Well-designed Pentamethylcyclopentadienyl Cobalt(Ⅱ/Ⅲ) Complexes:Synthesis And Reactivity Studies

The cobalt-catalyzed reaction systems have been attracting attention from synthetic chemists for its advantages such as low cost,high efficiency,and good selectivity.In traditional cobalt catalysis,low valent cobalt complexes are generally employed such as Co(0),Co(Ⅰ)catalyst,are associated with an oxidative addition step followed by reductive elimination reaction.Although such systems have played an important role in hydrogenation and hydroformylation of olefins,their difficult synthetic procedures and characterizations greatly limit their development and application.Therefore,there is considerable interest to design and synthesize novel Co(Ⅱ)or Co(Ⅲ)complexes,studying their reactivity and exploring their applications in organic synthesis,which will have great significance for expanding the application range of cheap metal catalysis.In this thesis,half-sandwich neutral Co(Ⅱ)complex Cp*Co(1,2-Ph2PC6H4S)and cationic Co(Ⅲ)complex[Cp*Co(1,2-Ph2PC6H4S)]+were designed and synthesized.We studied their reactivity toward halides,arylhydrazines and alkynes,etc.Based on the separation and characterization of related intermediates,we have developed several catalytic systems such as Co(Ⅱ)-catalyzed radical cyclization of isocyanides forming phenanthridine derivatives;Co(Ⅲ)-catalyzed regioselective hydrohydrazination of epoxides;Co(Ⅲ)-S cooperative activation of alkynes forming alkenyl-cobalt complexes.The results are summarized as follows:1)A well-defined neutral Co(Ⅱ)complexes Cp*Co(1,2-Ph2PC6H4S)(1),was synthesized and characterized.Because of good reducibility,complex 1 undergoes a single-electron transfer reaction with alkyl halides R-X,generating the neutral Co(Ⅲ)complexes([1-X])and the alkyl radical,R·.Aryl isonitriles can then undergo addition reactions with the generated alkyl radicals,and then undergo an intra-molecular cyclization and dehydrogenation to afford phenanthridine-fused polycyclic compounds Rapid ion exchange of the complex[1-X]with NaBPh4,in acetonitrile generates[Cp*Co(Ⅲ)(P-S)]BPh4,([1]+BPh4)and NaX.The resulting complexes[1]+BPh4 have a higher redox potential and shows better reversibility in comparison to[1-X].Using complex 1 as the catalyst,a series of phenanthridine-fused polycyclic compounds were obtained through radical cyclization of arylisonitriles with C4F9I and CH3CHBrCOOEt.After decarboxylation and protonation,these compounds show good fluorescent properties in the range of 409-472 nm with quantum yields up to 0.388.2)Complex 1(E[1]0/+＝-0.25 V)can undergo a reversible oxidation-reduction reaction under mild conditions.Cationic Co(Ⅲ)complexes[Cp*Co(1,2-Ph2PC6H4S)]+BF4,([1(NCMe)]+BF4)were synthesized by electrochemical oxidation in acetonitrile with ferrocene tetrafluoroborate.[1(NCMe)]1 BF4 reacts with phcnylhydrazine to fonn[Cp*Co(Ⅲ)(P-S)(NH2NHPh)]BF-4,[1(NH2NHPh)]+BF4.It was found that the nitrogen atom of the-NH2 group on phenylhydrazine had a selective coordination with Co(Ⅲ)center and an intramolecular hydrogen bond was established between the hydrogen atom of the-NH group and the S atom of the ligand.We speculate that Co(Ⅲ)-NH2NHPh coordination can effectively inhibit the nucleophilicity of the terminal-NH2 and allow the-NH group to participate in the regioselective synthesis.Complex[1(NH2NHPh)]+BF4 exhibits a highly regioselective nucleophilic ring-opening reaction with 1,2-epoxyethylbenzene to produce complex[1(NCMe)]’BF4 and 2-plienyl-2-(1-phenyl-hydrazyl)ethane-l-alcohol.Using[1(NCMe)]+BF4 as catalyst,we have achieved catalytic rcgioselective hydrohydrazination of epoxides to 1,1-hydrazinoalcohols in an atom-economic manner.In contrast to the reported Co(Ⅲ)complexes to activate epoxides,the lewis acidity of[1(NCMe)]+ BF4 is just enough to coordinate with-NH2 group of arylhydrazine but not with epoxides,which ensures the rcgioselective synthesis of 1,1-hydrazinoalcohols.3)A 16-electron unsaturated Co(Ⅲ)complex[Cp*Co(1,2-Ph2P6H4S)]+,([1]+)was obtained by electrochemical oxidation of the corresponding Co(II)complex in a non-coordinating solvent(such as dichloromethane).[1]+ can react with alkyne to form alkenyl-cobalt complex.For instance,[1]+can react with phenylacctylene and acetylene to form alkenyl-cobalt complexes[Cp*Co(Ⅲ)(P-S)(PhC=CH)]BF4,([14]+BF4)and[Cp*Co(Ⅲ)(P-S)(CH=CH)]BArF4,([15]+BArF4)respectively.Single crystal X-ray diffraction results show the formation of alkenyl-cobalt complexes.Bifunctional catalyst[1]+reacts directly with the phenylacetylene through cobalt-thiolatc cooperation and is capable of splitting C≡C bond,affording alkenyl-cobalt complex with Co-C-C-S quaternary ring structure.The study laid the foundation for studying the"metal-ligand cooperation" synergistic activation of alkynes and their transformation,such as cycloaddition and hydration of alkynes(especially acetylene).