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partially substituted furans in a simple system using Pd(OAc)
2
in DMSO-
AcOH mixture in the absence of metal salt additives
335
or the alkenylation of
coumarins taking place in pivalic acid solution under an oxygen atmos-
phere.
336
The key role of DMSO in the activation of palladium acetate
through cleavage of a trimeric structure and formation of more reactive
monomeric and dimeric species was elucidated by Le Bras and co-work-
ers.
337
Alkenylation of indoles at the 3-position was achieved with high yields
in a Pd(OAc)
2
(10 mol%)-H
3
PMo
12
O
40
-DMAP system under an oxygen at-
mosphere in DMF-DMSO solvent mixture.
338
Although still very rare, a few examples of oxidative Heck reactions not
with aromatic substrates but with alkenes have appeared, as in the report by
Gillaizeau and co-workers on the alkenylation of protected enamides
(Scheme 9.110).
339
The other important direction of development is the search for more
economical and practical systems for Pd(II) recycling. In many cases this
chemistry can employ oxygen gas, but often an elevated pressure is required,
which is both hazardous and prone to cause oxidative side reactions.
B¨ckvall and co-workers described an approach to solve these problems by
employing electron-transfer mediators, which dramatically increase the ef-
fectiveness of reoxidation, thus reducing the required pressure of O
2
to
ambient.
340
The system used relied upon iron phthalocyanine as the primary
oxidant passing electrons to dioxygen and benzoquinone as the secondary
oxidant taking electrons from Pd(0) and passing them to iron phthalocya-
nine. The system can affect both mono- and diarylation (Scheme 9.111).
Concerning olefins, the Fujiwara-Moritani reaction is usually applied to
the same manifold of alkenes as the Mizoroki-Heck reaction - acrylic acid
derivatives being by far the most popular, which is not surprising as the
mechanism of olefin binding is the same in both reactions. However, a
certain broadening of scope is evident, as in many of the published oxidative
olefination protocols the use of trisubstituted olefins is fairly widespread,
COOMe
COOMe
Pd(OAc)
2,
Cu(OAc)
2
O
2,
AcOH,DMA,70°C
+
NHAc
NHAc
N
SO
2
Ph
N
SO
2
Ph
70%
Scheme 9.110 Example of oxidative alkenylation of an enamide.
MeO
OMe
OMe
Pd(O Ac)
2
(5 mol%)
BQ (20 mol%), FePc (2 mol% )
O
2
(1 atm) , AcOH, 90°C, 24 h
COOBu
MeO
+
COOBu
70%
OMe
OMe
Scheme 9.111 Diarylation in a biomimetic system.
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