Grignard reagents, bonding reduction with, mechanism

The mechanism of the Heck reaction with Pd(OAc)2 probably involves the initial in situ reduction of the Pd " ion to Pd(0), which then inserts in the C-X bond (recall the formation of the Grignard reagent from Mg and halides). The species R-PdX that is formed adds to the double bond in the sense of R and PdX, which in an intramolecular reaction will lead to a cyclic product. The adduct eliminates H-PdX to regenerate a double bond. This proposed mechanism is illustrated with the synthesis of the benzofuran 4.36 in Scheme 4.39. 

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