product formation. The reaction was accelerated by the addition of 25 mol% H 2 SO 4 .
1 equiv. of sulfuric acid could increase the yield to 97%. The researchers also applied
the reaction to starch as a model carbohydrate. Here two reactions can be performed
in one pot as starch hydrolysis is achieved by sulfuric acid in water. After 50 h the
yield of gluconic acid was 50%, with 30% glucose and no side product formation. A
mechanism was proposed on the basis of ESI-MS experiments combined with DFT
calculations. Catalyst precursors containing sulfate or chlorides convert into the
active aqua catalyst on dissolution in water. The two water ligands are displaced
by glucose in its ring-opened form being bound as a chelate. A nucleophilic attack of
H 2 O on the aldehyde group of glucose leads to a gem-diol, which reacts via the
release of a proton and beta hydride elimination leading to the release of gluconic
acid and the generation of an iridium hydride complex, which takes up the proton
and releases hydrogen gas to reform the initial complex. The observed acceleration
when acid was added to the system is probably due to the faster protonation of the
catalyst in this last step.
Following on from this initial study, the same group reported a triazole carbene
catalyst [Ir-52] which could dehydrogenate a range of sugars to their corresponding
acids with high selectivity (Schem 19, bottom) [91]. This catalyst did not require
acid to work leading to a large improvement over the previous work. The authors
tested different N-substituents on the triazole ligand (Me, Bu, and glucose) of which
the methyl induced the highest yield: quantitative conversion of glucose to gluconic
acid could be achieved at a catalyst loading of 2% in water. The glucose substituted
catalyst gave high initial rates but then degraded at the optimum reaction temperature
of 110
C. Several other sugars were then screened as substrates and most were
converted to the corresponding acids in yields of >90%. Catalyst [Ir-51] from the
group’s previous work was screened alongside (with 0.25 eq H 2 SO 4 ) and in some
cases gave slightly higher yields than [Ir-52]. Additionally, the butyl version was
also slightly better in some cases. Progress of the reaction was monitored by NMR
and was determined to be first order in the catalyst. Again, the authors used ESI-MS
for mechanistic studies. Based on these results they postulated that the catalyst forms
a dicationic hydride bridged bimetallic complex, which they then synthesized and
obtained the X-ray structure for. This dimer appears to be the resting state for the
catalyst. The catalytic cycle proceeds as described in the previous publication above.
One notable difference is that for the triazolylidene ligand no extra acid is necessary.
This is rationalized by the fact that the proton which is released by the rearrangement
of the gem-diol can be transported intramolecularly via the N2 of the ligand. This is
not possible with NHC ligands and points to a reason for the necessity of acid in the
previous work.
6 Iridium-Catalyzed Conversion of Lignin
Lignin is the most abundant source of bio-derived aromatics on earth (see Fig. 1),
being on average 30% of lignocellulose by weight. However, due to its heterogeneity, it has so far been much more difficult to valorize it compared to the
Homogenous Iridium Catalysts for Biomass Conversion
379
1 equiv. of sulfuric acid could increase the yield to 97%. The researchers also applied
the reaction to starch as a model carbohydrate. Here two reactions can be performed
in one pot as starch hydrolysis is achieved by sulfuric acid in water. After 50 h the
yield of gluconic acid was 50%, with 30% glucose and no side product formation. A
mechanism was proposed on the basis of ESI-MS experiments combined with DFT
calculations. Catalyst precursors containing sulfate or chlorides convert into the
active aqua catalyst on dissolution in water. The two water ligands are displaced
by glucose in its ring-opened form being bound as a chelate. A nucleophilic attack of
H 2 O on the aldehyde group of glucose leads to a gem-diol, which reacts via the
release of a proton and beta hydride elimination leading to the release of gluconic
acid and the generation of an iridium hydride complex, which takes up the proton
and releases hydrogen gas to reform the initial complex. The observed acceleration
when acid was added to the system is probably due to the faster protonation of the
catalyst in this last step.
Following on from this initial study, the same group reported a triazole carbene
catalyst [Ir-52] which could dehydrogenate a range of sugars to their corresponding
acids with high selectivity (Schem 19, bottom) [91]. This catalyst did not require
acid to work leading to a large improvement over the previous work. The authors
tested different N-substituents on the triazole ligand (Me, Bu, and glucose) of which
the methyl induced the highest yield: quantitative conversion of glucose to gluconic
acid could be achieved at a catalyst loading of 2% in water. The glucose substituted
catalyst gave high initial rates but then degraded at the optimum reaction temperature
of 110
C. Several other sugars were then screened as substrates and most were
converted to the corresponding acids in yields of >90%. Catalyst [Ir-51] from the
group’s previous work was screened alongside (with 0.25 eq H 2 SO 4 ) and in some
cases gave slightly higher yields than [Ir-52]. Additionally, the butyl version was
also slightly better in some cases. Progress of the reaction was monitored by NMR
and was determined to be first order in the catalyst. Again, the authors used ESI-MS
for mechanistic studies. Based on these results they postulated that the catalyst forms
a dicationic hydride bridged bimetallic complex, which they then synthesized and
obtained the X-ray structure for. This dimer appears to be the resting state for the
catalyst. The catalytic cycle proceeds as described in the previous publication above.
One notable difference is that for the triazolylidene ligand no extra acid is necessary.
This is rationalized by the fact that the proton which is released by the rearrangement
of the gem-diol can be transported intramolecularly via the N2 of the ligand. This is
not possible with NHC ligands and points to a reason for the necessity of acid in the
previous work.
6 Iridium-Catalyzed Conversion of Lignin
Lignin is the most abundant source of bio-derived aromatics on earth (see Fig. 1),
being on average 30% of lignocellulose by weight. However, due to its heterogeneity, it has so far been much more difficult to valorize it compared to the
Homogenous Iridium Catalysts for Biomass Conversion
379