Heck reaction palladium chloride - tertiary

Scheme 2 shows a similar mechanistic pathway for a Heck reaction taking place on a Pd octahedral comer. This mechanism is based on that established for soluble Pd catalysts (ref. 5). Adsorption of the aryl halide (or aryl acid chloride after decarbonylation) gives the aryl Pd halide, 15, by way of the adsorbed intermediate, 14. Vinyl ether adsorption, as in 16, takes place as described in Scheme 1. Aryl insertion gives the halometalalkyl, 17, which on f) elimination to the available 4dxy orbital gives the aryl enol ether, 2 (or 1 depending on which hydrogen is eliminated in 17). The resulting halo palladium hydride, 18, then reacts with the tertiary amine to give the amine hydrochloride and regenerates the octahedral comer for further reaction.

On the other hand, true ligand acceleration (type 3 processes) shows preference for solvents of low polarity and lower Lewis basicity (toluene, dioxane and THF) with soluble tertiary amines as bases. In this respect, these Mizoroki-Heck reactions resemble cross-coupling processes, which also display strong preference for these solvents. Reactions in nonpolar solvents (toluene or xylene) have been known since Heck s seminal articles [8]. The halide remains a crucial subject of concern in reactions catalysed by phosphine complexes of palladium, aryl iodides prefer triarylphosphines and polar solvents, whereas reactions of aryl bromides and chlorides indeed prefer electron-rich trialkylphosphines and nonpolar solvents [63-65]. 

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