Study On Thermodynamic Properties Of C-M Bond Of Rh,Ru Complexes And Thiol-derived Peptide

Rhodium and ruthenium transition-metal-organic cooperative catalysis,which has been concentratedly studied by many chemists,represents a great success in C-H and C-C bond activation because of high effectiveness and selectivities.Typically,in the reaction mechanism of aldehyde and alkene catalyzed by Rh?I?complexes and2-amino-3-picoline and the hydroesterifification reaction of alkenes with aodium formate and alcohols under cooperative catalysis by Ru₃?CO?₁₂and 2-pyridinemethanol,three kinds of metala-cyclic transition-metal complexes of?iminoacyl?Rh?III?hydride,?iminoacyl?Rh?III?alkyl and cyclohexyl-Ru?II?intermediate are generally formed.The three complexes play an important role in the overall reaction,in which the Rh-C and Ru-C bond formations are involved.Secondly,native chemical ligation?NCL?and its modified versions?ligation-desulfurization chemistry?have revolutionized the way that large peptides and proteins are obtained by chemical synthesis.Ligation-desulfurization chemistry can generate thiol-derived peptide models,which can generate proteins through reductive desulfurization.The C-S bond cleavages are involved in the process.Typically,?iminoacyl?Rh?III?hydride,?iminoacyl?Rh?III?alkyl and cyclohexyl-Ru?II?intermediate play an important role in the overall transition-metal-organic cooperative catalysis,and thiol-derived peptide models also plays an important role in ligation-desulfurization chemistry.Therefore,the C-M bond?M=Ru,Rh,S?involved in the experimental research has become even more important,and it has become the focus of research by experimental chemists.We will study the C-M bond system from the perspective of thermodynamics,which can be measured by the homogeneous bond dissociation enthalpy?BDE?.The quantum chemistry calculation method offers a reliable theoretical basis for the C-M BDEs of the transition-metal intermediates and the thiol-derived peptide model.First,we choose a variety of different density functional methods?DFT?to calculate C-M BDEs.We use Mean Deviation?MD?,Mean Absolute Deviation?MAD?,and Root Mean Square Error?RMSE?to evaluate the accuracy of different DFT methods,and then find the DFT method suitable for calculating the Rh-C,Ru-C,and C-S bonds.Finally,the most accurate DFT method is used to systematically calculate the BDEs of the C-M bond.The influence of different factors such as the substituent effect,front-line orbital energy,ring tension effect and the substituents constant on the BDEs of the C-M bond were further analyzed.1.16 relative Rh-C BDEs of Tp'Rh?CNneopentyl?RH?Tp'=hydridotris-?3,5-dimethylpyrazolyl?borate?were calculated by 19 DFT methods.In addition,the 5absolute Rh-C BDEs of Rh transition-metal complexes were also calculated.The results exhibited that the B97D3 is the most precise method to predict the relative and absolute Rh-C BDEs and the corresponding RMSE values are the smallest of 2.8 and 3.3 kcal/mol respectively.Therefore,the Rh-C BDEs of?iminoacyl?Rh?III?hydride and?iminoacyl?Rh?III?alkyl as well as the substituent effects were studied by using the B97D3method.What's more,the analysis including the natural bond orbital?NBO?and the energies of frontier orbitals were performed in order to further comprehend the nature of the Rh-C BDE change patterns.2.Secondly,the 16 Ru transition-metal complexes which have experimentalRu-C BDE values,were selected as our training set.17 DFT methods were used to calculate 12 Ru-C BDEs.The results show that M05-2X has the highest accuracy.The minimum RMSE value is 2.0 kcal/mol,and the MD and MAD values are 0.5 kcal/mol and1.3 kcal/mol,respectively.Therefore,we use the M05-2X method to calculate Ru-C BDEs of large system cyclohexyl-Ru?II?intermediate.The calculation results show that R=cycloalkyl has a stronger substitution effect on Ru-C BDE of cyclohexyl-Ru?II?intermediate,that is,as the ring size increases,the Ru-C BDEs of cyclohexyl-Ru?II?intermediate become significantly smaller.From the perspective of thermodynamics,the Ru-C bond of cyclohexyl-Ru?II?intermediate where R=cyclopropyl is most likely to be formed,while the Ru-C bond of cyclohexyl-Ru?II?intermediate where R=cyclooctyl is difficult to form.The analysis of frontier orbital energy(E_HOMO-E_LUMO,E_SOMO)reveals the nature of the changes in Ru-C BDEs.3.Finally,We first selected 7 high-level ab initio calculation methods including G3,G3B3,G4,G4MP2,CBS-QB3,CBS-Q and CBS-4M to calculate C-S BDEs of 44organic sulfides to evaluate the accuracy of the experimental values.Subsequently,25 DFT methods were chose to calculate the C-S BDEs of the 55 organic sulfides.It is reliable to predict C-S BDEs by M05-2X method with the corresponding RMSE value of 6.2 k J/mol.Then,the C-S BDEs of the thiol-derived peptide model and the structure-activity relationships were systematically researched by using this method.The result shows that different Cys surrogates have influence on C-S bond cleavages of the thiol-derived peptide model during desulfurization.The largest C-S BDE?414.3 k J/mol?belongs to?-thiol Trp surrogates.With the attempt to explain the C-S BDEs change trend,the natural spin densities of radical centers C are discussed.Surprisingly,for?-thiol Trp surrogates,there is the smallest natural spin densities of radical centers C of 0.210 while the BDE is the largest.It can be interpreted with the“anti-spin-delocalization effect”.In addition,comparing with the C-S BDEs of the thiol-derived peptide of?-thiol-derived Phe surrogates,thiol-derived Val surrogates and?-thiol-derived Gln surrogates,the C-S BDEs of the thiol-derived peptide model of?-thiol-derived Trp surrogates are much larger,which shows that the thiol-derived peptide model of?-thiol-derived Trp surrogates are more difficult to be desulfurized.