Theoretical Studies On The Activation Of C-H And C=C Bonds By High-valent Transition Metal-oxo Species

As the core of organometallic chemistry,the transition metal complex has become a new research field between organic synthesis and theoretical computation,due to its well chemical behaviors in the activation of C-H and C=C bonds.Compared with experimental research of transition metal complex,the theoretical study lags behind.For some complicated reactions catalyzed by transition metal,the special results can't be explained by conventional chemical knowledge.In addition,the mechanism of reactions may not be clear,and the intermediates of the reaction can't be detected by means of experiment.These factors seriously restrict the development of transition metal catalysis.Therefore,theoretical study of transition metal catalysis is of great significance for revealing the microscopic mechanism hidden in the macroscopic reactions at molecular level and for the development of organometallic chemistry.In this paper,based on the related experiments,the activation of C-H and C=C bonds by transition metals is systematically studied by using density functional theory(DFT)and the natural bond orbital analysis(NBO).The theoretical calculation deepens the cognition of the phenomena of transition metal catalysis and provides important theoretical guidance for the design of new catalysts.The conclusions are as follows:(1)Recent experimental studies on the conversion of androgen to estrogen catalyzed by Cytochrome P450 19A1 showed the 18 O atom exclusively incorporates into the water,rather than into the formic acids,and the nonenzymic hydration reaction of the 19-aldehyde to the 19,19-gem-diol was about 4-fold faster than the enzymatic estrogen formation from 19-OH androgen.To reveal the mechanism of C-H bond activation,we performed some calculations on these reactions.The calculaiotns showed that,the barrier of rate-determining step for the formation of estrogen from 19-OH catalyzed by Fe(IV)=O,was 10.8 kcal/mol,however the barrier for the isomerization of 19-CHO to yield 19,19-gem-diol was 21.9 kcal/mol.This was inconsistent with the recent experimental findings that the enzymatic estrogen formation from 19-OH androgen was relatively slower than nonenzymic hydration of 19-aldehyde.Herein,we proposed a new mechanism with dual oxidants.In this mechanism,ferric-peroxo species acts in the oxidation of 19-OH androgen to yield 19,19-gem-diol intermediate and generate ferryl-oxo(Cpd I)species.The latter would actually affect the final step of H-abstraction of O-H from 19,19-gem-diol to give experimentally observed products.Our new mechanism scenario reasonably explains the latest experimental observations and provides a deep insight complementing the newly-accepted Cpd I mechanism.(2)Recent experimental studies showed that,under mild conditions with dioxygen as the sole terminal oxidant,ruthenium porphyrin catalyzed oxidation of aryl alkenes to aldehydes with high selectivity.We propose related mechanism of reactions.Calculations showed that,the results indicated that two reactive oxidants,ruthenium?oxo and ruthenium?superoxo,participated in the catalytic oxidation.[O=Ru(Por)O] reacted with styrene firstly,followed by two competing reaction pathways: C?O bond rebound vs 1,2?H shift,to generate the epoxide and aldehyde respectively,with the latter being more thermodynamically and kinetically favorable.Following this,O2-bonding with the resultant monooxoruthenium porphyrin produced the superoxo species,which subsequently reacted with the substrate to generate the epoxide and dioxoruthenium porphyrin [O=Ru(Por)O].Finally,[O=Ru(Por)O] may re-enter the reaction cycle to create a catalytic oxidation process.Meanwhile,the tandem epoxide isomerization reaction(E-I),mediated by species [O=Ru(Por)] was thermodynamically favorable,required an overall free activation energy of 23.9 kcal/mol.In addition,it was discovered that the ruthenium–oxo/superoxo reactivity was significantly influenced by the identity of the axial ligands.There was an increase in the electrophilicity of the ruthenium–oxo/superoxo unit upon replacement of oxygen with chlorine.