concerted proton-hydride abstraction takes place in an associative event implying
two molecules of donor. Further synthetic studies and DFT calculations pointed out
that the active species should imply the pyridinyl ligand acting as a non-innocent
platform.
10 Conclusions
Although the first examples of TH date back to the first quarter of the twentieth
century, the systematic development of this methodology started in the early 1970s.
Despite the inherent advantage that the use of a sacrificial hydrogen donor presents
over the employ of hazardous molecular hydrogen, the good results that were
obtained in hydrogenation placed TH as a non-competitive variant with respect to
H 2 -hydrogenation for the reduction of unsaturated substrates.
Two fundamental milestones changed the landscape of the TH in the 1990s. In
1991, Bäckwall, based on mechanistic proposals, introduced the use of bases as an
additive in TH reactions. Increases of several orders of magnitude in reaction rate
supported this innovation. In the second half of the 1990s, Noyori presented a new
catalytic system that meant a conceptual change in the mechanism of TH reactions
and allowed almost perfect enantioselectivities to be achieved. These events gave
rise to an explosive and fruitful development of the area in which HT has become a
powerful and efficient alternative in the field of the reduction of unsaturated species.
Iridium catalysts have played a fundamental role in this process since, after ruthenium catalysts, they have been the most widely used.
Most of the catalytic processes, including TH reactions, are carried out in organic
media. HT reactions in water can make the process cheaper and environmentally
more friendly. In this regard, some iridium catalysts show a high tolerance toward
water and acids. Therefore, they are appealing candidates to develop TH reactions in
water as a solvent and with formic acid as a hydrogen source.
Along with the search for more robust and efficient catalysts, the development of
environmentally benign catalytic systems is currently another key point. Iridium
catalysts have been shown to be compatible with the use of biomass-derived
compounds, such as glycerol or ethanol, as the solvents and the hydrogen donors
in TH reactions. Likewise, iridium-based catalysts have been successfully applied in
the transformation of this type of compounds into high added-value products.
Half-sandwich iridium compounds are among the most widely used as the
metallic part of metalloenzymes that have been successfully applied in TH reactions.
Artificial metalloenzymes have the unique characteristic of being able to be
improved both from a chemical and a genetic point of view. This feature provides
them with immense room for improvement in order to optimise them and obtain
catalysts with the desired capabilities. On the other hand, new anticancer strategies
associated with the use of NADH or NAD(P)H as hydrogen donor in TH reactions
mediated by iridium complexes have also been developed.
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
145
two molecules of donor. Further synthetic studies and DFT calculations pointed out
that the active species should imply the pyridinyl ligand acting as a non-innocent
platform.
10 Conclusions
Although the first examples of TH date back to the first quarter of the twentieth
century, the systematic development of this methodology started in the early 1970s.
Despite the inherent advantage that the use of a sacrificial hydrogen donor presents
over the employ of hazardous molecular hydrogen, the good results that were
obtained in hydrogenation placed TH as a non-competitive variant with respect to
H 2 -hydrogenation for the reduction of unsaturated substrates.
Two fundamental milestones changed the landscape of the TH in the 1990s. In
1991, Bäckwall, based on mechanistic proposals, introduced the use of bases as an
additive in TH reactions. Increases of several orders of magnitude in reaction rate
supported this innovation. In the second half of the 1990s, Noyori presented a new
catalytic system that meant a conceptual change in the mechanism of TH reactions
and allowed almost perfect enantioselectivities to be achieved. These events gave
rise to an explosive and fruitful development of the area in which HT has become a
powerful and efficient alternative in the field of the reduction of unsaturated species.
Iridium catalysts have played a fundamental role in this process since, after ruthenium catalysts, they have been the most widely used.
Most of the catalytic processes, including TH reactions, are carried out in organic
media. HT reactions in water can make the process cheaper and environmentally
more friendly. In this regard, some iridium catalysts show a high tolerance toward
water and acids. Therefore, they are appealing candidates to develop TH reactions in
water as a solvent and with formic acid as a hydrogen source.
Along with the search for more robust and efficient catalysts, the development of
environmentally benign catalytic systems is currently another key point. Iridium
catalysts have been shown to be compatible with the use of biomass-derived
compounds, such as glycerol or ethanol, as the solvents and the hydrogen donors
in TH reactions. Likewise, iridium-based catalysts have been successfully applied in
the transformation of this type of compounds into high added-value products.
Half-sandwich iridium compounds are among the most widely used as the
metallic part of metalloenzymes that have been successfully applied in TH reactions.
Artificial metalloenzymes have the unique characteristic of being able to be
improved both from a chemical and a genetic point of view. This feature provides
them with immense room for improvement in order to optimise them and obtain
catalysts with the desired capabilities. On the other hand, new anticancer strategies
associated with the use of NADH or NAD(P)H as hydrogen donor in TH reactions
mediated by iridium complexes have also been developed.
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
145