of TH (or ATH) of different C ¼ X (X ¼ C, N, O) and C C bonds catalysed by
iridium complexes using either alcohols or formic acid as the hydrogen donor.
In 2015 Xiao [212] explored the reaction mechanism of the TH of the C¼N bond
in N-heterocycles using κ
2 C,N-iridacycle catalysts and formic acid as the hydrogen
source. Using 2-methylquinoline as the model substrate, stoichiometric assays as
well as isotopic-labelling experiments allowed shedding light on the main steps of
the TH reaction (Scheme 74). Indeed, the authors clearly demonstrated that the ratelimiting step is the hydrogen transfer from the metal hydride to the substrate.
Furthermore, the occurrence of both the 1,2-addition (Scheme 74, bottom) and the
1,4-addition of hydrogen (Scheme 74, top) to the H-heterocycle was proved.
Also, Xiao and Catlow [240] have described the remarkable activity of κ
2 C,Niridacycles in reducing imino groups by TH with formic acid. In this case, based on a
combination of kinetic measurements, crystallographic studies and DFT calculations, the metal hydride route depicted in Scheme 75 was proposed, pointing out that
the rate-limiting step is the hydride formation rather than the hydrogen transfer to the
protonated substrate.
Stirling [241] reported the ATH of imines using formic acid as the hydrogen
source (in the presence of NEt 3 ) using an iridium CATHy catalyst (Scheme 76). On
one hand, under the experimental conditions, the imine is expected to be protonated
rendering the iminium ion, and, on the other one, the observed KIEs as well as the
NH 2
NH 2
Ir
O
X
O
NH 2
Ir
O
X
O
H
H
H H
H
H
H
H
H
H
NH 2
Ir
O
X
O
H
H
H
H
H
Ir
O
X
H
H
H
NH 2
Ir
O
X
H
H
H
NH
Ir
X
H
H
O
H H
NH 2
Ir
O
X
H
H
H
H 2
N
Ir
X
H
H
N
Ir
X
X = OH, SCH 3
+ CH 3 OH
+ CH 3 OH
+ CH 2 O
X = OH, SCH 3
- CH 2 O
Scheme 73 Intermediates and transition states for the TH of carbonyl compounds catalysed by
Ir(COD)(HL) (HL ¼ 2-amino ethanol, 2-aminoethylmethylsulfide) [239]
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
137
iridium complexes using either alcohols or formic acid as the hydrogen donor.
In 2015 Xiao [212] explored the reaction mechanism of the TH of the C¼N bond
in N-heterocycles using κ
2 C,N-iridacycle catalysts and formic acid as the hydrogen
source. Using 2-methylquinoline as the model substrate, stoichiometric assays as
well as isotopic-labelling experiments allowed shedding light on the main steps of
the TH reaction (Scheme 74). Indeed, the authors clearly demonstrated that the ratelimiting step is the hydrogen transfer from the metal hydride to the substrate.
Furthermore, the occurrence of both the 1,2-addition (Scheme 74, bottom) and the
1,4-addition of hydrogen (Scheme 74, top) to the H-heterocycle was proved.
Also, Xiao and Catlow [240] have described the remarkable activity of κ
2 C,Niridacycles in reducing imino groups by TH with formic acid. In this case, based on a
combination of kinetic measurements, crystallographic studies and DFT calculations, the metal hydride route depicted in Scheme 75 was proposed, pointing out that
the rate-limiting step is the hydride formation rather than the hydrogen transfer to the
protonated substrate.
Stirling [241] reported the ATH of imines using formic acid as the hydrogen
source (in the presence of NEt 3 ) using an iridium CATHy catalyst (Scheme 76). On
one hand, under the experimental conditions, the imine is expected to be protonated
rendering the iminium ion, and, on the other one, the observed KIEs as well as the
NH 2
NH 2
Ir
O
X
O
NH 2
Ir
O
X
O
H
H
H H
H
H
H
H
H
H
NH 2
Ir
O
X
O
H
H
H
H
H
Ir
O
X
H
H
H
NH 2
Ir
O
X
H
H
H
NH
Ir
X
H
H
O
H H
NH 2
Ir
O
X
H
H
H
H 2
N
Ir
X
H
H
N
Ir
X
X = OH, SCH 3
+ CH 3 OH
+ CH 3 OH
+ CH 2 O
X = OH, SCH 3
- CH 2 O
Scheme 73 Intermediates and transition states for the TH of carbonyl compounds catalysed by
Ir(COD)(HL) (HL ¼ 2-amino ethanol, 2-aminoethylmethylsulfide) [239]
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
137