Palladium-Catalyzed Coupling Reaction of 1-Bromoalkynes with Olefins to Synthesize Enynes

1,3-Enynes are very useful synthetic intermediates. Furthermore, enyne moiety is also an important unit often found in many naturally occurring and biologically active compounds. For these reasons, the design and synthesis of such compounds containing an enyne moiety have received considerable attention in organic synthesis and have been extensively studied. Among of these methods, the Sonogashira cross-coupling reaction for the synthesis of enynes between a vinyl halide and a terminal alkyne or organometallic alkyne using Pd-Cu catalyst has become an indispensable and powerful tool for the synthesis of enynes. However, limited attention has been turned to another alternative cross-coupling reaction between a terminal alkene and a haloalkyne which is highly valuable synthon and widely used in synthetic organic chemistry for constructing complex organic compounds due to their versatility. Using Pd(OAc)(2) as the catalyst and K2CO3 as the base in DMF, an efficient procedure for the stereoselective synthesis of a series of conjugated enynes by a simple cross-coupling reaction of alkenes and alkynyl bromides has been developed. The effects of catalyst, base and reaction temperature were studied. The result indicated that the reaction proceeds smoothly to give the corresponding products in good to excellent yields using 5 mol% Pd(OAc)(2) and 2.5 equiv. K2CO3 in DMF for 2 h at 80 degrees C for arylalkene compounds, room temperature for electron-deficient alkenes. The structure of enyne derivatives was confirmed by IR, H-1 NMR, C-13 NMR, MS and HRMS. In summary, an efficient and general catalytic system has been developed for the cross-coupling reaction of a wide range of alkenes and bromoalkynes in the absence of any stabilizing ligands or special additives. The process is regio- and stereoselective, and provides good access to a series of functionalized 1,3-enynes in good to excellent yields. Moreover, the reaction is a convenient and simple path for the synthesis of the 1,3-enyne, and tolerates several functional groups on both coupling partners.