Pincer compounds 69 (Scheme 37), equipped with different functional groups in
the secondary coordination sphere, were employed as the catalysts in the
chemoselective TH of nitroarenes (Scheme 43) to anilines. Reactions were carried
out in dimethoxyethane, at 60–80
C with a catalyst loading of 1 mol%, with 2 equiv.
of HCOOH/NEt 3 azeotrope as a hydrogen source. Although reduction of nitroarenes
to amines is often accompanied by incomplete reduction to hydroxylamines or
azocompounds, almost complete conversion of nitrobenzene into aniline (99%)
with excellent selectivity (99%) was achieved with catalyst 69a. The
OMe-modified catalyst 69b gave a more modest conversion but with the same
selectivity [180].
Methyl- (E4), amino- (E9) and chloro- (E10-E12) substituted nitrobenzenes were
fully converted into the corresponding anilines. Cyano- (E18), amido- (E16), ester(E17) and keto- (E15) substituted nitrobenzenes as well as 2-nitrostyrene displayed
moderate chemoselectivity owing to partial reduction of the functional group present
in the substrates [180].
In contrast with the well-established TH of aldehydes, ketones and imines, TH
reactions of nonactivated C-C multiple bonds have been much less studied. Huang’s
group has recently demonstrated that a series of N,C,P-pincer iridium(III) complexes
(70–72, Scheme 37) efficiently reduced a wide variety of alkenes and alkynes
[181, 182] (Scheme 6). Notably, ethanol was employed as a solvent and as a
hydrogen donor. Ethanol is an abundant, sustainable and environmentally benign
source of hydrogen. However, it has been seldom applied to TH reactions [38, 176,
177, 179, 183–184] because acetaldehyde, its dehydrogenation product, readily
undergoes metal-mediated decarbonylation leading to catalytically inactive metal
carbonyl species. In Huang’s system EtOAc was the only detectable byproduct.
Probably, the acetaldehyde resulting from EtOH dehydrogenation reacts with
another molecule of EtOH to yield hemiacetal, which is further dehydrogenated to
EtOAc thus eliminating the possibility of catalyst poisoning by the
acetaldehyde [181].
Complex 72 gave the best results. Thus, for example, upon activation with
tBuONa, this complex (1 mol%) completely reduced both cyclooctene and
1-octene at 60
C within 20 min. Under these conditions, other nonactivated (D17,
D25, D31, D33, D36, D37, D39-D48, D52-D56), aryl (D1, D5-D7, D11, D13, D18,
D23, D24) and electron-rich (D28, D38) alkenes were efficiently reduced. Yields
from 65 to >99% were obtained, within 0.5–12 h [181].
Complex 72 was also active for the full reduction of alkynes. Nonactivated
aliphatic alkynes, 6-dodecyne and alkynes bearing Cl (D128) or OH (D127) functionalities were reduced (94–97% yield, 1.5 h) to the corresponding alkanes using
1 mol% of 72, at 80
C. Ethyl 2-octynoate and internal alkynes (D57, D58, D88,
D90) gave the reduction products in >95% isolated yield, at 2 mol% of catalyst
loading. Reduction was always complete due the good activity of catalyst 72 for
alkene TH [181].
However, complex 72 can be used for the semi-hydrogenation of a range of
internal alkynes [182]. Ethanol was also employed as a solvent, and E-alkenes were
stereoselectively obtained. Transformations can be stopped at the alkene step
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
111
the secondary coordination sphere, were employed as the catalysts in the
chemoselective TH of nitroarenes (Scheme 43) to anilines. Reactions were carried
out in dimethoxyethane, at 60–80
C with a catalyst loading of 1 mol%, with 2 equiv.
of HCOOH/NEt 3 azeotrope as a hydrogen source. Although reduction of nitroarenes
to amines is often accompanied by incomplete reduction to hydroxylamines or
azocompounds, almost complete conversion of nitrobenzene into aniline (99%)
with excellent selectivity (99%) was achieved with catalyst 69a. The
OMe-modified catalyst 69b gave a more modest conversion but with the same
selectivity [180].
Methyl- (E4), amino- (E9) and chloro- (E10-E12) substituted nitrobenzenes were
fully converted into the corresponding anilines. Cyano- (E18), amido- (E16), ester(E17) and keto- (E15) substituted nitrobenzenes as well as 2-nitrostyrene displayed
moderate chemoselectivity owing to partial reduction of the functional group present
in the substrates [180].
In contrast with the well-established TH of aldehydes, ketones and imines, TH
reactions of nonactivated C-C multiple bonds have been much less studied. Huang’s
group has recently demonstrated that a series of N,C,P-pincer iridium(III) complexes
(70–72, Scheme 37) efficiently reduced a wide variety of alkenes and alkynes
[181, 182] (Scheme 6). Notably, ethanol was employed as a solvent and as a
hydrogen donor. Ethanol is an abundant, sustainable and environmentally benign
source of hydrogen. However, it has been seldom applied to TH reactions [38, 176,
177, 179, 183–184] because acetaldehyde, its dehydrogenation product, readily
undergoes metal-mediated decarbonylation leading to catalytically inactive metal
carbonyl species. In Huang’s system EtOAc was the only detectable byproduct.
Probably, the acetaldehyde resulting from EtOH dehydrogenation reacts with
another molecule of EtOH to yield hemiacetal, which is further dehydrogenated to
EtOAc thus eliminating the possibility of catalyst poisoning by the
acetaldehyde [181].
Complex 72 gave the best results. Thus, for example, upon activation with
tBuONa, this complex (1 mol%) completely reduced both cyclooctene and
1-octene at 60
C within 20 min. Under these conditions, other nonactivated (D17,
D25, D31, D33, D36, D37, D39-D48, D52-D56), aryl (D1, D5-D7, D11, D13, D18,
D23, D24) and electron-rich (D28, D38) alkenes were efficiently reduced. Yields
from 65 to >99% were obtained, within 0.5–12 h [181].
Complex 72 was also active for the full reduction of alkynes. Nonactivated
aliphatic alkynes, 6-dodecyne and alkynes bearing Cl (D128) or OH (D127) functionalities were reduced (94–97% yield, 1.5 h) to the corresponding alkanes using
1 mol% of 72, at 80
C. Ethyl 2-octynoate and internal alkynes (D57, D58, D88,
D90) gave the reduction products in >95% isolated yield, at 2 mol% of catalyst
loading. Reduction was always complete due the good activity of catalyst 72 for
alkene TH [181].
However, complex 72 can be used for the semi-hydrogenation of a range of
internal alkynes [182]. Ethanol was also employed as a solvent, and E-alkenes were
stereoselectively obtained. Transformations can be stopped at the alkene step
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
111