Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 432

N

Pd(OAc) 2 (10 mol%)
O 2, Bu 4 NBr, toluene-DMSO, 70°C, 12 h

Ar

A r

H

H

46-78%

sp 3 CH activation in an intramolecular oxidative Heck reaction.

Scheme 9.119

In addition to palladium, the oxidative alkenylation of arenes is also ef-
fectively catalysed by ruthenium complexes, reported first by Milstein and
co-workers 359 and comprehensively reviewed recently by Kozhushkov and
Ackermann. 360 Rhodium complexes are also known as catalysts for this re-
action, although in most cases only directed processes take place; however,
Glorius and co-workers recently reported a case of non-directed oxidative
alkenylation of bromobenzenes in the presence of Rh(III) catalyst. 361 The
bromine atoms in the substrates in this case take part in specific activation
of the rhodium catalyst, but play no role in directing the reaction.

9.3.1.1 Directed Fujiwara-Moritani Reaction
Palladation is well known to be directed by various proximal donor groups -
this phenomenon is the basis of popular and well-developed palladacycle
chemistry. 362 Palladacycles are well known to be reactive in reactions with
olefins under the conditions of the Mizoroki-Heck reaction - which is most
probably the process to account for palladacycle disassembly and release of
free catalytically active Pd species, and the operation of palladacycles as
precatalysts in Mizoroki-Heck reactions. 363,364 Seen from the other side,
these transformations account for the directive effect of various groups to
make the Fujiwara-Moritani reaction in such cases highly regioselective.
Moreover, directed palladation takes place at both electron-rich and elec-
tron-deficient aromatic rings, 365 hence the limitations of undirected palla-
dation are lifted.
It should be noted that the mechanism of directed palladation is likely to
involve a concerted attack of the Pd centre at an aromatic carbon with
simultaneous (but not necessarily synchronized) abstraction of proton either
by external base or a ligand, similar to what takes place in non-directed
palladation (see below). As in any other concerted reaction, electronic pref-
erences (nucleophile-electrophile dichotomy) of reactants are not strictly
set, but adapt themselves to a given situation, the nature of substituents,
ligands, etc.
An exemplary case of such substituent indiscriminacy can be found in e.g.
a detailed study devoted to the Fujiwara-Moritani reaction of phenylacetic
acids with terminal olefins reported by Yu and co-workers, which takes place
in a system containing N-acylamino acids as ligands, weak base KHCO 3 and
O 2 as terminal oxidant. 366 Both electron-donating and electron-withdrawing
substituents can be present in phenylacetic acid to afford the products in
high yields under the same conditions(Scheme 9.120).