the commercialization of 5-HMF is immense, based on the enormous number of
publications dealing with its synthesis and derived products [54]. One of the 5-HMF
products which has recently received high attention in the field of homogenous
iridium catalysis is 1-hydroxyhexane-2,5-dione (HHD, see Table 1), the hydrogenated ring-opened form of 5-HMF [61].
Only a few homogenous iridium catalysts are known for the synthesis of HHD,
even though homogenous catalysts seem generally to perform better than heterogenous systems [61] (Fig. 8). The group of Zhang and coworkers was the first to
report an iridium-catalyzed hydrogenation of 5-HMF in aqueous solution in order to
synthesize HHD [49]. Bipyridine Cp*-Ir complexes were found to be efficient
catalysts for the selective formation of HHD and [Ir-31] emerged as the best
candidate under their optimized conditions (Table 1, Entry 1). Subsequently,
Deng, Fu, and coworkers carried out an in-depth investigation on the different
parameters of the hydrolytic transfer hydrogenation of HMF, by exploring the effect
of electron-donating and electron-withdrawing groups on the bipyridyl ligand, the
influence of the temperature and of the pH [50]. The obtained results revealed that
temperature and pH are important parameters to maximize catalytic performance.
Particularly, full conversions and high selectivities up to 95% were obtained at
130
C in aqueous formate buffer solutions (FBS, pH ¼ 2.5) with catalysts [Ir-32]
and [Ir-33] (Table 1, Entries 2 and 4). Moreover, the authors were the first to isolate
HHD in 85% yield on a large scale [50].
Unfortunately, attempts to reuse the catalyst resulted in a decrease of catalytic
activity and only 70% yield was achieved. The same catalyst [Ir-32] was used by
Table 1 Iridium-catalyzed conversion of HMF to HHD
Entry Catalyst (loading) Conditions
Conv.
[%]
Yield
[%]
Ref.
1
[Ir-31] (0.26 mol
%)
120
C, H 2 O, 7 bar H 2 , 2 h
100
86
[49]
2
[Ir-32] (0.01 mol
%)
130
C, FBS (pH ¼ 2.5), 2 h
100
92(85)
a
[50]
3
[Ir-32] (0.10 mol
%)
130
C, H 2 O, 5 wt% Al 2 (SO 4 ) 3 , 30 bar
H 2 , 4 h
100
77
[51]
4
[Ir-33] (0.01 mol
%)
130
C, FBS (pH ¼ 2.5), 2 h
100
~ 95
[50]
5
[Ir-34] (0.0008
mol%)
120
C, H 2 O (pH ¼ 3.4), 35 bar H 2 , 6 h 88
67
[52]
6
[Ir-35] (0.50 mol
%)
120
C, H 2 O, 10 bar H 2 , 2 h
100
76(69)
a
[53]
7
[Ir-36] (0.075 mol
%)
140
C, PBS (pH ¼ 2.5), 60 bar H 2 , 1 h 100
71
a
[53]
a Isolated yield
362
S. Kirchhecker et al.
publications dealing with its synthesis and derived products [54]. One of the 5-HMF
products which has recently received high attention in the field of homogenous
iridium catalysis is 1-hydroxyhexane-2,5-dione (HHD, see Table 1), the hydrogenated ring-opened form of 5-HMF [61].
Only a few homogenous iridium catalysts are known for the synthesis of HHD,
even though homogenous catalysts seem generally to perform better than heterogenous systems [61] (Fig. 8). The group of Zhang and coworkers was the first to
report an iridium-catalyzed hydrogenation of 5-HMF in aqueous solution in order to
synthesize HHD [49]. Bipyridine Cp*-Ir complexes were found to be efficient
catalysts for the selective formation of HHD and [Ir-31] emerged as the best
candidate under their optimized conditions (Table 1, Entry 1). Subsequently,
Deng, Fu, and coworkers carried out an in-depth investigation on the different
parameters of the hydrolytic transfer hydrogenation of HMF, by exploring the effect
of electron-donating and electron-withdrawing groups on the bipyridyl ligand, the
influence of the temperature and of the pH [50]. The obtained results revealed that
temperature and pH are important parameters to maximize catalytic performance.
Particularly, full conversions and high selectivities up to 95% were obtained at
130
C in aqueous formate buffer solutions (FBS, pH ¼ 2.5) with catalysts [Ir-32]
and [Ir-33] (Table 1, Entries 2 and 4). Moreover, the authors were the first to isolate
HHD in 85% yield on a large scale [50].
Unfortunately, attempts to reuse the catalyst resulted in a decrease of catalytic
activity and only 70% yield was achieved. The same catalyst [Ir-32] was used by
Table 1 Iridium-catalyzed conversion of HMF to HHD
Entry Catalyst (loading) Conditions
Conv.
[%]
Yield
[%]
Ref.
1
[Ir-31] (0.26 mol
%)
120
C, H 2 O, 7 bar H 2 , 2 h
100
86
[49]
2
[Ir-32] (0.01 mol
%)
130
C, FBS (pH ¼ 2.5), 2 h
100
92(85)
a
[50]
3
[Ir-32] (0.10 mol
%)
130
C, H 2 O, 5 wt% Al 2 (SO 4 ) 3 , 30 bar
H 2 , 4 h
100
77
[51]
4
[Ir-33] (0.01 mol
%)
130
C, FBS (pH ¼ 2.5), 2 h
100
~ 95
[50]
5
[Ir-34] (0.0008
mol%)
120
C, H 2 O (pH ¼ 3.4), 35 bar H 2 , 6 h 88
67
[52]
6
[Ir-35] (0.50 mol
%)
120
C, H 2 O, 10 bar H 2 , 2 h
100
76(69)
a
[53]
7
[Ir-36] (0.075 mol
%)
140
C, PBS (pH ¼ 2.5), 60 bar H 2 , 1 h 100
71
a
[53]
a Isolated yield
362
S. Kirchhecker et al.