6.1 Transfer Hydrogenation of α,β-Unsaturated Aldehydes
A variety of α,β-unsaturated aldehydes (Scheme 50) have been reduced employing
iridium compounds as the catalysts (Scheme 51).
Compound 57a catalyses the chemoselective reduction of cinnamaldehyde to the
corresponding alcohol in 90% yield, in water, using HCOOH (5 equiv) as a
hydrogen donor [113].
Under neutral conditions, complex 79 is an efficient catalyst for chemoselective
TH of unsaturated aldehydes using iPrOH as a solvent and a hydrogen donor
[198]. Cinnamaldehydes F1, F2, F4, F11-F13, F22 and F23, 3-(9-anthryl)
acrylaldehyde, 3-(2-furyl)acrylaldehyde and aliphatic α,β-unsaturated aldehydes
F29 and F30 were reduced in 80–97% yield. A metal-ligand bifunctional mechanism
was proposed for the catalytic TH reaction [198].
The polypyridyl iridium(III) 67a in EtOH/H 2 O, at 100
C, at 0.2 mol% of catalyst
loading, in the presence of 4.5 equiv. of HCOONa, reduced cinnamaldehyde. A 1/1
mixture of the allylic and saturated alcohol was obtained. However, using 9 equiv. of
HCOONa (initial pH of the reaction, 9) full reduction to the saturated alcohol was
achieved. Similar results were obtained for α-methyl cinnamaldehyde and
4-methoxy cinnamaldehyde. Alternatively, when HCOOH was used to maintain
acidic the reaction medium, unsaturated alcohol was predominantly obtained [179].
The Luo’s group investigated the TH of a wide variety of α,β-unsaturated
aldehydes, including cinnamaldehydes F1, F3, F6- F11 and F13-F23, furansubstituted aldehyde F27 and aliphatic aldehydes F31-F35, mediated by catalyst
55 h. All the reactions were carried out at 0.1 mol% of catalyst loading, in H 2 O at
80
C. Nonetheless, a set of reactions was performed using HCOONa (5 equiv) as a
hydrogen donor, and another set was carried out with HCOOH (5 equiv). Full
reduction to the saturated alcohol was achieved with HCOONa, and semi-reduction
to the unsaturated alcohol was obtained with the acid. Typically, yields higher than
90% were achieved, in both cases [199]. Control experiments showed that
3-phenylpropanal was reduced to the saturated alcohol under the same conditions,
whereas the C¼C bond of cinnamyl alcohol was not reduced by complex 55h, when
using HCOONa as a hydrogen donor. These results indicated that the formation of
the fully reduced product starts with the reduction of the C¼C bond to give the
saturated aldehyde intermediate and follows with the reduction of the C¼O bond to
give the final product [199].
6.2 Transfer Hydrogenation of α,β-Unsaturated Ketones
A range of α,β-unsaturated ketones (Scheme 52) have been reduced employing
iridium catalysts (Scheme 53).
The dimer [Cp*IrCl 2 ] 2 chemoselectively reduced α,β-unsaturated ketones such as
(E)-chalcones G1-G3, G6, G8-G17, G21, and G28, heterocyclic α-enones G29 and
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
119
A variety of α,β-unsaturated aldehydes (Scheme 50) have been reduced employing
iridium compounds as the catalysts (Scheme 51).
Compound 57a catalyses the chemoselective reduction of cinnamaldehyde to the
corresponding alcohol in 90% yield, in water, using HCOOH (5 equiv) as a
hydrogen donor [113].
Under neutral conditions, complex 79 is an efficient catalyst for chemoselective
TH of unsaturated aldehydes using iPrOH as a solvent and a hydrogen donor
[198]. Cinnamaldehydes F1, F2, F4, F11-F13, F22 and F23, 3-(9-anthryl)
acrylaldehyde, 3-(2-furyl)acrylaldehyde and aliphatic α,β-unsaturated aldehydes
F29 and F30 were reduced in 80–97% yield. A metal-ligand bifunctional mechanism
was proposed for the catalytic TH reaction [198].
The polypyridyl iridium(III) 67a in EtOH/H 2 O, at 100
C, at 0.2 mol% of catalyst
loading, in the presence of 4.5 equiv. of HCOONa, reduced cinnamaldehyde. A 1/1
mixture of the allylic and saturated alcohol was obtained. However, using 9 equiv. of
HCOONa (initial pH of the reaction, 9) full reduction to the saturated alcohol was
achieved. Similar results were obtained for α-methyl cinnamaldehyde and
4-methoxy cinnamaldehyde. Alternatively, when HCOOH was used to maintain
acidic the reaction medium, unsaturated alcohol was predominantly obtained [179].
The Luo’s group investigated the TH of a wide variety of α,β-unsaturated
aldehydes, including cinnamaldehydes F1, F3, F6- F11 and F13-F23, furansubstituted aldehyde F27 and aliphatic aldehydes F31-F35, mediated by catalyst
55 h. All the reactions were carried out at 0.1 mol% of catalyst loading, in H 2 O at
80
C. Nonetheless, a set of reactions was performed using HCOONa (5 equiv) as a
hydrogen donor, and another set was carried out with HCOOH (5 equiv). Full
reduction to the saturated alcohol was achieved with HCOONa, and semi-reduction
to the unsaturated alcohol was obtained with the acid. Typically, yields higher than
90% were achieved, in both cases [199]. Control experiments showed that
3-phenylpropanal was reduced to the saturated alcohol under the same conditions,
whereas the C¼C bond of cinnamyl alcohol was not reduced by complex 55h, when
using HCOONa as a hydrogen donor. These results indicated that the formation of
the fully reduced product starts with the reduction of the C¼C bond to give the
saturated aldehyde intermediate and follows with the reduction of the C¼O bond to
give the final product [199].
6.2 Transfer Hydrogenation of α,β-Unsaturated Ketones
A range of α,β-unsaturated ketones (Scheme 52) have been reduced employing
iridium catalysts (Scheme 53).
The dimer [Cp*IrCl 2 ] 2 chemoselectively reduced α,β-unsaturated ketones such as
(E)-chalcones G1-G3, G6, G8-G17, G21, and G28, heterocyclic α-enones G29 and
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
119