Synthesis And Catalytic Properties Of Iron And Cobalt Hydrides By Selective Activation Of C-F/C-H Bonds

With the continuous development and progress of organic chemistry,more and more attention from the academic community has been paid to the research on the inert bond reactions widely existing in various compounds.One of the goals of the development of organic chemistry is to change the chemical structure of existing compounds to obtain new compounds.If a molecule has both C-F and C-H bonds,then when the metal center interacts with the molecule,the choice to activate C-F or C-H bond is involved in selectivity issues.In the vast majority of reactions,researchers mainly use the precious metals of the second and third transition series to achieve the selective activation of C-F/C-H bond.Relative investigation about cheaper and more readily available 3d metals,such as cobalt and iron complexes is rare.In addition,the anchor group capable of forming coordination bonds with the central metal atom can effectively assist the metal atom to undergo an oxidative addition reaction of the inert bond and in turn improve the selectivity of the reaction and reduce the activation energy of the reaction.In this dissertation,fluorinated Schiff bases with anchoring groups were selected as raw materials,and a series of studies on the selective activation of C-F/C-H bonds were carried out by using the complexes of low-valent cobalt and iron supported by trimethylphosphine.The functionalization of C-H bond was also studied.The details are mainly as follows:(1)Synthesis of four hydrido cobalt complexes,namely(2,4,5-R1,R2,R3-C6H-HC=N-1-C10H6)Co(H)(PMe3)2[R1= F,R2 = F,R3 = H(4),R1=F,R2 = H,R3 = F(5),Ri = F,R2 = F,R3 = F(6)]and(F4C6-HC=N-1-C10H6)Co(H)(PMe3)2(8)by using the reaction between mono-valent cobalt coordinated with trimethylphosphine and fluorinated Schiff base.Their catalytic properties for the hydrosilylation reduction of aldehydes and ketones were investigated and the corresponding catalytic mechanism was discussed.The reaction condition was optimum.Under such condition,a series of reduction products of aldehydes and ketones were synthesized and characterized.Investigation revealed that fluorinated Schiff base hydrido cobalt complexes exhibited excellent catalytic activity and selectivity towards the hydrosilylation of aldehydes and ketones.Under the same conditions,the complex 8 has the best catalytic activity among the four hydrido cobalt complexes.When the amount of the catalyst was 2 mol%,triethoxysilane and tetrahydrofuran were used as the hydrogen source and solvent respectively,and the reaction temperature was 80?,the isolated yield of primary alcohol was up to 92%,and the isolated yield of secondary alcohol was up to 87%after 4 h of reaction.In particular,the selective hydrosilation of carbonyl groups rather than C=C double bond in a,?-unsaturated aldehydes and ketones was also realized.(2)The hydrosilylation reduction of amides was investigated by using the above four fluorinated Schiff base hydrido cobalt complexes and the catalytic mechanism was discussed.The optimum conditions for the hydrosilylation reduction of amides were explored.Under such condition,a series of reduction products of amides were synthesized and characterized.It has been found that fluorinated Schiff base hydrido cobalt complexes also exhibited excellent catalytic activity and selectivity towards the hydrosilylation reduction of amides.Under the same conditions,complex 5 has the best catalytic activity among the four hydrido cobalt complexes.When the amount of the catalyst was 2 mol%,triethoxysilane and tetrahydrofuran were used as the hydrogen source and solvent respectively,and the reaction temperature was 60?,the isolated yield of nitriles was up to 89%after 24 h of reaction.The experimental results showed that when an aldehyde or ketone carbonyl group was in the substrate,the aldehyde or ketone carbonyl group is preferentially reduced,followed by the amide group.In addition,to the best of our knowledge,this is the first report on the successful reduction of amides to nitriles with hydrido cobalt complexes as catalysts.(3)The selective activation of C-F/C-H bond in Schiff base ligands was realized by using the reaction between zero-valent iron coordinated with trimethylphosphine and fluorine-containing Schiff base.Two novel fluorine-containing Schiff base hydrogen iron complexes,namely 2,4,5-R1,R2,R3-C6H-HC=N-CH2C6H5)Fe(H)(PMe3)3[R1 = F,R2= H,R3 = F(16),Ri = F,R2 = F,R3 = F(17)]were designed and synthesized.Their catalytic performance and the catalytic mechanism for transfer hydrogenation reduction of ketone were discussed.The optimum conditions for the transfer hydrogenation reduction of ketones were explored,and a series of ketone reduction products were synthesized under these conditions.It has been found that the fluorine-containing Schiff base hydrogen iron complexes has excellent catalytic activity and selectivity for the transfer hydrogenation reduction reaction of the ketone.Under the same conditions,complex 16 has better catalytic activity.When the amount of catalyst is 1 mol%,isopropanol is used as solvent and hydrogen source,and the reaction was at 80? for 24 h,the alcohol has an isolated yield of up to 97%.In the hydrogenation of aldehydes and ketones catalyzed by transition metal,the transfer hydrogenation with alcohol as hydrogen source and solvent has the advantages of mild reaction conditions,good safety and easy operation,which does not need additional hydrogen source.And therefore,the reaction process was simplified and the reaction efficiency was improved.In addition,the effect of selective activation of C-F/C-H bonds in fluorine-containing Schiff base by zero-valent iron coordinated with trimethylphosphine was investigated with the aid of Lewis acids.The experimental results showed that Lewis acid played a significant role in the reaction process.In the absence of Lewis acid,the product of C-H bond activation is obtained.When Lewis acid is involved,activation of the C-F bond is achieved.(4)In the functionalization study of C-H bond,the optimal conditions for the oxidative cross-dehydrogenation of 2H-chromenes and terminal alkynes were investigated and a series of alkynylation products of 2H-chromenes were prepared under the optimal conditions.The results show that DDQ can be used as an oxidant to achieve highly efficient copper-catalyzed cross-dehydrogenation coupling of 2H-chromenes and terminal alkynes,and the isolated yield was up to 92%.Protic additive EtOH proved to be crucial to harmonizing the oxidation with subsequent alkynylation step by retaining the oxidation state of oxocarbenium ion in the form of acetal.The copper-catalyzed alkynylation of 2H-chromene acetals with terminal alkynes were also explored.Both 2H-chromene and alkynyl groups are common dominant skeletons in bioactive molecules,which exist in many natural products and synthetic drugs.As a direct method for dehydrogen coupling to form C-C bonds by using C-H bonds in different reaction components,cross dehydrocoupling reaction has good atomic economy and step economy.