Greener Approaches to Cross-Coupling - New Trends in Cross-Coupling: Theory and Applications - page 654

bromides in water at 100 1C. 38 The catalyst can be precipitated by addition of
acid and reused in four cycles with a slow degradation of the reaction yield.
Reusing the catalyst-containing aqueous layer gave improved catalyst re-
cycling, particularly when TBAB was used as an additive. Very high catalyst
productivity in the Suzuki coupling of aryl bromides was obtained with a
palladium-pyridyl-bis(NHC) pincer complex (16). 39 Catalyst loadings as low
as 8 ppm provided quantitative conversion of 4-bromoacetophenone to
product. The aqueous catalyst solution could be used for three reaction
cycles with no loss in activity. Attempted poisoning with mercury and
polyvinylpyridine showed no effect on catalyst activity, which suggests that
the active species is a molecular catalyst, rather than heterogeneous
palladium particles. This catalyst effectively catalyzed the Heck coupling of
aryl iodides (0.005 mol% Pd). 40
NHC ligands supported on neutral hydrophilic polyether and polyol
derivatives provide active catalysts for cross-coupling reactions in water.
An N-PEG-substituted imidazolium salt in combination with Pd(OAc) 2
(0.5 mol%) afforded high yields in the Suzuki coupling of aryl bromides in
5-15 min. 41 Good yields were obtained in Suzuki couplings of aryl bromides
using a bis(NHC)-palladium catalyst supported on hyperbranched poly-
glycerol (17, Scheme 14.14). 42 The catalyst could be separated by dialysis
with approximately 80% recovery. The catalyst was successfully used for four
reaction cycles with 497% yield, but further recycling was not possible
owing to the loss of catalyst in each dialysis cycle.
14.2.1.3 Palladium-Catalyzed Cross-Coupling with Hydrophilic
Nitrogen Ligands
Nitrogen-based ligands have received less attention than phosphines or
NHCs in palladium-catalyzed cross-coupling reactions. The lower cost of
nitrogen-based ligands compared with phosphines makes them attractive
alternatives. Hydrophilic versions of nitrogen ligands have been reported to
be effective in aqueous-phase cross-coupling reactions. 4,4 0 -Trimethyl-
ammonium-2,2 0 -bipyrdine (18) in combination with palladium provides
active catalysts for Suzuki, 43 Hiyama 44 and Heck 45 couplings of aryl brom-
ides and iodides (Scheme 14.15). Recycling of the aqueous catalyst solution
was demonstrated for the Suzuki coupling of aryl bromides at 80 1C. Good
yields were obtained for five reaction cycles, but the reaction time increased
with each cycle, indicating a decrease in catalytic activity. Alkylammonium-
substituted 2-(2-pyridyl)imidazole-palladium complex 19 (Scheme 14.16)
catalyzes the Suzuki coupling of aryl bromides and chlorides in TBAB-water
at low catalyst loading (0.1 mol%), but high temperature (120 1C). 46 TEM
analysis showed that palladium nanoparticles form during the reaction,
which the authors suggested are stabilized by the hydrophilic ligand.
Sulfonated aniline derivatives are readily available and provide easy access
to sulfonated imine-based ligand systems. Excellent yields for Suzuki
couplings were obtained using sulfonated b-ketoimine and b-diimine