Historical Perspective of the Heck Reaction

All of the palladacycles shown in the Scheme promote the Heck reaction of aryl
iodides with acrylic esters (Table 1). In this reaction, the highest catalytic activity
observed to date was achieved with the palladacycle 6 (Table 1, Entry 9). It is
interesting to note that this palladacycle only promotes the Heck reaction between aryl
iodides and acrylic esters.
Only a few palladacycles are active catalyst precursors for electron-rich aryl bromides
(Table 1). Moreover, in the case of aryl chlorides, complexes 1, 7, 8, and 11 were
found to be the only active palladacycles (Entries 24-30, Table 1).
In particular, 7 promotes the Heck reaction of electron-rich aryl chlorides, such as 4-
chloroanisole (Entry 29, Table 1).
In contrast with the other catalyst systems presented in Table 1, the PCP-pincer complex 7 does
not require the use of additives to show good activity, even with electronically, deactivated aryl
chlorides. This may imply that here the active catalyst is not colloidal palladium but rather is a
well defined molecular species, and currently it is not possible to rule out a Pd(II)/Pd(IV)
catalytic manifold. It was proposed a catalytic cycle that proceeds via the C- H activation of the
olefin at a Pd(II) centre(s) followed by oxidative addition.
Complex 7
Proposed mechanism for Heck
coupling catalysed by complex
7.
11