The Mechanisms For N-heterocyclic Olefin-catalyzed Formation Of Cyclic Carbonate From CO₂ With Propargylic Alcohols And Epoxide

Carbon dioxide is considered as the main source of the greenhouse effect,rational utilization of carbon dioxide can not only reduce environmental problems,and is of great significance to alleviate the energy crisis.But chemical inertness of carbon dioxide hinders its application in the chemical reaction.In recent years,more and more people began to concern and explore the catalyst that can activate carbon dioxide,which can be used as a raw material react with other simple organic materials for the synthesis of various valuable organic compounds.In this paper,all calculations were performed with the Gaussian 09 software program package using the B3 LYP means of density functional theory?DFT?.Detailed studies and discussions on the mechanisms for N-heterocyclic Olefin-catalyzed formation of cyclic carbonate from CO₂ with propargylic alcohols or epoxide have been done.The results are shown below:1)The mechanisms for N-heterocyclic Olefin-catalyzed formation of cyclic carbonate from CO₂ and propargylic alcoholsThe mechanisms of cyclic carbonate formation from CO₂ and propargyl alcohols catalyzed by NHO have been studied by means ofcomprehensive density functional theory calculations at the B3LYP/6-311+G?d,p?level.For inclusion of salvation effect,geometries of all the stationary points were fully optimized without any constrains and were further proved by vibrational analysis in DCM solvent using the integral equation formalism polarizable continuum model?IEF-PCM?with radius and the static relationship taken from SMD model,in the corporation dispersion correction with D3 model.Six mechanisms for the formation of cyclic carbonate were fully investigated.The energy profiles in DCM showed that,for the four mechanisms leading to five-membered ring product P1,the highest saddle points of M-A,M-B,M-B' and M-C were located 45.37,31.99,57.07 and 35.11 kcal/mol higher than the initial reactants,respectively.For the two mechanisms leading to six-membered ring product P2,M-D and M-E,the highest saddle points along the two paths were located 35.18 and 59.61 kcal/mol that higher than the initial reactants,respectively.So M-B is the predominant one with the lowest barrier of 31.99kcal/mol,should be the more preferable mechanism,while M-C and M-D may be kinetically competitive to M-B.Very high activation energy of 45.37,57.07 and 59.61kcal/mol for M-A,M-B' and M-E,respectively,suggest that they are of less importance in the whole mechanisms.Meanwhile,P1 is indeed 2.43 kcal/mol more stable than P2.This result is in consistent with the experimental fact that P1 is the predominant product.2)The mechanisms for N-heterocyclic Olefin-catalyzed formation of cyclic carbonate from CO₂ and epoxideThe mechanisms for N-heterocyclic Olefin-catalyzed formation of cyclic carbonate from CO₂ and epoxide were studied by means of comprehensive density functional theory calculations at the B3LYP/6-311G?d,p?level.For inclusion of salvation effect,single point energy calculations were performed using the integral equation formalism polarizable continuum model?IEF-PCM?with radius and the static relationship taken from SMD model.In addition,the empirical dispersion correction recommended by Grimme was added to the B3 LYP energies.The formation of cyclic carbonate from CO₂ and epoxide contains two mechanisms?M-1 and M-2?.The difference between two reaction mechanisms is that epoxide adds to NHO first then CO₂ in M-1,while CO₂ addition is first,then epoxide addition in M-2.M-1 is composed of three steps,the step of CO₂ addition in M-1 is barrierless,and the final step with the highest barrier?40.33 kcal/mol?is the rate determining step of the reaction.M-2 includes four reaction steps,the third step with a barrier of 32.74 kcal/mol,is the highest saddle point of the reaction path.M-1 and M-2 may play as kinetically competing mechanisms,M-2 is the predominant one with the lower barrier for the rate controlling step.