Vinyl triflates organopalladium catalysts

The Heck reaction consists in the Pd(0)-catalysed coupling of alkenes with an aryl or alkenyl halide or triflate in the presence of a base to form a substituted alkene (Scheme 7.1). The reaction is performed in the presence of an organopalladium catalyst. The halide or triflate is an aryl or a vinyl compound and the alkene contains at least one proton. 

The mechanism involves the oxidative addition of the vinyl or aromatic triflate or halide to give an organopalladium intermediate. TransmetaUation with the organostannane forms another organopalladium intermediate with two Pd—C a bonds. A reductive elimination step releases the product and thereby regenerates the palladium(0) catalyst. Vinyl triflates can be made from enolizable aldehydes or ketones and aryl triflates from phenols, but the reaction also works with vinyl and aryl halides. 

Terminal alkynes can be coupled directly to aryl, heteroaryl, and vinyl halides or triflates in the presence of a Pd-catalyst and a base, which frequently is an amine acting both as solvent and as scavanger for the respective acid formed in the reaction. The mechanism appears to involve oxidative addition of the sp2-halide or triflate to Pd(0), followed by alkynylation of the intermediate organopalladium complex and reductive elimination of the substituted alkyne. Copper(I) iodide is a particular effective cocatalyst, allowing the reaction to proceed at room temperature (91MI2). 

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