Study On Copper-catalyzed Semihydrogenation And Homocoupling Reactions Of Terminal Alkynes

Terminal alkynes have been widely applied in organic synthesis, which can be convertedto a variety of compounds under suitable reaction conditions, such as, olefins bysemihydrogenation, alkanes by fully hydrogenation,1,3-diynes by homocoupling and theconjugated enynes by dimerization. Among these transformations, the semihydrogenation ofalkynes was often conducted using palladium catalysts with hydrogen as a reducing reagent.Now, a lot of researches have focused on semihydrogenation of interal alkynes, while fewstudies on selective semihydrogenation of terminal alkynes have been reported.Homocoupling reaction of terminal alkynes is realized mainly through copper-catalyzed,generally requiring base as co-catalyst. This thesis aims to develop a safe catalytic system forselective semihydrogenation of terminal alkynes with a liquid substance as a source ofhydrogen, cheap copper salts as catalysts. Another is the realization of the copper catalyzedhomocoupling of terminal alkynes to1,3-diynes without any co-catalyst in simple conditions.The main research results are as follows:(1) Using the copper citrate as catalyst, hexamethylenetetramine as co-catalyst,50%hypophosphorous acid aqueous solution as hydrogen source, terminal alkynes could beefficiently and highly selectively converted to terminal olefins. The effects of the reactionconditions (hydrogen sources, solvents, reaction temperature and reaction time) have beensystematically investigated. With the semihydrogenation of phenylacetylene to styrene as amodel reaction, the results showed that the conversion rate of the reaction close to100%, andthe selectivity is ca.90%under the optimal reaction conditions (5mol%copper citrate,2.0equiv50%of hypophosphorous acid aqueous solution in DMF at130°C for6h). Botharomatic and alkyl aklyne could be selectivly hydrogenated smoothly under this optimalreaction conditions to give corresponding olefins in high yield. The catalytic system wastolerant to a varety of functional groups such as alkyl, halogen atoms, methoxy, acetyl,carboxyl, ester, acid amides.(2) Using CuBr2as catalyst, terminal alkynes could be efficiently converted to1,3-diynesin high yields in DMF. To find the reaction conditions, the reaction temperature, reaction time,solvents and additives have been systematically screened. The results showed that theconversion rate close to100%and the selectivity is ca.95%under the optimal reactionconditions (5mol%CuBr2in DMF at130°C for12h). This homocoulpling reaction couldtake place for both terminal aryl alkynes and alkyl terminal alkynes to furnish the corresponding1,3-diynes in high yields. Many alkynes bearing various functional groupssuch as halogen groups, methoxyl, hydroxyl, carbonyl, ether and ester, could survive in thiscatalytic system.