conversion was slower (85%, 24 h), but it was completely selective to benzyl
alcohol. In the absence of a base, other aryl aldehydes such as A24, A29 and A35
were also selectively converted to the corresponding alcohol. Both electron-donating
and electron-withdrawing substituents at the phenyl group of the PYA ligand slow
down the catalysis. In contrast to the behaviour of ketones,
2-pyridinecarboxaldehyde and the aliphatic aldehyde A54 (Scheme 3) were not
hydrogenated in the absence of a base. Probably, under base-free conditions, coordination of the pyridine nitrogen to the iridium centre is favoured thus preventing the
catalytic TH [72].
Chiral amidoiridium complexes 19–21 (Scheme 7) catalyse the asymmetric TH of
acetophenone in 2-propanol, in the absence of a base, with a substrate/catalyst (S/C)
ratio of 200 at 30
C [73]. The mononuclear amido complexes 20 and 21 reached
almost complete conversion within 30 min. The catalytic activity of the dinuclear
complex 19 is remarkably lower. Catalytic rates can be related to the relative ability
of complexes 19–21 to dehydrogenate 2-propanol. The bridging complex 19 is inert
in 2-propanol, but the mononuclear complexes 20 and 21 were completely converted
into diastereomeric mixtures of the amino complexes A and B, respectively, by
addition of excess of 2-propanol (Scheme 8). The (S)-product was obtained in all
cases with an e.r. of about 80/20.
The C carborane -cyclometallated complex 22 and the B carborane -cyclometallated
complexes 23 and 24 reduce acetophenone to 2-phenylethanol, in the presence of
tBuOH, with 2-propanol as a hydrogen source and at a catalyst loading of
0.5–1.0 mol%. Conversions greater than 90% were achieved after 1 h of reaction.
Cyclometallation of the carborane moiety enhances catalysis greatly compared to the
non-cyclometallated counterparts. The B-cyclometallated complex 23 is more active
than the corresponding C-cyclometallated 22 [74].
Cp*Ir(III) complexes (25–27) bearing imidazolium ion-tethered TsDPEN ligands
(Scheme 7) are efficient catalysts for asymmetric TH of α-ketophosphonates B137B148 (Scheme 4) in water. At RT, moderate yields (44–78%, 4–8 h of reaction) and
good to excellent e.r.’s (up to >99.5/0.5) were obtained by using 1 mol% of
[Cp*IrCl 2 ] 2 , 2 mol% of the ligand and HCOONa as hydrogen donor [75].
Half-sandwich iridium complexes bearing pyridinesulfonamide ligands (28–34,
Scheme 7) were assessed as precatalysts for TH of methyl aryl (B1, B3, B10, B15,
B18, B22, B27) diaryl (B66), methyl alkyl (B96, B103) and cyclic (B117-B119)
ketones (Scheme 4) [76]. In general, good conversions are obtained after 3 h of
reaction, in refluxing 2-propanol, at 1 mol% of catalyst loading and in the absence of
a base. With electro-donating groups, on the 4-substituted acetophenone substrate,
the observed conversion after 6 h drops in comparison with substrates with electronwithdrawing substituents. On the other hand, precatalysts bearing electron-rich
substituents on the pyridinesulfonamide ligand (complexes 28, 31 and 33) exhibited
the highest conversion of 4-nitroacetophenone to the corresponding alcohol, while
precatalysts 29, 32 and 34, which possess electron-withdrawing substituents on the
ligand, afforded only moderate conversions. Under the same conditions, trials
conducted using precatalyst 28 reduced benzophenone (82% conversion, 24 h
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
83
alcohol. In the absence of a base, other aryl aldehydes such as A24, A29 and A35
were also selectively converted to the corresponding alcohol. Both electron-donating
and electron-withdrawing substituents at the phenyl group of the PYA ligand slow
down the catalysis. In contrast to the behaviour of ketones,
2-pyridinecarboxaldehyde and the aliphatic aldehyde A54 (Scheme 3) were not
hydrogenated in the absence of a base. Probably, under base-free conditions, coordination of the pyridine nitrogen to the iridium centre is favoured thus preventing the
catalytic TH [72].
Chiral amidoiridium complexes 19–21 (Scheme 7) catalyse the asymmetric TH of
acetophenone in 2-propanol, in the absence of a base, with a substrate/catalyst (S/C)
ratio of 200 at 30
C [73]. The mononuclear amido complexes 20 and 21 reached
almost complete conversion within 30 min. The catalytic activity of the dinuclear
complex 19 is remarkably lower. Catalytic rates can be related to the relative ability
of complexes 19–21 to dehydrogenate 2-propanol. The bridging complex 19 is inert
in 2-propanol, but the mononuclear complexes 20 and 21 were completely converted
into diastereomeric mixtures of the amino complexes A and B, respectively, by
addition of excess of 2-propanol (Scheme 8). The (S)-product was obtained in all
cases with an e.r. of about 80/20.
The C carborane -cyclometallated complex 22 and the B carborane -cyclometallated
complexes 23 and 24 reduce acetophenone to 2-phenylethanol, in the presence of
tBuOH, with 2-propanol as a hydrogen source and at a catalyst loading of
0.5–1.0 mol%. Conversions greater than 90% were achieved after 1 h of reaction.
Cyclometallation of the carborane moiety enhances catalysis greatly compared to the
non-cyclometallated counterparts. The B-cyclometallated complex 23 is more active
than the corresponding C-cyclometallated 22 [74].
Cp*Ir(III) complexes (25–27) bearing imidazolium ion-tethered TsDPEN ligands
(Scheme 7) are efficient catalysts for asymmetric TH of α-ketophosphonates B137B148 (Scheme 4) in water. At RT, moderate yields (44–78%, 4–8 h of reaction) and
good to excellent e.r.’s (up to >99.5/0.5) were obtained by using 1 mol% of
[Cp*IrCl 2 ] 2 , 2 mol% of the ligand and HCOONa as hydrogen donor [75].
Half-sandwich iridium complexes bearing pyridinesulfonamide ligands (28–34,
Scheme 7) were assessed as precatalysts for TH of methyl aryl (B1, B3, B10, B15,
B18, B22, B27) diaryl (B66), methyl alkyl (B96, B103) and cyclic (B117-B119)
ketones (Scheme 4) [76]. In general, good conversions are obtained after 3 h of
reaction, in refluxing 2-propanol, at 1 mol% of catalyst loading and in the absence of
a base. With electro-donating groups, on the 4-substituted acetophenone substrate,
the observed conversion after 6 h drops in comparison with substrates with electronwithdrawing substituents. On the other hand, precatalysts bearing electron-rich
substituents on the pyridinesulfonamide ligand (complexes 28, 31 and 33) exhibited
the highest conversion of 4-nitroacetophenone to the corresponding alcohol, while
precatalysts 29, 32 and 34, which possess electron-withdrawing substituents on the
ligand, afforded only moderate conversions. Under the same conditions, trials
conducted using precatalyst 28 reduced benzophenone (82% conversion, 24 h
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
83