Science of Synthesis

Shibasaki, M.; Ohshima, T.; Itano, W., Science of Synthesis: Stereoselective Synthesis, (2011) 3, 483.

The mechanism of the Mizoroki–Heck reaction is generally thought to follow the five-step catalytic cycle illustrated in ‌Scheme 1‌, with the individual steps being (a) oxidative addition of the aryl or alkenyl (pseudo)halide ‌2‌ (R1X) to the palladium(0) complex ‌1‌ to afford the organopalladium(II) species ‌3‌, followed by (b) π-complex formation (‌3‌ + ‌4‌ → ‌5‌ or ‌9‌), then (c) syn-insertion of the alkene substrate into the Pd—R1 bond to furnish a σ-alkylpalladium(II) complex (‌5‌ → ‌6‌, or ‌9‌ → ‌10‌), which undergoes (d) syn-β-hydride elimination to give either the α-product ‌8‌ or the β-product ‌11‌ [E-isomer (E)-‌11‌ through βE-hydride elimination and Z-isomer (Z)-‌11‌ through βZ-hydride elimination], and finally (e) regeneration of the palladium(0) species ‌1‌ by reductive elimination of HX from HPd(II)XL2 (‌7‌). The regioselectivity of the insertion into the Pd—R1 bond (step c) is heavily dependent upon the stereoelectronic environment provided by the substituents of the alkenes and the metal center. Control of selectivity, which has tended to limit the scope of the reaction, can be accomplished with the judicious choice of ligands and optimizing the reaction parameters.

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