coupled to the desired α-arylated ketones in high yields with high
enantioselectivities in the presence of a chiral modifier 30 or 31 without leaching
of the metals. STEM analysis and EDS mapping revealed that alloy nanoparticles
formed, and aggregation of Rh nanoparticles was not observed in PI/CB
Rh/Ag. PI/CB Rh/Ag could be reused while keeping high yields and high
enantioselectivities for 14 times.
In 2015, Kobayashi et al. improved their chiral Rh nanoparticle systems
by employing a bifunctional chiral diene bearing a secondary amide moiety (Scheme
42) [116]. The new modifier 32 was supposed to possess a diene part for coordination to metal and a secondary amide group to interact with carbonyl substrates
through hydrogen bonding. While the previous chiral diene 30 gave a low
enantioselectivity (33% ee) for an asymmetric 1,4-addition to an α,β-unsaturated
ester, chiral diene 32-modified Rh nanoparticle systems dramatically improve the
yield and enantioselectivity. Control studies using a tertiary amide-substituted chiral
diene that gave a lower enantioselectivity (66% ee) suggested that the secondary
amide moiety in diene 32 played an important role in achieving the excellent yield
and enantioselectivity. Moreover, the amount of a diene loading could be reduced to
0.05–0.1 mol%, and broad substrate scope including formal synthesis of biologically
important compounds was achieved with outstanding enantioselectivities. The Rh
catalyst could be reused with the addition of a new portion of the chiral ligand, and
no significant loss of the catalytic activity and enantioselectivity was observed over
six runs. No metal leaching was confirmed by ICP-AES analysis of the crude
Scheme 41 Chiral diene-modified Rh nanoparticle catalysts for asymmetric 1,4-addition of
arylboronic acids to enones
Scheme 42 Secondary amide-substituted chiral diene 32 modified Rh nanoparticle catalysts for
asymmetric 1,4-addition of arylboronic acids to unsaturated esters
Chiral Metal Nanoparticles for Asymmetric Catalysis
303
enantioselectivities in the presence of a chiral modifier 30 or 31 without leaching
of the metals. STEM analysis and EDS mapping revealed that alloy nanoparticles
formed, and aggregation of Rh nanoparticles was not observed in PI/CB
Rh/Ag. PI/CB Rh/Ag could be reused while keeping high yields and high
enantioselectivities for 14 times.
In 2015, Kobayashi et al. improved their chiral Rh nanoparticle systems
by employing a bifunctional chiral diene bearing a secondary amide moiety (Scheme
42) [116]. The new modifier 32 was supposed to possess a diene part for coordination to metal and a secondary amide group to interact with carbonyl substrates
through hydrogen bonding. While the previous chiral diene 30 gave a low
enantioselectivity (33% ee) for an asymmetric 1,4-addition to an α,β-unsaturated
ester, chiral diene 32-modified Rh nanoparticle systems dramatically improve the
yield and enantioselectivity. Control studies using a tertiary amide-substituted chiral
diene that gave a lower enantioselectivity (66% ee) suggested that the secondary
amide moiety in diene 32 played an important role in achieving the excellent yield
and enantioselectivity. Moreover, the amount of a diene loading could be reduced to
0.05–0.1 mol%, and broad substrate scope including formal synthesis of biologically
important compounds was achieved with outstanding enantioselectivities. The Rh
catalyst could be reused with the addition of a new portion of the chiral ligand, and
no significant loss of the catalytic activity and enantioselectivity was observed over
six runs. No metal leaching was confirmed by ICP-AES analysis of the crude
Scheme 41 Chiral diene-modified Rh nanoparticle catalysts for asymmetric 1,4-addition of
arylboronic acids to enones
Scheme 42 Secondary amide-substituted chiral diene 32 modified Rh nanoparticle catalysts for
asymmetric 1,4-addition of arylboronic acids to unsaturated esters
Chiral Metal Nanoparticles for Asymmetric Catalysis
303