Adduct Of N-Heterocyclic Carbene And Nitrile Oxide: Structure And Catalytic Property

N-heterocyclic carbenes（NHC）as a strong nucleophile to activate small electrophilic molecules such as CO₂ and N₂O has attracted extensive attentions.Meanwhile,adducts formed by NHC and CO₂ have been widely used as organocatalysts to promote the cycloaddition reaction of epoxides and CO₂,as well as the carboxylative cyclization of propargyl alcohol and CO₂.The nitrile oxide（RCNO）molecule contains a linear CNO triatomic sequence,in which the carbon atom shows strong electrophilic reactivity and the terminal oxygen atom is linked to a sp hybrid nitrogen atom.The reaction between RCNO and nucleophile is similar to that of small molecules such as CO₂ and N₂O.In this dissertation,we have synthesized a series of adducts of NHC and RCNO（NHC-RCNO adducts）,and systematically studied their structural properties by X-ray single crystal diffraction,nucleophilic reactivity and stability.On this basis,the application of these adducts and their alkylation derivatives as organocatalysts in the cycloaddition reaction of epoxides and CO₂has been investigated.（1）A series of adducts have been synthesized by the reactions between NHC and RCNO.These adducts have been further characterized by NMR,IR,HRMS.X-ray single crystal diffraction analysis of 11 kinds of NHC-RCNO adducts shows that the CNO part in adducts is bent,and the bond lengths between CNO atoms are larger than that of the linear free RCNO.In addition,the bond length of NHC carbon atom to RCNO carbon atom in the NHC-RCNO adducts is significantly shorter than those in NHC-CO₂ adducts and NHO-CO₂ adducts,which indicates that the stability of the NHC-RCNO adducts is higher than that of NHC-CO₂ and NHO-CO₂ adducts.The bond length of N-O in RCNO part in NHC-RCNO adducts is longer than that of nitrogen atom to oxygen atom in NHC-N₂O.Unlike NHC-CO₂ and NHO-CO₂adducts in which the imidazole ring plane is nearly perpendicular to the carbon dioxide plane,the angles of the imidazole ring plane to the nitrile oxide plane in the NHC-RCNO adducts are from 15.6^o to 37.5^o,which is similar to that of the imidazole ring plane to the nitrous oxide plane in NHC-N₂O adducts.（2）The NHC-RCNO adducts can react with electrophilic reagents such as iodomethane,iodohexane or benzyl bromide to obtain the oxygen-alkylated product.X-ray single crystal diffraction analysis shows that when the NHC-RCNO adducts react with iodomethane to obtain the methylated derivatives,the bond lengths of NHC carbon atom and RCNO carbon atom obviously increase,however,the bond lengths of C2-N1 in RCNO part decrease and the bond length of N1-O in RCNO part increase.The bond angles of C2-N1-O decrease,and the bond angles of N1-C2-C3 increase.The bond lengths of the RCNO part in the methylated product of the NHC-RCNO adducts are the same as that of the FLP-RCNO adducts reported in the literature.（3）The cross reaction of NHC-RCNO adducts with free NHC or RCNO proves that the NHC-RCNO adducts are not easy to dissociate in solution and shows high stability.Thermogravimetric analysis shows that the decomposition temperature of NHC-RCNO adducts ranges from 270 ^oC to 370 ^oC,and the decomposition temperature of the alkylated derivatives is from 310 ^oC to 440 ^oC.The thermal stability of the NHC-RCNO adducts is higher than that of NHC-CO₂,NHO-CO₂ and NHC-N₂O adducts.（4）The NHC-RCNO adducts and its alkylated derivatives show catalytic activity in the cycloaddition reactions of epoxides and CO₂ to synthesize cyclic carbonate.The catalytic activity of the alkylated derivatives is higher than that of NHC-RCNO adducts.The electronic effect of substituent on the ring of epoxides has an important effect on the yield catalyzed by the derivatives,and epoxides with electron-withdrawing groups give high yields.