Greener Approaches to Cross-Coupling - New Trends in Cross-Coupling: Theory and Applications - page 675

P t -Bu 2

N Pd(OAc) 2

P t -Bu 2

O

P t -Bu 2

O
5

N Pd(OAc) 2

γ -Fe 2 O 3

H

P t -Bu 2

P t -Bu 2

N Pd(OAc) 2

64

P t -Bu 2

Scheme 14.53

Scheme 14.53) attached to a polymer-coated g-Fe 2 O 3 particle was applied to
the aqueous-phase Sonogashira coupling of aryl bromides. 159 A surfactant
was necessary to achieve high yields in water with Triton-X405 (2.1 wt%)
providing the best results. The catalyst could be recycled for 10 reaction
cycles with a slow decrease in reaction yield. In contrast, in methanol the
product yields decreased precipitously after cycle six. Fe 3 O 4 particles surface
functionalized with imidazolium salts were used to immobilize a palladium
bis(NHC) complex. 160 The catalyst was applied to the Suzuki coupling of aryl
bromides in water-TBAB at 75 1C. The catalyst could be separated mag-
netically and reused for five reaction cycles with minimal decrease in re-
action yield. Palladium leaching was 10 ppm in the first cycle, but decreased
to

1 ppm for subsequent cycles. The relatively high leaching rates may be
due to the fact that the catalyst was not covalently attached to the magnetic
particle.

B

14.3 Other Potentially Green Alternatives to
Traditional Organic Solvents
Replacement of the traditional volatile organic solvents used in organic
synthetic methods represents one of the significant improvements that can
be made to these methods. Water as an alternative to traditional organic
solvents has attracted significant attention because of the beneficial prop-
erties that it offers as a solvent. Although water has garnered the most
attention, other potentially environmentally benign alternative solvents can
be considered. Ideally, these alternative solvents would be of low toxicity,
non-volatile and derived from renewable resources.

14.3.1 Poly(Ethylene Glycol)
Poly(ethylene glycol) (PEG) oligomers are non-toxic, low-volatility liquids that
are approved as food additives. PEG is miscible with water and polar
organics, but immiscible with non-polar organic solvents. PEG is commonly
used as an additive in aqueous-phase cross-coupling reactions to improve
the interaction between water-soluble catalysts and organic substrates.
Mixed PEG-400-water solvents with PEG:water ratios ranging from 2 : 1 to
1 : 2 give optimal results in the PdCl 2 -catalyzed Suzuki coupling of aryl
bromides at room temperature. 161 Water alone or PEG-400 alone as solvent