| For the exhaustion of traditional fossil fuel energy and subsequent serious environmental pollution,researchers in chemical industry try to seek for alternative substrates and synthetic methods in an atom-efficient and environment-friendly manner.In this respect,synthesis of carboxylic acid derivatives using the low-cost and renewable alcohol as raw materials,acceptorless dehydrogenation coupling(ADC)catalyzed by transition metal complexes attracted much more attention,due to the byproducds are water and H2.With highly stable and controllable steric hindrance,pincer complexes generally exhibit excellent catalytic activity during thus process,however most of reported pincer complexes were constructed by at least phosphine or N-heterocyclic carbene ligands,which exist some disadvantages such as complicated synthetic methodologies and the moisture-or air-sensitive nature of phosphine or N-heterocyclic carbene ligands so urge to be developed new pincer system containing only alternative N or 0 atoms.In this paper,three phosphine-free Ru(Ⅱ)-NNN type pincer complexes were prepared and used as the catalysts for ADC reaction of benzyl alcohol and aromatic diamines.In addition,synthesis of various substituted quinoxalines started from vicinal diols coupling with benzene-1.2-diamines by ADC strategy in the presence of commercial available ruthenium complexes as catalyst,has also been developed The main results are listed follows:(1)Three Ru(II)-NNN pincer complexes(Cat.l-Cat.3)were synthesized from the reaction 2,6-bis(1H-imidazole-2-yl)pyridine derivatives(L1-L2)with Ru precursors([RuCl2(p-cymene)]2 or RuCl2(PPh3)3),respectively.These three complexes were fully characterized and applied to the homogeneous catalytic system to efficiently couple the aromatic diamines with alcohols to 1H-benzo[d]imidazoles accompanied with the liberation of H2.When phosphine ligands,such as PPh3 or dppe were coordinated to the Ru center in the catalytic system,the catalytic activity was significantly improved.Under the optimal condition,93%of 1H-benzo[d]imidazoles were obtained in 4.5 h,and two equivalent hydrogens were collected.With the ratio of alcohol/amine =3:1,(mol/mol),we tested the substrates scope catalyzed by Cat.1/dppe.The results reveal that Cat.l has excellent tolerance with electron-donating groups or weak electron-withdrawing groups on diamines or alcohols,while the decomposition of NaBPh4 to NaB(OR)4(R = alkyl or H)plays an important role during the catalytic cycle which was verified by 11B NMR spectrum and experimental data.The high turnover number(～6000)with achieved when lower catalyst loading(0.01 mol%)was used.These results give new thread to design highly efficient and green catalysts for synthesis of N-heterocycles.(2)Acceptorless dehydrogenative condensation of diols and functionalized benzene-1,2-diamine to quinoxalines catalyzed by commercial available Ru complexes[RuC13(Cat.4)、RuCl2(PPh)3(Cat.5)、RuClHCO(PPh3)2(Cat.6)and[RuCl2(p-cymene)]2(Cat.7)]were investigated.Addition of strong electron-donating phosphine ligands[triphenylphosphine(L4,PPh3),1,2-Bis(diphenylphosphino)ethane(L5,dppe)and 1,4-Bis(diphenylphosphino)butane(L6,dppb)]to the catalytic system greatly improve the catalytic reactivity,due to the coordination of phosphine ligands to the Ru center.Comparing to L4,L5,L6 gives the best yield.Under the optimal conditions,the catalytic reactivity follows the order:Cat.7>Cat.6>Cat.4>Cat.5 Compared to traditional metal complexes required functional and modified ligands,Cat.7 can also efficiently catalyze the reaction of coupling alcohols with amines to N-heterocyclic compounds by ADC strategy,circumventing the need for the preparation of ligands. |
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