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equivalent of ArX), which may become a serious cause of the formation of
unexpected side products.
The reactions obeying this paradigm should be called reductive Heck re-
actions, which can be formally considered as the opposite case to oxidative
Heck reactions.
Examples of reductive Heck reactions can be found in intramolecular
transformations of formidable utility and complexity. Thus, Lautens et al.
observed a stereoselective ring closure (Scheme 9.4), corresponding to
pathway B in Scheme 9.2.
60
The protocol was extended to intermolecular reactions.
61,62
This version is
interesting because as Pd(0) cannot be regenerated via the regular pathway
by PdH deprotonation, the reaction could be considered as a base-free Heck
reaction and the amine used in the catalytic system probably served as a
reducing agent for Pd(II).
Another example of pathway B shows that the methodology can come
close to performing miracles! The C-F bond is among the least reactive
bonds. However, the case of termination by Pd-F elimination was reported
by Ichikawa et al. in an intramolecular reaction (Scheme 9.5).
63
In the ab-
sence of reducing agents the reaction is apparently stoichiometric in Pd(0)
complex.
The last stoichiometric type corresponding to pathway D in Scheme 9.2 is
so far very rare. Zaitsev and Daugulis showed in 2005 that acetanilides can be
made to react with 2-bromoacrylates in the presence of a Pd catalyst and
silver salt to afford the same cinnamates as in the directed oxidative Heck
reaction terminated not by PdH elimination but by Pd-Br elimination, with
silver triflate apparently serving the role of removal of the strongly bonded Br
ligand, which would otherwise suppress the reaction
64
(e.g. Scheme 9.6).
Pd
2
(dba)
3
(5 mol %)
4(o-Tol)
3
P, BuNMe
2
MeCN-H
2
O (10:1)
reflux, 6 h
I
76%
H
Ph
OAc
H
Ph
Scheme 9.4
Intramolecular arylation terminated by b-elimination of acetate.
OTf
Pd(P Ph
3
)
4
(10 0 mol%) , PPh
3
DM A, 110° C, 9 h
F
F
PdF(OTf)
F
F
O
F
H
2
O
PdO Tf
Scheme 9.5
Intramolecular
arylation
terminated
by
fluoride
elimination.
A stoichiometric case.
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