Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 396

OO OO O O

Cl Cl Cl

N N N N N N

NPdN NPdN NPdN

Cl Cl Cl

(11) (12) (13)

Scheme 9.58 Palladium complexes with macromolecular chelators (11-13)usedas
precatalysts for Mizoroki-Heck reactions of chloro- and bromoarenes.

Ph

Ph

N N

N N

CF 3

N

Ph

O N Pd N N Pd N O

O

Ph

N Ph

N

N

Ph

(14) (15)

Scheme 9.59 Heteroleptic complex (14) and homoleptic complex (15) used as
precatalysts by Lee and co-workers. 221

bromobenzene reacted with standard olefins in the presence of such com-
plexes (0.1-0.01 mol% Pd, Et 3 NorK 2 CO 3 ,Bu 4 NBr, DMF, 140 1C, 2-72 h) to
give good to high yields (56-93% for PhBr) or modest to high yields (20-89%,
for PhCl) of the respective products. TON and TOF values for PhBr were up to
7300 and 600 h 1 and for PhCl up to 6800 and 120 h 1 , respectively, re-
vealing the good activity and stability of systems based on such precatalysts.
A further important achievement in the design of precatalysts was re-
ported by Lee and co-workers. 221 They created a series of sophisticated
heteroleptic complexes containing two N,N-bidentate ligands - one based on
a heterocyclic carbene with attached flexible amide arm being strongly
bonded, and the other being more weakly bonded rigid 2-(3-pyrazolyl)pyr-
idine (Scheme 9.59).
This complex, if used in a system with an ionic liquid medium (0.1-
0.5 mol% Pd, NaOAc, Bu 4 NBr, 140 1C), provides an ecient precatalyst for
the arylation of standard olefins by aryl bromides and chlorides, including
electron-rich and hindered 2,6-disubstituted compounds, with good to high
yields. Similar results (a slightly lower yield in half the time) were obtained
with homoleptic complexes (the trans-complex 15 is shown; the cis-isomer
exhibited the same performance) of the same ligand 222 (Scheme 9.60).
This system could even be considered as a true ligand-accelerated case
with heterocyclic carbene as a stable ancillary, but the behaviour - high
reaction temperature and induction period revealing a characteristic pre-
activation phenomenon - indicates

that the complex works as a

typical SRPC.