Historical Perspective of the Heck Reaction

Aryl and vinyl halides- Aryl and vinyl chlorides are most reluctant to undergo Pdcatalyzed
activa-tion. Heck reactivity - as expected from the C-X bond dissociation
energies (Figure 1) - increases in the order CI « Br < I, with fluorides being
com-pletely unreactive with any of the known catalysts.
The ideal substrates for coupling reactions are aryl chlorides since they tend to be
cheaper and more widely available than their bromide or iodide counterparts.
Unfortunately the high C-Cl bond strength compared with C-Br and C-I bonds
disfavours oxidative addition, the first step in catalytic coupling reactions, making the
coupling of such substrates far more challenging. Therefore, there is currently much
interest in the synthesis of catalysts that are able to activate aryl chloride substrates at
ever lower catalyst loading
The activation of chlorohydrocarbons is of major industrial interest. Progress was
made by introducing a bimetallic Ni/Pd catalyst which converts the aryl-CI in a first
step into the more reactive aryl-Br bond (NiBr2)' followed by Pd-catalyzed C-Ccoupling
(Pd(dbab)2 dba = dibenzylidene-acetone, (C6H5CH=CH)2C=O).
).
Figure 1. C-X bond dissociation energies (X = F, CI, Hr, I) of aryl halides. 1 kcal
mol -1 = 4,184 kJ mol -1 .
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