Syntheses and catalytic activities of Pd(II) dicarbene and hetero-dicarbene complexes |
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Authors: | Han Vinh Huynh Ramasamy Jothibasu |
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Institution: | Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore |
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Abstract: | A series of palladium(II) complexes (1-6) bearing cis-chelating homo-dicarbene ligands with varying alkyl bridges (C1-C3) and N-heterocyclic backbones (imidazole and benzimidazole) have been synthesized by reaction of Pd(OAc)2 with the respective diazolium bromides (A·2HBr - F·2HBr) in DMSO. A comparative catalytic study employing aryl chlorides in the Mizoroki-Heck reaction revealed the superiority of methylene- and propylene-bridged dibenzimidazolin-2-ylidenes over their imidazole-derived analogues. Based on these results, two new propylene-bridged hetero-dicarbene complexes (7 and 8) were designed containing a mixed benzimidazole/imidazole-derived NHC-donor set. Notably, both complexes outperformed their homo-dicarbene analogues, which may be due to the electronic asymmetry induced by hetero-dicarbene ligands. The molecular structures of complex 6 and 8 are also presented. |
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Keywords: | N-Heterocyclic carbene Hetero-dicarbene Palladium Mizoroki-Heck coupling Electronic asymmetry |
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