Mechanistic Study On Deoxygenation Of Epoxides And Synthesis Of Polycaprolactone Based On C-O Bond Cleavage

The selective activation and cleavage of C-O bond is not only the key to efficient conversion of biomass to produce high value-added chemicals,but also one of the important means to achieve the diversity-oriented synthesis of organic molecules.Ether group and ester group are two types of common functional groups containing C-O bond.Epoxides are ternary cyclic ethers with high reactivity.Due to the strong polar C-O bond and high ring tension,the epoxides exhibit high reactivity and become an important synthetic intermediate in organic reactions.Deoxygenation of epoxides to alkenes is a fundamental and important reaction in organic chemistry and biochemistry.In 2020,Professor Coates has reported significant progress on deoxygenation of epoxide with excellent stereoselectivity:various alkyl substituted cis-and trans-2,3disubstituted epoxides can be deoxygenated to their corresponding E-or Zalkenes in a system consisting of CO and bimetallic catalysts.On the other hand,the lactones containing C-O bond are also another important multifunctional intermediate.Lactones with weak electrophilicity are vulnerable to strong nucleophiles causing the cleavage of C(sp2)-O bond.Polycaprolactone(PCL)produced byε-Caprolactone(ε-CL)via ring-opening polymerization is one of the most promising and widely used biodegradable polyesters.An effective Sn(IV)catalytic system developed recently not only achieves the synthesis of high-performance PCL,but also inhibits the formation of oligomers with uncertain molecular weight,without any additional initiators.Important progresses have been made in the above-mentioned experimental studies based on C-O bond cleavage.However,the detailed mechanisms especially the origins of selectivity are still unclear.Computational studies are carried out in this thesis for the above two kinds of reactions:In the first part of this thesis,we studied the mechanism of the stereoinvertive deoxygenation reaction of cis-2,3-disubstituted epoxides.DFT calculation is performed on the three reaction paths proposed in experimental literature,and we proposed a new reaction pathway 4.It was found that the activation barrier of pathway 1 was as high as 56.2 kcal/mol,and this reaction pathway was ruled out.Although pathway 2 and 3 are dynamically feasible,the calculated results showed that the reaction barrier of the new pathway 4 was the lowest among all the pathways.Therefore,we believed that pathway 4 was the most reasonable pathway for cis-2,3-dissubstituted epoxides deoxygenation.Based on the detailed reaction mechanism,the difference of deoxygenation rate between cis-epoxides and trans-epoxide is attributed to that the overlap of substituents on the trans-epoxide compounds resulted in a higher overall activation barrier of deoxygenation.In addition,we explained that electronic effects and steric hindrance were the controlling factors for the selective attack of Mn(CO)5-on the cis/trans-2,3-dissubstituted oxirane carbon atom.In the second part of this thesis,we studied the mechanism of Sn(IV)catalyzed synthesis of PCL,and revealed the reason that why the transesterification side reaction competing withε-CL ring-opening polymerization was not easy to occur.Theoretical studies demonstrated that the reaction followed the currently widely accepted coordination-insertion mechanism.To initiate the polymerization,the Ph4Sn precatalyst needed to be converted to the active species(tin-alkoxide complex).Ph4Sn reacts with 6-hydroxy hexanoic acid,a hydrolyzed product ofε=Cl at high temperature to form tin-carbonate complex,and then combined with anotherε-CL molecule via the coordination-insertion mechanism to form a tin-alkoxide complex(Sn-OR)with an anhydride bond at the end.Subsequently,the polymer chain grew by inserting the monomer into the Sn-O bond of the tin-alkoxide complex.An anhydride bond was formed around the terminal hydroxyl group of the PCL chain,this special chemical structure could effectively prevent the transesterification.The difference of the activation barrier between the transesterification side reaction and the synthesis of PCL also indicated that the Ph4Sn catalytic system could effectively inhibit the formation of oligomers.