electrochemical reduction of the corresponding salts. Interestingly, 3,000 ppm of
metal within these NPs was sufficient to catalyze Heck couplings of bromides
under close to the same conditions as used for Au-PdNPs-1 (entry 2). The authors
did not perform control experiments with only Au NPs or Pd NPs; hence, the
need for a bimetallic system is still an open question. Experiments that test
the extent of leaching of metals, however, were carried out to establish the
heterogeneous nature of these reactions.
De Souza et al. [80] have used 3 mol% (30,000 ppm) gold nanoparticles
supported on silica (Au/SiO 2 ) for copper-free Sonogashira couplings of iodobenzene
with phenylacetylene under microwave conditions in DMF (entry 3). This educt
gave an 87% yield of the alkyne within 1 h, while bromobenzene afforded a 61%
yield and required a longer reaction time (3.5 h). Recently, Nafria et al. [81] used
gold nanoparticles supported on Pd 2 -Sn nanorods (Au/Pd 2 -Sn NRs), prepared by
growing gold dots on nanorods for Sonogashira coupling between iodobenzene
and phenylacetylene in hot DMF (entry 4). Apart from the desired product, both
partial and complete reduction of the initially coupled product alkyne was observed,
where the hydrogen for reduction was coming from DMF.
Three-component couplings that rely on gold NPs involve 1-pot, 2-bond (C-C/
C-N) formations leading to highly valued propargylamines [82]. As examples,
this reaction represents a key step in the synthesis of an intermediate toward the
naamine family of natural products [83], as well as a lactone intermediate used
to prepare other natural products (Fig. 33, left) [84]. The mechanism of this
reaction is believed to be similar to that of other metal-catalyzed A
3 couplings.
Aurophilic interaction of Au NPs with the alkyne leads to formation of a
Au-acetylide complex, which then attacks an iminium ion formed from the amine
and aldehyde to give the desired product, releasing the Au NPs. In the process,
a molecule of water is lost (Fig. 33, right).
N
OCH 3
R 1
R 2
R 3
R 4
Naamine family
natural products
N
TBDPSO
OMOM
lactone
intermediate
OTBS
R 1 CHO
R 2
N
H
R 4
R 3
R 1
N
R 3
R 4
R 2
Au NPs
Au NPs
R 2
H
R 1 N
R 4
R 3
- OH
H 2 O
R 2
Au NPs
R 1 N +
R 4
R 3
Fig. 33 Targeted products using Au NPs (left) and a proposed mechanism of A
3
-couplings (right)
Earth-Abundant and Precious Metal Nanoparticle Catalysis
107
metal within these NPs was sufficient to catalyze Heck couplings of bromides
under close to the same conditions as used for Au-PdNPs-1 (entry 2). The authors
did not perform control experiments with only Au NPs or Pd NPs; hence, the
need for a bimetallic system is still an open question. Experiments that test
the extent of leaching of metals, however, were carried out to establish the
heterogeneous nature of these reactions.
De Souza et al. [80] have used 3 mol% (30,000 ppm) gold nanoparticles
supported on silica (Au/SiO 2 ) for copper-free Sonogashira couplings of iodobenzene
with phenylacetylene under microwave conditions in DMF (entry 3). This educt
gave an 87% yield of the alkyne within 1 h, while bromobenzene afforded a 61%
yield and required a longer reaction time (3.5 h). Recently, Nafria et al. [81] used
gold nanoparticles supported on Pd 2 -Sn nanorods (Au/Pd 2 -Sn NRs), prepared by
growing gold dots on nanorods for Sonogashira coupling between iodobenzene
and phenylacetylene in hot DMF (entry 4). Apart from the desired product, both
partial and complete reduction of the initially coupled product alkyne was observed,
where the hydrogen for reduction was coming from DMF.
Three-component couplings that rely on gold NPs involve 1-pot, 2-bond (C-C/
C-N) formations leading to highly valued propargylamines [82]. As examples,
this reaction represents a key step in the synthesis of an intermediate toward the
naamine family of natural products [83], as well as a lactone intermediate used
to prepare other natural products (Fig. 33, left) [84]. The mechanism of this
reaction is believed to be similar to that of other metal-catalyzed A
3 couplings.
Aurophilic interaction of Au NPs with the alkyne leads to formation of a
Au-acetylide complex, which then attacks an iminium ion formed from the amine
and aldehyde to give the desired product, releasing the Au NPs. In the process,
a molecule of water is lost (Fig. 33, right).
N
OCH 3
R 1
R 2
R 3
R 4
Naamine family
natural products
N
TBDPSO
OMOM
lactone
intermediate
OTBS
R 1 CHO
R 2
N
H
R 4
R 3
R 1
N
R 3
R 4
R 2
Au NPs
Au NPs
R 2
H
R 1 N
R 4
R 3
- OH
H 2 O
R 2
Au NPs
R 1 N +
R 4
R 3
Fig. 33 Targeted products using Au NPs (left) and a proposed mechanism of A
3
-couplings (right)
Earth-Abundant and Precious Metal Nanoparticle Catalysis
107