Selective Activation Mechanism Of Rhodium-Catalyzed Unsaturated Molecules

Transition metal catalyzed activation of unsaturated molecules is an efficient and atom-economic synthesis method.Owing to its very important role in the synthesis of natural products,drugs,and compounds with biologically active skeletons,it has been widely studied by organic chemists.Rhodium catalysts are often used for the activation of unsaturated molecules because of their high reactivity,wide substrate application range,good selectivity,and good functional group compatibility.There are many kinds of unsaturated molecules,and their high-efficiency and high-selectivity functionalization reactions are still the focus of research in organic synthetic chemistry.Usually,the activation or functionalization of unsaturated molecules requires the coupling reaction.However,the investigation of the reaction mechanism in the experiment is not clear enough.With the rapid development of computational chemistry,we often use the method of combining experiments and theoretical calculations to understand the mechanism of each step in the reaction process more comprehensively,and use various analytical methods to clarify the reasons for the chemoselectivity,regioselectivity,stereoselectivity.In this paper,the mechanism of rhodium-catalyzed activation of unsaturated molecules is studied by theoretical calculation methods,and the activation of olefins,alkynes and other unsaturated molecules catalyzed by rhodium is mainly discussed.Based on previous research reports,dinuclear rhodium mechanism only occurs in Rh(II)reaction,while Rh(I)or Rh(III)species undergo a mononuclear rhodium process.We guess whether Rh(I)or Rh(III)species can also undergo dinuclear rhodium mechanism,and the dinuclear configuration remains unchanged throughout the catalytic cycle.Similar to the dinuclear Rh(II)species,the two rhodium atoms are not equivalent.Therefore,we studied these reactions in detail by theoretical calculation and analyzed their regioselectivity.The main research results are as follows:(1)The mechanism of rhodium-catalyzed olefin terminal arylation reaction was studied by B3 LYP and M06-L calculation methods.The calculation results show that the dinuclear pathway is more favorable than the mononuclear pathway,and both undergo four processes: oxidation addition,olefin insertion,Rh-walking,and reductive elimination.The dinuclear pathway is mainly carried out through the Rh(III)-Rh(III)redox catalytic cycle,the rate-determining step of the reaction is the process of reductive elimination.The regioselectivity of the reaction is controlled by steric effects,and finally generate the δ-arylation product.Mononuclear pathway is affected by ring tension and tether effect,and the β-arylation product is major product,which is inconsistent with the experimental results.In addition,C-H alkylation at the C7 position is more favorable than that at the C2 position in the dinuclear pathway.(2)The mechanism of rhodium-catalyzed hydroformylation of alkynes was studied by density functional theory(DFT).This reaction involves dehydroformylation and hydroformylation.The dehydroformylation includes five processes: oxidative addition,deprotonation,decarbonylation,β-H elimination and ligand exchange.The hydroformylation includes four processes: alkyne insertion,CO insertion,protonation and reductive elimination.The calculation results show that the rate-determining step of the reaction is the process of alkyne insertion,and the(E)-α,β-unsaturated aldehyde is finally obtained.The stereoselectivity of the reaction is determined by the alkyne insertion mode.