Research On Palladium-catalyzed Cross Coupling Reactions Of Alkynes With Alkenes In Ionic Liquids

Transition metal-catalyzed carbon-carbon and carbon-heteroatom bond formations hasattracted considerable attention over the past decades, owing to its easy access to highlyfunctionalized complicated molecules in a rather efficient, atom-and step-economical way.Prominently, palladium-catalyzed coupling reactions is now establish as a captivating branchof organic chemistry for the construction of both carbon-carbon and carbon-heteroatom bonds,since they usually proceed under mild reaction conditions, tolerant of many functional groupsand also fall under the umbrella of green chemistry. From the point of reaction mechanism,palladium-catalyzed coupling reactions containing two elementary chemical reactions: oneelementary reaction is forming the C-PdIIbond intermediate, which can be conveniently gotthrough nucleopalladation of alkynes, then alkenes inserting into the carbon-palladium, or theoxidative addition of Ar-X (X=halide and OTf) to Pd(0), or transmetallation oforganometallic reagents to PdIIcenter; the other is generally required to break thecarbon-palladium bond and to regenerate the activated palladium species. Some of thesereactions are β-H elimination, β-heteroatom elimination, reductive elimination, oxidativecleavage, reductive cleavage, and protonolysis. And β-H elimination is one of the mostfundamental reaction classes in organometallic chemistry and how to suppress the β-Helimination and functionalize the alkyl-PdIIintermediate is of great importance.Moreover, ionic liquids (ILs) are regarded as “green” solvents which provide anexcellent medium for performing transition metal catalyzed reactions. Their negligible vapourpressure, ease of handling and potential for recycling, circumvent many of the problemsassociated with volatile organic solvents. Furthermore, their high compatibility with transitionmetal catalysts and limited miscibility with common solvents, enables easy product andcatalyst separation with the retention of the stabilised catalyst in the ionic phase. Because ofthese properties, these novel solvents have shown great promise in transition metal-catalyzedcoupling reactions.In this context, we have preliminarily studied the palladium-catalyzed coupling reactionsof alkynes with alkenes in ionic liquids, and developed a series of powerful cascadeapproaches to afford highly functionalized tetrasubstituted alkenes in good yields withexcellent control of regio-and stereoselectivity. The details are summarized as following:(1) A Pd-catalyzed haloallylation of alkynes with allyl alcohols in ionic liquids has been reported. Both chloroallylation and bromoallylation can be easily carried out with highselectivity. A variety of1,4-dienes were formed in moderate to excellent yields. The reactionsystem of the Pd catalyst as well as the ionic liquid can be recycled for several times. And theionic liquid acts as not only a solvent in the reaction, but also provides the excess halide ionsto control Z/E selectivity and acts as a ligand inhibit the β-hydride elimination.(2) A palladium-catalyzed regio-and stereoselective intermolecular tandem reaction ofalkynes and unactivated1,n-enols in ionic liquids is described, providing a practical, efficient,and versatile method for the synthesis of functionalized1,n-dienes in moderate to good yields.The present reaction has high functional-group tolerance and gives products on a gram scale.Mechanistic studies indicate that the reaction might proceed via a chain-walking mechanism.(3) Carbonylation processes using carbon monoxide (CO) as a C1building block havebeen studied extensively, because of the direct formation of complicated molecules fromreadily accessible starting materials. In particular, transition metal-catalyzed carbonylation ofaromatic halides with CO in the presence of various nucleophiles has undergone rapiddevelopment. we have developed a practical, efficient, and versatile method for the synthesisof functionalized saturated γ-lactones. This novel and general methodology may open up anew viewpoint on the carbonylation of the C(sp3)–palladium bond. A mechanism involvinghalopalladation of alkynes, followed by insertion of homoallylic alcohol, migratory insertionof CO into the palladium–carbon and a reductive elimination gives the target product.(4) A highly efficient and mild palladium-catalyzed, one-pot, four-step cascadeannulation has been developed to afford functionalized β-or γ-lactones in moderate to goodyields with high regio-and diastereoselectivities in ionic liquids. The employment of ionicliquids under mild conditions makes this transformation green and practical. Especially, thisreaction provided a novel and convenient methodology for the construction of naturallyoccurring biologically active β-or γ-lactone skeletons.