A bis-cyclometallated iridium(III) complex (74, Scheme 44) with metal-centred
chirality catalyses the enantioselective TH of ketones in the presence of an additional
pyrazole ligand. Reactions were carried out in THF/H 2 O 1/1 (v/v) mixtures, at 40 or
60
C, with HCOONH 4 (9 equiv) as the hydrogen source. The best results were
obtained by adding to the catalyst precursor 74 (0.2 mol%) 10 equiv. of
5-(4-methoxyphenyl)-3-methyl-1H-pyrazole. Under these reaction conditions, a
Ir
N
S
NCMe
NCMe
Ir
R
1
R
2
R 1 = R 2 = PPh 3 (75a)
R
1 = R
2
= PBn 3 (75b)
R 1 = PBn 3 , R 2 = PPh 3 (75c)
Ir
N
N
P(nBu) 3
Cl
S
73
S
N
74
PF 6
tBu
tBu
Ir
PBn 3
Bn 2 P
H
76
PF 6
PF 6
Ir
Cl
N
N
78
Ir
Cl
N
N
Y
Y
R
Ir
Cl
N
N
R
N
Y
Y
H
H
Benzyl
Benzyl
Mesityl
Mesityl
Pentamethylbenzyl
Pentamethylbenzyl
H
Me
H
Me
H
Me
H
Me
77a
77b
77c
77d
77e
77f
77g
77h
R
Y
Scheme 44 Other iridium complexes employed as TH catalysts (73–78)
HN
NH
HN
NH
HN
NH
OMe
MeO
HN
NH
NH HN
NH
NH 2
Scheme 45 Chiral amino ligands employed in combination with [IrCl(COD)] 2 in the TH of
ketones
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
115