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A
n eutr al p athway
Ar
Ar
R
L
Pd
L
Pd
Ar
L
X
X
R
L
Pd
L
X
R
Ar
Ar
X
-
L
Pd
L
L
P
d
L
polar pathway
R
B
Ar
Ar
L
L
Pd
X
Pd
X
n eutr al p athway
L
L
C
Ar
Ar
L
Pd
X
L
Pd
pola r p athway
X
-
L
L
Scheme 9.30 Dominant scenarios of dependence of pathway type on stable mono-
and bidentate ancillary ligands.
pathway requires the dissociation of a ligand that is favoured by the steric
bulk. Such a pathway can occur either with monodentate ligands or un-
symmetrical bidentate ligands involving two coordination sites - a stronger
one, usually phosphine, and a weaker one, such as N or O atoms, electron-
rich aryls furnishing the Z
2
-binding mode, and even the second but more
bulky phosphine centre. Such unsymmetrical bidentate ligands, developed
particularly by the Buchwald, Beller and Hartwig groups, achieved great
success in the cross-coupling chemistry (reviewed recently
118
), but their
impact on Heck chemistry has so far been modest.
True bidentate ligands have a separate and very profound impact on Heck
chemistry. Whereas all other ligands can be involved in both polar and
neutral pathways, true bidentate ligands form complexes which can operate
only via the polar pathway, so their effect is to direct the process into a
definite route. This property is used in two major branches of Heck chem-
istry - the regioselective arylation of electron-rich olefins
119
and enantiose-
lective Heck reaction.
120-123
The detailed analysis and the classification of the roles of ancillary ligands
in the Mizoroki-Heck reaction
124
resulted in the conclusion that practically
all the data accumulated on this reaction can be fitted into four major types
of catalytic systems according to the role of the ancillary ligands. The de-
tailed analysis and arguments can be found in Ref. 124 and here only a brief
description of each is given. It should be noted that the Mizoroki-Heck re-
action, particularly the reactions of iodoarenes and selected activated bro-
moarenes with standard olefins (acrylates and styrene), was for a decade and
a half used as a testing ground for developing new catalytic systems, pre-
catalysts and ligands and new ways of performing reactions,
125
and many of
the thus developed systems were tried with variable success and with un-
certain trends for less reactive bromo- and even chloroarenes. Such a huge
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