PROCEEDINGS OF THE SHEVCHENKO SCIENTIFIC SOCIETY

Chemical Sciences

Archive / Том LVI 2019

Nikolai KOROTKIKH1, Gennady RAYENKO2, Vagiz SABEROV2, Vasyl YENYA2, Nataliya GLINYANAYA2, Maxim NECHITAILOV1, Оles SHVAIKA2

1Institute of Organic Chemistry of UNAS, Murmanskaya, 5, 02094 Kyiv, Ukraine
2L.M. Litvinenko Institute of Physical Organic and Coal Chemistry of UNAS, Kharkiv road, 50, 02160 Kyiv, Ukraine

DOI: https://doi.org/10.37827/ntsh.chem.2019.56.023

DIMERIZATION ENERGIES AS AN IMPORTANT FACTOR OF CARBENE STABILITY. II. N,N′-DISUBSTITUTED AZOLYLIDENES AND SYSTEMS WITH INCREASED ELECTRON DONATING ABILITY

The dimerization energies (Edim, ESP) have been calculated using the DFT method (B3LYP5, 3-21G та 6-31G, RHF) to estimate stabilities of N,N′-disubstituted azolylidene systems of symmetrical imidazol-2-ylidenes, 1,3,4-trisubstituted 1,2,4-triazol-5-ylidenes and 1,4-disubstituted tetrazol-5-ylidenes. Based on the values of the dimerization energies, the conclusions are made regarding the effect of the heterocyclic nuclei on the persistence of carbenes. The systems of imidazol-2-ylidenes and 1,2,4-triazol-5-ylidenes are shown to have the highest stability to dimerization. A series of substituent effects on the stability of symmetric imidazol-2-ylidenes was proposed to be as follows: Dipp (ttt, 178,7) > t-Bu (cct, 145,2) > 1-Ad (cct, 141,8) > Mes (ttt, 55,7) > п-Me2NC6H4 (ttt, 37,2) > Np (ttt, 33,1) > i-Pr (ttt, -29,8) > Ph (cct, -34,3) > Me (ttt, -36,0). The electron donating substituents and their steric shielding increase the values of ESP and the carbene stability. Conjugated bis-1,2,4-triazol-5-ylidenes are more stable than the respective monocyclic carbenes. The steric influence of the substituents affects not only the kinetic but also the thermodynamic stabilization of the carbenes. The structural condition of stability for the systems of pyridin-2-ylidenes, pyrazol-5-ylidenes, pyrrolidin-2-ylidenes, cyclic and acyclic diaminocarbons is determined to be the presence of sterically shielding N-substituents. Pyridin-2-ylidenes, pyrazol-5-ylidenes, pyrrolidin-2-ylidenes are relatively destabilized with respect to imidazol-2-ylidenes and can be more persistent with sterically shielding substituents. The amino groups in the 3,5-positions stabilize pyridin-2-ilidenes, but the related C-substituents are much weaker. Stabilization of pyrrolidin-2-ylidenes is strongly promoted by spirocyclic protection of the carbene center from the neibour carbon atom and steric protection from the N-substituent. Aromatic groups near the nitrogen atoms stabilize diaminocarbens. The steric influence of the substituents affects not only the kinetic buta also the thermodynamic stabilization of the carbenes, increasing it.

Keywords: dimerization energies, heterocyclic carbenes, effect of structure.

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How to Cite

Korotkikh N., Rayenko G., Saberov V., Yenya V., Glinyanaya N., Nechitailov M., Shvaika О. DIMERIZATION ENERGIES AS AN IMPORTANT FACTOR OF CARBENE STABILITY. II. N,N′-DISUBSTITUTED AZOLYLIDENES AND SYSTEMS WITH INCREASED ELECTRON DONATING ABILITY Proc. Shevchenko Sci. Soc. Chem. Sci. 2019 Vol. LVI. P. 23-34.

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