A new designed hydrazine group-containing ruthenium complex used for catalytic hydrogenation of esters

Article abstract of DOI:10.1039/c5cc04242a

A hydrazine group-containing nitrogen-phosphine ligand and corresponding ruthenium complexes were synthesized. When these complexes were used for hydrogenation of esters, excellent performance was observed (TON up to 17 200). A wide substrate scope was suitable for this catalytic system.

Full text of DOI:10.1039/c5cc04242a

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A New Designed Hydrazine Group-contained
Ruthenium Complex Used for Catalytic Hydrogenation
of Esters
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Xuefeng Tan, Qingli Wang, Yuanhua Liu, Fangyuan Wang, Hui Lv*, and Xumu
Zhang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
9
efficiency of catalytic ester hydrogenation. Very recently, Zhou and
our group respectively reported the bipyridyl-containing tetradentate
A hydrazine group-contained nitrogen-phosphine ligand
and correlated ruthenium complexes were synthesized.
When these complexes were used for hydrogenation of
esters, excellent performance was observed (TON up to
5
l
ligands and a pyridin-2-ylmethanamine-containing tetradentate
ligand, both of them exhibited excellent catalytic activities in ester
5m
reduction. However, many of the research works centered on the
1
7200). A wide substrate scope was suitable for this electron and stereo-properties of ligands or other additives, and scare
catalytic system.
new motifs have been investigated for ligand designing. Therefore,
to develop new and efficient ester reduction catalyst is highly
The reduction of esters to their corresponding alcohols is one of the desirable.
most important chemical transformations in organic synthesis.
Traditionally, this reaction was commonly performed with LiAlH
4
1
and NaBH
4
, which are hazardous to operate and have tedious
2
workups. Due to the poor reactivity of esters relative to ketones,
catalytic hydrogenation of esters mainly proceed with heterogeneous
3
catalysts under harsh conditions (200-300 ºC, 200-300 atm), which
is an industrially important process to obtain long-chain alcohols. In
order to reduce the high energy consumption and high cost
associated with heterogeneous catalyst, developing highly active
homogeneous catalytic systems to replace the traditional way has
become one of themost important targets for chemists.
We notice that most of homogenous ester reduction catalysts
bearing the N-H groups, which can activate carbonyl groups by
Giving the earlier stages of homogeneous catalytic hydrogenation hydrogen bond and provide a proton to carbonyl oxygen atom.
1
0
of esters to alcohols, many more effective catalysts with novel ligand Based on these facts and mechanism studies, we envision that
structures are discovered other than the few patented systems and replacing the N-H group to a NH-NH group (hydrazine group)
introduction of new catalysts can promote broad usages in organic whether can make sense for ester reduction as the N-H bond density
chemistry. Although great efforts have been devoted to this in per unit increased greatly and electronic property on N-H group
4
-5
transformation, progresses have been made until recent years.
Representative ester reduction catalysts are listed in Figure 1. In group contained ligand L based on the model of Takasago’s Ru-PNP
006, Milstein and co-workers developed the PNN pincer-type catalyst III. The novel ligand L can be obtained by a short synthetic
changed a lot in hydrazine group. Hence we designed a hydrazine
2
6
ruthenium complex I (Figure 1), which undergoes an aromatization- route described in Scheme 1. Several ruthenium precursors were
dearomatizaiton process when used to reduce esters under a used for the coordination reaction under different conditions as the
relatively mild condition (5 atm H
co-workers (in Firmenich SA) reported and patented RuCl
and a tetradentate RuCl (PNNP) catalyst which could efficiently method, firstly precipitated the complexes with ethyl ether and then
2
, 115 ºC). In 2007, Saudan and table in Scheme 1 illustrated.
2
(P-N) (II)
2
All the preparations of A-G were handled with a commonly used
2
catalyze the hydrogenation of aromatic and aliphatic esters in the washed with ether for several times. Most of the directly obtained
7
presence of a base. A good chemo-selectivity can be obtained when crystalline products were miscellaneous and structures were difficult
it used in hydrogenation of esters and alkenes. In 2011, Kuriyama to be confirmed by NMR or other spectra analysis, which may be
and co-workers from Takasago company reported a simple tridentate due to the coordinating diversity of the hydrazine group. In order to
8
ruthenium complex III, which can be used in hydrogenation of a evaluate the catalytic hydrogenation efficiency of complex A-G,
wide substrate scope of esters, especially for methyl (R)-lactate methyl benzoate was used as standard substrate, with 2.5 mol%
8
d
t
(
TON ≈ 2000 by a 2200 kg scale). Subsequently, Gusev developed KO Bu as additive and under 50 atm of H at 80 ºC . We set the
2
an electron-rich ruthenium complex IV which greatly improved the substrate/catalyst ratio as per milligram catalyst react with 5 mmol
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substrate (Table 1). Several ruthenium precursors, such as conversion was observed (96%), accompanied with 9% of alkenyl
RuCl (PPh [RuCl (p-cymene)] and RuCl (DMSO) were group was hydrogenated.
2
3
)
3
,
2
2
2
4
capable of obtaining high efficient catalysts for methyl benzoate
reduction, while CO containing precursor RuHCl(CO)(PPh ) gave
3
3
unsatisfied result. Interestingly, for the same ruthenium precursor,
e.g. RuCl (PPh [RuCl (p-cymene)] the higher reaction
2
3
)
3
,
2
2
,
temperature for catalyst preparation, the better catalytic activity
performed (Table 1, entries 1-8). In order to finding out the optimal
catalyst precursor, we maximized the S/C ratio of catalysts C, D and
E (Table 1, entries 4, 6 and 7), and found that catalyst E gave the
best result.
In order to further optimize the reaction conditions, a series of
solvents, bases and catalyst loading (in situ form, e.g., 1 mmol
[
2 2
RuCl (p-cymene)] and 1.1 mmol L were used for catalyst
preparation according to Scheme 1, if we need 0.001 mmol catalyst,
then 1/1000 weight of the total catalyst would be weighed) were
investigated. As shown in Table 2, different solvents have great
influence on this reaction and dioxane gave excellent performance
(
Table 2, entries 1-9). Subsequently, we screened a number of bases
as additives to activate the catalyst. Results showed that inorganic
bases (e.g. NaOH, Cs CO , Table 2, entries 19-20) gave unsatisfied
2
3
results, while alkali metal alkoxides were capable of this reaction.
Interestingly, the alkali metal sodium performed better than lithium
and potassium bases. At last, we chose NaOEt with a 5 mol%
loading as the standard base additive (Table 2, entries 10-18). Under
the optimized reaction conditions, a turnover number of 17200
(Table 2, entry 15) was obtained for methyl benzoate hydrogenation,
compared with Takasago’s catalyst III of a 1000 turnover number
8
d
ever reported, which greatly demonstrated the hydrazine group has
positiveeffects on ester reduction.
Then we investigated the substrate scope under the optimized
reaction conditions in the presence of E (Table 3). It’s obvious that
aromatic esters gave more satisfied results than aliphatic esters,
except for methyl 4-fluorobenzoate (Table 3, entries 1-3). Most of
the aliphatic esters catalyzed by E with moderate turnover numbers,
except for ethyl acetate obtained a TON of 9100 (Table 3, entries 6-
9
). A comparison of dimethyl esters, dimethyl o-phthalate and
dimethyl terephthalate (Table 3, entries 10-11), we can see that
In our attempts to obtain the catalysts structures of A-E, many
ortho-substituted diesters hardly converted to diols with equal crystallization experiments had been tried, but failed to obtain a
activities as other substrates, which may due to the product crystal structure. Then we carried out tracking experiments to
inhibition because ortho-position substitution in favor of the analyze the reaction mixture of the preparation of catalysts A-E,
chelating with the metal center. For alkenyl group containing esters, monitoring the disassociated ligand PPh
3
and p-cymene (GC
such as methyl 3-cyclohexenecarboxylate (Table 3, entry 12), good analysis, n-tridecane as internal standard). For catalysts A-C, the
2
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substitution ratio (actually, no H NMR signals of p-cymene can be
seen at this stage). These experiments showed the exact high activity
3
species are the complexes that PPh and p-cymene disassociated
1
31
completely (catalysts C and E). H and P NMR spectra showed the
structure of catalysts C - G are extremely complicated, and high-
2
resolution mass spectra revealed there existed RuCl (L) and its
dimer and trimer (see Supporting Information). This may be due to
the deficient coordinating atoms to fulfill the 18 electron
compositions and solvents participated the coordination or even one
hydrazine group coordinated with two ruthenium to form the dimer,
trimer or oligomer.
Conclusions
In conclusion, we have synthesized a hydrazine group contained
nitrogen-phosphine ligand and its ruthenium complexes. These
catalysts were successfully used in catalytic hydrogenation of esters
to alcohols. A variety of esters have been reduced with high
efficiency, TON up to 17200 for reduction of aromatic esters. This
new motif can help us to develop more efficient catalysts for ester
reduction in thefield.
Acknowledgements
We are grateful for the financial support by a grant from
Wuhan University (203273463), Natural Science Foundation of
Hubei Province (2014CFB181), “111” Project of the Ministry
of Education of China and the National Natural Science
Foundation of China (Grant No. 21372179, 21432007).
Notes and references
Key Laboratory of Biomedical Polymers of Ministry of Education &
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan,
Hubei 430072, China
Email: huilv@whu.edu.cn , xumu@whu.edu.cn
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
1
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3
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3
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3
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3
1
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3
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3
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