Imidazo[1,5-a]pyridin-3-ylidenes as π-accepting carbene ligands: substituent effects on properties of N-heterocyclic carbenes
| Autor: | Toshiaki Murai, Fumitoshi Shibahara, Yuma Koto |
|---|---|
| Rok vydání: | 2017 |
| Předmět: |
010405 organic chemistry
Ligand Stereochemistry Chemical shift Organic Chemistry Substituent Carbon-13 NMR 010402 general chemistry 01 natural sciences Biochemistry Medicinal chemistry 0104 chemical sciences chemistry.chemical_compound chemistry Phenylacetylene Pyridine Moiety Physical and Theoretical Chemistry Carbene |
| Zdroj: | Organic & Biomolecular Chemistry. 15:1810-1820 |
| ISSN: | 1477-0539 1477-0520 |
| DOI: | 10.1039/c6ob02827f |
| Popis: | Various 1-substituted-imidazo[1,5-a]pyridin-3-ylidenes were prepared and characterized. The fundamental character and the effects of substituents on the imidazo[1,5-a]pyridine backbone on the electronic character of carbenes were evaluated using rhodium complexes and selenium adducts. 13C NMR chemical shifts of carbene carbons in imidazo[1,5-a]pyridin-3-ylidene and 1H and 13C NMR spectra of olefin moieties trans to the carbene in Rh(nhc)(cod)Cl complexes shifted relatively downfield compared to those for the corresponding complexes bearing conventional N-heterocyclic carbenes (NHCs). The IR peak of the trans-carbonyl moiety of Rh(nhc)(CO)2Cl complexes appeared at a higher wavenumber than for conventional NHCs. These observations suggest that imidazo[1,5-a]pyridin-3-ylidene ligands have strong π-accepting character. Rh(nhc)(CO)2Cl complexes were further characterized by X-ray diffraction analyses. To obtain further insight into the electronic character of imidazo[1,5-a]pyridin-3-ylidenes, selenoureas were prepared. We found that a combination of both the 77Se NMR chemical shift and 1JC–Se coupling constant of C–Se bonds are appropriate for a rigorous evaluation of the electronic characters of imidazo[1,5-a]pyridin-3-ylidenes, but did not correspond to those for conventional NHCs. DFT calculations for imidazo[1,5-a]pyridin-3-ylidenes revealed that a hybrid accepting orbital comprised of a vacant p-orbital of carbene and a π* orbital of the pyridine ring newly occurred to result in significantly increased π-accepting character. Finally, the strong π-accepting character of the ligand was demonstrated by the Rh-catalyzed polymerization of phenylacetylene. |
| Databáze: | OpenAIRE |
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