RESUMO
Bis-N-heterocyclic carbene (NHC) aminopincer ligands were successfully applied for the first time in the catalytic hydrogenation of esters. We have isolated and characterized a well-defined catalyst precursor as a dimeric [Ru2(L)2Cl3]PF6 complex and studied its reactivity and catalytic performance. Remarkable initial activities up to 283,000 h(-1) were achieved in the hydrogenation of ethyl hexanoate at only 12.5 ppm Ru loading. A wide range of aliphatic and aromatic esters can be converted with this catalyst to corresponding alcohols in near quantitative yields. The described synthetic protocol makes use of air-stable reagents available in multigram quantities, rendering the bis-NHC ligands an attractive alternative to the conventional phosphine-based systems.
RESUMO
We examine important reactivity pathways relevant to stoichiometric and catalytic C-H amination via isolable ß-diketiminato dicopper alkylnitrene intermediates {[Cl2NN]Cu}2(µ-NR). Kinetic studies involving the stoichiometric amination of ethylbenzene by {[Cl2NN]Cu}2(µ-N(t)Bu) (3) demonstrate that the terminal nitrene [Cl2NN]CuâN(t)Bu is the active intermediate in C-H amination. Initial rates exhibit saturation behavior at high ethylbenzene loadings and an inverse dependence on the copper species [Cl2NN]Cu, both consistent with dissociation of a [Cl2NN]Cu fragment from 3 prior to C-H amination. C-H amination experiments employing 1,4-dimethylcyclohexane and benzylic radical clock substrate support a stepwise H-atom abstraction/radical rebound pathway. Dicopper nitrenes [Cu]2(µ-NCHRR') derived from 1° and 2° alkylazides are unstable toward tautomerization to copper(I) imine complexes [Cu](HNâCRR'), rendering 1° and 2° alkylnitrene complexes unsuitable for C-H amination.
Assuntos
Cobre/química , Iminas/química , Compostos Organometálicos/síntese química , Aminação , Cristalografia por Raios X , Cinética , Modelos Moleculares , Conformação Molecular , Compostos Organometálicos/químicaRESUMO
Kinetic analysis of the reaction of the copper(I) ß-diketiminate [Cl(2)NN]Cu ([Cu(I)]) with (t)BuOO(t)Bu to give [Cu(II)]-O(t)Bu (1) reveals first-order behavior in each component implicating the formation of free (t)BuO(â¢) radicals. Added pyridine mildly inhibits this reaction indicating competition between (t)BuOO(t)Bu and py for coordination at [Cu(I)] prior to peroxide activation. Reaction of [Cu(I)] with dicumyl peroxide leads to [Cu(II)]-OCMe(2)Ph (3) and acetophenone suggesting the intermediacy of the PhMe(2)CO(â¢) radical. Computational methods provide insight into the activation of (t)BuOO(t)Bu at [Cu(I)]. The novel peroxide adduct [Cu(I)]((t)BuOO(t)Bu) (4) and the square planar [Cu(III)](O(t)Bu)(2) (5) were identified, each unstable toward loss of the (t)BuO(â¢) radical. Facile generation of the (t)BuO(â¢) radical is harnessed in the catalytic C-H etherification of cyclohexane with (t)BuOO(t)Bu at rt employing [Cu(I)] (5 mol %) to give the ether Cy-O(t)Bu in 60% yield.
Assuntos
Cobre/química , Éteres/síntese química , Compostos Organometálicos/química , Óxidos/química , Peróxidos/química , Catálise , Éteres/química , Radicais Livres/química , Estrutura Molecular , Compostos Organometálicos/síntese químicaRESUMO
Aniline joins the club: A ß-diketiminato copper(I) catalyst enables C-H amination of anilines employing low catalyst loadings to preclude oxidation to the diazene ArN=NAr. Electron-poor anilines are particularly resistant towards diazene formation and participate in the amination of strong and unactivated C-H bonds. N-alkyl anilines also take part in C-H amination.