Greener Approaches to Cross-Coupling - New Trends in Cross-Coupling: Theory and Applications - page 676

gave lower yields. The catalyst was poorly recyclable since the palladium
nanoparticles were not well stabilized by the PEG. Similar results were seen
in a Pd(OAc) 2 -catalyzed Suzuki coupling using PEG as a co-solvent. 162 The
best results were obtained with water as a co-solvent, whereas organic
solvents gave much lower yields. Higher molecular weight PEG-2000 gave
better results than PEG-600. In the Suzuki coupling of aryl bromides cata-
lyzed by palladium supported on BaSO 4 , good yields could be obtained in
PEG-300 at 80 1C, but the reaction could be carried out at room temperature
with 40% aqueous PEG 300. 163 The catalyst was used for three reaction cycles
with no decrease in product yield. Good activity using PEG-300 alone as
solvent was achieved in the Suzuki coupling of aryl iodides using 0.1 mol%
Pd 2 (dba) 3 , but aryl bromides and chlorides gave low conversion. 164

14.3.2 Glycerol
Glycerol is a widely used starting material in the drug, food, beverage and
chemical industries. It is also a by-product produced in biodiesel pro-
duction, which has led to an oversupply on world markets. As a result, there
is interest in finding alternative uses for glycerol. Like water, glycerol has
many attractive properties as a green solvent. 165 Glycerol is essentially non-
toxic, non-flammable and non-volatile. It is also a relatively cheap and
renewable resource as a by-product of biodiesel production. It has similar
polarity and hydrogen bonding properties to water, which allows it to be
used in biphasic reaction media. The high viscosity of glycerol along with the
potential reactivity of the alcohol groups are challenges to be overcome in
its use.
Wolfson and Dlugy reported the first study of glycerol as a solvent for
palladium-catalyzed cross-coupling reactions. 166 Suzuki and Heck couplings
of iodobenzene occurred in high yield using Pd(TPPTS) 2 Cl 2 (2 mol%) as the
precatalyst at 80 1C. Although higher initial yields were obtained with
Pd(TPPTS) 2 Cl 2 , the resulting catalyst solution rapidly lost activity when re-
cycled. In contrast, Pd(DPPF)Cl 2 , consistently gave 80% yields in three cycles
of the coupling of iodobenzene and phenylboronic acid. Palladium nano-
particles supported on aminopolysaccharide (AP) is an effective catalyst for
the Heck b,b-diarylation of acrylates in glycerol (Scheme 14.54). 167 The or-
ganic products could be extracted from the glycerol phase using continuous
extraction with supercritical CO 2 . The system showed poor recyclability
owing to the accumulation of by-products in the glycerol catalyst phase.

NH 2

H 2 N I O

Pd/AP (0.9 mol %)
Et 3 N, glycerol
120 °C, 30 h

O

+

H 2 N

O

O

3 equiv

93%

Scheme 14.54