8 Earth-Abundant d-Block Metal Nanocatalysis for Coupling …
269
[114]. Notably, the cyclization only takes place in an intramolecular fashion and no
Cu leaching was detected in the EG reaction medium after catalyst filtration (ICP
analyses).
Preformed Cu 2 O nanoparticles (mean diameter: ca. 5 nm, Fig. 8.9) were applied
in the coupling between iodoaryl derivatives and different nitrogen-based reagents,
such as aryl and alkyl amines, but also aqueous ammonia, in glycerol [24]. This
nanocatalyst was also efficient for the synthesis of thioethers through C–S couplings.
This catalytic system has also been applied for the activation of terminal
alkyne groups toward azide–alkyne cycloaddition reaction (CuAAC) in glycerol
(Scheme 8.17) [24].
Furthermore, Sharghi and Aberi reported the application of Cu 2 O NPs in the
synthesis of indazole derivatives through a three-component strategy in PEG300
(Scheme 8.18) [115].
Cu-based catalysts in polyol medium have also found interesting applications in
C–S bond formation processes, as proved by our group using Cu 2 O nanoparticles
[24]. Primo, García, and coworkers reported the synthesis of Cu-based nanoparticles
stabilized by chitosan, mainly constituted of Cu(0) but surrounded by a thin layer
of copper oxides as proven by XPS [49]. This nanocatalyst was applied in the C–
S coupling of aryl halides and thiophenol, being more active for iodo than bromo
and chloro arenes, for the latter ones they were only active for aryl halides containing electron withdrawing substituents (Scheme 8.19). Authors observed that halide
anions released to the medium during the catalytic reaction can act as poison for
CuNPs, in addition to promoting metal leaching.
Generally speaking, C–O couplings are more challenging reactions mainly due
to the limited functional compatibilities and the need of activated substrates [77,
118]. Besides, the use of polyol medium adds another difficulty because the solvent
can compete with the substrate. In this frame, Biegi and Ghiasbeigi have recently
reported the synthesis and full characterization of functionalized magnetite with
isonicotinic acid with the aim of coordinating Cu(I) precursors (Fig. 8.10) [53]. The
resulting catalytic material was efficiently applied to the synthesis of phenol and
aniline derivatives (TOF up to 4494 h
−1 ), using a mixture of PEG and water as
solvent (PEG:H 2 O = 2:1) (Scheme 8.20). The catalytic phase was recycled up to 5
times with slight loss of yield, although the 30% copper loss reported by the authors
after the fourth run was substantial (Cu content of catalyst before use: 57,037.8 ppm;
after 4th run: 40,297.3 ppm).
8.4 Conclusions and Outlook
This chapter describes the use of nanocatalysts from Earth-abundant metals in polyol
media applied in C–C and C–heteroatom coupling reactions. Despite the interesting reports mentioned in this contribution, nanocatalysis based on d-block transition
metals in polyols is still in its infancy. This is an exponentially growing research
269
[114]. Notably, the cyclization only takes place in an intramolecular fashion and no
Cu leaching was detected in the EG reaction medium after catalyst filtration (ICP
analyses).
Preformed Cu 2 O nanoparticles (mean diameter: ca. 5 nm, Fig. 8.9) were applied
in the coupling between iodoaryl derivatives and different nitrogen-based reagents,
such as aryl and alkyl amines, but also aqueous ammonia, in glycerol [24]. This
nanocatalyst was also efficient for the synthesis of thioethers through C–S couplings.
This catalytic system has also been applied for the activation of terminal
alkyne groups toward azide–alkyne cycloaddition reaction (CuAAC) in glycerol
(Scheme 8.17) [24].
Furthermore, Sharghi and Aberi reported the application of Cu 2 O NPs in the
synthesis of indazole derivatives through a three-component strategy in PEG300
(Scheme 8.18) [115].
Cu-based catalysts in polyol medium have also found interesting applications in
C–S bond formation processes, as proved by our group using Cu 2 O nanoparticles
[24]. Primo, García, and coworkers reported the synthesis of Cu-based nanoparticles
stabilized by chitosan, mainly constituted of Cu(0) but surrounded by a thin layer
of copper oxides as proven by XPS [49]. This nanocatalyst was applied in the C–
S coupling of aryl halides and thiophenol, being more active for iodo than bromo
and chloro arenes, for the latter ones they were only active for aryl halides containing electron withdrawing substituents (Scheme 8.19). Authors observed that halide
anions released to the medium during the catalytic reaction can act as poison for
CuNPs, in addition to promoting metal leaching.
Generally speaking, C–O couplings are more challenging reactions mainly due
to the limited functional compatibilities and the need of activated substrates [77,
118]. Besides, the use of polyol medium adds another difficulty because the solvent
can compete with the substrate. In this frame, Biegi and Ghiasbeigi have recently
reported the synthesis and full characterization of functionalized magnetite with
isonicotinic acid with the aim of coordinating Cu(I) precursors (Fig. 8.10) [53]. The
resulting catalytic material was efficiently applied to the synthesis of phenol and
aniline derivatives (TOF up to 4494 h
−1 ), using a mixture of PEG and water as
solvent (PEG:H 2 O = 2:1) (Scheme 8.20). The catalytic phase was recycled up to 5
times with slight loss of yield, although the 30% copper loss reported by the authors
after the fourth run was substantial (Cu content of catalyst before use: 57,037.8 ppm;
after 4th run: 40,297.3 ppm).
8.4 Conclusions and Outlook
This chapter describes the use of nanocatalysts from Earth-abundant metals in polyol
media applied in C–C and C–heteroatom coupling reactions. Despite the interesting reports mentioned in this contribution, nanocatalysis based on d-block transition
metals in polyols is still in its infancy. This is an exponentially growing research