Mechanism Study Of NHC-Iron Complexes Catalyzed Hydrosilylation Of Methyl Phenylacetate And Diethylsilane

The hydrosilylation of carbonyl compounds is of great value in synthetic organic chemistry.This hydrosilylation reaction has the advantages of easy operation,mild conditions,and so on,to obtain silicon monomer or polymer that is difficult in other methods.New silicone materials with the valuable properties,such as high and low temperature resistance,ozone resistance,moisture proof,corrosion resistance,non-toxic and tasteless,and physiological inertia,etc.,then synthesized.In recent decades,researchers have developed many transition metal catalysts for the hydrosilylation of carbonyl compounds.Especially,high active,high selective,cheap catalyst systems have made in great progress by employing ligands with excellent properties and transition metals.At the same time,with the rapid development of theoretical calculations and the progress of experimental means,the catalytic mechanism of transition metal complexes has also made a breakthrough.The purpose of this work is to clarify the detailed mechanism of iron–catalyzed hydrosilylation of methyl phenylacetate and Et₂SiH₂ at B3 LYP level of density functional theory.The important intermediates and transition states are given.The kinetic and thermodynamic data of various possible reaction paths were compared and analyzed,and the most favorable reaction channel was proposed.This study has provided a reasonable theoretical basis and guidance for the experimental study of this kind of chemical reaction.The main contents and results are as follows.（1）The advances in new reaction mechanisms of carbonyl compounds hydrosilylation catalyzed by group Ⅷ transition metal complexes are reviewed.In particular,we highlight the different mechanistic pathways of Rh,Ru,Fe,Ir etc complexes.Some key intermediates and transition states and related energetic parameters are illustrated.（2）The catalytic activity and isomers of electron deficient compounds（NHC）Fe（CO）₃ and（NHC）Fe（CO）₂,which is obtained through the CO dissociation of（NHC）Fe（CO）₄,are investigated and analyzed.The reactions of complexes（NHC）Fe（CO）₃ and（NHC）Fe（CO）₂ with substrate diethyl silane or methyl phenylacetate are studied by theoretical computations.It is found that the oxidative addition of silane by active species is favorable thermodynamically.（3）We have explored two kinds of reaction pathways initiated by（NHC）Fe（CO）₂.Path I is started with the oxidative addition of Et₂SiH₂ to（NHC）Fe（CO）₂ to form（NHC）Fe（CO）₂（H）（SiHEt₂）.Then PhCH₂CO₂ Me coordinates to Si center of（NHC）Fe（CO）₂（H）（SiHEt₂）,followed by the insertion of C=O to Fe-Si via the five-membered transition state leading to the formation of Fe-C bond.This step is the rate determining step that needs to overcome the energy barrier of 20.90 kcal/mol.Finally,C-H reductive elimination occurs and catalyst regenerates.Path II consists of three parts: the coordination of PhCH₂CO₂ Me and oxidative addition of Si–H bond to Fe center;then the insertion of C=O double bond into the Fe-Si bond through the four-membered transition state,leading to the formation of Fe-C and O-Si bonds.This step is the rate determining step in the whole reaction and needs to overcome a high energy barrier of 44.22 kcal/mol.Finally,C-H reductive elimination and catalyst regeneration take place.In summary,the energy barrier in path II is higher by 23.32 kcal/mol than path I.Path I is more favorable than path II.（4）The pathway initiated by（NHC）Fe（CO）₃ is put forward as follows.The oxidative addition of Et₂SiH₂ to（NHC）Fe（CO）₃ to form（NHC）Fe（CO）₃（H）（SiHEt₂）.Then another CO ligand is released from（NHC）Fe（CO）₃（H）（SiHEt₂）and（NHC）Fe（CO）₂（H）（SiHEt₂）is formed.This structure is very similar to the related intermediate in path I,so the reaction will proceed along the correlative steps in path I.We verified and corrected the mechanism proposed by Darcel’s group.First,the oxidative addition of Et₂SiH₂ to（NHC）Fe（CO）₃ affords（NHC）Fe（CO）₃（H）（SiHEt₂）,followed by the dissociation of another CO and（NHC）Fe（CO）₂（H）（SiHEt₂）formed.Then since the electrostatic interaction between carbonyl O and Si center,the insertion of C=O to Fe-Si takes place via a special five-membered transition state leading to the formation of Fe-C bond.Finally,C-H reductive elimination occurs and product regenerates.