Inorganic Chemistry
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hydrogenation of CAL, along with the expected product
cinnamyl alcohol (COL), the byproducts hydrocinnamalde-
hyde (HCAL) and hydrocinnamic alcohol (HCOL) are often
observed (Scheme 1b).
that a fast hydride-transfer reaction may occur from the Ru-
metal center to the carbonyl group of CAL, even under low H2
pressure. The reactivity of catalyst 1 was further investigated
with an increased S/C of 5000 under optimal reaction
conditions (Table 1, entry 16). A high TON of 2750 was
obtained after 3.0 h with a very high TOF of 920 h−1, and the
selectivity of the allylic alcohol product was maintained at 98%,
demonstrating that catalyst 1 was durable and active. However,
the initial TOF after 9% conversion under the similar
conditions of Table 1, entry 16, was determined to be 380 h−1.
Additionally, the homogeneous catalyst Ru-Macho had
shown hydrogenation activity superior to that of catalyst 1;
however, because of the high reactivity in the homogeneous
conditions, the selectivity for COL was only 83% and a 17%
HCOL formation was observed (Table 1, entry 17).
After exploration of the catalytic activity of catalyst 1 for the
selective hydrogenation of CAL, the scope and limitations of
catalyst 1 for the selective hydrogenation of carbonyl groups in
the presence of various other reducible functional groups, such
as nitrile, nitro, halogen, ester, carboxylic acid, and olefin, were
also evaluated under the optimized conditions. The results are
As seen in Table S1, the heterogenized catalyst 1 exhibited
high chemoselectivity for hydrogenation of the carbonyl group
over nitrile, carboxylic ester, and carboxylic acid functional
groups (Table S1, entries 1−3). Both the nitrile and ester
groups were tolerated during hydrogenation of the carbonyl
group with excellent conversion rates (Table S1, entries 1 and
2). Although the homogeneous Ru-Macho complex was
reported to be an active catalyst for the hydrogenation of
nitrile and ester functional groups, the soothingly mild reaction
conditions required for the reduction of aldehyde and keto
groups inherited high chemoselectivity in the case of catalyst 1.
The carboxylic group was not affected under the hydro-
genation conditions; however, it was shown to affect the
reaction rate (64%) for hydrogenation of the carbonyl group of
4-carboxybenzaldehyde (Table S1, entry 3).
However, when 4-bromobenzaldehyde was employed as a
substrate, hydrodebromination also occurred along with the
reduction of a carbonyl group (Table S1, entry 4). Similarly, in
the case of 4-nitrobenzaldehyde as the substrate, hydro-
genation of both the nitro and carbonyl functional groups
occurred (Table S1, entry 5). Thus, carbonyl compounds
containing the highly labile arylhalogen and nitro groups are
unsuitable substrates for the selective hydrogenation by
catalyst 1. Furthermore, p-anisaldehyde and o-tolualdehyde
are amenable substrates and are completely converted to the
corresponding alcohols (Table S1, entries 6 and 7).
Acetophenone, a ketone, was also smoothly hydrogenated to
1-phenylethanol by catalyst 1 (Table S1, entry 8). The
heterocyclic carbonyl compound furfural was hydrogenated to
furfuryl alcohol by catalyst 1 with a 100% conversion rate
Initially, the catalytic ability of catalyst 1 for the chemo-
selective hydrogenation of CAL to COL was tested in water
(H2O) with a substrate-to-catalyst ratio (S/C) of 200 in the
presence of 100 mol % potassium hydroxide (KOH) at 40 °C
under 1.0 MPa of H2 pressure for 1 h. However, the use of
H2O as a solvent results in negligible formation of COL,
although with high selectivity (99%; Table 1, entry 1), which is
possibly due to the low dispersibility of catalyst 1 in H2O
arising from its low density. Therefore, other organic solvents
were tested for their suitability in the chemoselective
hydrogenation of CAL to COL using catalyst 1 (Table 1,
entries 2−6). As seen in Table 1, catalyst 1 was inactive in the
nonpolar solvent toluene. Notably, the catalytic conversion
considerably increases in polar solvents, such as alcohols and
tetrahydrofuran (THF). Among the tested alcoholic solvents,
methanol (MeOH) showed the highest conversion (98%) and
selectivity (99%) toward COL with a high turnover number
(TON) of 190. The higher solubility of the base and H2 gas in
MeOH might be responsible for this high reactivity. Addition-
ally, the aprotic polar solvent THF showed a reactivity similar
to that of MeOH; however, the latter was preferred as a solvent
in this study because the simple alcohol is an environmentally
benign and greener solvent compared to THF.
The influence of a base on the selective hydrogenation of
CAL to COL by catalyst 1 was then studied using a series of
inorganic and organic bases, such as Et3N, KHCO3, K2CO3,
K3PO4, and KOH (Table 1, entries 2 and 7−10). It is observed
that the addition of a strong base is favorable for the
conversion of CAL to COL because in the presence of the
organic base Et3N or the mild inorganic base KHCO3 catalyst
1 shows negligible reactivity in the hydrogenation of CAL
(<5% conversion; Table 1, entries 7 and 8).
The role of a base in the activation of Ru-Macho and related
PNHP-Ru complexes has been reported in the litera-
ture.25,34,58−80 The base assists in the initial formation of the
“activated Ru-Macho” via dehydrochlorination, as shown in
Scheme S1. In the case of 1, a series of dehydrochlorination
and hydrogenation steps may be required to form the
catalytically active Ru−H intermediate (Scheme S2), which
necessitates the presence of a base in the current study. In line
with this, in the absence of a base additive, only the acetal
product formed from the reaction of CAL and MeOH was
detected, and no COL formation was observed (Table 1, entry
11).
The reaction temperature significantly affects the outcome of
the CAL reduction by catalyst 1. At a higher temperature of 80
°C, 17% HCOL was formed as a result of additional reduction
of the resulting COL (Table 1, entry 12). The selectivity for
COL was only 83% in this case. However, decreasing the
reaction temperature to 20 °C showed a reduction in the
reaction rate, although the selectivity for COL was maintained
over 99% (Table 1, entry 13). Therefore, the optimum
temperature was maintained at 40 °C in this study.
Interestingly, catalyst 1 shows similar reactivity under
different H2 pressures ranging from 0.5 to 2.0 MPa (Table 1,
entries 2, 14, and 15). Under a higher pressure of 2.0 MPa,
only a negligible amount of HCOL was observed, whereas
employing lower H2 pressures of 1.0 and 0.5 MPa has shown
similar TON and selectivity for COL formation. This indicates
The chief benefit of a heterogeneous catalyst entails its usage
in multiple-recycling runs with appreciable reactivity. There-
fore, the recyclability of catalyst 1 in the chemoselective
reduction of CAL was evaluated. As shown in Figure 2, catalyst
1 had shown recyclable catalytic activity at a reduced S/C of
100 for at least four cycles with high chemoselectivity.
Markedly, after the fourth cycle, the conversion of CAL
remained at 94% with a 97% chemoselectivity. Notably, the
TEM−EDX analysis of the spent catalyst 1 suggested that Ru
was maintained in the POMP support (Figure S6), which
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Inorg. Chem. XXXX, XXX, XXX−XXX