| The olefin hydrogenation and hydrosilylation reactions catalyzed by transition metal complexes are two important reactions among organic synthesis.The alkene hydrogenation has been widely used in academic and industrial fields due to its characteristic of atom economy.The olefin hydrosilylation can synthesize a large number of commercial silicon resins,which is suitable for personal care,agricultural chemicals,silicon rubber,anti sticking coating and adhesive and so on.Moreover,many commercial silicone resins derived from the anti-Markovnikov addition of tertiary silane to terminal alkenes.This stimulated the scientists to find the catalysts with uniquely terminal selectivity.So far,precious metals Pt,Rh,Ru,Pd,Co and other catalysts have been largely reported to have high efficiency,high selectivity.However,the high cost of precious metal and heavy metal toxicity make researchers develop the low cost,safe and environmental friendly metal catalysts.Recently,many researchgroups focus on synthetizing high activity,high selectivity,air stable and cheap catalyst systems by using transition metal iron and the excellent ligands.It makes iron catalysts become a hot spot of research.The purpose of this work is to study the detailed reaction mechanism of iron-catalyzed 1-butene hydrogenation and hydrosilylation deeply at B3 LYP level of density functional theory.Our results have clarified the formation of key intermediates and transition states,and provide the very favorably possible reaction paths.Moreover,it provides theoretical basis and guidance for the experimental investigations.The main contents and conclusions for 1-butene hydrogenation are as follows.?1?The precatalyst(MePDI)Fe?N2?2is easily dissociated from N2 to generate triplet state active catalytic species(MePDI)Fe?N2?and(MePDI)Fe.The hydrogenation of 1-butene catalyzed by(MePDI)Fe?N2?2mainly includes four steps: the formation of active species(MePDI)Fe?H2??1t-H2?,followed by 1-butene coordination,the bound-H2 oxidative addition,and the C–H reductive elimination.?2?Notably,in the hydrogenation of 1-butene,the crucial intermediate is iron dihydrogen complex(MePDI)Fe?H2??C4H8?not iron?II?dihydride complex(MePDI)Fe?H?2?C4H8?.The coordination of1-butene to(MePDI)Fe?H2?contributes to the rate-determining step and the whole energy barrier is 16.8 kcal/mol referenced to(MePDI)Fe?N2?2.?3?Alternatively for 1-butene isomerization,the triplet(MePDI)Fe is predicted to be the active species.The rate-determining step is associated with the coordination of 1-butene to(MePDI)Fe to afford?CH2=CHCH2CH3?(MePDI)Fe?1t-C4H8?,followed by spin-state transformation,and the hydrogen shift leading to ?3-allyl hydride species??3-C4H7?(MePDI)FeH.The energy barrier is predicted to be17.7 kcal/mol referenced to(MePDI)Fe?N2?2.?4?In the presence of H2 and 1-butene,only occurrence of alkene hydrogenation is due to the large percentage of 1t-H2 over 1t-C4H8.In addition,we infer that high H2 pressure can accelerate the formation of(MePDI)Fe?H2?but slightly retard the olefin hydrogenation.The main contents and conclusions for 1-butene hydrosilylation are as follows.?1?As a catalytically reactive species,(MePDI)Fe completed the CH mechanism.Firstly,the silane coordination compound(MePDI)Fe??2-H-Si?OMe?3?is more stable than the olefin coordination compound(MePDI)Fe?C4H8?by 12.1 kcal/mol,so the first key intermediate is the triplet state species(MePDI)Fe??2-H-Si?OMe?3?in the catalytic cycle.Noticely,since the transition state TS2/3 about H migration from Si to C2 has similar stability with 1-butene coordinatedcomplex 2,olefin coordination and the hydrogen migration of silane is a concerted reaction process.Then the reaction proceeds via N2 coordination,the isomerization of the Si–H oxidative addition product3,and Si–C2 reductive elimination.It is found that 1-butene coordination is pivotal and endergonic by 22.6 kcal/mol,and?N2?[Fe]?Si?OMe?3??C4H9?is an important intermediate.The rate determining step of is associated with Si–C reductive elimination,and the corresponding energy barrier is 22.9 kcal/mol referenced to complex 3.?3?Alternatively,the ionic mechanistic catalytic cycle from(MePDI)Fe?N2?is initiated by silane coordination to form(MePDI)Fe?N2???1-H[Si]?6.Then the nucleophilic attack of by alkene substrate to the silicon center of 6,leading to the heterolytic cleavage of the Si?H bond and generation of species 7 which is composed of a silycarbenium ion paired [?Me O?3SiC4H8]+and an anionic iron hydride [?N2?(MePDI)FeH]–.Finally,intermediate 8 formed by means of N2 dissociation occurred C–H reductive elimination to yield the alkylsilane product.The activation energy of the turnover-limiting step which featuring an SN2–Si structure of the transition state was calculated to be 66.7 kcal/mol.?4?By comparing the potential energy surface curves of two reaction paths,we found that the(MePDI)Fe?N2?2catalytic olefinhydrogenation reaction follows the CH mechanism,and the ionic outer mechanism is not competitive.Since the coordination ability of silane to(MePDI)Fe is much better than that of the olefin,only the olefin hydrosilylation can take place,and the side reaction of1-butene isomerization cannot occur.The hydrosilylation of 1-butene to form the butyl silane is exergonic by 9.7 kcal/mol. |
PDF Full Text Request