The Mechanism Of Transition Metal Palladium Catalyzed Dimerization Of Terminal Alkynes

Enynes is a kind of important compounds, widely found in a variety of natural products as well as many biologically active compounds. Meanwhile it is also a kind of an important intermediate in organic synthesis. Among all the methods for synthesis of enynes, transition metal palladium-catalyzed dimerization of terminal alkynes is a highly efficient and regio- and stereo-selective controlled way. For the dimerization of terminal alkynes, a commonly proposed mechanism is that the dimerization reaction undergoes a key hydridopalladium intermediate, which could be formed through the oxidative addition（OA） of terminal alkyne C_sp-H bond into the Pd（0） centre. Subsequently, hydridopalladium intermediate coordinated with second alkyne molecule could result in either the carbopalladation or the hydropalladation followed by reductive elimination（RE） to afford the dimerization product. The commonly proposed mechanism is not suitable for all dimerization systems. When there is a kind of Br?nsted acid（Ph₂P（O）OH） existing in the system, instead of the terminal alkyne C_sp-H bond undergoing through oxidative addition and insertion into the Pd（0） centre. The formation of alkenylpalladium complex would occur through the proton transfer process. The possible reaction pathways were studied computationally,（I） OA of terminal alkyne C_sp-H bond,（II） OA of Ph₂P（O）OH and（III） proton transfer process. As the results shown in hereinafter, the formation of alkenylpalladium complex through the proton transfer process is the most favorable pathway. Subsequently, a ligand-exchange reaction is followed to yield an alkenyl（alkynyl）palladium complex. Finally, a RE reaction takes place to afford the desired product. Such head-to-tail regio-selectivity could be rationalized by the charge analysis. For the alkynes, the terminal carbon has more negative charge than the internal carbon. Therefore, it is more favourable for a positive proton to transfer to the terminal carbon than to the internal carbon of the terminal alkyne in the proton transfer process. On the basis of the theoretical calculated results above, the role of water in the transition metal palladium-catalyzed dimerization of terminal alkynes was experimentally studied. According to the results of the experiments we found that when water is absent in the system the yield of the desired product is only 39%, while the volume of added water is 1.8μL（0.1mmol）, the yield increases to 79%. So H2 O can promote the transition metal palladium-catalyzed dimerization of terminal alkynes.