Selective Hydrogenation of Cyclic Imides to Diols and Amines and Its Application in the Development of a Liquid Organic Hydrogen Carrier

Article abstract of DOI:10.1021/jacs.8b04581

Direct hydrogenation of a broad variety of cyclic imides to diols and amines using a ruthenium catalyst is reported here. We have applied this strategy toward the development of a new liquid organic hydrogen carrier system based on the hydrogenation of bis-cyclic imide that is formed by the dehydrogenative coupling of 1,4-butanediol and ethylenediamine using a new ruthenium catalyst. The rechargeable system has a maximum gravimetric hydrogen storage capacity of 6.66 wt%.

Full text of DOI:10.1021/jacs.8b04581

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Communication
Selective Hydrogenation of Cyclic Imides to Diols and Amines and its
Application in the Development of a Liquid Organic Hydrogen Carrier
Amit Kumar, Trevor Janes, Noel Angel Espinosa-Jalapa, and David Milstein
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04581 • Publication Date (Web): 29 May 2018
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Selective Hydrogenation of Cyclic Imides to Diols and Amines and its
Application in the Development of a Liquid Organic Hydrogen Carrier.
Amit Kumar, Trevor Janes, Noel Angel Espinosa-Jalapa and David Milstein.*
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Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel.
Supporting Information Placeholder
ABSTRACT: Direct hydrogenation of a broad variety of cyclic
imides to diols and amines using a ruthenium catalyst is reported
here. We have applied this strategy towards the development of a
new Liquid Organic Hydrogen Carrier (LOHC) system based on
the hydrogenation of bis-cyclic imide that is formed by the
dehydrogenative coupling of 1,4-butanediol and ethylenediamine
using a new ruthenium catalyst. The rechargeable system has a
maximum gravimetric hydrogen storage capacity of 6.66 wt%.
Catalytic hydrogenation of polar bonds, in particular organic
carbonyl groups, offers a green and sustainable strategy to access
synthetically important building blocks such as alcohols and amines. We
and others have previously reported the catalytic hydrogenation of
ketones, esters, acids, anhydrides, amides and even more challenging
carbonyl groups, such as of carbonates, carbamates and urea derivatives,
to the corresponding alcohols and amines, as reviewed.1-9 Much
attention has also been paid to the hydrogenation of cyclic imides;
Ikariya10-11 and Bergens12 have reported the partial hydrogenation of
cyclic imides to hydroxyamides. Hydrogenative C-O bond cleavage of
cyclic imides to form lactams or cyclic amines has also been reported
independently by Drago,13 Bruneau,14 Beller15 and Agbossou-
Niedercorn.16 However, selective hydrogenation of cyclic imides to diols
and amines has been hitherto limited to a single substrate (N-
benzylphthalimide), recently reported by Lv and Zhang.17
Figure1. LOHCs reported by us based on the amide bond formation and
hydrogenation (top), selective hydrogenation of cyclic imides to diols and
amines; and the LOHC based on the formation of bis-cyclic imide and its
hydrogenation as described in this work (bottom).
Hydrogenation/Dehydrogenation of small organic molecules has
also attracted significant interest in recent years because of its potential
application in the development of Liquid Organic Hydrogen Carriers
(LOHCs).18-20 Taking advantage of the favorable thermodynamics in
amide bond formation by dehydrogenating coupling of alcohols and
amines,21 we recently reported LOHC systems based on the formation
of amide bonds from cheap and easily available starting materials such
as ethanolamine or ethanol/ethylenediamine and their hydrogenation
under mild conditions (Figure 1).22-23
Figure 2. Ruthenium complexes used in this report.
We commenced our investigation by exploring the catalytic utility
of complexes
1
-
3
(Figure 2) for the hydrogenation of N-
(1
benzylphthalimide. Under the catalytic conditions of complex
1
mol%), KOtBu (1 mol% ) in THF under 20 bar H2 at 110oC, 67%
conversion of N-benzylphthalimide was observed in 24 h with the
concomitant formation of benzylamine in 65% yield and 1,2-
benzenedimethanol in 60% yield. Formation of the cyclic ester
phthalide was also observed in 5% yield by 1H NMR spectroscopy and
GC-MS (See Table S1 for the optimization details). Under the same
Herein we report the first example of selective hydrogenation of a
broad variety of cyclic imides to diols and amines, using a ruthenium
catalyst under mild conditions. The rare selectivity of this
hydrogenation reaction allowed us to develop a new reversible hydrogen
storage system based on the formation of bis-cyclic imides (spent fuel)
from inexpensive and easily available starting materials 1,4-butanediol
conditions, precatalyst , which has been previously used for the
2
efficient hydrogenation of amides gave similar yields of the products as
24
that of precatalyst
1
.
Interestingly, employing complex (1 mol%) in
3
presence of 1 mol% KOtBu under the same reaction conditions as used
for resulted in better conversion of N-benzylphthalimide (84%
and ethylenediamine (charged fuel). Using ruthenium complexes
3
and
4
we show that both the dehydrogenation and hydrogenation processes
1
can occur to make a reversible hydrogen storage system with the high
conversion) to 1,2-benzenedimethanol and benzylamine. Finally, when
theoretical hydrogen storage capacity of 6.66 wt %.
3
the amount of base was raised to 3 mol%, complex gave almost
complete conversion of N-benzylphthalimide, with quantitative
formation of 1,2-benzenedimethanol and benzylamine as detected by
GC and NMR spectroscopy. We believe that the higher catalytic activity
of complex
relative to and is due to its ability to exhibit dual modes
3 1 2
of metal-ligand cooperation i.e. via H-M/N-H and via
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aromatization/dearomatization of the lutidine backbone. We have
previously demonstrated that addition of two equivalents of base or
more to complex results in the formation of a double deprotonated
monoanionic enamido complex which serves as a highly active catalyst
for the unprecedented hydrogenation of ester at room temperature and
low pressure (5at.).25
hydrogenated in good yield and 78% of 1,4-butanediol was observed by
1H NMR spectroscopy (entry 7). Under the same conditions,
hydrogenation of N-phenylmaleimide and N-benzylmaleimide resulted
in high yields of aniline and benzylamine, respectively, in addition to 1,4-
butanediol yields (entries 8,9); both C=O and C=C hydrogenation took
place. N-benzylglutarimide was also successfully hydrogenated to afford
1,6-hexanediol and benzyl amine in 90% yield (entry 10).
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3
Table 1. Catalytic hydrogenation of phthalimides using complex 3 a
.
Table 2. Catalytic hydrogenation of cyclic imides using complex 3 a
.
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Upon optimizing the catalytic conditions for efficient
hydrogenation of N-benzylphthalimide, we explored the hydrogenation
of other phthalimde substrates. As shown in Table 1, phthalimdes of
benzylamines bearing both electron withdrawing and electron donating
substituents were efficiently hydrogenated. N-phenylphthalimide and
N-hexylphthalimde were also completely hydrogenated to afford 1,2-
benzenedimethanol and the corresponding amine in quantitative yield.
In order to gain mechanistic insight into the hydrogenation
process, we carried out the hydrogenation of N-benzylphthalimide
under conditions described in Table 1, except that the catalysis was
stopped after 6 h. Analysis of the reaction mixture by 1H NMR
spectroscopy and GC showed approximately 50% conversion of N-
benzylphthalimide. Other than the expected 1,4-benzenedimethanol
and benzylamine, phthalide was observed in approximately 15% yield.
We suggest that the hydrogenation process takes place as outlined in
Scheme 1. In presence of the ruthenium catalyst, the cyclic imide first
undergoes hydrogenation of one of the carbonyl groups to form
Encouraged by the efficient catalytic hydrogenation of N-
substituted phthalimides, we screened other cyclic imides towards
hydrogenation. However, under the catalytic conditions described in
Table 2, only 30% conversion of N-benzylsuccinimide to 1,4-butanediol
and benzylamine was observed. Increasing the reaction temperature to
135oC improved the hydrogenation yield such that 75% conversion of
N-benzylsuccinimide to 1,4-butanediol and benzylamine was observed.
Finally, changing the reaction time from 24 h to 40 h and using 1,4-
dioxane as a solvent resulted in almost complete hydrogenation of N-
benzylsuccinimide and the quantitative formation of 1,4-butanediol and
intermediate
hydroxyamide B. Upon intramolecular cyclization,
hemiaminal intermediate which can eliminate the amine and form a
A
,
followed by C-N hydrogenolysis forming the
B
forms
a
C
lactone (phthalide). The lactone then undergoes ruthenium catalyzed
hydrogenation to form the corresponding diol product. Indeed, the
catalytic hydrogenation of phthalide under the reaction conditions
1
benzylamine was observed by H NMR spectroscopy and GC. Other
succinimides bearing either electron rich or electron deficient benzyl
groups, phenyl and hexyl groups were successfully hydrogenated to
afford 1,4-butanediol and the corresponding amines in excellent yield
(Table 2, entries 1-6). Unsubstituted succinimide was also
described in Table
1 resulted in complete conversion to 1,2-
benzenedimethanol in quantitative yield.
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and bis-cyclic imide (Table 3, entry 2-3). We then synthesized a new
complex 4, which is the PPh2 analogue of complex (See SI for the
synthesis and characterization details). Interestingly, use of complex
under identical reaction conditions as that of resulted in 70%
1
2
3
3
4
3
formation of bis-cyclic imide (isolated yield 60%) with the concomitant
release of 82 mL of hydrogen gas, corresponding to 3.36 mmol at 25oC,
i.e. 84% of the expected 4 mmol H2 for quantitative formation of the
imide (Table 3, entry 4). Considering the yield of the imide (70%),
formation of the lactone and oligoamide contributes to approximately
14% of the produced H2. Decreasing the solvent amount from 2 mL to
1 mL lowered the yield of hydrogen gas (Table 3, entry 5). When the
reaction was carried out without adding any solvent under the reaction
conditions of entry 4 (Table 3), 5 mmol of 1,4-butanediol and 3 mmol
of ethylenediamine produced only 150 mL of hydrogen gas resulting in
only 30% efficiency of the hydrogen storage capacity. Changing the
solvent from dioxane to toluene and base from KOtBu to KH also
resulted in the lower amount of hydrogen gas production (Table 3,
entries 7 and 8).
4
5
6
7
8
9
Scheme 1. Proposed mechanism for the hydrogenation of cyclic imides to diols
and amines.
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The unique selectivity of the catalytic hydrogenation of
succinimides inspired us to develop an LOHC system based on the
formation of butanediol and ethylenediamine. Hong has reported the
synthesis of cyclic imides from simple diols and amines using a
ruthenium catalyst.26 We envisioned that the bis-cyclic imide formed
from the dehydrogenative coupling of 1,4-butanediol and
ethylenediamine can serve as a potentially clean reversible LOHC if the
spent fuel, i.e. bis-cyclic imide can be hydrogenated back to the charged
fuel, i.e. 1,4-butanediol and ethylenediamine (Scheme 2). The proposed
system would have a maximum hydrogen capacity of 6.67 wt% which is
higher than the current DOE target for 2020 of 5.5 wt%.27
After successful dehydrogenative synthesis of bis-cyclic imide, we
explored the reverse reaction i.e. the hydrogenation of bis-cyclic imide
to 1,4-butanediol and ethylenediamine. Under the catalytic conditions
described in Table 2, 99% conversion of bis-cyclic imide was observed
and the corresponding 1,4-butanediol and ethylenediamine were
detected by 1H NMR in more than 90% yield (Scheme 3). Using
Entry Cat
Base (mol%)
Solvent (mL)
temp
H2 (mL)
Diol
Product yield
conversion
95%
1
2
3
4
5
6
7
8
1
2
3
4
4
4
4
4
KOtBu (1 mol%)
KOtBu (1 mol%)
KOtBu (2 mol%)
KOtBu (2 mol%)
KOtBu (2 mol%)
KOtBu (2 mol%)
KH (2 mol%)
Dioxane (2 mL)
Dioxane (2 mL)
Dioxane (2 mL)
Dioxane (2 mL)
Dioxane (1 mL)
Dioxane (1 mL)
Dioxane (2 mL)
Tol (2 mL)
120oC
120oC
120oC
120oC
120oC
135oC
120oC
120oC
70 mL
62 mL
58 mL
82 mL
81 mL
82 mL
68 mL
74 mL
Imide (55%) + lactone (20%) + oligoamide
Imide (40%) + lactone (25%) + oligoamide
Imide (34%) + lactone (30%) + oligoamide
Imide (70%) + lactone (12%) + oligoamide
Imide (63%) + lactone (24%) + oligoamide
Imide (60%) + lactone (20%) + oligoamide
Imide (58%) + lactone (27%) + oligoamide
Imide (65%) + lactone (15%) + oligoamide
90%
99%
99%
99%
99%
99%
99%
KH (2 mol%)
complex
4
under identical catalytic conditions resulted in only 65%
conversion of the bis-cyclic imide and the desired 1,4-butanediol was
detected in only 50% yield. Use of complexes and also resulted in a
1 2
lower yield of 1,4-butanediol and ethylenediamine as detected by the 1H
NMR spectroscopy (See SI, Table S3).
Scheme 2. Dehydrogenative coupling of 1,4-butanediol and ethylenediamine.
We started our investigation by screening ruthenium catalysts
for the dehydrogenative coupling of 1,4-butanediol (1 mmol) and
ethylenediamine (0.6 mmol). Using 1 mol% of complex and 1 mol%
1
KOtBu, almost complete conversion of 1,4-butanediol was observed in
24 h but the bis-cyclic imide was formed in only 55% yield as observed
by 1H NMR spectroscopy (Table 3, entry 1). Remaining side products
were characterized as lactone (20% yield) and linear oligoamides28 (See
SI for characterization details). 70 mL of hydrogen gas was also collected
using a gas-burette method. Analysis of the gas evolved during the
dehydrogenation reaction by GC showed the presence of only H2 gas.
CO which is harmful for the fuel cell was not detected. Under similar
Scheme
3
. Hydrogenation of bis-cyclic imide to form 1,4-butanediol and
ethylene diamine.
Furthermore, we explored the interconversion cycle between
the spent fuel bis-cyclic imide and the charged fuel i.e. 1,4-
butanediol/ethylenediamine mixture. As mentioned above, the bis-
cyclic imide (1 mmol) was hydrogenated using 1 mol% and 3 mol%
3
KOtBu in 2 mL of 1,4-dioxane using 40 bar H2 to produce 1,4-butanediol
and ethylenediamine in more than 90% yield. The resulting reaction
reaction conditions, complexes and gave slightly lower yields of H2
2 3
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mixture was charged with 1 mol%
4
and 3 mol% KOtBu and refluxed at
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85% yields respectively under the catalytic conditions of 2 mol% , 6
3
mol% KOtBu, 50 bar hydrogen and the reaction time of 40 h (See SI,
Table S4 for optimization details). A further dehydrogenation step
(conditions: Table 3, entry 4) resulted in the formation of 71 mL
hydrogen gas and the expected bis-cyclic imide in ~64% yield.
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In conclusion, we have reported a rare, efficient and selective
catalytic hydrogenation of
a broad variety of cyclic imides to
synthetically and industrially useful diols and amines building blocks.
The unique selectivity of succinimide hydrogenation allowed to pursue
a fundamentally new LOHC system based on the hydrogenation of bis-
cyclic imide and its formation from the dehydrogenative coupling of 1,4-
butanediol and ethylenediamine, which are inexpensive and extensively
produced by industry. This hydrogen storage system has a theoretically
high gravimetric storage capacity of 6.66 wt%. Further studies are aimed
at increasing the efficiency of this novel LOHC system.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and spectroscopic details of the catalytic reactions. The
Supporting Information is available free of charge on the ACS Publications
website.
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Chem. Soc. 2014, 136 (13), 4829–4832.
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AUTHOR INFORMATION
Corresponding Author
* david.milstein@weizmann.ac.il
ACKNOWLEDGMENT
This research was supported by the European Research Council (ERC AdG
692775). D. M. holds the Israel Matz Professorial Chair of Organic Chemistry.
A.K. is thankful to the Israel Planning and Budgeting Commission (PBC) for a
fellowship. T.J. thanks the Azrieli Foundation for a postdoctoral fellowship. N. A.
E.-J. thanks Mr. Armando Jinich for a postdoctoral fellowship.
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