ChemCatChem
10.1002/cctc.201900932
COMMUNICATION
In a finalizing set of experiments, a first insight towards a
catalytic system with integrated hydrogen management should
be generated. Thus, in a recycling experiment the reaction
mixture after transfer hydrogenation was re-pressurized with
molecular hydrogen and the formed ethyl acetate was
hydrogenated to ethanol in almost quantitative yield (Table 4,
entry 1,2). The increased amount of methanol after the recycling
cooled in an ice bath and the pressure was carefully released. The
resulting crude, clear reaction mixture was analyzed by 1H-NMR
spectroscopy/gas chromatography, using mesitylene as an internal
standard.
Keywords: molecular catalysis · transfer hydrogenation ·
carbon dioxide utilization · methanol
2
experiment can be explained by residual CO and ethyl formate
in the solvent. Furthermore, these experiments clearly
corroborate the stability of the catalytic system and pave the way
to an efficient process concept. In details running the reaction in
higher boiling alcohols like nBuOH and could clearly facilitate the
isolation of volatile MeOH.[5b] The hydrogenation of ethyl acetate
was not further optimized as already intensive investigations on
ester reduction with similar ruthenium catalyst systems are
reported.[17]
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2
Table 4. Catalytic transfer hydrogenation of CO to methanol with consecutive
rehydrogenation of ethyl acetate to ethanol.[a]
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entry
TONMeOH[b]
TONMeOH
b]
TONEtOAc [b]
conv. EtOAc
[%]
H [
2
+
1
2
14
17
45
42
89
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> 97
> 97
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[
a] reaction conditions:
n
[Ru] = 20.0 µmol,
nHNTf = 20 µmol (1.0 eq.),
2
5
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V
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H
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1
[4]
[5]
1818.
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HNTf = 20 µmol, VEtOH = 1.0 mL.
n[Ru] = 20.0 µmol,
n
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2
In summary, a molecular Ru/triphos/Lewis Acid catalyst
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ethanol as the hydrogen donor and solvent in combination with
Ru(triphosXyl)(tmm)] 3 and Lewis acid Zn(NTf
, resulted in
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2 2
)
TONMeOH up to 121. Detailed investigations allowed proposing
acceptorless dehydrogenation of ethanol, followed by the
6
187.
reduction of CO
2
via ethyl formate, as mechanistic basis.
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2
lead structure for CO -hydrogenation. Finally, this approach
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5
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1
Experimental Section
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General procedure for the transfer hydrogenation of carbon dioxide
to methanol: Under argon atmosphere the Ru/triphos catalysts (15.6 mg,
0.02 mmol), additives (0.02 mmol) were weighted in a glass insert,
equipped with a stir bar and subsequently placed in a high-pressure steel
autoclave. The alcohol (2.0 mL) was added through a cannula under
argon. Once the autoclave was sealed, and pressurized with 50 bar of
1
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2
CO , it was stirred for 22 hours at 140 ˚C. Afterwards the reaction was
1
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