Investigating The Mechanism Of Palladium Catalyzed Unsaturated Bond Activation

Unsaturated molecules are the most basic and abundant compounds in organic chemistry,because of their unsaturated bond properties,they are often used in organic synthesis.In these organic synthesis reactions,transition metal palladium is often used as a catalyst for the conversion of unsaturated molecules,so that unsaturated molecules can be transformed into functional molecules with specific functions.There are various forms of palladium catalyzed unsaturated molecular transformation,and its mechanism process is also complex and changeable,so the research on the mechanism of palladium catalyzed unsaturated molecular transformation has always been a hot spot for researchers.In recent years,with the development of science and technology,computing methods have been continuously developed,and computing devices have been constantly updated and improved,computational chemistry plays a more and more important role in mechanism exploration,Through theoretical calculation,we can speculate the possible reaction mechanism and explore the source of reaction selectivity,which plays a very important role in guiding the further experiment.In this paper,the activation mechanism of unsaturated bond catalyzed by palladium was discussed by theoretical calculation,and the activation reactions of unsaturated molecules such as carbon monoxide,butenolactone and vinyl lactone catalyzed by palladium were studied.These unsaturated molecules and palladium catalysts form rapid and reversible complexes and many transformations take place.The reactions involved are oxidative carbonylation,cycloaddition and so on.In these reactions,carbonylation products or cyclization products are obtained by activating or functionalizing unsaturated bonds in carbon monoxide,butenolactone and vinyl lactone.From the point of view of the reaction mechanism,the reaction usually starts from zero-valent palladium,and then the zero-valent palladium catalyst reacts with the reaction substrate to form a divalent palladium intermediate,followed by migration,insertion or protonation,and finally the product was obtained by reduction elimination.Because there is no strong oxidant in the reaction conditions,the tetravalent palladium intermediates are not involved in these reactions.Through theoretical calculation,we speculate the dominant paths of these reactions,exclude the possible sub-paths,and explain the causes of regioselectivity and stereoselectivity.The specific research contents include the following four aspects:(1)The mechanism of C(sp~3)-H carbonylation catalyzed by palladium was studied by M06-L and B3LYP calculation method.The reaction was carried out through the radical addition of alkyl radical with CO,then react with palladium catalyst to form acyl palladium species,and finally occurs reducation elimination.In addition,we also investigated the regioselectivity of C(sp~3)-H carbonylation.The results show that the regioselectivity is controlled by both the BDEs of the C-H bond of the reaction and the stability of the generated carbon free radicals.(3)The asymmetric cycloaddition reaction of vinyl formate with butenolide catalyzed by palladium was studied by density functional theory(DFT).The site selectivity of butenolide in the reaction was mainly explored by calculation.The dominant path of the reaction includes oxidation addition,decarboxylation,proton transfer,reduction elimination and other steps.It is found that theαsite of butenolide reacts more easily thanγsite.In addition,we also explore the stereoselectivity of the reaction,and the calculated results are consistent with the experimental results.(2)The mechanism of palladium-catalyzed cycloaddition of vinyl carbamate with ketene was studied by density functional theory(DFT).When the protective group on the N atom of vinyl carbamate is donating electron group,the dominant path of the reaction will not undergo the process of decarboxylation.When the protective group on the N atom is withdrawing electron group,the dominant path of the reaction will undergo the process of decarboxylation,so the electronic effect has a great influence on the reaction system.The rate-determining step of the reaction is the process of oxidation addition.(4)The cycloaddition reaction of indoledione andα-diazoketone catalyzed by palladium was investigated by B3LYP and M06-2X calculation methods.We mainly explored the two regioselectivity of the reaction,one is that the carbon anion attack of allyl palladium intermediate is easier or the oxygen anion attack of allyl palladium intermediate is easier,the other is the selectivity of allyl straight chain carbon and branched chain carbon allyl when reduction elimination occurs.The calculated results are consistent with the selectivity in the experiment.The rate-determining step of the whole reaction is the process of nucleophilic attack.