more stable than the gamma. Based on the calculations a mechanism was proposed
which proceeds via abstraction of a hydrogen from the carbon by the iridium atom
and abstraction of the hydrogen of the hydroxyl group by one of the carbonyl groups
of the bipyridine ligand. Following the release of the product, the catalyst returns to
its original state by the release of a molecule of H 2 . The authors then applied the
catalyst to real lignin, which they extracted with acidic dioxane from birch wood, a
source high in β-O-4 linkages. To confirm that the gas released from the reaction is
hydrogen, they channeled it into another reaction using a Ru catalyst for the
hydrogenation of decene to decane, which was then detected.
In order to depolymerize the Ir-pretreated lignin, first the authors tested a 2-step
protocol on the model compound, where the depolymerization was achieved efficiently by Zn/NH 4 Cl. A formate buffer system was also tested, but due to the
efficient dehydrogenation of formic acid by the Ir catalyst remaining in solution
this was not usable. This procedure was then also applied to real lignin. GPC analysis
of lignin samples before and after the treatment steps confirmed that lignin could not
be depolymerized by the Zn system without the iridium pretreatment, while after the
two steps it showed a decrease in molecular weight.
Two years later the group of Bruijnincx employed the anionic version of the
bipyridone catalyst [Ir-49], [Ir-2], to catalyze the conversion of lignin into aromatic
diols and diacids via dehydrogenation of the primary (β) hydroxyl followed by retroaldol Cα – Cβ bond cleavage (Scheme 20b) [95]. This catalyst, [Ir-2], was chosen as
it was known to be active in primary alcohol dehydrogenation (for example EG, see
Sect. 2.1.2). In a basic water/dioxane mixture (pH 11) [Ir-49] converted spontaneously into its anionic form [Ir-2], and this catalyst system was able to cleave several
lignin model compounds, also including phenol-containing ones, efficiently within
1 h. Next, the authors applied their method to real lignin (softwood enzyme), which
resulted in an oil in 82% yield, which was confirmed by GPC to have much lower
molecular weights. The other 18% were higher molecular weight material. A 2D
HSQC NMR analysis showed that there were hardly any β-O-4 linkages left in this
material and also the β-5 (phenylcoumaran) linkages were highly reduced. Another
type of common linkage in lignin, the resinol (β-β) unit was not touched. Analysis of
the oil fraction by GC confirmed the presence of the monomers expected from the
model compound experiments. The yield of identified monomers was 3.8 wt%
which is about 40% of the theoretical yield. Dioxasolv lignin was also tested and
gave a slightly higher yield, mainly due to vanillin derived from the end groups. To
simplify the product mixture, the authors refluxed the reaction mixture in air for a
further 16 h, which led to the partial conversion of the monomers into a diacid
derivative (see Scheme 16) in 2.4% yield, demonstrating that it can be possible to
tune the depolymerization reaction to give a few products.
De Vries, Barta, and coworkers investigated iridium-catalyzed decarbonylation
as one of the several strategies to prevent recondensation reactions of the aldehyde
fragments which are formed by acidolysis of the β-O-4 linkage (Scheme 21a) [96]. In
a one-pot reaction they achieved acid-catalyzed cleavage of β-O-4 model compounds by triflic acid and decarbonylation of the resulting aldehydes to methyl
aromatics by [Ir(cod)Cl] 2 with triphenylphosphine ligand [Ir-53]. They then applied
Homogenous Iridium Catalysts for Biomass Conversion
381
which proceeds via abstraction of a hydrogen from the carbon by the iridium atom
and abstraction of the hydrogen of the hydroxyl group by one of the carbonyl groups
of the bipyridine ligand. Following the release of the product, the catalyst returns to
its original state by the release of a molecule of H 2 . The authors then applied the
catalyst to real lignin, which they extracted with acidic dioxane from birch wood, a
source high in β-O-4 linkages. To confirm that the gas released from the reaction is
hydrogen, they channeled it into another reaction using a Ru catalyst for the
hydrogenation of decene to decane, which was then detected.
In order to depolymerize the Ir-pretreated lignin, first the authors tested a 2-step
protocol on the model compound, where the depolymerization was achieved efficiently by Zn/NH 4 Cl. A formate buffer system was also tested, but due to the
efficient dehydrogenation of formic acid by the Ir catalyst remaining in solution
this was not usable. This procedure was then also applied to real lignin. GPC analysis
of lignin samples before and after the treatment steps confirmed that lignin could not
be depolymerized by the Zn system without the iridium pretreatment, while after the
two steps it showed a decrease in molecular weight.
Two years later the group of Bruijnincx employed the anionic version of the
bipyridone catalyst [Ir-49], [Ir-2], to catalyze the conversion of lignin into aromatic
diols and diacids via dehydrogenation of the primary (β) hydroxyl followed by retroaldol Cα – Cβ bond cleavage (Scheme 20b) [95]. This catalyst, [Ir-2], was chosen as
it was known to be active in primary alcohol dehydrogenation (for example EG, see
Sect. 2.1.2). In a basic water/dioxane mixture (pH 11) [Ir-49] converted spontaneously into its anionic form [Ir-2], and this catalyst system was able to cleave several
lignin model compounds, also including phenol-containing ones, efficiently within
1 h. Next, the authors applied their method to real lignin (softwood enzyme), which
resulted in an oil in 82% yield, which was confirmed by GPC to have much lower
molecular weights. The other 18% were higher molecular weight material. A 2D
HSQC NMR analysis showed that there were hardly any β-O-4 linkages left in this
material and also the β-5 (phenylcoumaran) linkages were highly reduced. Another
type of common linkage in lignin, the resinol (β-β) unit was not touched. Analysis of
the oil fraction by GC confirmed the presence of the monomers expected from the
model compound experiments. The yield of identified monomers was 3.8 wt%
which is about 40% of the theoretical yield. Dioxasolv lignin was also tested and
gave a slightly higher yield, mainly due to vanillin derived from the end groups. To
simplify the product mixture, the authors refluxed the reaction mixture in air for a
further 16 h, which led to the partial conversion of the monomers into a diacid
derivative (see Scheme 16) in 2.4% yield, demonstrating that it can be possible to
tune the depolymerization reaction to give a few products.
De Vries, Barta, and coworkers investigated iridium-catalyzed decarbonylation
as one of the several strategies to prevent recondensation reactions of the aldehyde
fragments which are formed by acidolysis of the β-O-4 linkage (Scheme 21a) [96]. In
a one-pot reaction they achieved acid-catalyzed cleavage of β-O-4 model compounds by triflic acid and decarbonylation of the resulting aldehydes to methyl
aromatics by [Ir(cod)Cl] 2 with triphenylphosphine ligand [Ir-53]. They then applied
Homogenous Iridium Catalysts for Biomass Conversion
381