Iron And Nickel Catalyzed Intermolecular Cyclotrimerization Of Alkynes To Polysubstituted Benzenes In The Presence Of Nitrogen Ligands

Polysubstituted benzenes are important synthetic intermediates. A large number of polysubstituted benzenes are crucial building blocks of some pharmaceuticals, biologically active compounds and and polymers. So polysubstituted benzenes are highly useful compounds which are widely used in industry as well as in the laboratory. The transition-metal-catalyzed [2+2+2] cyclotrimerization reaction of alkynes is a powerful approach to the synthesis of polysubstituted benzenes in a single operation. Because of its atom economy and convergent nature, the cyclotrimerization reaction has long been of great interest, which is currently one of the hotspots in organic chemistry.Since the first discovery by Reppe and co-workers, various transition metal (Rh, Co, Pd, Ru, Ni, Ti, Ir, Mo, Zr and Fe) catalytical systems for cyclotrimerization of alkynes have been developed. However, regioselectivity of catalytic intermolecular cyclotrimerization of alkynes has remained a vital issue. In this dissertation, we were devoted to the development of novel catalytic systems containing nitrogen ligands for intermolecular cyclotrimerization of alkynes to polysubstituted benzenes in high yield with excellent regioselectivity.Firstly, a series of iron (II) complexes bearing bidentate N,N-ligands, tridentate N,N,O- and N,N,N-ligands were synthesized according to reported methods. The iron complexes ligated by tridentate N,N,N-ligands were characterized by IR and elemental analysis. Several aromatic terminal alkynes were synthesized via Sonogashira reaction.Then, the iron complexes were utilized for intermolecular cyclotrimerization of alkynes. A novel iron catalytic system, which consisted of (E)-2,6-dimethyl-N-(1-(6-(1-methyl-1H-benzo[d]imidazol-2-yl)pyridine-2-yl)ethylidene)benzenaminedichloroiron, activated zinc powder, zinc iodide and acetonitrile, was developed by exploring and optimizing the reaction conditions of cyclotrimerization of phenylacetylene. It could effectively catalyze intermolecular cyclotrimerization of aromatic terminal alkynes to afford 1,2,4-trisubstituted benzene derivatives in good yield with high selectivity. This catalytic system showed moderate activity for alkyl substituted terminal alkynes. The intermolecular cyclotrimerization reaction was easy to handle and carried out under mild conditions.Furthermore, nickel catalyzed intermolecular cyclotrimerization reaction of alkynes in the presence of nitrogen ligands and zinc iodide was studied and a newfashioned nickel catalytic system was exploited. This catalytic system, which was composed of NiCl₂·6H₂O, 1,10-phenanthroline monohydrate, activated zinc powder, zinc iodide and acetonitrile, demonstrated excellent catalytic performance for intermolecular cyclotrimerization aryl substituted terminal alkynes. In most case, the 1,2,4-trisubstituted benzene derivatives were obtained in high yield with good selectivity. Its catalytic activity on cyclotrimerization of alkyl substituted alkynes was proven low. This catalytic system was inexpensive and easy to use.Finally, another nickel catalytic system NiCl₂·6H₂O/2-(pyridin-2-yl)quinoxaline /activated zinc powder/CH₃CN was developed. Catalyzed by this system various para-substituted phenylacetylenes could be completely converted in short time. However, 1,3,5-trisubstituted benzenes were formed in moderate yield as the main products due to the possible formation of linear oligomers of the corresponding alkynes.