Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 423

sensitive. Particularly sensitive are phosphines, which are known to be
catalytically oxidized by oxygen. 311
2. In the absence of electron-rich ancillaries, PdH intermediates are likely
to be highly acidic to release proton spontaneously to weakly basic
components of the reaction system (e.g. solvent), thus permitting base-
free systems. There is some controversy regarding this subject, how-
ever, as strong donor ligands were also observed to enhance the acidity
of PdH (for a discussion, see Ref. 124).
3. Direct recycling of catalytically active Pd species by oxygen without the
need for bases and other oxidants is possible, although the effective-
ness of such pathways critically depends on other ligands at the Pd
centre. Underligated complexes bearing highly labile ligands and cat-
ionic palladium centres are good candidates for such systems. A single
stable N,N-bidentate ligand can be present to ensure overall stability of
the catalytic species.

The rest of both processes involve exactly the same chemistry, hence
common trends in reactivity and selectivity should also be the same, which
makes the comparisons between reactions of the two types a promising
source of helpful conclusions.

9.3.1 The Fujiwara-Moritani Reaction
Direct C-H bond activation without the need to use various leaving groups is
the ultimate goal of any synthetic strategy based on substitution reactions.
Only such an approach would meet the atom economy principle, 312 as
otherwise the material of the leaving group is waste. Therefore, the possi-
bility of using benzene and other aromatic substrates for the arylation of
olefins, discovered in 1967 by Fujiwara, Moritani and co-workers, 7 was
highly motivating. Nevertheless, in reality, the reactions discovered simul-
taneously by the Heck and Mizoroki teams and using the worst possible
(from the economy principles viewpoint) leaving groups, heavy and expen-
sive iodine, mercury, etc., progressed rapidly and captured the area, whereas
the original Fujiwara-Moritani reaction had been practically abandoned for
quite a period.
The original method suffered from at least two serious drawbacks - the
need to use a stoichiometric amount of Pd(II) salt and low yields and se-
lectivity. Substituted benzenes gave mixtures of isomers (Scheme 9.103). 13

OMe Ph

Pd(OA c) 2
AcOH, 120°C

+

MeO

o:m:p = 30 :5:48

Scheme 9.103 Low regioselectivity of the Fujiwara-Moritani reaction of simple
benzene derivatives.