Electrochemical Oxidative Carbonylation And Mechanistic Study

As for one of the important methods of inorganic-organic transformation,the carbonylation reaction using CO as carbonyl source has been widely used in industrial production.In recent years,chemists pay attention to the oxidative C-H carbonylation because it conforms to the concept of green catalysis and sustainable development.However,the consumption of equivalent oxidants makes oxidative C-H carbonylations uneconomical.Oxygen,as a large amount of green oxidant in nature,is also used for oxidative carbonylation,but the combination of oxygen and carbon monoxide has a wide range of explosion limits(12.5%-74%)that deters application because safety could be an important issue in industrial processes.Electrochemical C-H bond activation has become a hot topic in organic synthesis methodology.The anodic oxidation method can avoid the use of external oxidants,and protons are reduced to produce clean energy-hydrogen.In this paper,we develop three types of oxidative carbonylations through the combination of transition metal catalysis and electrochemical synthesis(or addition of oxidants).The main results of this paper are as follows:1.A palladium-catalyzed electrochemical oxidative C(sp)-H/N-H carbonylation was developed.A variety of amine sources(primary amines,secondary amines and ammonium salts)reacted with terminal alkynes smoothly to form 2-ynamides.Compared with previous reports which generally required high temperature,high pressure,explosive gas mixture and other harsh conditions,these electrochemical conditions were equipped with safe and mild conditions including atmospheric pressure and room temperature.Our discovery had potential for industrial application.The applicability of this reaction was also extended to the primary amines and ammonium salts.Cyclic voltammetry(CV)indicated the phosphine ligands,n-Bu OH and CO play important roles in oxidation and reduction potentials for the Pd-catalyst.Electrochemical in-situ infrared spectroscopy(in-situ IR)was used to prove that the electrode oxidation processes might be the rate-determining step of this catalytic cycle.Quick-scanning X-ray absorption fine structure spectroscopy(QXAFS)revealed that the carbamoyl intermediate was formed initially after amine interacts with Pd species and CO insertion.An alkyne then reacted with carbamoyl intermediate to give the desired product.The possible structure and coordination environment of reactive palladium species were predicted by EXAFS fitting.2.A cobalt-catalyzed electrochemical oxidative C(sp²)-H/N-H carbonylation was developed.Using 8-aminoquinoline as the directing group,Aryl or Acryl amides reacted with carbon monoxide to construct phthalimide under electrochemical conditions.Phthalamide derivatives can be constructed by highly selective intermolecular coupling with external aliphatic amines.This reaction avoided the use of external oxidants and could be scaled up to the gram level.Cyclic voltammetry(CV)indicated that Co^? species were more likely to be generated through the oxidation of coordinated Co^? species instead of simple Co(acac)₂ by anodic oxidation.X-ray absorption near edge structure(XANES)were used to demonstrate a proposed mechanism through Co^?/Co^?/Co^? catalytic cycle.3.We developed a methodology of Co-catalyzed C(sp³)-H/N-H carbonylation.Succinimides were obtained by the addition of Ag₂CO₃ as an oxidant.Carbon monoxide reacted selectively with C(sp³)-H bond of?-methyl group.Interestingly,Cycloheptanecarboxamide and cyclooctanecarboxamide furnished the carbonylation products,further afforded cycloheptane-1,2-dicarboxylic acid and cyclooctane-1,2-dicarboxylic acid by hydrolysis and deprotection.X-ray absorption near edge structure(XANES)showed that the cobalt catalyst first coordinated with 8-aminoquinoline of amides then went through the Co^?/Co^?/Co^? catalytic cycle.