| Transition metal catalyzed C–H and C–C bond activation,as two very effective methods to construct new C–C bonds and carbon heteroatom bonds,have been widely concerned and developed by organic chemists.Among these transition metals,rhodium is widely used in the activation of C–H and C–C bonds due to its unique double electron redox properties and high reactivity.The activation modes of Rh-catalyzed C–H bond activation can be divided into nucleophilic and electrophilic C–H bond activation mode,respectively.The nucleophilic C–H bond activation modes include concerted metalation-deprotonation,σ-complex-assisted metathesis and Friedel-Crafts-type electrophilic aromatic substitution.The electrophilic activation modes are oxidation addition and insertion of the Rh-carbene or Rh-nitrene into the C–H bond.There are three strategies for Rh-catalyzed C–C bond activation: oxidation addition,β-carbon elimination and retro-allylation.The main work of this paper is to investigate the mechanism of several Rh-catalyzed C–H and C–C bond activation reactions.This paper mainly includes the following two parts:1.The mechanism of a series of Rh(Ⅲ)-catalyzed C–H bond activation reactions was studied through theoretical calculations.The mechanism of Rh(Ⅲ)-catalyzed C–H bond activation reactions would not always be same because of different substrate,reaction condition and reaction strategy.The mechanism of Rh(Ⅲ)-catalyzed C–H bond activation can be only Rh(Ⅲ)catalytic cycle.The mechanism also can be a Rh(Ⅰ)-Rh(Ⅲ)catalytic cycle or Rh(Ⅲ)-Rh(Ⅴ)catalytic cycle.1.1 M11-L calculation method was used to investigate the mechanism of the carbonylation reaction of phenylimine catalyzed by Rh(Ⅲ).The calculated results show that the catalytic cycle includes isomerization of phenylimine,cleavage of nitrogen hydrogen bond,C–H bond activation of phenyl,insertion of carbonyl and reductive elimination.It is also found that the formation of C–Rh bond can promote the phenyl C–H bond activation and reduce the activation free energy of C–H bond activation.The C–H bond activation was completed by the stratery of oxidative coupling.The mechanism of this reaction is a Rh(Ⅰ)-Rh(Ⅲ)catalytic cycle.1.2 Using M11-L calculation method,the mechanism of Rh-catalyzed ketene dithioacetal C–H bond activation and cyclization with α-diazo ketone was studied.The calculated results show that the phenyl C–H bond activation is prior to the alkenyl C–H bond activation.The rate-determining step in the catalytic cycle is the carbonation step rather than the phenyl C–H bond activation.The following carbene insertion and dehydration would generate the alkyl-Rh(Ⅲ)intermediate,which would conduct Tsα-elimination and intramolecular 1,4-migration to give the formal intramolecular Ts1,3-migration product.The C–H bond activation was completed by the stratery of intermolecular dehydration cross-coupling.The mechanism of this reaction is a Rh(Ⅲ)catalytic cycle.1.3 M06 and B3-LYP-D3 calculation methods are used to study the mechanism of Rh-catalyzed C–H bond activation of phenacyl ammonium salts.Theoretical calculation indicated that the catalytic cycle includes the formation of Rh–C compounds,C–H bond activation,denitrogenation,the first carbene insertion,removal of triethyl amine,the second carbene insertion and proton exchange.In this reaction,The C–H activation was calculated to be the rate-limiting step in the catalytic cycle.In this reaction,the quaternary ammonium salt was used as internal oxidant in the substrate to realize the cross-coupling,and the reaction was a Rh(Ⅲ)catalytic cycle.1.4 M11-L calculation method was used to investigate the mechanism of Rh-catalyzed C–H bond activation and annulation of o-pivaloyl oxime with ketene.The calculated indicated that the catalytic cycle for this reaction include Rh(Ⅲ)-catalyzed phenyl C–H bond activation,insertion of C=C bond in ketene into aryl C–Rh(Ⅲ)bond,and concerted N–O bond oxidation addition/C–N bond reductive elimination.The valence state of Rh center remains trivalent in the catalytic cycle.The IRC calculation confirmed the validity of the concerted N–O bond oxidation addition/C–N bond reductive elimination transition state.In this reaction,the oxime ester was used as internal oxidant in the substrate to realize the cross-coupling,and the reaction was a Rh(Ⅲ)catalytic cycle.1.5 M11-L calculation method was used to study the mechanism of Rh(Ⅲ)-catalyzed arene azidation reaction.It is found that the mechanism of this azidation reaction is oxidation of Rh(Ⅲ)to Rh(Ⅴ)by trivalent iodine,Rh(Ⅴ)-mediated aryl C–H bond activation,and reductive elimination.The calculated results indicated that the Rh(Ⅴ)-catalyzed aryl C–H bond activation is the rate-determining step in the catalytic cycle.Moreover,the natural population analysis(NPA)of the oxidation process clarified that Rh(Ⅲ)is oxidized to Rh(Ⅴ)by trivalent iodine.The reaction is an oxidative coupling with high iodine as a strong oxidant,and the reaction is a Rh(Ⅲ)-Rh(Ⅴ)catalytic cycle.2.The mechanism of Rh(Ⅰ)-catalyzed C–C bond activation and intramolecular [3 +2] cycloaddition reaction was studied by theoretical calculations.Through calculation,we found two new modes of C–C bond activation modes,which are retro-propargylation and retro-Aldol-type addition.2.1 We have used DFT method,M11-L,to study the mechanism of Rh catalyzed intramolecular [3 + 2] cycloaddition reaction,which gives [3.3.0] bicyclic ring systems with two vicinal quaternary stereocenters at the bridgehead positions.Through theoretical calculation we found that the catalytic cycle involves two possible pathways.Actually,the selection of which pathway was determined by the chirality of the starting materials.The stereoselectivity of the product is determined by the α-carbon in the substrate.The computational results are also well verified by the experimental results.2.2 M11-L calculation method was used to study the competition of retro-propargylation and retro-Aldol in Rh(Ⅰ)-catalyzed intramolecular [3 + 2]cycloaddition reaction.The calculated results indicated that the C–C bond activation in the six membered ring substrate was carried on through a new C–C bond cleavage mechanism(retro-Aldol-type reaction).It is found that the favorable pathway for this Rh(Ⅰ)-catalyzed intramolecular [3 + 2] cycloaddition reaction is retro-Aldol-type reaction to generate a rhodium enolate intermediate,intramolecular Michael-type addition of acetylene to the enal moiety,a Conia-ene type reaction between the enolate moiety and allene,and protonolysis. |
PDF Full Text Request