Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 405

Heck chemistry. Moreover, diazonium salts are notably different from all
other types of organic electrophiles used for Mizoroki-Heck reactions with
respect to both reactivity and tolerance to base-free conditions, allowing for
other means of regeneration of the active catalytic species. In this respect,
the reactions of diazonium salts bear parallels with oxidative Heck reactions.
Ancillary ligands in these reactions play different roles than in more com-
mon Mizoroki-Heck reactions.
The main problem with the reactions of arenediazonium salts is their low
stability in complex reaction multicomponent systems used for catalytic
processes. Uncontrolled decomposition of arenediazonium salts leading to
various by-products and coloured tars is known to be triggered by heating,
even gently, in the presence of bases, nucleophiles and particularly transi-
tion metals. Therefore, the choice of composition of a catalytic system
suitable for carrying out useful and high-yield catalytic transformations
is a challenging task in itself, which explains why so useful and reactive
substrates were only introduced into Heck chemistry very recently. However,
the arenediazonium salts are attracting more and more interest as highly
reactive and readily available electrophiles for transition metal-catalysed
reactions, and in the course of such studies many prejudices against these
compounds, routinely believed to be highly unstable, dangerous, prone to
undergoing undesirable side-reactions, etc., are being revoked one by one.
New methods of arenediazonium salt generation are being discovered to
contribute to the realization by synthetic chemists that these reagents
are readily available, safe and definitely worth trying in the pursuit of
sophisticated synthetic goals. Most remarkably, Filimonov et al. discovered
that pure tosylates of arenediazonium salts, prepared in pure form by
diazotization with nitrite-exchanged anion-exchange resin, are astonishingly
thermally stable with distinct melting points above 100 1C without
decomposition. 260 Such salts are very reactive in nucleophilic substitution
reactions, e.g. to give the respective substitution products in high yields not
only with iodide, but also with bromide and nitrite without copper catalysis.
This study clearly shows that the resources of arenediazonium salts are
impressive and much yet remains to be discovered.
The stability of arenediazonium salts depends on the substituents on the
benzene ring, with both electron-donating and electron-withdrawing sub-
stituents stabilizing the salt against spontaneous thermal decomposition
(but not necessarily towards side-reactions induced by bases, nucleophiles
and metals). As a result, the parent benzenediazonium salt often fails to
behave and give good yields of target products of arylation, thus requiring
special tricks such as addition in small portions (rarely used in catalytic
reactions overall, which are customarily performed by charging all com-
ponents at once into a reaction vessel). This is likely to become most im-
portant when scaling-up and ensuring reproducibility of procedures initially
developed for micro- or millimolar scales.
In other cases, amazing effects are observed. For instance, phenoldiazo-
nium salts are somewhat exotic as they are not easily prepared by common