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centre at a C-H bond is initiated by coordination involving fairly weak,
probably agostic interactions, hence in the non-directed intermolecular case
the probability of bonding two reagents in the reacting complex is too small.
In directed palladation, the attack of the bonded Pd centre at the proximal
C-H bond is more facile, hence no restrictions due to substituent effects apply.
Recently, it was discovered that electron-deficient polyfluorinated mol-
ecules are activated towards reactions involving direct or ipso-palladation.
The use of polyfluorobenzenes for palladium-assisted arylation (oxidative
cross-coupling) was described by Fagnou and co-workers.
319-321
This re-
action involves the attack of an arylpalladium intermediate formed by oxi-
dative addition of a Pd-phosphine complex on an electron-deficient
polyfluoroarene (theoretical analysis of the reaction mechanism by high-
level DFT calculations was performed
322
). The palladium centre bearing
s-aryl and electron-rich phosphine ligands apparently prefers the attack of
an electron-deficient substrate.
The use of electron-deficient polyfluoroarenes in the Fujiwara-Moritani
reaction was observed only fairly recently. Zhang and co-workers demon-
strated that arylation of terminal alkenes takes place in the presence of a
palladium catalyst and 2 equiv. of Ag
2
CO
3
, which apparently has a dual role,
being both a terminal oxidant and very probably the base required for
deprotonation (Scheme 9.105).
323
This study explored how many fluorine atoms are required for reaction to
take place. Just two fluorine atoms are sucient to activate the molecule
towards palladation, in which case the major product is the expected ''be-
tween-fluorines'' isomer, whereas the isomer in which palladation takes
place is the ''ortho-to-a-single-fluorine'' site (Scheme 9.106).
F
F
R'
F
F
R'
Pd(OAc)
2
(10 mol% )
Ag
2
CO
3
DMSO-DMF, 120°C, 12 h
+
R
R
F
F
F
F
F
F
R' = H, R = CO
2
Me, CO
2
nBu, CO
2
tBu, CONMe
2,
CN, COEt, COPh,
Ph,tBu,nBu;R'=Me,R=CO
2
Me , Ph
Scheme 9.105
Intermolecular oxidative arylation by electron-deficient arenes.
F
Pd(OAc)
2
(10 mol% )
Ag
2
CO
3,
Me
3
CCOOH
DMF, 120°C, 24 h
+
CO
2
Me
F
F
F
CO
2
Me
CO
2
Me
+
F
F
40%
20%
Scheme 9.106 Regioselectivity of oxidative arylation by 1,3-difluorobenzene.
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