reaction starting from glucose via acid hydrolysis. The GVL yield over two steps
was 57% using hydrogen gas and 44% under TH conditions using only the FA
generated in the reaction as the hydrogen donor.
The Nielsen group investigated the hydrogenation of alkyl levulinates to GVL
[73]. These ethyl or methyl esters of LEV are obtained when alcohols are used as the
reaction medium for the acid-catalyzed conversion of carbohydrates instead of water
and are also produced as a side stream in some processes [76]. Among the range of
PNP and SNS pincer catalysts tested, [Ir-38] based on the MACHO ligand, earlier
developed by Takasago, was the best, able to convert ethyl levulinate to GVL under
very mild conditions (20 bar hydrogen and 60
C, neat with only small amounts of
alcohol added) (Table 5, Entry 6). In contrast to the previous catalysts, this pincer
catalyst requires 5 mol% of base (NaOEt) for activation. The highest TON reached
was 9,300. Methyl levulinate reacted faster than the ethyl ester under the same
conditions. In a small scale-up the authors tested the catalyst under very mild
conditions – 25 bar hydrogen and 25
C. Here they could achieve full conversion
with 0.05 mol% of catalyst after 72 h.
Zhou and coworkers screened a range of PNP, PNN, and NNN pincer ligands
with iridium in the presence of base. The best catalyst was PNP complex [Ir-44],
with the nitrogen analogue also giving high yields [74]. In the optimization of the
reaction conditions it was found that base was necessary for the reaction to occur;
when it was lowered from 1.2 equiv. to 0.5 equiv., the yield decreased to below 10%.
With optimized reaction conditions LEV could be converted to GVL in 98% yield
(Table 5, Entry 7). A TON of 71,000 was measured, albeit at quite high hydrogen
pressure of 100 bar.
Sakthivel and coworkers synthesized several iridium porphyrin complexes and
tested them in the hydrogenation of LEV to GVL and 1,4-pentanediol under basefree conditions [75]. The catalyst with the best performance and selectivity for LEV
was [Ir-45] (Table 5, Entry 8). The carboxylated version of the catalyst, [Ir-46], was
not as efficient as [Ir-45]. However, when the authors performed reuse experiments
by adding more batches of LEV into the reaction mixture, [Ir-46] produced mostly
1,4-pentanediol (1,4-PDO), the further hydrogenation product of GVL, and the
yields of 1,4-PDO increased with each cycle, reaching a selectivity of 81% after
five runs. For [Ir-45] the yield of 1,4-PDO also increased with each run so that the
selectivity for LEV was only 66% for run four. The authors attributed this to the
buildup of water in the system, but did not perform any control experiments to
confirm this or to exclude the formation of nanoparticles.
The group of Voutchkova-Kostal employed TH for the hydrogenation of LEV to
the potassium salt of gamma-hydroxyvaleric acid (GHV) (Scheme 16) [77]. Under
most reaction conditions, even in the presence of base, this compound immediately
cyclizes to form the lactone GVL. Based on their efficient NHC iridium complex for
the acceptorless dehydrogenation of glycerol (vide supra), glycerol was investigated
as the hydrogen donor and a range of NHC-based complexes with pendant sulfonate
groups for better water solubility were tested. In the screening, the same complex
used in the previous publication, carbonyl complex [Ir-10] with two sulfonated
NHC ligands, was found to be the most active. Two equivalents of KOH per mol
372
S. Kirchhecker et al.
was 57% using hydrogen gas and 44% under TH conditions using only the FA
generated in the reaction as the hydrogen donor.
The Nielsen group investigated the hydrogenation of alkyl levulinates to GVL
[73]. These ethyl or methyl esters of LEV are obtained when alcohols are used as the
reaction medium for the acid-catalyzed conversion of carbohydrates instead of water
and are also produced as a side stream in some processes [76]. Among the range of
PNP and SNS pincer catalysts tested, [Ir-38] based on the MACHO ligand, earlier
developed by Takasago, was the best, able to convert ethyl levulinate to GVL under
very mild conditions (20 bar hydrogen and 60
C, neat with only small amounts of
alcohol added) (Table 5, Entry 6). In contrast to the previous catalysts, this pincer
catalyst requires 5 mol% of base (NaOEt) for activation. The highest TON reached
was 9,300. Methyl levulinate reacted faster than the ethyl ester under the same
conditions. In a small scale-up the authors tested the catalyst under very mild
conditions – 25 bar hydrogen and 25
C. Here they could achieve full conversion
with 0.05 mol% of catalyst after 72 h.
Zhou and coworkers screened a range of PNP, PNN, and NNN pincer ligands
with iridium in the presence of base. The best catalyst was PNP complex [Ir-44],
with the nitrogen analogue also giving high yields [74]. In the optimization of the
reaction conditions it was found that base was necessary for the reaction to occur;
when it was lowered from 1.2 equiv. to 0.5 equiv., the yield decreased to below 10%.
With optimized reaction conditions LEV could be converted to GVL in 98% yield
(Table 5, Entry 7). A TON of 71,000 was measured, albeit at quite high hydrogen
pressure of 100 bar.
Sakthivel and coworkers synthesized several iridium porphyrin complexes and
tested them in the hydrogenation of LEV to GVL and 1,4-pentanediol under basefree conditions [75]. The catalyst with the best performance and selectivity for LEV
was [Ir-45] (Table 5, Entry 8). The carboxylated version of the catalyst, [Ir-46], was
not as efficient as [Ir-45]. However, when the authors performed reuse experiments
by adding more batches of LEV into the reaction mixture, [Ir-46] produced mostly
1,4-pentanediol (1,4-PDO), the further hydrogenation product of GVL, and the
yields of 1,4-PDO increased with each cycle, reaching a selectivity of 81% after
five runs. For [Ir-45] the yield of 1,4-PDO also increased with each run so that the
selectivity for LEV was only 66% for run four. The authors attributed this to the
buildup of water in the system, but did not perform any control experiments to
confirm this or to exclude the formation of nanoparticles.
The group of Voutchkova-Kostal employed TH for the hydrogenation of LEV to
the potassium salt of gamma-hydroxyvaleric acid (GHV) (Scheme 16) [77]. Under
most reaction conditions, even in the presence of base, this compound immediately
cyclizes to form the lactone GVL. Based on their efficient NHC iridium complex for
the acceptorless dehydrogenation of glycerol (vide supra), glycerol was investigated
as the hydrogen donor and a range of NHC-based complexes with pendant sulfonate
groups for better water solubility were tested. In the screening, the same complex
used in the previous publication, carbonyl complex [Ir-10] with two sulfonated
NHC ligands, was found to be the most active. Two equivalents of KOH per mol
372
S. Kirchhecker et al.