give 97% yield. After this time, maintaining the reaction conditions, hydrogenation
of the C¼O groups gave the saturated alcohol in 63% yield within 4 h [69].
6.3 Transfer Hydrogenation of Non-conjugated Unsaturated
Aldehydes and Ketones
A handful of non-conjugated unsaturated aldehydes and ketones (Scheme 54) have
been subjected to TH protocols based on iridium catalysts (Scheme 55).
In iPrOH, under a N 2 atmosphere, non-conjugated unsaturated aldehydes
3-cyclohexene-1-carboxaldehyde and 5-norbornadiene-2-carboxaldehyde were
chemoselectively converted into the corresponding unsaturated alcohols in 93 and
97% yield using the half-sandwich bipyridonate complex 79 (2 mol%) [198].
The non-conjugated ketone 5-hexen-2-one was chemoselectively transformed
into 5-hexen-2-ol in 6 h with the dinuclear di(N-heterocyclic carbene) iridium
complex 6 (0.5 mol% of catalyst loading, iPrOH, iPrONa (3 mol%), 82
C) in 96%
yield. Under the same conditions, 86% yield was obtained with catalyst 7 within
10 h [67].
The same ketone and the related non-conjugated ketone 6-methyl-5-hepten-2-one
were hydrogenated to the corresponding unsaturated alcohol by the bis(N-heterocyclic carbene) complex 9a (2 mol% of catalyst loading, iPrOH, KOH (10 mol%),
95
C). Yields of 83 and 96% were obtained within 6 h. Notably, the same catalyst
under similar conditions hydrogenated selectively the C¼C bond of the conjugated
α,β-unsaturated ketone (E)-chalcone [67].
H
Ir
Cp*
N
N
Cl
N
N
Cl
9a
Cl
Ir
Cp*
N
Cl
N
N H
63
Ir
N
N
N
Bu
Cl
MeO
Ir
PBn 3
PPh 3
75c
PF 6
55a
PCy 2
Cy 2 P
+
80
Ir
N
N
Cl
O
O
[IrCl(COD)] 2
Scheme 53 Catalysts for the reduction of α,β-unsaturated ketones
124
M. Pilar Lamata et al.