Theoretical Study On Synthesis Of New Carbon - Carbon Bond By Catalytic Catalytic Synthesis Of Alkynes By Palladium And

In this thesis, we have studied the mechanisms of the following two works with the M06 and B3 LYP level of density functional theory（DFT）. The purpose of our study is to present the reasonable reaction mechanisms, explain the experimental phenomena and the experimental results, and offer guidance for further study of related reactions. Two specific work is as follows:（1） With the aid of density functional theory calculations, We have carried on the theoretical study of the Pd-catalyzed reactions of aryl iodide styrene（R1） with unsymmetrical alkyne（R3）（Scheme1, the third chapter） leading to two products containing C（sp3）-I bond（P3 and P4） and one product containing a three-membered carbocyclic unit（P5）, have been studied theoretically. It is found that both the alkyne insertion and the subsequent C=C bond insertion involved in the reaction are the major thermodynamic driving forces. The alkyne insertion instead of the C（sp3）-I reductive elimination is predicted to be rate-determinant. Similar barrier heights calculated for the two insertion modes of unsymmetrical internal alkyne（TS3-4 and TS3’-4’） lead to the products P3（47.2%） and P4+P5（48.8%） having similar product yields. The intriguing formation of the product containing a three-membered carbocyclic unit（P5） was investigated in details. The second alkene insertion is found to be kinetically more favored than the C（sp3）-I reductive elimination, leading to product P5（39.0%） more productive than P4（9.8%）. The remarkably thermodynamically favored b-H elimination is the key factor enabling formation of P5. Why significant bulky phosphine ligand such as P（t-Bu）3 instead of small one such as P（Me）3 was employed experimentally have also been rationalized based on our calculation results.（2） With the aid of density functional theory calculations, we have investigated the mechanism of the Ni-catalyzed [2+2+2]and[2+2+1+1]cycloaddition of diketene with alkynes（Scheme 2, the fourth chapter）. The product of reaction only get only P1（98%） in the presence of Et₂Al（OEt） with the efficient regioselectivity. Our DFT results did not support the proposed mechanism in the experiments, which involves oxidative coupling in the beginning of the reaction and form a ternary ring intermediate when breaking C=C bond（formation of product P2 or P3）. Instead, our calculations supported a mechanism which involves C-O oxidative addition of diketene to a Ni（0） center to form a five-membered ring intermediate having both Ni-C and Ni-O bonds in the beginning of the reaction and form a four-membered ring intermediate when breaking C=C bond（formation of product P2 or P3）. We also studied the Et₂Al（OEt） and HCl on the influence of the reaction. We found that the Et₂Al（OEt） and HCl both have to speed up the reaction rate, but their essence is different. At the same time we also reveals the Et₂Al（OEt） and HCl for the cause of the high selectivity of the reaction.