Communications
a was reduced by HCOONH , although at a much lower rate
which led to quantitative formation of 1,4-phenylenedimetha-
nol in 10 h (Table 2, entry 7). Interestingly, with a lower amount
4
than by the catalytic pathway. The use of HCOOH/NEt =5:2
3
and 1:1 mixtures (1.5 and 4 equiv. of formic acid, respectively)
in the presence of 2 at S/C=5000 and 2000 gave 48–50% con-
version to the alcohol after 24 h (Table 1, entries 10 and 11),
whereas with HCOONa the conversion was 2% in 14 h (Table 1,
entry 12). These results indicated that inexpensive HCOONH4
could be employed as a practical hydrogen donor for the se-
lective reduction of a catalyzed by robust pincer complexes 1–
of HCOONH (1.5 equiv.), the TH of f afforded the reduction
4
products 4-(hydroxymethyl)benzaldehyde/1,4-phenylenedime-
thanol in a 9:1 molar ratio (Table 2, entry 8). Also, heteroaro-
matic 2-formylfuran (g) and 2-formylthiophene (h) were selec-
tively reduced to alcohols with conversions of 98 and 97% at
S/2=10000 in 20 and 24 h, respectively (Table 2, entries 9 and
10). Unsaturated trans-cinnamaldehyde (i) gave almost com-
plete conversion (97%) to trans-cinnamol (91%) and 3-phenyl-
propan-1-ol (6%) at S/2=2000 in 10 h, whereas at a lower
loading (S/2=5000), formation of 85% trans-cinnamol and
12% saturated alcohol was observed in 48 h, which indicated
that higher selectivity was achieved at a higher catalyst load-
ing and within a shorter reaction time (Table 2, entries 11 and
12). On the other hand, trans-a-methylcinnamaldehyde (j) was
chemoselectively transformed into trans-a-methylcinnamol
and was isolated in 88% yield at S/2=5000 in 20 h with no re-
duction of the C=C bond (Table 2, entry 13). In addition, the ali-
phatic aldehydes hexanal (k) and rac-citronellal (l) were re-
duced to 1-hexanol and rac-citronellol with conversions of 95
and 99% with 2 at S/C=5000 (Table 2, entries 14 and 15). By
contrast, with 2 (S/C=2000) the TH of vanillin and pyrrole-2-
carboxaldehyde, with relatively acidic hydrogen atoms, failed,
which led to unreacted starting material. Notably, cis-Ru-
3
. Preliminary experiments with other media different than the
toluene/water system, such as methanol/water mixtures or
pure methanol, led to poor conversions and poor selectivities
owing to the formation of aminative condensation/reduction
side products.
To broaden the scope of the TH of aldehydes with
HCOONH , aromatic, aliphatic, conjugated, and heteroaromatic
4
aldehydes were studied with complex 2. Reduction of 4-bro-
mobenzaldehyde (b) (2m in toluene) with HCOONH (4 equiv.)
4
resulted in conversions of 97 and 98% to the corresponding al-
cohol in 10 and 24 h by using 2 at S/C=2000 and 10000, re-
spectively (Table 2, entries 1 and 2), whereas 75% conversion
to the alcohol was obtained at S/C=20000 (Table 2, entry 3).
Table 2. TH of aldehydes (2m in toluene) with HCOONH
water) catalyzed by complex 2 in toluene/water 908C.
4
(4 equiv., 2m in
Cl (ampy)(dppf) [ampy=2-(aminomethyl)pyridine; dppf=1,1’-
2
[
a]
bis(diphenylphosphino)ferrocene; S/C=2000], which is a com-
plex related to 3 and known to catalyze the TH of aldehydes
with 2-propanol efficiently, was significantly less active in the
TH of benzaldehyde (18% conversion, 15 h) with HCOONH4
(4 equiv.). All spectral data were in the agreement with the lit-
erature, as all the obtained compounds are known.
Entry
Aldehyde
S/C
2000
t [h]
Conversion [%]
1
2
3
4
5
6
7
8
9
b
b
b
c
d
e
f
10
24
48
15
6
3.5
10
4
20
24
10
48
20
8
97
98
75
[29]
10000
20000
5000
2000
2000
2000
2000
10000
10000
2000
5000
5000
5000
5000
96
99, 87
99, 70
[
[
b]
b]
[
c]
As regards the mechanism, it is likely that the
98
99
98
97
97
97
98, 88
95
99
Ph
[
d]
f
RuCl(CNN )(PP) pincer complex in the presence of HCOONH
4
Ph
g
h
i
i
j
leads to the Ru(O CH)(CNN )(PP) formate complex with the
2
[
[
[
e]
f]
10
Ph
[30]
formation of the RuH(CNN )(PP) hydride by elimination of
1
1
g]
CO . Subsequent reaction with the RCHO substrate gives the
2
12
13
14
15
Ph
[
e]
[b]
Ru(OCH R)(CNN )(PP) alkoxide, which is protonated by
2
k
l
HCOONH to afford the alcohol product, ammonia, and the for-
4
[
e]
18
mate complex to close the cycle, as also inferred from thermal
conductivity detector (TCD) gas analysis, which showed that
CO and NH were evolved during the reaction. Given that the
[
1
a] The conversion and purity were determined by GC analyses and
H NMR spectroscopy. [b] Yield of isolated product. [c] Only the double-re-
duction product was detected. [d] With HCOONH (1.5 equiv.) a mixture
of 4-(hydroxymethyl)benzaldehyde/1,4-phenylenedimethanol in 9:1
ratio was observed. [e] [HCOONH ]=4m in water. [f] 91% of trans-cinna-
2
3
4
performance of HCOONH was better than that of HCOONa
4
a
and that of the HCOOH/NEt system, it is reasonable that the
3
4
mol and 6% of the saturated alcohol 3-phenylpropan-1-ol. [g] 85% of
elimination of NH during the catalysis has a positive effect
3
trans-cinnamol and 12% of 3-phenylpropan-1-ol.
and shifts the reaction toward the alcohol product, which pre-
À
vents a significant increase in the HO concentration, thus dis-
favoring base-catalyzed aldehyde side reactions. Control ex-
periments performed during the catalytic reduction of benzal-
dehyde and rac-citronellal showed that the pH values of the
aqueous phase were in the 7.5–8.5 range.
Electron-rich 4-(dimethylamino)benzaldehyde (c) was effi-
ciently reduced to the alcohol (96%) with S/2=5000 in 15 h
(Table 2, entry 4). Conversely, the TH of electron-poor 4-nitro-
benzaldehyde (d) and 4-cyanobenzaldehyde (e) resulted in
quantitative formation of the corresponding alcohols, isolated
in yields of 87 and 70% at S/2=2000, without reduction of
In conclusion, simple and functionalized aldehydes were
chemoselectively reduced to primary alcohols by using
HCOONH4 as a hydrogen donor with the benzo[h]quinoline
Ph
the NO and CN functionalities or deactivation of the catalyst,
RuCl(CNN )(PP) pincer complexes at substrate to catalyst
2
that is, by coordination at the metal center (Table 2, entries 5
molar ratios up to 20000. This straightforward reaction per-
formed with aldehydes of commercial-grade purity at a high
and 6). Double TH was observed for 4-formylbenzaldeyde (f),
ChemCatChem 2016, 8, 1 – 5
3
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