Imidazoles thermodynamic stability

These considerations clearly suggest that kinetic stabilization is a far better way to stabilize the triplet states of carbenes than thermodynamic stabilization. It is also important to note that thermodynamic stabilization usually results in the perturbation of electronic integrity of the reactive center, as has been seen in the case of phosphinocarbene and imidazol-2-ylidene (112). " On the other hand,... 

Nesset MJM, Shokhirev NV, Enemark PD, Jacobson SE, Walker FA (1996) Models of the cytochromes. Redox properties and thermodynamic stabilities of complexes off hindered iron(III) and iron(II) tetra-phenylporphyrinates with substituted pyridines and imidazoles. Inorg Chem 35 5188-5200... 

Theoretical approaches have been applied to understand the features that are responsible for the stability of imidazole (in)-2-ylidine. Density Functional theory (DFT) was applied in this respect. Studies examining the relationship between structure and reactivity have shown that thermodynamic stabilization (electronic conjugation effect) usually plays an important role in stabilizing the singlet state and it can become less reactive to such an extent that it can be isolated under ambient conditions. 

Is the stability of 8Ad due to unfavorable kinetics, i.e., the bulky adamantyl groups blocking reaction, or to unfavorable thermochemistry, i.e., loss of aromaticity of the imidazole ring as a result of reaction, or both The distinction is potentially important as understanding could assist in designing stable carbenes. To decide, compare the kinetics and thermodynamics of the insertion of 8Ad into the central CH bond in propane with reactions of 8Me, which should also be aromatic but lacks shielding groups , and 9, which is neither aromatic nor crowded. 

Stability constant measurements have shown that imidazoles form some of the most stable complexes of all N-heterocyclic ligands. Some thermodynamic data are given in Table 12. 

The relative aromaticities of isomers of oxygen and sulfur heterocycles can be predicted in a similar way, e.g., thiatriazoles . Of course, the most stable isomer of a pair, as measured by heat of formation, is not necessarily the most aromatic in fact, imidazole (A7/f = 132.9 kj mol ) is thermodynamically more stable than pyrazole (Hf= 179.4 kj mol-1) <1999JPCA9336>. Nevertheless, the empirical rule that 1,2-nitrogen interactions are more favorable for aromaticity than 1,3-nitrogen interactions is a convenient guide to the relative stabilities of closely related azole isomers in the gas phase <2010T2695>. 

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