were obtained when 0.1 mol% of [Ir-32] was used in combination with 40 wt% of
γ-Al 2 O 3 as a Lewis acid cocatalyst at 130
C under 30 bar of H 2 for 4 h.
In addition to 5-HMF as a starting material (Table 2, Entry 1), other bio-derived
furan compounds like 5-methyl furfural (Table 2, Entries 2–3) and furfural (Table 2,
Entries 4–5) could be hydrogenated in the same manner to 3-methyl cyclopentenone
and cyclopentenone in 30% and 60% yield, respectively (Table 2). Under Brønsted
acid catalysis using a phosphate buffer solution (PBS) 5-methyl furfural (Entry 3)
and furfural (Entry 5) were converted to the straight chain ketones 2,5-hexanedione
and levulinic acid in moderate yields.
When the intramolecular aldol condensation of HHD is carried out under basic
conditions, 8 is formed. The latter is a flavoring agent which is commonly synthesized from adipic acid in a multistep reaction that uses toxic and carcinogenic
reactants [61]. For this reason, a catalytic protocol for the synthesis of 8 from benign
bio-based materials would be highly desired. The base-promoted formation of 8 by
using HHD as a starting material was firstly discovered by Deng, Fu, and coworkers
[51]. Later, de Vries and coworkers carried out an in-depth study on the role of the
base and of the solvent [53].
As a conclusion, an isolated yield of 72% of 8 in an aqueous KOH solution at
60
C after 15 min was achieved. Furthermore, the authors decided to combine their
previous procedure for the synthesis of HHD mediated by [Ir-35] with the
KOH-mediated aldol condensation reaction in order to synthesize 8 in a one-pot
reaction. Based on this methodology, an overall yield of 55% of 8 was achieved.
Table 2 Effect of Lewis acid or Brønsted acid on the iridium-catalyzed conversion of furfural
compounds
Entry Substrate
Acid
Conv.
[%]
Yield
[%]
Products
1
40 wt%
γ-Al 2 O 3
100
a
2/82
a
2
45 wt%
γ-Al 2 O 3
100
a
30/24
a
3
PBS
(pH ¼ 1.5)
100
a
39
a
4
52 wt%
γ-Al 2 O 3
100
a
60/~2
a
5
PBS
(pH ¼ 1.0)
100
b
41/9
b
a 5-HMF, 5-methyl furfural or furfural aq. (0.067 M, 3 mL), [Ir-32] (0.1 mol%), H 2 30 bar, 130
C,
4 h
b
Furfural (0.60 mmol), 2 mL pH ¼ 1.0 phosphate buffer solution (0.1 M), [Ir-32] (0.0083 mol%),
10 bar H 2 , 120
C, 4 h
Homogenous Iridium Catalysts for Biomass Conversion
365
γ-Al 2 O 3 as a Lewis acid cocatalyst at 130
C under 30 bar of H 2 for 4 h.
In addition to 5-HMF as a starting material (Table 2, Entry 1), other bio-derived
furan compounds like 5-methyl furfural (Table 2, Entries 2–3) and furfural (Table 2,
Entries 4–5) could be hydrogenated in the same manner to 3-methyl cyclopentenone
and cyclopentenone in 30% and 60% yield, respectively (Table 2). Under Brønsted
acid catalysis using a phosphate buffer solution (PBS) 5-methyl furfural (Entry 3)
and furfural (Entry 5) were converted to the straight chain ketones 2,5-hexanedione
and levulinic acid in moderate yields.
When the intramolecular aldol condensation of HHD is carried out under basic
conditions, 8 is formed. The latter is a flavoring agent which is commonly synthesized from adipic acid in a multistep reaction that uses toxic and carcinogenic
reactants [61]. For this reason, a catalytic protocol for the synthesis of 8 from benign
bio-based materials would be highly desired. The base-promoted formation of 8 by
using HHD as a starting material was firstly discovered by Deng, Fu, and coworkers
[51]. Later, de Vries and coworkers carried out an in-depth study on the role of the
base and of the solvent [53].
As a conclusion, an isolated yield of 72% of 8 in an aqueous KOH solution at
60
C after 15 min was achieved. Furthermore, the authors decided to combine their
previous procedure for the synthesis of HHD mediated by [Ir-35] with the
KOH-mediated aldol condensation reaction in order to synthesize 8 in a one-pot
reaction. Based on this methodology, an overall yield of 55% of 8 was achieved.
Table 2 Effect of Lewis acid or Brønsted acid on the iridium-catalyzed conversion of furfural
compounds
Entry Substrate
Acid
Conv.
[%]
Yield
[%]
Products
1
40 wt%
γ-Al 2 O 3
100
a
2/82
a
2
45 wt%
γ-Al 2 O 3
100
a
30/24
a
3
PBS
(pH ¼ 1.5)
100
a
39
a
4
52 wt%
γ-Al 2 O 3
100
a
60/~2
a
5
PBS
(pH ¼ 1.0)
100
b
41/9
b
a 5-HMF, 5-methyl furfural or furfural aq. (0.067 M, 3 mL), [Ir-32] (0.1 mol%), H 2 30 bar, 130
C,
4 h
b
Furfural (0.60 mmol), 2 mL pH ¼ 1.0 phosphate buffer solution (0.1 M), [Ir-32] (0.0083 mol%),
10 bar H 2 , 120
C, 4 h
Homogenous Iridium Catalysts for Biomass Conversion
365