(Cp* ¼ pentamethylcyclopentadienyl; bpym ¼ bipyrimidine) led to yields which
were two times higher than the ones obtained using the analogous rhodium complex
[Cp*Rh(bpym)]Cl 2 [11]. For this reason, the authors further explored the reactivity
of the iridium complex by replacing the ligands, and when the complex [Cp*Ir
(bpyO)]OH
À (bpyO ¼ α,α
0 -bipyridonate) [Ir-2] was used, an excellent conversion
of 93% and a GA yield of 76% were obtained (Scheme 2). In order to maximize the
concentration of the active dehydrogenation catalyst [Ir-2], the pH of the solution
was adjusted with a phosphate buffer and additional NaOH was added to compensate the acidifying effect of the produced GA. The best results were obtained
(GA yield: 81.5%) when a stochiometric amount of NaOH was sequentially added
to the reaction. Tang and coworkers were also able to reuse the [Ir-2] for several
catalytic runs, although after the sixth run the conversion of EG and the yield of GA
slightly dropped.
2.2 Iridium-Catalyzed Hydrogen Transfer Initiated
Dehydration of 1,3-Propanediol to Aldehydes
Aldehydes like propionaldehyde are important intermediates for the production of
fragrances, agrochemicals, plasticizers, or drugs [13]. 1,3-Propanediol (1,3-PDO)
has been one of the first renewable platform chemicals which is produced by
bacterial fermentation of sugars [6, 13]. Due to the relevance of 1,3-PDO, Marr
and coworkers became interested in finding new chemocatalytic transformations of
this bio-based alcohol into value-added chemicals. During their attempt to aminate
1,3-PDO with a Cp*Ir(III) N-heterocyclic carbene complex they discovered that, by
working in ionic liquids, dehydration of 1,3-PDO occurred [14]. For that reason, the
authors decided to use the same iridium catalyst ([Ir-3], Fig. 2) in ionic liquids for
the selective dehydration of 1,3-PDO (1) to propionaldehyde (2) [13]. This interesting conversion proceeds via a dehydrogenation to 3-hydroxypropionaldehyde, dehydration to acrolein, and hydrogenation to propionaldehyde. The authors refer to the
sequence as a hydrogen transfer initiated dehydration (HTID). The reaction was
initially tested in the presence of two different ionic liquids (Fig. 2), EmmimNTf 2 or
N 1,8,8,8 NTf 2 , respectively, and at temperatures varying between 80
C and 150
C for
6 h. Next to the desired product 2, side products 3, 4, and 5 were found in varying
amounts (Scheme 3). The formation of 3 and 4 can be explained by the Aldol
reaction of 2 followed by dehydration and consecutive hydrogenation. Hydrogenation of 2 leads to the formation of 5. In order to prevent the occurrence of aldol
condensation reactions, a reduced pressure (0.35 bar) was used to remove the highly
volatile propionaldehyde (Schemes 3, 2) which was collected in a cold trap.
The use of EmmimNTf 2 resulted in higher yields and selectivities towards
2 compared to N 1,8,8,8 NTf 2 . Furthermore, the use of ionic liquids led to better results
for the dehydration with respect to neat conditions. Finally, it was shown that the
base (K 2 CO 3 , KOH, Cs 2 CO 3 ) itself had a minor effect on the product distribution
Homogenous Iridium Catalysts for Biomass Conversion
347
were two times higher than the ones obtained using the analogous rhodium complex
[Cp*Rh(bpym)]Cl 2 [11]. For this reason, the authors further explored the reactivity
of the iridium complex by replacing the ligands, and when the complex [Cp*Ir
(bpyO)]OH
À (bpyO ¼ α,α
0 -bipyridonate) [Ir-2] was used, an excellent conversion
of 93% and a GA yield of 76% were obtained (Scheme 2). In order to maximize the
concentration of the active dehydrogenation catalyst [Ir-2], the pH of the solution
was adjusted with a phosphate buffer and additional NaOH was added to compensate the acidifying effect of the produced GA. The best results were obtained
(GA yield: 81.5%) when a stochiometric amount of NaOH was sequentially added
to the reaction. Tang and coworkers were also able to reuse the [Ir-2] for several
catalytic runs, although after the sixth run the conversion of EG and the yield of GA
slightly dropped.
2.2 Iridium-Catalyzed Hydrogen Transfer Initiated
Dehydration of 1,3-Propanediol to Aldehydes
Aldehydes like propionaldehyde are important intermediates for the production of
fragrances, agrochemicals, plasticizers, or drugs [13]. 1,3-Propanediol (1,3-PDO)
has been one of the first renewable platform chemicals which is produced by
bacterial fermentation of sugars [6, 13]. Due to the relevance of 1,3-PDO, Marr
and coworkers became interested in finding new chemocatalytic transformations of
this bio-based alcohol into value-added chemicals. During their attempt to aminate
1,3-PDO with a Cp*Ir(III) N-heterocyclic carbene complex they discovered that, by
working in ionic liquids, dehydration of 1,3-PDO occurred [14]. For that reason, the
authors decided to use the same iridium catalyst ([Ir-3], Fig. 2) in ionic liquids for
the selective dehydration of 1,3-PDO (1) to propionaldehyde (2) [13]. This interesting conversion proceeds via a dehydrogenation to 3-hydroxypropionaldehyde, dehydration to acrolein, and hydrogenation to propionaldehyde. The authors refer to the
sequence as a hydrogen transfer initiated dehydration (HTID). The reaction was
initially tested in the presence of two different ionic liquids (Fig. 2), EmmimNTf 2 or
N 1,8,8,8 NTf 2 , respectively, and at temperatures varying between 80
C and 150
C for
6 h. Next to the desired product 2, side products 3, 4, and 5 were found in varying
amounts (Scheme 3). The formation of 3 and 4 can be explained by the Aldol
reaction of 2 followed by dehydration and consecutive hydrogenation. Hydrogenation of 2 leads to the formation of 5. In order to prevent the occurrence of aldol
condensation reactions, a reduced pressure (0.35 bar) was used to remove the highly
volatile propionaldehyde (Schemes 3, 2) which was collected in a cold trap.
The use of EmmimNTf 2 resulted in higher yields and selectivities towards
2 compared to N 1,8,8,8 NTf 2 . Furthermore, the use of ionic liquids led to better results
for the dehydration with respect to neat conditions. Finally, it was shown that the
base (K 2 CO 3 , KOH, Cs 2 CO 3 ) itself had a minor effect on the product distribution
Homogenous Iridium Catalysts for Biomass Conversion
347