Efficient Hydration of Nitriles Promoted by Gallic Acid Derived from Renewable Bioresources

Article abstract of DOI:10.1002/cctc.201700016

An efficient gallic acid promoted nitriles hydration at room temperature with ethanol/water as a solvent has been developed. The present protocol offers a wide range of amides in moderate to good yields. Moreover, galla chinensis extract can serve as the promoter to perform the hydration, which also shows the potential utilization of natural feedstocks.

Full text of DOI:10.1002/cctc.201700016

Accepted Article
Title: An efficient hydration of nitriles promoted by gallic acid derived
from renewable bio-resources
Authors: Tao Deng
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10.1002/cctc.201700016
ChemCatChem
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An efficient hydration of nitriles promoted by gallic acid derived
from renewable bio-resources
Tao Deng,[a,b] and Cheng-Zhang Wang*[a,b]
Dedication ((optional))
but also because of the lower cost and lesser energy.[13]
Abstract: An efficient gallic acid promoted nitriles hydration at room
temperature with ethanol/water as solvent has been developed. The
present protocol offers a wide range of amides in moderate to good
yields. Moreover, galla chinensis extract can serve as the promoter to
perform the hydration, which also shows the potential utilization of
natural feedstocks.
Among them, gallic acid is a widespread natural polyphenolic
compound with strong antioxidant activity,[14-16] and it can be
extracted from vegetal sources such as fruits, nuts, and
flowers. Free radicals can be effectively scavenged by the
phenolic hydroxyl group, by which can also act as a hydrogen
donor.[17]
Introduction
Amide is a useful and prevalent key group in nature due to its
ubiquitous in proteins, pharmaceuticals, biomolecules, constitute
excellent intermediates, and other raw materials for synthetic
organic chemistry (Figure 1).[1] Typical conventional reactions for
amide formation have been extensively investigated in the past
decades.[2] Among them, the hydration of nitriles seems to provide
a direct and efficient approach, as it offers the most atom-
economical access to valuable amide molecules.[3] However,
traditional procedures requires strong acids and bases which
results in high cost and by-products along with large amount of
chemical waste (Scheme 1a).[4] Meanwhile, numerous metal
catalysts such as Ru,[5] Au,[6] Rh,[7] Pd,[8] Pt,[9] Cu,[10] Mn[11] have
been proven to achieve this transformation, along with one or
more drawbacks, such as high temperatures, long reaction times,
large excess of reagents and sometimes extra steps preparing
catalyst (Scheme 1b). Hence, challenges still remain, but it is still
highly desirable to develop more efficient, environmentally-benign
and sustainable methods to obtain amides from nitriles.
Figure 1. Representative pharmaceuticals and biomolecules containing a
primary amide functionality
On the other hand, the employment of reactive oxygen species
(ROS) is commonly used in numerous fields.[18] The
photoirradiation of gallic acid was demonstrated for augmentation
of the antimicrobial activity along with formation of reactive
oxygen species.[19] In fact, the auto-oxidation process can be
achieved through both the thermal and the photochemical routes.
Although well-documented procedures on bactericidal activity are
available in the literature, the auto-oxidation issues associated
with the generation of H2O2 still remains as one of the main
bottlenecks current methodology, and there is little information
available about the organic reaction with H2O2 generated from
gallic acid as catalyst. Inspired by the above work, we envisioned
that gallic acid would be oxidized with O2 in air under mild
conditions to form hydrogen peroxide (H2O2), which would
participate in the next nitrile hydration.
With the growing emphasis on the applications of green
principles in organic synthesis, significant effort has been
dedicated to develop some green reaction media, catalysts, and
reaction conditions.[12]
An increasing number of chemists are
drawing significant awareness to utilize natural feedstocks as
catalyst or promoter in organic synthesis not only because of
the abundance and benign nature of the renewable source
which are the cause for the majority of waste and pollution
[a]
[b]
Dr. T Deng, Dr. C-Z Wang
Institute of Chemical Industry of Forest Products, CAF
Nanjing, Jiangsu 210042, P. R. China.
E-mail: wangczlhs@sina.com
Dr. T Deng, Dr. C-Z Wang
Department Key Lab. of Biomass Energy and Material
Nanjing, Jiangsu 210042, P. R. China.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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challenging processes can be effected in the presence of light. [19]
And therefore we next performed the reaction with gallic acid and
KOH under light at room temperature. When, to our delight, the
reaction was completed in 15 h to afforded 2a in 93% yield (Table
1, entry 22). Additionally, similar results were obtained with open
air and O2 (Table 1, entries 20 and 23). The reaction failed when
under the rigorous exclusion of oxygen (Table 1, entry 24).
Considering that yields were not significantly different whether the
reaction was conducted under light (Table 1, entries 21 and 22),
thus the optimal conditions were identified as nitrile (1.0 equiv.),
gallic acid (2.0 equiv.), KOH (5% in H2O) in EtOH/H2O at room
temperature for 24 hours.
Table 1. Optimization of the reaction conditions for transformation nitriles to
amides[a]
Scheme 1. Comparison of previous work and this work
Results and Discussion
Gallic acid
(equiv.)
T
(oC)
Yield[b]
(%)
Entry
Base
Solvent
In order to examine this design, benzonitrile 1a was treated with
NaHCO3 in the absence of gallic acid in EtOH/H2O at 70 oC under
air atmosphere. Only with NaHCO3 as the additive cannot
produce any product (Table 1, entry 1). To our delight, performing
the reaction using 1.0 equiv. of gallic acid afforded benzamide 2a
in 15% yield (Table 1, entry 2). Subsequently we discovered that
increases in amount of gallic acid have an accountable effect on
the yield. Among them, the employment of gallic acid (2.0 equiv.)
as the promoter led to a proportionate increase in yield (Table 1,
entry 4).
1
--
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
Na2CO3
K2CO3
KH2PO4
KF
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
Acetone/H2O
THF/H2O
H2O
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
50
r.t.
r.t.
NR
15
2
1.0
1.5
2.0
2.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
3
16
4
22
5
20
6
37
7
39
8
22
We then screened the reaction in the presence of various
additives including inorganic and organic bases. The reaction
proceeded smoothly with inorganic base tested in each case
(Table 1, entries 6-11) whereas the reaction failed when organic
base (e.g. Et3N, DBU and DMAP) was used as the additive (Table
1, entries 12-14). Inorganic bases such as Na2CO3, K2CO3 and
NaOH only generate the product in very low to moderate yields
(Table 1, entries 6-11). When choosing KOH, yield of the product
was among the best (Table 1, entry 10). Notably, the additive also
played a key role in the present reaction, and no reaction took
place when no suitable additive were used (Table 1, entry 15).
Further, the experimental outcome showed that other mixed
solvent such as acetone/H2O and THF/H2O were not all efficient
(Table 1, entries 16 and 17). Lower yields (53% and 29%) of the
product were observed, while single solvent H2O could not
product any hydration product (Table 1, entry 18).
9
19
10
11
12
13
14
15
16
17
18
19
20
21[c]
KOH
88
NaOH
Et3N
73
NR
NR
NR
NR
53
DBU
DMAP
--
KOH
KOH
29
KOH
NR
72
KOH
EtOH/H2O
EtOH/H2O
EtOH/H2O
KOH
67
KOH
89(85)[d]
In particular, the catalytic system afforded the desired product
in 72% yield at 50 oC (Table 1, entry 19). Performing the reaction
at room temperature afforded 2a in 67% yield while extending the
reaction time to 24 h further improved the yield of 2a to 89%
(Table 1, entries 20 and 21). It is known that for the production of
H2O2 from catechol moiety, both thermodynamic and kinetic
light
(r.t.)
22
2.0
KOH
EtOH/H2O
93
23[e]
24[f]
2.0
2.0
KOH
KOH
EtOH/H2O
EtOH/H2O
r.t.
r.t.
70
NR
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10.1002/cctc.201700016
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[a] Reactions were carried out with benzonitrile (1a, 0.5 mmol), gallic acid,
additive (if any, 5% in H2O), 1.5 mL of solvent (v/v=1:2) in air under the
corresponding temperature for 15 h. [b] Yields were determined by GC using
naphthalene as an internal standard. [c] Reaction time: 24 h. [d] Isolated yield.
[e] Under O2. [f] Under N2.
[a] Reaction conditions: nitrile 1 (1 mmol), gallic acid (2 eq), KOH (5% in
H2O), 3 mL of EtOH/H2O (v/v=1:2), room temperature, corresponding time
in air. [b] Isolated yield. [c] At 50 oC.
Furthermore, a variety of nitriles having heterocycle, biphenyl as
well as other aryl groups was explored (Table 2, 2m-2q). The
biphenyl derivative (1m) had similar activity on the product yield
relative to the electron-neutral substrate (1a). Nitriles bearing
heterocycle substituent, cyanopyridine (1n) and sulphur-
containing heterocycle (1o) gave their desired products
respectively, in relatively lower yields to that of the phenyl
analogue (2m). Moreover, 2-arylacetonitrile was less reactive and
required 50 oC to obtain good results (2p, 61%). This may be due
to the presence of the adjacent aryl group, which led to certain
steric hindrance as in the case of ortho-sustituted benzonitrile.
Similarly, reaction of naphthonitrile also required 50 oC (2q).
Gratifyingly, four aliphatic nitriles also worked well, allowing easy
access to the corresponding products 2r-2u in moderate yields.
On the basis that many biomolecules possess primary amide
substituents, we set out to improve the potential practicality of the
reaction conditions to enable this methodology to be used in the
pharmaceutical industry. The hydration of 2-amino-3-
phenylpropanamide was tested in a gram scale (Scheme 2). In
this instance, the reaction was still very efficient under the
standard conditions, albeit with extended reaction time to 30 h,
and produced good yield of the product 2r (59% yield). Product 2r
can be further transformed to a cathepsin K inhibitor 3 through
known procedures,[20] which also showed excellent selectivity
against the cathepsins B, L, and S. The result showed that this
protocol was applicable to the late-stage hydration of medicinally
relevant compounds.
With the optimized reaction conditions in hand, we next
examined the scope of the method with different nitriles. As
presented in Table 2, various functional groups commonly utilized
in organic synthesis were fully compatible with the simple
hydration method. Substrates bearing electron-withdrawing
groups, irrespective of their positions of attachment, underwent
the hydration to give the corresponding amides 2b, 2c, 2d, 2g and
2h in good yields. Note that benzonitriles bearing electron-
donating functional groups such as methyl (1e and 1j), methoxyl
(1f) and ethyoxyl (1i) at different positions were also suitable
substrates for this process to provide corresponding products.
Accordingly, steric effects of substituents on the phenyl ring were
evaluated. In comparison to benzonitrile (1a), the presence of
either a meta- or a para-group, did not show any significant effect
on the product yields ranging from 80–91%. Likewise,
comparable yields of the products (2g) and (2h) were obtained
when the phenyl ring is substituted at the ortho-position. However,
the presence of two groups in the ring resulted in an obvious
decrease in the product yield (2k and 2l), along with the
requirement of longer reaction time.
Table 2. Substrate Scope[a]
2a, 85%
2e, 84%
2b, 89%
2f, 80% (30 h)
2j, 81%
2c, 85%
2g, 82%
2d, 91%
2h, 77% (30 h)
2i, 71% (30 h)
2m, 83%
2k, 56% (30 h)
2l, 79% (30 h)
2o, 75%
Scheme 2. Gram-scale synthesis of cathepsin K inhibitor precursor
2n, 76%
On the basis of the experiment results and previous reports, a
plausible mechanism for this hydration was proposed in Scheme
3. H2O2 can be produced through both thermal and photochemical
routes. For the thermal process, gallic acid can be oxidized in
couple with the intermediate I formation. Intermediate I was
followed by reoxidation, releasing quinone product II and
2p, 61%[c]
2q, 80%[c]
2t, 68%
2r,79%
-
regenerating O2 from O2. After gallic acid oxidation was complete,
-
HO2 , O2- and H+ can easily react to form H2O2. For minor part, it
2s, 77% (30 h)
2u, 71%
has been shown earlier that, as expected, both two hydrogen
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bond interactions and captodative effect can effectively stabilize
the radical at 4OH in galloyl radical.[13c, 21] The initiation step would
be the photo-oxidation of gallic acid by O2 under light to generate
a superoxide anion (O2-) and a semiquinone radical. The
semiquinone radical is more susceptible to be oxidized by O2 than
tolerance make this protocol flexible and particularly suitable for
preparing primary amides. Gallic acid is the simplest prototype of
tannin. In terms of the abundant renewable source, this protocol
also shows the potential utilization of natural feedstocks.
-
gallic acid itself. In consideration of O2 as a stronger one-
electron oxidant than O2, it can be easily reduced to H2O2.
Consequently, H2O2 can be readily catalyze the hydration of nitrile
to afford the desired amide.
Experimental Section
General Information
All reagents were purchased from commercial sources and used without
treatment, unless otherwise indicated. The products were purified by
column chromatography over silica gel. NMR spectra were recorded on a
Bruker AMX500 (500 MHz) spectrometer and tetramethylsilane (TMS) was
used as a reference. Data were reported as follows: chemical shift,
multiplicity (s = singlet, d = doublet, t = triplet, quin = quintuplet, sex =
sextet and br = broad), coupling constant (J values) in Hz and integration.
The visible light irradiation upon the tube reactor was provided by a 300W
xenon lamp (PLS-SXE300C, Beijing Perfect Light Co.) equipped with a
360-580 nm filter. Gallic acid from ALDRICH were used. Galla
chinensis was purchased from the local Chinese market. All of the
products were known compounds and were identified by comparison of
their physical and spectra data with those of authentic samples.
General procedure for the photocatalytic nitriles hydration reactions
A mixture of gallic acid (2 mmol) and 2.0 mL of 5% KOH was stirred open-
flask at room temperature for 15 min. Then, 1 mmol of benzonitrile in EtOH
(1 mL) was added. The progress of the reaction was monitored by TLC,
until the starting material disappeared. The mixture was poured into
saturated aqueous NH4Cl (10 mL, saturated solution) and extracted with
ethyl acetate (3 × 10 mL). The combined extract was dried over anhydrous
MgSO4. After evaporation of the solvent under reduced pressure, the
residue was chromatographed by silica gel to obtain the desired product
2a as a white solid. 1H NMR (500 MHz, CDCl3) δ 7.82 (d, J = 7.3 Hz, 2H),
7.55-7.52 (m, 1H), 7.47-7.44 (m, 2H), 6.20 (s, 2H).
Scheme 3. Proposed reaction mechanism
Subsequently, experiment directly with galla chinensis extract
was performed to get a better understanding on the reaction, as
shown in Scheme 4. The dry galla chinensis was purchased from
the local Chinese market, and other detailed procedure can be
seen in the SI. The reaction proceeded smoothly under the
optimized condition, which thereby indicated that other complex
compounds in the extract had no influence on this process.
Procedure with galla chinense extract
The dry galla chinensis was purchased from the local Chinese market, and
ground to powder with a mini electric grinder. Of this raw powder, 5 g was
extracted with 50 mL of the mixture of ethanol/water/HCl (37%) (1:1.9:0.1)
at 70 oC for 5 h. The resulting solution was filtered to remove the solid
residue. To 3 mL of such extract, KOH (268 mg) was added. The reaction
vessel was stirred open-flask at room temperature for 15 min. Then,
benzonitrile (0.1 mmol) was added. The resulting mixture was stirred for
15 h and the precipitate was poured into saturated aqueous NH4Cl (10 mL,
saturated solution) and extracted with ethyl acetate (3 × 10 mL). The
combined extract was dried over anhydrous MgSO4. After evaporation of
the solvent under reduced pressure, the residue was chromatographed by
silica gel to obtain the desired product.
Scheme 4. Hydration of benzonitrile by galla chinensis extract
Conclusions
Acknowledgements
Taken together, we have developed a general and efficient
transformation of nitriles to primary amides. The methodology is
clearly advantageous not only because of the use of inexpensive
renewable promoter and metal-free mild conditions but also
because it employs ethanol/water as solvent. Gram-scale
reactions can be easily performed. The broad substrates scope
applicability together with the wide range of functional groups
Authors gratefully appreciate the financial supports from the
Jiangsu Key Lab. of Biomass Energy and Material Foundation of
China (JSBEM-S-201608), National Key Research Development
Program of China (2016YFD0600805), Beijing Institute of New
Technology of Forestry of China (CAFYBB2016SY030) and thank
all reviewers for their valuable technical suggestions.
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
Tao Deng,Cheng-Zhang Wang*
Page No. – Page No.
An efficient gallic acid promoted
nitriles
hydration
under
mild
conditions in ethanol/water is
developed for the formation of
amines. The use of renewable bio-
resources (gallic acid and galla
chinensis extract) makes the reaction
environmentally friendly, which also
offers an alternative method of
reutilization of natural feedstocks.
An efficient hydration of nitriles
promoted by gallic acid derived
from renewable bio-resources
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