Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 427

palladation of an electron-deficient arene (C 6 H 5 CF 3 ) by a cationic palladium
complex, which shows the formation of meta þ para isomers without ortho
substitution, thus being a clear prototype of this alkenylation method.
However, even in this case, undoubtedly showing that electrophilic palla-
dation of electron-deficient arenes is indeed possible, the distribution of
isomers was different from what could be reasonably explained by the
interplay of a static inductive effect and steric bulk. 327 We have paid con-
siderable attention to this case because it explicitly reveals that the trends in
reactivity of palladium complexes in palladation reactions are not properly
understood and interesting chemistry promising substantial widening of the
reaction scope is most likely still hidden. More research is required to
understand better the reactivity of various palladium complexes in reactions
involving undirected palladation.
However, although electron-deficient arenes are reactive, the majority of
arenes used in the intermolecular Fujiwara-Moritani reaction are electron
rich and the positions attacked are the same as in common electrophilic
substitution reactions The problem of low regioselectivity is commonly
overcome by using multisubstituted compounds in which only a single option
for the substitution is possible, as in, for example, the arylation of substituted
fluorofurans developed by Zhao and co-workers (Scheme 9.108), 328 reaction of
2-substituted thiazoles and oxazoles with standard olefins in the presence of
10-20 mol% Pd(OAc) 2 and silver acetate in the presence of carboxylic acids, 329
reaction of 1,2,3-triazoles with standard olefins realized in a Pd/Cu/O 2 sys-
tem, 330 the alkenylation of imidazo[1,2-a]pyridines by standard olefins taking
place in the presence of a Pd/Cu system in a non-polar solvent, 331 alkenylation
of 5-pyrazolones under mild conditions using K 2 S 2 O 8 as reoxidant 332 and the
arylation of quinones. 333
In other cases, the reactions of less substituted heterocyclic compounds
are selective enough to afford definite products, as e.g. in the alkenylation of
indolizines taking place in a complex system using a silver salt, a base and
the bidentate ligand 2,2 0 -bipyridyl (Scheme 9.109), 334

the arylation of

Z

F F

Pd(OA c) 2 (10 mol %)
Cu( OAc ) 2, LiCl
DMF, 120ºC, 24 h

+

Z

R 1 R 2 R 1 R

O O

Z = COOR, Ar; R 1 ,R 2 =Alk,Ar

Scheme 9.108 Oxidative arylation by trisubstituted furans.

A r

P d(OAc) 2 (1 0 mol%), bipy
Ag 2 CO 3, KOA c, DMF, 100°C

Ar

N

+

N

Scheme 9.109 Alkenylation of indolizines in the presence of a Pd complex of 2,2 0 -
bipyridyl ligand.