Application Of Electron-rich Cobalt Complexes Supported By Trimethylphosphine In C-X (X=F, Cl) Bond Activation

Aromatic halides (ArX) are of genuine importance in synthetic organic chemistry. However, the chloro- and fluoroarenes exhibit much lower reactivity due to the inertness of their C-X bonds. It is not surprising, therefore, that attention of numerous research groups has been focused for many years on activation of the C-Cl and C-F bond. A variety of C-Cl and C-F activation reactions that employ electron-rich transition-metal complexes, require comparatively mild conditions, are selective, and afford isolable products are nowknown and catalytic systems have been achieved.Cobalt show their attraction owing to the low prices and envirment friendly. In this doctoral dissertation, application of cobalt complexes supported by trimethylphosphine in C-X(X=F,Cl) bond activation are studied. The mail contents are described as follows:Reactions of cobalt complexes with ortho-chlorobenzaldehydesChloroarenes are certainly the most attractive aryl halides for synthetic applications on an industrialscale, because they are inexpensive and readily available in bulk quantities. Chlorinated organic compounds have been used on a large scale in the chemical, petrochemical, and electronic industries. The disposal of organic wastes containing halogen has become a major environmental and social problem, because most of them are toxic and thermally stable, accumulating in the surroundings for long time periods. The development of a highly efficient,safe alternative technology for activation of C-Cl bonds has been anticipated. The study of activation of C-Cl bond has important significance and value. Mastery of the activation of C-Cl bond will help us to find a new chlorination, dechlorination and C-Cl bond key functional way, and help us study the new chlorinated organic synthesis. Moreover, the chlorine compounds help solve the increasingly serious problem of environmental pollution.Reactions of CoMeL₄,CoL₄,CoL₃X (X=Cl, Br, I) with ortho-chlorinated substrates containing imine anchoring groups are selected to study the reactivity of cobalt complexes supported by trimethylphosphine,and the novel [C-Co-Cl] complexes are expected to be afforded.Reactions of CoMeL₄ with ortho-chlorinated substrates containing imine anchoring groups (1-5), proceed by oxidative addition of the C-Cl bond to give rise to the mono-ortho-metalated diorgano cobalt(â…¢) chlorides 6-10, respectively. The new ortho-chelated cobalt complexes formed by oxidative addition of an aromatic C-Cl bond were isolated and characterized. C,C-coupling reactions have been observed in reactions of [CoMe(PMe₃)₄], LC-MS analysis verified the formation of the respective methyl arene derivatives via C, C-coupling through reductive elimination.Steric hindrance has a marked effect on reactions of CoL₃X (X=Cl, Br, I) with ortho-chlorinated substrates containing imine anchoring groups (2-5). Reactions of reactants containing small groups afforded mono-ortho-metalated cobalt(â…¢) chlorides (11, 12, 15, 16), while Co(â…¡) complexes (13, 14, 17, 18) were formed by reactions with reactants containing bulky groups. C0L3I does not carry out any reaction with 3-5.Reactions of C0L4 with N-phenyl-2-chlorobenzaldehydeamine 21, proceed by oxidative addition of the C-Cl bond to give rise to the mono-ortho-metalated diorgano cobalt(â…¡) chloride 22.Functionalizations of the novel complexes containing [C-Co-Cl] fragmentThe functionalization of carbon-chlorine bonds for converting organic chlorides intoindustrially useful materials, is an interesting area of research activity. Enhancementof the poor reactivity of aryl chlorides toward nucleophiles has been one of the most challenging problems and desirable goals for chemists. In this doctoral dissertation, we attempted to study functionalizations of the novel complexes containing [C-Co-Cl] fragment.The mono-ortho-metalated cobalt(â…¢) chloride 12 did not carry out any reaction with ethynyltrimethylsilane, while in the reaction with n-butyl lithium, the cholrine in the phenyl ring was replaced by butyl group and afforded the cobalt(â…¢) chloride 23. Through the reaction of 12 or 22 with phenylmagnesium, the chlorine was replaced by phenyl group toward nucleophilic substitution and in the reaction of 22, the phenyl cobalt comgplex 24 was isolated.Synthesis of the novel complexes containing [C-Co-F] fragmentThe activation of carbon-fluorine bonds is of great importance in organometallic chemistry and catalyst development because this type of reaction contributes to the fundamental understanding of the reactivity of stable bonds and the selective replacement of F atoms. The use of transition-metal complexes offers another means with which to activate C-F bonds. Reports on C-F activation by firstrow transition metals are rare, and only one example of organocobalt fluorides containing a C-M-F moiety has been described.We have recently reported a cyclometalation reaction involvingcarbon-fluorine bond activation at a cobalt(â… ) center with azine as an anchoring group, which, for the first time, affordsan ortho-chelated cobalt(â…¢) complex containing a C-Co-F fragment.Reactions of CoMeL₄ and CoL₄ with ortho-fluorinated substrates containing imine anchoring groups are selected to study the reactivity of cobalt complexes supported by trimethylphosphine, and the novel [C-Co-F] complexes are expected to be afforded.Reactions of CoMeL₄ with ortho-fluorinated substrates containing imine anchoring groups (24-27), proceed by oxidative addition of the C-F bond to give rise to the mono-ortho-metalated diorgano cobalt(â…¢) chlorides 28-31, respectively. The new ortho-chelated cobalt complexes formed by oxidative addition of an aromatic C-F bond were isolated and characterized. No reaction of with reactants containing bulky groups has been carried out. Under the experimental conditions, a C=N bondÏ€-coordinated Co(O) complexes were obtained when C0L4 was emplyed.Reactions of CoL₄ with octafluorotolueneMost of the reported C-F activation of reactions by transition metals occur at electron-rich dⁿ (n≥6) metal centers via an oxidative addition process. Therefore, reaction of CoL₄ with octafluorotoluene is selected to study the reactivity of cobalt complexes supported by trimethylphosphine with perfluorinated aromatics, and the novel [C-Co-F] complexes are expected to be afforded. Two kinds of products were isolated from the reaction of CoL₄ with octafluorotoluene. It generated one Co (â…¡) 39 formed by activation of two C-F bonds in the octafluorotoluene and one Co (â… ) 38 which was obtained as a intermediate.Functionalizations of octafluorotolueneWhen we treated the Co (â… ) 38 with bromobenzene, fluoro substituents in trans position of the CF₃ group was substituted with phenyl group and the expected C-C coupling product was isolated. At the same time the other product Co(â…¡) 41 was obtained. It was a stoichiometric reaction, however, it may be a synthetically useful route for the generation of fluoroorganics.All of the now complexes are characterized by IR, ¹H, ¹³C and ³¹P NMR. Structures of some single crystals have been confiemed by X-ray diffraction techniques.