Study On Synthesis Of Alkynes Participated By Gem-dibromoalkenes

The gem-dibromoalkene can be easily synthesized by Wittig reaction.It has both a carbon-carbon double bond and two reactive carbon-bromine bonds,of which the trans-C-Br bond is more active.As one type of organic synthetic materials,gem-dibromoalkenes have been widely researched in C-C,C-N,C-O,C-P,and C-S bond formations.In the basic medium,gem-dibromoalkene can generate bromoalkyne or terminal alkyne in situ,which can be used to synthesize internal alkynes,alkynyl amines,and alkyne ethers.Noted that gem-dibromoalkenes have cost advantage compared with teriimal alkynes.More and more organic chemists have focused on the construction of carbon skeletons in which gem-dibromoalkenes are involved in recent years,ascribed to its unique molecule structure.Internal alkynes have been extensively applied as intermediates for the synthesis of bioactive compounds,natural products,pharmaceuticals,and functional materials.Among other methods,the Sonogashira coupling reaction is considered to be a premium tool for the preparation of internal alkynes from aryl or alkenyl（pseudo）halides and terminal alkynes.However,for the synthesis of particular alkynes,the use of alternatives to terminal alkynes is essential.In this paper,a one-pot two-step process was used to prepare internal alkynes with gem-dibromoalkenes and halobenzenes by C-C cross-coupling reaction.Through changing the amount of base,the type of catalyst,the reaction temperature,the molar ratio of starting materials,etc.,the optimal reaction conditions was founded as follow:gem-dibromoalkene and halobenzene（with a molar ratio of 1.7:1）as the starting materials,cesium carbonate（6.0 eq.）as the base,5 mol%Pd/C or 5 mol%Pd（OAc）₂and 10 mol%PPh₃ as the catalyst,DMSO as the solvent,and the reaction was carried out at 60 or 80 ^oC for 25 h under N₂ atmosphere.Based on the mechanistic study,it was suggested that terminal alkyne was generated in situ through a tandem elimination-hydrodebromination process,and the final product,internal alkyne was generated by Sonogashira coupling reaction of terminal alkyne with halobenzenes in one pot.The reaction proceeded under the simple,facile,and classic Cu-free Sonogashira coupling reaction conditions with good to excellent yields.In addition,the sythesis of asymmetric 1,3-diyne with two different gem-dibromoalkenes was also discussed in this paper.