[1] In their original 1977 publication, Tamejiro Hiyama and Hitoshi Nozaki[2] reported on a chromium(II) salt solution prepared by reduction of chromic chloride by lithium aluminium hydride to which was added benzaldehyde and allyl chloride: Compared to Grignard reactions, this reaction is very selective towards aldehydes with large tolerance towards a range of functional groups such as ketones, esters, amides and nitriles.
[4] It was observed that the success of the reaction depended on the source of chromium(II) chloride and in 1986 it was found that this is due to nickel impurities.
[6] In the same year Yoshito Kishi et al. independently discovered the beneficial effects of nickel in his quest for palytoxin:[7] Palladium acetate was also found to be an effective cocatalyst.
The next step is oxidative addition of nickel into the carbon to halide bond forming an alkenylnickel R–Ni(II)–X intermediate followed by a transmetallation step exchanging NiX with a Cr(III) group to an alkenylchromium R–Cr(III)–X intermediate and regenerating Ni(II).
The amount of nickel used should be low as a direct alkene coupling to a diene is a side reaction.