Studies Towards Co(Ⅲ)-Catalyzed C-H Bond Activations Directed By Monodentate Directing Groups

Transition metal-catalyzed C–H activation has become an important synthetic tool in recent decades,and this strategy is extensively used in the synthesis of natural products and drug molecules.Most of the C–H bond activation reactions required expensive noble metal catalysts.From the viewpoint of practicality,the precious prices and high toxicity of noble metals may limit their wide applications.Therefore,developing cheap,less toxic 3d transition metal catalysts for C-H functionalization has received widespread attention.Recently,the high valent Cp*Co（Ⅲ）catalyst pioneered by Matsunaga and Kanai has gained much attention due to its high activity for C–H activation.Major progress in this area has been made by many other groups.Herein,we discussed the development of Cp*Co（Ⅲ）-catalyzed C–H bond activation.On the basis of these results,we have developed four Co（Ⅲ）-catalyzed C-H bond activation reactions directed by monodentate directing groups.This thesis is divided into five chapters.Chapter 1 commenced with a review on the development of Cp*Co（Ⅲ）-catalyzed C–H activation.For clarity,this review is classified by the type of bond-formations,including C–C,C–N,C–O,C–S and C–X（X=Br,I）bond-formations.Chapter 2 demonstrated a practical approach for the synthesis of benzofulvenes through Cp*Co（Ⅲ）-catalyzed sequential C–H activation,addition,annulation,and dehydration tandem reactions between aromatic ketones and alkynes,and H₂O is the only by-product in the reaction.The reaction tolerates various functional groups,and a series of benzofulvenes were synthesized in 42-92%yields.Competition experiment between electronically different acetophenones suggested that more electron-rich substrate was favored for the reaction.Chapter 3 discussed an efficient and practical synthesis of 1-naphthols through Cp*Co（Ⅲ）-catalyzed C–H activation and tandem[4+2]annulation between sulfoxonium ylides and alkynes.This approach features operational simplicity,good substrate compatibility and excellent regioselectivity,and various 1-naphthols were synthesized in 36-92%yields.Chapter 4 demonstrated a novel synthesis of benzoxazinones through Cp*Co（Ⅲ）-catalyzed C–H activation and tandem cyclization between sulfoxonium ylides and dioxazolones.This transformation tolerates a variety of functional groups.Mechanistic experiments showed that Ag Sb F₆ and Cp*Co（CO）I₂ may serve as Lewis acids to activate the carbonyl of sulfoxonium ylides.KIE values indicated that C–H activation may not be involved in the rate-limiting step.Chapter 5 focused on a practical approach for the highly monoselective ortho-amidation of 2-arylimidazoheterocycles.Cost-effective and air-stable Cp*Co（Ⅲ）was used as the catalyst and dioxazolones was utilized as the amide source.H/D exchange experiment showed the C–H activation step is reversible,and KIE values indicated that C–H activation might be involved in the turnover-limiting step of the reaction.