Under the optimal conditions (0.02 mol% of catalyst loading, 4 equiv. of
HCOOH, 2 mL of water, 80
C) high conversion (91–99%) was obtained for most
of the substrates tried, in about 15 min [110].
The catalytic process is chemoselective. Thus, for example, only the aldehyde
group of the ketonic substrate A34 was reduced. The system also works at a catalyst
loading as low as 0.005 mmol %, and, notably, a TOF value of 73,800 h
À1 , at 49% of
conversion, was achieved for 4-methoxybenzaldehyde [110].
However, the group of Tang and Xu demonstrated that properly adjusting the
reaction conditions, ketones can be efficiently reduced employing the same type of
imidazolyl-pyridine iridium complexes. The most important changes in the reaction
conditions were that the amount of formic acid was increased up to 12 equiv. and that
the catalytic reactions were carried out under inert atmosphere [111]. Under these
conditions, aryl alkyl (B1, B3-B5, B9, B10, B12-B15, B20-B26, B27, B37, B47),
methyl alkyl (B100, B104, B107), methyl heterocyclic (B38, B41) and cyclic (B118,
B119, B121) ketones were hydrogenated. Yields between 57 and >99% were
obtained after 4–12 h of reaction. The reduction rates are dependent on the pH
values of the reaction media, conversions dramatically decreasing when pH
increases. The standard S/C ratio employed was 10,000 but catalysis also occurs at
an S/C ratio of 100,000. A TOF of 26,000 h
À1 , at 52% of conversion, was achieved
in the reduction of acetophenone, employing catalyst 55a [111]. Furthermore, a wide
range of ketones containing a variety of functional groups such as aryloxy (B106),
halogens (B57, B64), cyano (B63–65) or ester (B54, B61, B108) was also reduced.
Typically, conversions higher than 90% were obtained, at 80
C, within
3–12 h [111].
The Fu’s and Zhou’s group reported on the asymmetric TH of non-orthosubstituted-2-pyridylketone N-oxides (Scheme 4) using chiral diamine-derived iridium complexes (56) (Scheme 14). A H 2 O/iPrOH mixture and sodium formate were
employed as the solvent and the hydrogen source, respectively. The N-oxide function was removed by treating the resulting alcohols with Zn/NH 4 Cl [112].
After screening, by examining the reduction of 2-(4-chlorobenzoyl)pyridine Noxide (B80), complex 56a was selected as the best catalyst, in terms of yield and
enantioselectivity. Under the optimised conditions (5 mol% of catalyst loading,
10 equiv. of HCOONa, H 2 O/iPrOH, v/v:1/1, RT, 24 h), a wide variety of
2-pyridyl ketone N-oxides with different functionalities and electronic properties
(B74-B90) were reduced. Yields from 56 to 90% and e.r.’s from 78.7/26.3 to
99.1/0.9 were obtained [112].
Structurally diverse aldehydes and ketones were reduced, in water, with formic
acid as a hydrogen source, by using half-sandwich iridium complexes 57 as catalyst
precursors (Scheme 14). Under aerobic conditions, the reduction of benzaldehyde
was completed with catalyst 57a within 9 min using an S/C ratio of 2,000. At 5,000
and 10,000 S/C ratios, reduction of benzaldehyde was not observed. However, under
nitrogen atmosphere, in degassed water, good results were obtained at S/C ratios as
high as 20,000 and 10,000 for aldehydes (including benzaldehyde) and ketones,
respectively.
92
M. Pilar Lamata et al.
HCOOH, 2 mL of water, 80
C) high conversion (91–99%) was obtained for most
of the substrates tried, in about 15 min [110].
The catalytic process is chemoselective. Thus, for example, only the aldehyde
group of the ketonic substrate A34 was reduced. The system also works at a catalyst
loading as low as 0.005 mmol %, and, notably, a TOF value of 73,800 h
À1 , at 49% of
conversion, was achieved for 4-methoxybenzaldehyde [110].
However, the group of Tang and Xu demonstrated that properly adjusting the
reaction conditions, ketones can be efficiently reduced employing the same type of
imidazolyl-pyridine iridium complexes. The most important changes in the reaction
conditions were that the amount of formic acid was increased up to 12 equiv. and that
the catalytic reactions were carried out under inert atmosphere [111]. Under these
conditions, aryl alkyl (B1, B3-B5, B9, B10, B12-B15, B20-B26, B27, B37, B47),
methyl alkyl (B100, B104, B107), methyl heterocyclic (B38, B41) and cyclic (B118,
B119, B121) ketones were hydrogenated. Yields between 57 and >99% were
obtained after 4–12 h of reaction. The reduction rates are dependent on the pH
values of the reaction media, conversions dramatically decreasing when pH
increases. The standard S/C ratio employed was 10,000 but catalysis also occurs at
an S/C ratio of 100,000. A TOF of 26,000 h
À1 , at 52% of conversion, was achieved
in the reduction of acetophenone, employing catalyst 55a [111]. Furthermore, a wide
range of ketones containing a variety of functional groups such as aryloxy (B106),
halogens (B57, B64), cyano (B63–65) or ester (B54, B61, B108) was also reduced.
Typically, conversions higher than 90% were obtained, at 80
C, within
3–12 h [111].
The Fu’s and Zhou’s group reported on the asymmetric TH of non-orthosubstituted-2-pyridylketone N-oxides (Scheme 4) using chiral diamine-derived iridium complexes (56) (Scheme 14). A H 2 O/iPrOH mixture and sodium formate were
employed as the solvent and the hydrogen source, respectively. The N-oxide function was removed by treating the resulting alcohols with Zn/NH 4 Cl [112].
After screening, by examining the reduction of 2-(4-chlorobenzoyl)pyridine Noxide (B80), complex 56a was selected as the best catalyst, in terms of yield and
enantioselectivity. Under the optimised conditions (5 mol% of catalyst loading,
10 equiv. of HCOONa, H 2 O/iPrOH, v/v:1/1, RT, 24 h), a wide variety of
2-pyridyl ketone N-oxides with different functionalities and electronic properties
(B74-B90) were reduced. Yields from 56 to 90% and e.r.’s from 78.7/26.3 to
99.1/0.9 were obtained [112].
Structurally diverse aldehydes and ketones were reduced, in water, with formic
acid as a hydrogen source, by using half-sandwich iridium complexes 57 as catalyst
precursors (Scheme 14). Under aerobic conditions, the reduction of benzaldehyde
was completed with catalyst 57a within 9 min using an S/C ratio of 2,000. At 5,000
and 10,000 S/C ratios, reduction of benzaldehyde was not observed. However, under
nitrogen atmosphere, in degassed water, good results were obtained at S/C ratios as
high as 20,000 and 10,000 for aldehydes (including benzaldehyde) and ketones,
respectively.
92
M. Pilar Lamata et al.