Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 404

X

Ar I Ar Cu I

Z

CuX
-HI

Z

X

Ar

HCu I

Cu I

X

Z

Ar

Scheme 9.71 Hypothetical mechanism of base-free copper-catalysed Heck reaction.

This interesting transformation apparently ignores the build-up of acidity,
which is not tolerated in Pd-catalysed regular Mizoroki-Heck reactions using
aryl halide substrates. This is most probably somehow related to the
mechanism of regeneration of the catalytically active low-valent state of Cu.
If we presume that the mechanisms overall are similar, the difference in the
Cu version is probably due to spontaneous decomposition of a copper(III)
hydride intermediate. PdH intermediates in the Mizoroki-Heck reaction also
decompose spontaneously, which allows a few turns of the catalytic cycle,
but this reaction is choked by increasing acidity, whereas the Cu-catalysed
counterpart apparently is not (Scheme 9.71). In this respect, copper-
catalysed Heck reaction appears similar to palladium-catalysed arylation by
arenediazonium ions (see below).
Diaryliodonium salts except iodoarenes have also been used in copper-
catalysed Heck reactions. 256-258
Hence these new achievements with copper promise important new
findings and as such should be thoroughly investigated further.

9.2.6 Arylation by Arenediazonium Salts, the Matsuda-Heck
Reaction
Among the less usual substrates for the Mizoroki-Heck reaction, arenedia-
zonium salts 244 occupy a special place. These compounds are by default the
most reactive aromatic electrophiles, with reactivity far exceeding that of all
common substrates. The other important feature of diazonium salts is the
absolutely non-nucleophilic leaving group, hence the oxidative addition of
Pd(0) to these compounds always gives cationic Pd complexes, so the polar
pathway would always be involved in the reactions of arenediazonium salts,
unless strong anionic ligands (such as halides) are deliberately added.
Arenediazonium salts came rather late in palladium catalysis, and at first
were viewed with some scepticism. The development was very slow (a few
initial steps were treated in Ref. 87) until recently. However, in the last 3-4
years, studies have begun to multiply and we can now see that arylation by
arenediazonium salts is among the most fruitful source of new useful
protocols (for a recent review, see Ref. 259), an intensely growing branch of