Generation of Hydrogen from Water: A Pd-Catalyzed Reduction of Water Using Diboron Reagent at Ambient Conditions

Article abstract of DOI:10.1021/acs.orglett.6b02508

Production of hydrogen from renewable sources, particularly from water, is an intensive area of research, which has far-reaching relevance in hydrogen economy. A homogeneous catalytic method is presented for producing clean hydrogen gas from water, in a reaction of water with a diboron compound as the reductant, under ambient reaction conditions. The Pd-catalytic system is stable in water and displays excellent recyclability. Hydroxy analogues such as alcohols are compatible with the Pd/B₂Pin₂ system and generate hydrogen gas efficiently. The B₂Pin₂-H₂O system, in the presence of palladium, is an excellent catalytic system for selective hydrogenation of olefins.

Full text of DOI:10.1021/acs.orglett.6b02508

Letter
pubs.acs.org/OrgLett
Generation of Hydrogen from Water: A Pd-Catalyzed Reduction of
Water Using Diboron Reagent at Ambient Conditions
Devi Prasan Ojha, Karthik Gadde, and Kandikere Ramaiah Prabhu*
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
S
* Supporting Information
ABSTRACT: Production of hydrogen from renewable sources, partic-
ularly from water, is an intensive area of research, which has far-reaching
relevance in hydrogen economy. A homogeneous catalytic method is
presented for producing clean hydrogen gas from water, in a reaction of
water with a diboron compound as the reductant, under ambient reaction conditions. The Pd-catalytic system is stable in water
and displays excellent recyclability. Hydroxy analogues such as alcohols are compatible with the Pd/B2Pin2 system and generate
hydrogen gas efficiently. The B2Pin2−H2O system, in the presence of palladium, is an excellent catalytic system for selective
hydrogenation of olefins.
he quest for developing renewable energy production and
their storage feed stocks has become one of the major
Scheme 1. Pd-Catalyzed Cleavage of B−B Bond Using H2O
T
and MeOH
scientific goals in recent years. In this direction, production of
hydrogen from water has direct impact on hydrogen economy
and its feasibility.1 However, these expeditions are challenging,
particularly for the discovery of efficient and cost-effective
catalytic processes. A vast majority of research on the generation
of hydrogen gas has been focused on the transition metal driven
catalytic processes by harnessing a photochemical or electro-
chemical method.2 Generally, transition metal catalyzed
dehydrocoupling reactions, involving p-block elements, are
effective methods for the generation of hydrogen gas.3 In this
regard, considerable efforts have been devoted to designing
ammonia−borane complexes and related compounds as hydro-
gen storage materials via dehydrocoupling reactions.4 Beller et al.
have reported a methanol dehydrogenation process using Ru-
catalysts and dehydrogenation of formic acid using iron
catalysts.5 Effective utility of a variety of hydroxy systems such
as sorbitol, glycerol, ethylene glycol, and methanol with Pt/Al2O3
at 500 K has also been explored by Dumesic.6
Most of the methods, which use transition metal catalyzed
strategies, efficiently convert water into hydrogen gas via
hydrogen evolution reaction (HER). Generally, these methods
employ a variety of metal hydrides such as AlH3, R3SiH, BH3, etc.,
which are potentially unstable and moisture sensitive.3i One of
the main features of transition metal salts is their ability to
activate nonpolar bimetallic bonds via an oxidative addition
process.7 For example, the activation of B−B, B−X, and Si−B
bonds, assisted by transition metals, has led to several useful
polymetallic organic compounds.8 However, to the best of our
knowledge, the metal catalyzed hydrolytic cleavage of the B−B
bond using water is unknown and these reagents have never been
employed in metal catalyzed transformations for producing
hydrogen gas (Scheme 1). Palladium catalysts in combination
with other metal systems are well-known for their reaction with
HCO2H to form hydrogen gas.9 The utility of the homogeneous
monometallic transition metal catalysts for the generation of
hydrogen gas using formic acid is limited due to poisoning of
metals by CO.10 Nevertheless, there are few reports using
methane (with Pd−Ag membrane), and cyclohexane (with Pt or
Pd clusters) dehydrogenation strategies for generating pure
hydrogen gas using heterogeneous catalysts.11In continuation of
our work on Pd-catalyzed reactions, herein, we present a
homogeneous catalytic method for producing clean hydrogen
gas using water as a hydrogen source and a diboron compound as
the reductant at ambient reaction conditions. The present
homogeneous catalytic method for producing hydrogen gas
under ambient reaction conditions involves three key reaction
parameters: (i) the catalytic amount of Pd(OAc)2 trimer, (ii)
B2Pin2, and (iii) water. The salient feature of this method is that it
leads to an evolution of hydrogen gas, and oxygen evolution
(OER) was not observed at all. This process is very rapid and
generates an accurate amount of gas on demand at ambient
temperature. Besides, this method is also compatible with
alcohols such as MeOH, tert-BuOH, and trifulroethanol and
leads to efficient generation of hydrogen gas exclusively. The
only limitation of this process is the use of a stoichiometric
amount of bis-(pinacolato)diboron, which helps in trapping the
oxygen. Besides, the mild and unreactive nature of these bis-
boron reagents makes the process more attractive.
We began our studies with serendipitous formation of the H2
gas in a reaction designed to reduce an olefin in a reaction of 4-
allyl-1,2-dimethoxybenzene (4a) with [Pd(OAc)2]3, B2Pin2,
PCy3, water, and toluene (Scheme 2). Besides, the preliminary
investigation revealed that the similar reaction of [Pd(OAc)2]3,
B2Pin2 (1), and water, in the absence of olefin, can lead to the
Received: August 22, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02508
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Organic Letters
Letter
a
Scheme 2. Transfer Hydrogenation of Olefins
Scheme 3. Working Hypothesis
formation of hydrogen gas and 2,2′-oxybis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolane) (2) as a byproduct (Table 1). As can be
a b
,
Table 1. Optimization Studies
a
Catalytic cycle for oxidative cleavage of B2Pin2 with (a) H2O or (b)
MeOH.
good purity of hydrogen gas generated in the reaction (97%; see
SI for more details). The formation of PinB-O-BPin (2) and OH-
BPin (IV) was confirmed by boron NMR studies (SI Figure 3).
Further, all the measurements were correlated with the starting
material (B2Pin2) used. As seen in entry 1 of Table 1, using 1
equiv of B2Pin2 (762 mg, 3 mmol) and 0.5 equiv of water (27 mg,
1.5 mmol) resulted in the formation of hydrogen gas in 39% yield
(28 mL). A remarkable result was obtained by increasing the
amount of water to 3 mmol, which resulted in the generation of
hydrogen gas in 84% yield (1 equiv of H2 per 1 equiv of B2Pin2;
entry 2, Table 1). The experiment with 1 mL of H2O has also
shown efficient generation of H2 gas in good yield (83%, entry 3).
Based on these results, we envisioned the necessity of a hydroxy
functional group, which can possibly facilitate the hydrogen
generation process. Hence, the subsequent experiments with
alcohols, instead of water, led to efficient generation of hydrogen
gas. Thus, the reaction of MeOH (2 equiv) with B2Pin2 in the
presence of a catalytic amount of a Pd-reagent ([Pd(OAc)2]3)
produced hydrogen gas in excellent yield (91%, entry 4, Table 1).
The reaction with higher stoichiometry of methanol (1 mL) has
also generated a good amount of hydrogen gas (90%, entry 5).
tert-Butanol (2 equiv) as a H2-source displayed a remarkable
result by producing hydrogen gas in 91% yield (entry 6), whereas
85% of hydrogen gas was obtained using 1 mL of tert-butanol
(entry 7, Table 1). The reaction with MeOH was exothermic,
and the gas collected showed the presence of methanol vapors
(1H NMR). This problem was circumvented by performing the
reaction at 0 °C, which showed only the presence of hydrogen
gas (MeOH vapors were not detected in 1H NMR; see SI Figure
4). Similarly, trifluoroethanol (CF3CH2OH) also effectively
produced 84% of hydrogen gas (entry 8, Table 1). A similar
reaction using 1 mL of trifluoroethanol generated hydrogen gas
in 83% yield (entry 9, Table 1).
In successive studies, recycling experiments of the catalyst led
to interesting results. Thus, the Pd-catalyst can be effectively
reused several times for generation of hydrogen gas. As observed,
the reaction mixture produced 90% of hydrogen gas in the
second run, but required addition of a stoichiometric amount of
B2Pin2 and H2O. With this interesting outcome, we performed
five consecutive reactions, wherein the yields were consistent for
the next 5 times with variable metal catalyst loadings. In the first
set of experiments, the reaction with 5 mol % palladium acetate
generated hydrogen gas in 90%, 85%, 85%, 80%, and 70% yield,
respectively (Figure 1). Similarly, experiments using 2.5 mol % of
the Pd-catalyst under optimal conditions for five consecutive
runs generated hydrogen gas in 95%, 90%, 90%, 80%, and 80%
yields, respectively (Figure 1). Interestingly, loading a lower
amount of metal catalyst (1.5 mol % of [Pd(OAc)2]3 was also
effective and produced yields of 95%, 90%, 85%, 85%, and 75%,
respectively in consecutive runs. Similar sets of reactions were
B2Pin2
time
H2
(BPin)2O
H
b
2b
entry
1
H2 source
H2O
(mmol)
(min)
(mL)
(%)
(%)
3
60
28
44
39
(1.5 mmol)
2
H2O
(3 mmol)
3
60
61
92
84
3
4
H2O (1 mL)
3
3
30
30
60
66
92
84
83
c
MeOH
(6 mmol)
91
91
91
85
84
83
c
c
c
c
c
5
6
7
8
9
MeOH
(1 mL)
tBuOH
(6 mmol)
tBuOH
(1 mL)
3
3
3
3
3
15
30
15
30
15
65
66
63
61
60
85
96
91
92
89
TFE
(6 mmol)
TFE (1 mL)
a
Reaction conditions: B2Pin2 (3 mmol), [Pd] (1 mol %), H2-source,
b
c
rt, 1 h. Isolated yields Reactions were performed at 0 °C.
seen from the initial experiments, we found that the Pd-catalyst is
essential for this reaction (see Supporting Information (SI)
Table 1). Hence, the efficacy of this reaction has been tested with
variable amounts of both a boron source and water. In a reaction
of bis(pinacolato)diboron (1, 1 equiv) and water (1 equiv), it was
found that the formation of hydrogen gas depends upon the
stoichiometry of both the boron source and water, and there was
no formation of hydrogen in the absence of either the Pd-catalyst
or B2Pin2 (see SI Table 1). As can be seen in Table 1 (entries 1−
3), 1 equiv of B2Pin2 is needed for effective generation of
hydrogen gas (see SI for more details). Indeed, the reaction was
exothermic and reached 27 °C → 38 °C in 15 min. Under similar
conditions, alcohols such as MeOH, tert-BuOH, or CF3CH2OH
were also found to be useful hydrogen sources and successfully
led to the efficient generation of hydrogen gas (entries 4−9,
Table 1). We believe that the oxidative insertion of Pd(0) with
B2Pin2 led to the formation of metal species I.8 Further, an aquo-
complex can be formed due to interaction of water with [Pd−B]
to the form II.12 Species II then can decompose to form PinB-
Pd(H)L2 (III). The OH-BPin (IV) generated from the above
process eventually was trapped by intermediate III to form PinB-
O-BPin (2) producing dihydrogen and regenerating the Pd-
catalyst for further catalytic cycling (Scheme 3a; SI Figure 1).13
A
similar explanation can be conceived using alcohols as shown in
Scheme 3b. The volume of the hydrogen gas generated has been
quantified by measuring the amount of water displaced in a
reverse buret apparatus. The hydrogen gas generated was passed
through CDCl3, and the 1H NMR was recorded for the
qualitative analysis (SI Figure 1). This has been further
confirmed by gas chromatographic analysis, which showed the
B
DOI: 10.1021/acs.orglett.6b02508
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Organic Letters
Letter
a b
,
Table 2. Scope of the Reaction
Figure 1. Recyclability of the catalyst; reactions were performed with
Pd-catalyst, B2Pin2, and H2O (1 mL) up to 5 consecutive runs.
carried out to examine the recyclability of MeOH under the
reaction conditions. Using MeOH (1 mL) with a low catalyst
loading ([Pd(OAc)2]3, 1.5 mol %) with a stoichiometric amount
of B2Pin2 (1 equiv) in successive experiments, we were delighted
to obtain 91%, 83%, and 76% yields for the consecutive runs (see
SI Table 5). Subsequent experiments revealed significant activity
by the catalyst. As shown in Figure 2, the catalyst exhibited a
a
Reaction conditions: 4a (0.5 mmol), B2Pin2 (1 mmol), [Pd] (1.5 mol
%), ligand (10 mol %), condition A (H2O, 1 mmol), condition B
(MeOH, 1 mmol), toluene (1.5 mL), rt, 12 h. Isolated yields.
b
was also efficient in reducing N-methyl-N-phenylmethacryl-
amide to its corresponding hydrogenated product N-methyl-N-
phenylisobutyramide (5j) in good yield (66%). In this example,
the reduction of conjugated olefin proceeded well under the
present catalytic palladium system. Next, we employed a variety
of N-allyl derivatives such as 1-allyl-1H-indole-5-carbonitrile and
5-nitro-1-(prop-1-en-1-yl)-1H-indole, which were chemoselc-
tively reduced into 1-propyl-1H-indole-5-carbonitrile (5k) and
5-nitro-1-propyl-1H-indole (5l) in good yields (87% and 63%,
respectively), wherein sensitive nitrile and nitro groups were
intact during the reaction conditions. Similarly, 1-allyl-5-
methoxy-1H-indole underwent a smooth reduction to 5m
(64%) and a N-allyl derivative of benzamide furnished N-
propylbenzamide (5n) in excellent yield (83%).
Figure 2. Efficacy of Pd-catalytic system; Vgas = volume of the gas; the
TON (turnover number) and TOF (turnover frequency) of the catalytic
process were calculated using the volume of the gas formed in 5 min
intervals of the reaction.
turnover number (TON) of 31.3 per 0.03 mmol of catalyst after
15 min. The catalyst also exhibited a turnover frequency (TOF)
of 6.26 per min per 0.03 mmol of catalyst after 15 min. This value
corresponds to 1.04 × 106 TON per mole of catalyst used (more
details in SI).
To gain insight into the mechanism, we designed a few
controlled experiments (Scheme 4). Reaction of N-allyl
a b
,
Scheme 4. Deuterium Labeling Studies
Transition metal catalyzed homogeneous transfer hydro-
genation reactions are highly exploited synthetic strategies for
reducing C−C multiple bonds, which also exhibit excellent
chemo-, regio-, and stereoselectivity.14 Therefore, the reduction
of a variety of functional groups, especially alkenes, has been
reported using various hydrogen transfer sources such as
HCO2H, N2H4, NH3−BH3, etc.15 On the basis of current
study, optimization studies for the hydrogenation of alkenes were
performed (SI Table 6, Supporting Information). Based on this
study, it was found that the catalytic amount of Pd-catalyst, 2
equiv of boron source, 2 equiv of water, and 10 mol % of ligand
(PCy3) were necessary for the reduction to complete.
The hydrogenation reaction to obtain a saturated carbon chain
was studied under these homogeneous catalytic conditions for a
variety of olefins (Table 2). Olefins such as aryl allyl benzene
derivatives (4a−4e) were reduced to furnish corresponding
saturated products 5a−5e in excellent yields (84−95%) at
ambient temperature. As can be seen, the reduction of 4f led to a
chemoselective reduction of olefin in the presence of a keto
group furnishing the reduced product 5f in 74% yield. Further,
O-silyl protected homoallylic alcohols (4g and 4h) were
effectively converted to their saturated forms (5g and 5h) in
excellent yields (98% and 89%, respectively). Unlike unactivated
olefins, the reduction of stilbene was smooth and afforded the
reduced product 5i in excellent yield (94%). The catalytic system
a
Reaction conditions: 4a (0.5 mmol), B2Pin2 (1 mmol), [Pd] (1.5 mol
%), ligand (10 mol %), D2O (1 mmol), toluene (1.5 mL), rt, 12 h.
b
Isolated yields.
benzamide (4n), under optimal conditions, formed the expected
product 5n in 83% yield. Interestingly, the reaction of N-(prop-1-
en-1-yl)benzamide (4p), an isomer of 4n, constituting a N-vinyl
internal double bond furnished the product 5n in good yield.
Extension of the study, for comparing the reactions of E- and Z-
isomers of 4p, furnished reduced product 5n in 81% and 85%,
respectively. As can be noted from this experiment, the reaction
of 4a with [Pd(OAc)2]3, B2Pin2, PCy3, and D2O in toluene
furnished a mixture of deuterated compounds 5a′ and 5a″ in
95% yield, in almost equal ratio (52:48, Scheme 4; for NMR
studies, see SI). This observation clearly indicates the formation
of intermediates A and B during the reaction, which is shown in
Scheme 5. Further, these intermediates were hydrogenated by
C
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Organic Letters
Letter
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Scheme 5. Proposed Mechanism
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In summary, we have shown for the first time that
[Pd(OAc)2]3 (1 mol %) can be efficiently employed to generate
hydrogen gas using water as the hydrogen source and B2Pin2 as
the scavenger of oxygen. The major advantage of the protocol is
oxygen evolution is not observed in the reaction and pure
hydrogen gas is evolved in the process under ambient reaction
conditions. Additionally, alcohols such as MeOH, tert-butanol,
and CF3CH2OH can also be utilized under the reaction
conditions. The only limitation of the process is that it requires
a stoichiometric amount of the boron source. This protocol has
been successfully demonstrated for the reduction of a variety of
olefins. Further work is underway in our laboratory for
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02508.
(8) (a) Takaya, J.; Iwasawa, N. ACS Catal. 2012, 2, 1993. (b) Ojha, D.
P.; Prabhu, K. R. Org. Lett. 2016, 18, 432. (c) Neeve, E. C.; Geier, S. J.;
Mkhalid, I. A. I.; Westcott, S. A.; Marder, T. B. Chem. Rev. 2016, 116,
9091.
(9) (a) Tedsree, K.; Li, T.; Jones, S.; Chan, C. W. A.; Yu, K. M. K.;
Bagot, P. A. J.; Marquis, E. A.; Smith, G. D. W.; Tsang, S. C. E. Nat.
Nanotechnol. 2011, 6, 302. (b) Yu, W.-Y.; Mullen, G. M.; Flaherty, D. W.;
Mullins, C. B. J. Am. Chem. Soc. 2014, 136, 11070.
Experimental procedures, characterization data, and
spectra for all compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
(10) Crabtree, R. H. Chem. Rev. 2015, 115, 127.
*E-mail: prabhu@orgchem.iisc.ernet.in.
Notes
(11) Wang, Y.; Shah, N.; Huffman, G. P. Energy Fuels 2004, 18, 1429.
(12) Rossin, A.; Bottari, G.; Lozano-Vila, A. M.; Paneque, M.;
Peruzzini, M.; Rossi, A.; Zanobini, F. Dalton Trans. 2013, 42, 3533−
3541.
The authors declare no competing financial interest.
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Chem. Soc. Rev. 2013, 42, 497.
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manuscript, the following papers appeared employing heterogeneous
catalysis and B2(OH)4 and water for transfer hydrogenation of olefins:
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P.; Negri, J.
ACKNOWLEDGMENTS
■
This work was supported by the Indian Institute of Science. We
are thankful to Prof. G. Madras, Mr. Ravi Kiran (CE, IISc), Prof.
B. Jagirdhar, Mr. Sourav Ghosh (IPC, IISc), Dr. S. Raghothama
(NRC, IISc), and Dr. A. R. Ramesha (RL Fine Chem) for useful
discussions. D.P.O. thanks UGC, New-Delhi for a fellowship.
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