Study On The C-H Bond Activation Directed By S-Triazine Functional Group

Chelation-assisted C-H bond functionalization is one of the indispensable synthetic methods in organic chemistry with the advantage of atomic economy and low toxicity.Hence,the exploration on removable and/or modifiable directing groups for C-H bond functionalizations is one of hot topics.However,there have been fewer reports about s-triazinyl as a directing group.According to the literature,s-triazinyl has the potential of removability and modification.Coupled with the wide application in medicine,optoelectronics,biotechnology and other fields of s-triazines substances,we envisioned that s-triazinyl could be used as a guiding group to participate in the C-H bond activation reactions.By screenings of the structures of s-triazinyl,we found that 4,6-dimethoxy-1,3,5-triazine units were readily synthesized,and could be removed and modified.Therefore,it was available as directing groups in this paper.In this thesis,We synthesized nine kinds of 2-aryloxy-4,6-dimethoxy-1,3,5-triazine derivatives bearing electron-donating groups and electron-withdrawing groups,including-Me,-OMe,-OEt,-Cl,-F,-Br,etc.And the yields were 39%-67%.As there were no reports on the C-H bond activation reactions directed by s-triazine,we explored the types of C-H functionalizations,including Pd/Ru-catalyzed C-H alkenylation,arylation and acetoxylation.The results showed that the acetoxylation of 2-aryloxy-4,6-dimethoxy-1,3,5-triazine was carried out smoothly in the presence of Pd(OAc)2,Ph I(OAc)2 and Ac2O/AcOH,heated for 20 h at 120 ?.Using 2-aryloxy-4,6-dimethoxy-1,3,5-triazine derivatives as the reaction partners,various reaction parameters including catalysts,oxidants,quantity of the solvent,temperature and gas atmosphere were screened.We were pleased to obtain the acetoxylation product in 53% yield by using Pd(OAc)2,PhI(OAc)2 and Ac2O/AcOH(4 mL)at 150 ? within 24 h under the atmosphere of N2.Then,we investigated the scope of this reaction with respect to the heteroaryl ether substrates.It was found that the electron effect of substituents in the substrates had an influence on the reaction.The results showed that the acetoxylations of electron-donating groups were more efficiently achieved.Finally,we sought to speculate the mechanistic steps of C-H bond acetoxylation directed by s-triazinyl.The key steps involved:(i)ligand coordination to the Pd catalyst to generate intermediate complex.(ii)cyclometalation to form palladacycle complex.(iii)oxidation of palladacycle complex by Ph I(OAc)2 to afford PdIV intermediate.(iv)C-O bond-forming reductive elimination to generate PdII complex.(v)ligand exchange to release the product and to coordinate a new substrate to the metal center.