Therefore, the combination of both MCR and non-noble nanoparticle catalysis,
as well as neat reaction conditions, represents a synthetic strategy as a means to
“green-up” an approach to functionalized, complex cores. A recent example of
the combination of MCR and copper(I) oxide NPs was described by Sharghi et al.
for construction of functionalized benzofurans [62]. This involved a three-step,
one-pot process, wherein a secondary amine condenses onto a salicylaldehyde,
forming an iminium intermediate in the presence of tetra-n-butylammonium
bromide (TBAB), followed by 1,2-addition of a copper-(NP)-acetylide to form the
corresponding tertiary amine (Fig. 28).
Key to this sequence is the combination of the deprotonation of the phenol along
with activation of the alkyne by the copper nanoparticles. Subsequent 5-exo-dig
cyclization affords the immediate precursor to the final aromatic benzofuran product
(Fig. 29).
Optimization of the catalyst for this transformation led to copper(I) NPs as the
best source of copper, compared to copper(II) salts. The nanoparticles were then
prepared using a route developed by Tang et al. [63, 64]. Screening solvents
showed that the reaction run neat at 100
C gave the best result for the cyclized
product (91% yield) in less than 2 h. Recycling of these nanoparticles for the same
reaction reduced efficiency in product formation by only 8% after five consecutive
runs (90% yield for the first run, 82% for the fifth run). Analysis of the catalyst
showed loss of copper of 7% after five runs. Ultimately, this reaction provides
an example of the combination of two “green” technologies: multicomponent
reactions and low-cost copper NP catalysis.
6 Gold
Heterogeneous gold catalysts as used in organic synthesis have appeared in three
main forms (Fig. 30). These are not only different in appearance and reactivity
but also in their preparation. The first two, metal clusters and Au NPs on a
Fig. 28 Reaction between 2-hydroxybenzaldehyde, a secondary amine, and an alkyne catalyzed by
Cu(I) oxide NPs
Fig. 29 Solvent-free synthesis of substituted benzofurans catalyzed by Cu(I) oxide NPs
Earth-Abundant and Precious Metal Nanoparticle Catalysis
103
as well as neat reaction conditions, represents a synthetic strategy as a means to
“green-up” an approach to functionalized, complex cores. A recent example of
the combination of MCR and copper(I) oxide NPs was described by Sharghi et al.
for construction of functionalized benzofurans [62]. This involved a three-step,
one-pot process, wherein a secondary amine condenses onto a salicylaldehyde,
forming an iminium intermediate in the presence of tetra-n-butylammonium
bromide (TBAB), followed by 1,2-addition of a copper-(NP)-acetylide to form the
corresponding tertiary amine (Fig. 28).
Key to this sequence is the combination of the deprotonation of the phenol along
with activation of the alkyne by the copper nanoparticles. Subsequent 5-exo-dig
cyclization affords the immediate precursor to the final aromatic benzofuran product
(Fig. 29).
Optimization of the catalyst for this transformation led to copper(I) NPs as the
best source of copper, compared to copper(II) salts. The nanoparticles were then
prepared using a route developed by Tang et al. [63, 64]. Screening solvents
showed that the reaction run neat at 100
C gave the best result for the cyclized
product (91% yield) in less than 2 h. Recycling of these nanoparticles for the same
reaction reduced efficiency in product formation by only 8% after five consecutive
runs (90% yield for the first run, 82% for the fifth run). Analysis of the catalyst
showed loss of copper of 7% after five runs. Ultimately, this reaction provides
an example of the combination of two “green” technologies: multicomponent
reactions and low-cost copper NP catalysis.
6 Gold
Heterogeneous gold catalysts as used in organic synthesis have appeared in three
main forms (Fig. 30). These are not only different in appearance and reactivity
but also in their preparation. The first two, metal clusters and Au NPs on a
Fig. 28 Reaction between 2-hydroxybenzaldehyde, a secondary amine, and an alkyne catalyzed by
Cu(I) oxide NPs
Fig. 29 Solvent-free synthesis of substituted benzofurans catalyzed by Cu(I) oxide NPs
Earth-Abundant and Precious Metal Nanoparticle Catalysis
103