Synthesis and characterization of [Cu(NHC)2]X complexes: catalytic and mechanistic studies of hydrosilylation reactions |
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Authors: | Díez-González Silvia Stevens Edwin D Scott Natalie M Petersen Jeffrey L Nolan Steven P |
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Institution: | 1. Institute of Chemical Research of Catalonia (ICIQ), Av. Pa?sos Catalans, 16, 43007 Tarragona, Spain, Fax: (+34)?977‐920‐244;2. Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148, USA;3. C. Eugene Bennet Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506‐6045, USA |
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Abstract: | The preparation of two series of Cu(NHC)2]X complexes (NHC=N-heterocyclic carbene, X=PF6 or BF4) in high yields from readily available materials is reported. These complexes have been spectroscopically and structurally characterized. The activity of these cationic bis-NHC complexes in the hydrosilylation of ketones was examined, and both the ligand and the counterion showed a significant influence on the catalytic performance. Moreover, when compared with related Cu(NHC)]-based systems, these cationic complexes proved to be more efficient under similar reaction conditions. The activation step of Cu(NHC)2]X precatalysts towards hydrosilylation was investigated by means of 1H NMR spectroscopy. Notably, it was shown that one of the N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) ligands in Cu(IPr)2]BF4 is displaced by tBuO(-) in the presence of NaOtBu, producing the neutral Cu(IPr)(OtBu)]. This copper alkoxide is known to be a direct precursor of an NHC-copper hydride, the actual active species in this transformation. Furthermore, reagent loading and counterion effects have been rationalized in light of the species formed during the reaction. |
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Keywords: | copper hydrosilylation ketones N‐heterocyclic carbenes reaction mechanisms |
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