additive, solvent and fine chemical intermediate [218]. However, reports dealing
with homogeneous TH of LA are scarce. In particular, the generation of GLV from
aqueous mixtures of LA and HCOOH catalysed by the half-sandwich iridium
complex 84 (Scheme 65) has been reported [219].
More recently, Fischmeister and co-workers reported on the solvent free reduction of LA to GVL (Scheme 66) mediated by the dipyridylamine iridium complexes
85. Reactions were performed at 120–150
C, employing mixtures of HCOOH/NEt 3
as a hydrogen donor. Yields from 22 to 98% were obtained within 16 h. Complex
85c with the bulkiest ligand was found to be the less active catalyst [220].
The half-sandwich complex 28 (Scheme 7) catalysed the transformation of LA to
GVL. Under standard TH conditions (1 mol% of catalyst, iPrOH, 85
C), 69% yield
was obtained within 12 h [76].
Furfural is one of the organic compounds readily available from nonedible
biomass (corncobs, oat hulls, bagasse, etc.) [221]. Furfural is mainly used as a raw
material for the synthesis of a multitude of important nonpetroleum-derived
chemicals such as furfuryl alcohol, methyltetrahydrofuran and furan [222]. In this
line, Grushin and van Leeuween et al. reported a highly efficient chemo- and
stereoselective synthesis of chiral hydrofuroins via asymmetric TH of racemic furoin
that is prepared from furfural (Scheme 67) using the half-sandwich iridium complex
[Cp*IrCl((S,S)-TsDPEN)] as catalyst precursor [223].
On the other hand, solvents often account for most of the mass wasted in
syntheses and processes [224]. The utilisation of biogenetic alcohols such as glycerol as a H-donor solvent in catalytic TH reactions is an interesting contribution to
the field of sustainable chemistry. Glycerol is a non-toxic and non-flammable
84
H 2 O Ir
Cp*
N
N
SO 4
OMe
MeO
Cl
Ir
Cp*
N
N
SO 2 Me
28
Scheme 65 Iridium
catalysts for the TH of
levulinic acid
H 2 O Ir
Cp*
N
N
SO 4
R
R
O
OH
O
O
O
HCOOH/NEt 3
85
LA
GVL
R = H (85a), CH 2 Ph (85b), CH(CH 3 )Ph (85c)
Scheme 66 Transfer
hydrogenation of
levulinic acid
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
131
with homogeneous TH of LA are scarce. In particular, the generation of GLV from
aqueous mixtures of LA and HCOOH catalysed by the half-sandwich iridium
complex 84 (Scheme 65) has been reported [219].
More recently, Fischmeister and co-workers reported on the solvent free reduction of LA to GVL (Scheme 66) mediated by the dipyridylamine iridium complexes
85. Reactions were performed at 120–150
C, employing mixtures of HCOOH/NEt 3
as a hydrogen donor. Yields from 22 to 98% were obtained within 16 h. Complex
85c with the bulkiest ligand was found to be the less active catalyst [220].
The half-sandwich complex 28 (Scheme 7) catalysed the transformation of LA to
GVL. Under standard TH conditions (1 mol% of catalyst, iPrOH, 85
C), 69% yield
was obtained within 12 h [76].
Furfural is one of the organic compounds readily available from nonedible
biomass (corncobs, oat hulls, bagasse, etc.) [221]. Furfural is mainly used as a raw
material for the synthesis of a multitude of important nonpetroleum-derived
chemicals such as furfuryl alcohol, methyltetrahydrofuran and furan [222]. In this
line, Grushin and van Leeuween et al. reported a highly efficient chemo- and
stereoselective synthesis of chiral hydrofuroins via asymmetric TH of racemic furoin
that is prepared from furfural (Scheme 67) using the half-sandwich iridium complex
[Cp*IrCl((S,S)-TsDPEN)] as catalyst precursor [223].
On the other hand, solvents often account for most of the mass wasted in
syntheses and processes [224]. The utilisation of biogenetic alcohols such as glycerol as a H-donor solvent in catalytic TH reactions is an interesting contribution to
the field of sustainable chemistry. Glycerol is a non-toxic and non-flammable
84
H 2 O Ir
Cp*
N
N
SO 4
OMe
MeO
Cl
Ir
Cp*
N
N
SO 2 Me
28
Scheme 65 Iridium
catalysts for the TH of
levulinic acid
H 2 O Ir
Cp*
N
N
SO 4
R
R
O
OH
O
O
O
HCOOH/NEt 3
85
LA
GVL
R = H (85a), CH 2 Ph (85b), CH(CH 3 )Ph (85c)
Scheme 66 Transfer
hydrogenation of
levulinic acid
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
131