molecule as it easily releases CO2 under protolysis conditions comparable with the vitamin B6 reaction mode. Interesting five‐membered heterocycles were isolated after reaction of 4 with isocyanates followed by hydrolysis and oxidation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

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“…[28] Furthermore, the chemical literature has examples of exchange of neutral substituted guanidine derivatives interpreted as rotation. [33,35,36] The kinetic barriers to exchange are pertinent to the stability imparted by S(6) because S(6) has to break for the exchanges to occur. [3] In a hydrogen-bonding medium the transition states of the rotations are not devoid of hydrogen bonds.…”

Section: Solution-state Studiesmentioning

confidence: 99%

In Search of the Weak, Six‐Membered Intramolecular Hydrogen Bond in the Solution and Solid States of Guanidinobenzimidazole

et al. 2004

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The chemical literature presents evidence for the nonexistence of the intramolecular hydrogen bond in neutral 2‐guanidinobenzimidazole, a result that defies chemical intuition. In the current study, analyses of substituted 2‐guanidinobenzimidazoles by dynamic 1H NMR, IR, and X‐ray diffraction unveiled the contribution of the intramolecular hydrogen bond to the overall structure and conformational equilibria. The presence of the intramolecular hydrogen bond in this work and its absence in previous studies of the unsubstituted parent compound is reconciled by the fact that intramolecular hydrogen bonds between the imidazole moieties and guanidino NH2 protons were weak. The intramolecular hydrogen bonds were more apparent in derivatives with guanidino NHR. The behavior of the latter indicated competition between and coexistence of inter‐ and intramolecular hydrogen bonding. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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Structural organic chemistry by nmr. III. Isomerization of compounds containing the carbon-nitrogen double bond

Cited by 20 publications

References 15 publications

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

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

Metal 4‐Alkylidene‐4H‐pyridin‐1‐ides and 2H‐Imidazol‐4‐ones from Novel HighlyN‐(Pyridin‐4‐yl)methyl‐Substituted Azomethines

In Search of the Weak, Six‐Membered Intramolecular Hydrogen Bond in the Solution and Solid States of Guanidinobenzimidazole

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