4 Chiral N-Heterocyclic Carbene-Modified Nanoparticle
Catalysts
N-Heterocyclic carbenes (NHCs) have been widely studied not only as ligands for
metal complexes and organocatalysts but also as stabilizers for metal nanoparticles
because of their strong surface binding [98, 99].
In 2010, Glorius et al. reported the first successful application of chiral NHC for
metal nanoparticle-catalyzed asymmetric reactions [100]. Chiral NHC 25 bearing
hydroxy groups was utilized to modify Pd nanoparticles supported on magnetite
(Fe 3 O 4 /Pd) (Scheme 34), and this system showed good catalytic performance for
asymmetric α-arylation of ketones with aryl halides (Scheme 35). Various types of
cyclic ketones and aryl halides could be employed to afford the corresponding
α-arylated ketones in moderate enantioselectivities. Intramolecular α-arylation reactions of amide substrates could also proceed smoothly, and relatively high
enantioselectivities were observed (Scheme 36). In the latter reaction (when
X ¼ Cl, R
1
¼ Bn, R
2
¼ Et, Ar ¼ Ph), the corresponding homogeneous Pd complex
catalyst system (Pd(allyl)Cl) 2 and chiral NHC 25) gave a poor result (32% yield,
23% ee), indicating that an activity principle of the chiral nanoparticle system is
different from that of a homogeneous complex system. The heterogeneous nature of
the active species was further confirmed by a hot filtration test, a mercury-poisoning
test, and trace-metal analysis by ICP-OES. The paramagnetic catalyst could be
recovered using a magnet and reused for five cycles without significant loss of
activity and selectivity.
Several characterization of the obtained NHC-modified Pd catalyst (Fe 3 O 4 /Pd/25)
was conducted. XPS analysis confirmed that zero valency of Pd was maintained
even after the surface modification. Attenuated total reflection infrared spectroscopy
(ATR-IR) analysis could detect the NHC-modified surface by comparison of the
spectra obtained from the free salt 25 and that of Fe 3 O 4 /Pd/25, and marked differences were observed. Different BET surfaces were obtained for Fe 3 O 4 , Fe 3 O 4 /Pd,
and Fe 3 O 4 /Pd/25.
Scheme 33 Tao-Phos 24-Cu nanoparticles catalyzed asymmetric Huisgen alkyne-azide cycloaddition of bisalkynes
Chiral Metal Nanoparticles for Asymmetric Catalysis
299
Catalysts
N-Heterocyclic carbenes (NHCs) have been widely studied not only as ligands for
metal complexes and organocatalysts but also as stabilizers for metal nanoparticles
because of their strong surface binding [98, 99].
In 2010, Glorius et al. reported the first successful application of chiral NHC for
metal nanoparticle-catalyzed asymmetric reactions [100]. Chiral NHC 25 bearing
hydroxy groups was utilized to modify Pd nanoparticles supported on magnetite
(Fe 3 O 4 /Pd) (Scheme 34), and this system showed good catalytic performance for
asymmetric α-arylation of ketones with aryl halides (Scheme 35). Various types of
cyclic ketones and aryl halides could be employed to afford the corresponding
α-arylated ketones in moderate enantioselectivities. Intramolecular α-arylation reactions of amide substrates could also proceed smoothly, and relatively high
enantioselectivities were observed (Scheme 36). In the latter reaction (when
X ¼ Cl, R
1
¼ Bn, R
2
¼ Et, Ar ¼ Ph), the corresponding homogeneous Pd complex
catalyst system (Pd(allyl)Cl) 2 and chiral NHC 25) gave a poor result (32% yield,
23% ee), indicating that an activity principle of the chiral nanoparticle system is
different from that of a homogeneous complex system. The heterogeneous nature of
the active species was further confirmed by a hot filtration test, a mercury-poisoning
test, and trace-metal analysis by ICP-OES. The paramagnetic catalyst could be
recovered using a magnet and reused for five cycles without significant loss of
activity and selectivity.
Several characterization of the obtained NHC-modified Pd catalyst (Fe 3 O 4 /Pd/25)
was conducted. XPS analysis confirmed that zero valency of Pd was maintained
even after the surface modification. Attenuated total reflection infrared spectroscopy
(ATR-IR) analysis could detect the NHC-modified surface by comparison of the
spectra obtained from the free salt 25 and that of Fe 3 O 4 /Pd/25, and marked differences were observed. Different BET surfaces were obtained for Fe 3 O 4 , Fe 3 O 4 /Pd,
and Fe 3 O 4 /Pd/25.
Scheme 33 Tao-Phos 24-Cu nanoparticles catalyzed asymmetric Huisgen alkyne-azide cycloaddition of bisalkynes
Chiral Metal Nanoparticles for Asymmetric Catalysis
299