NH3?H2O: The Simplest Nitrogen-Containing Ligand for Selective Aerobic Alcohol Oxidation to Aldehydes or Nitriles in Neat Water

Article abstract of DOI:10.1002/open.201800196

Aqueous ammonia (NH₃?H₂O) has been shown to serve as the simplest nitrogen-containing ligand to effectively promote copper-catalyzed selective alcohol oxidation under air in water. A series of alcohols with varying electronic and steric properties were selectively oxidized to aldehydes with up to 95 % yield. Notably, by increasing the amount of aqueous ammonia in neat water, the exclusive formation of aryl nitriles was also accomplished with good-to-excellent yields. Additionally, the catalytic system exhibits a high level of functional group tolerance with ?OH, ?NO₂, esters, and heteroaryl groups all being amenable to the reaction conditions. This one-pot and green oxidation protocol provides an important synthetic route for the selective preparation of either aldehydes or nitriles from commercially available alcohols.

Full text of DOI:10.1002/open.201800196

DOI: 10.1002/open.201800196
NH3·H2O: The Simplest Nitrogen-Containing Ligand for
Selective Aerobic Alcohol Oxidation to Aldehydes or
Nitriles in Neat Water
Guofu Zhang,[a] Danting Ma,[a] Yiyong Zhao,[a] Guihua Zhang,[a] Guangyao Mei,[b]
Jinghui Lyu,[a] Chengrong Ding,*[a] and Shang Shan*[a]
Aqueous ammonia (NH3·H2O) has been shown to serve as the
simplest nitrogen-containing ligand to effectively promote
copper-catalyzed selective alcohol oxidation under air in water.
A series of alcohols with varying electronic and steric proper-
ties were selectively oxidized to aldehydes with up to 95%
yield. Notably, by increasing the amount of aqueous ammonia
in neat water, the exclusive formation of aryl nitriles was also
accomplished with good-to-excellent yields. Additionally, the
catalytic system exhibits a high level of functional group toler-
ance with ÀOH, ÀNO2, esters, and heteroaryl groups all being
amenable to the reaction conditions. This one-pot and green
oxidation protocol provides an important synthetic route for
the selective preparation of either aldehydes or nitriles from
commercially available alcohols.
1. Introduction
The selective aerobic oxidation of alcohols to carbonyl com-
pounds has emerged as one of the most important synthetic
strategies in organic chemistry,[1] with significant effort being
devoted to the development of new and efficient oxidative
systems for effective transformations. Among these strategies,
transition-metal-catalyzed selective alcohol oxidations have at-
tracted much attention in the chemical community over the
past few years. Generally, noble metal catalysts such as those
based on palladium,[2] platinum,[3] gold,[4] ruthenium,[5] and rho-
dium,[6] together with various ligands, can effectively facilitate
the oxidation of alcohols; however, their large-scale application
has been hampered by the high cost and rarity of the metal
species, as well as by their poor compatibility with heterocy-
cles and other heteroatom-containing functional groups. Re-
cently, inexpensive first-row transition metals such as Co,[7]
Fe,[8] and Cu[9] have been shown to exhibit broader functional
group tolerance in aerobic oxidations. A catalytic system con-
sisting of a copper catalyst and TEMPO is an effective combina-
tion for alcohol oxidations, employing molecular oxygen or air
as an ideal oxidant and a commercially available copper source
as a catalyst. Since the earliest example of the use of a
CuCl/TEMPO-catalytic system for the oxidation of benzylic and
allylic alcohols reported by Semmelhack in 1984,[9a] significant
progress has been made in this field. Due to the low electrode
potential of copper (Cu/Cu2+ Eox = +0.34 V),[10] ligands such
as pyridine,[9a] 2,2-bipyridine,[9c,f] phenanthroline,[9b] N-heterocy-
clic carbene,[9d,i] and amino acids[9k,l] or their expensive deriva-
tives,[9g,h] are usually required in order to promote smooth oxi-
dative transformation. Generally, these ligands possess com-
plex structures and most derivatives are not commercially
available for direct use. Additionally, such ligands are not easily
removed from the reaction and so complicate purification of
the reaction mixture. To the best of our knowledge, aqueous
ammonia, a readily available commodity source, has not been
used as an N-containing ligand in aerobic oxidation reactions
of alcohols. Its low boiling point and good solubility in water
may allow it to be more easily removed from the reaction mix-
ture and thus simplify the purification process.
[a] G. Zhang, D. Ma, Y. Zhao, G. Zhang, J. Lyu, Dr. C. Ding, Dr. S. Shan
College of Chemical Engineering
In addition to the above, we were also interested in explor-
ing whether it was possible to achieve a selective synthesis of
aryl nitriles from commercially available aryl alcohols using am-
monia as a potential nitrogen source and by carrying out the
reaction in water. Although there are a considerable number
of protocols that employ ammonia as a nitrogen source for cy-
anation reactions, only a few examples of the exclusive forma-
tion of both aryl aldehyde and aryl nitriles have been reported
in aqueous media.[11] In this scenario, ammonia may act as an
effective ligand to facilitate the copper-catalyzed aerobic oxi-
dation, as well as function as a cyanide source for aryl nitrile
formation. Herein, we report an efficient copper-catalyzed
aerobic oxidative system for the selective preparation of both
aldehydes and nitriles in water. By using ammonia as the sim-
Zhejiang University of Technology
Hangzhou 310014 (P. R. China)
E-mail: dingcr@zjut.edu.cn
shanshang2001@163.com
[b] G. Mei
Zhejiang Hongyuan Pharmaceutical Co. Ltd
Taizhou 317016 (P. R. China)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/open.201800196.
ꢀ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
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plest N-containing ligand, a series of alcohols could be reacted
with excellent reactivity and selectivity. In addition, the demon-
strated system exhibits a high level of functional group
tolerance.
Considering ammonia can be formed in situ via the decompo-
sition of ammonium salts at high temperature, the influence of
ammonium acetate on the catalytic reaction was investigated.
However, the aldehyde was obtained with 77% conversion
and was accompanied by 2% of the corresponding benzoni-
trile (Table 1, entry 11). The effect of different copper salts was
next examined (Table 1, entries 12–20). The results show that
almost all of the copper salts, besides CuO, were effective in
the reaction. CuI proved to be the optimal copper source
giving the desired aldehyde with quantitative conversion. The
conversion decreased when the catalyst loading was reduced
to 3 mol% (Table 1, entry 21). Control experiments showed
that both the copper catalyst and TEMPO are essential for this
oxidative reaction (Table 1, entries 22, 23). Thus, the optimal re-
action conditions for the aerobic alcohol oxidation to alde-
hydes were established as the following: alcohol (1.0 mmol),
ammonia (7.9 mol%), catalyzed by CuI (5 mol%)/TEMPO
(5 mol%), heating at reflux condition in water under an air at-
mosphere. Notably, by simply increasing the amount of ammo-
nia to 8.0 equiv, we were excited to see that the benzonitrile
was formed exclusively (see the SI).
2. Results and Discussion
At the beginning of our studies, we embarked on reaction con-
dition optimization using benzyl alcohol 1a as a model sub-
strate and by conducting the reaction in water under an air at-
mosphere (Table 1). Initial studies showed that only 39%
benzyl alcohol was converted to the desired aldehyde using
CuI/TEMPO in the absence of any additive and heating under
reflux conditions in water (Table 1, entry 1). Gratifyingly, the
conversion was significantly increased to 75% via the addition
of 1.98 equiv of ammonia as a ligand. Notably, the side-prod-
uct, benzonitrile, was also produced in the reaction, implying
the high possibility of nitrile formation using ammonia as a
sole ligand (Table 1, entry 2). The ammonia loading was further
decreased in order to further improve the reactivity and selec-
tivity of aldehyde formation (Table 1, entries 3–10). With
7.9 mol% ammonia, the desired product 2a was exclusively
formed and no benzonitrile was detected (Table 1, entry 8).
Under the optimal conditions, we investigated the generality
of our aerobic alcohol oxidation (Table 2). Generally, all the aryl
alcohols bearing various electron-donating or electron-with-
drawing groups on the aromatic rings, were oxidized selective-
ly to the corresponding aldehydes with almost quantitative
conversion and up to 95% isolated yield (Table 2, 2a–k and
2r). Compared to electronic-deficient substrates, electronic-rich
substrates were more efficient and able to complete the trans-
formation in a shorter time. Notably, the system exhibited a
high level of chemoselectivity. For temperature-sensitive sub-
strates such as 4-(hydroxymethyl)benzoic acid, methyl 4-(hy-
Table 1. Reaction condition optimization for selective aerobic alcohol oxi-
dation of benzyl alcohol to benzaldehyde·.[a]
Entry
NH3·H2O
[x mmol]
Catalyst
Conv.[b]
[%]
Yield[b] [%]
3a
2a
1
2
–
1.98
CuI
CuI
39
75
39
55
–
20
Table 2. Direct conversion of primary alcohols into aldehydes.[a]
3
4
1.32
0.79
CuI
CuI
70
64
60
60
10
4
5
0.66
CuI
72
69
3
6
0.39
CuI
65
65
–
7
0.15
CuI
CuI
CuI
CuI
97
100
87
65
79
81
89
88
92
73
74
72
92
14
97
trace
trace
97
100 (94)
87
65
77
81
89
88
92
73
74
72
91
14
97
trace
trace
<1
–
–
–
2
–
–
–
–
–
–
–
–
–
–
–
–
8[c]
9
0.079
0.039
0.016
1.5
10
11[d]
12
13
14
15
16
17
18
19
20
21[e]
22[f]
23[g]
CuI
0.079
0.079
0.079
0.079
0.079
0.079
0.079
0.079
0.079
0.079
0.079
0.079
CuBr
CuBr2
CuCl
CuCl2
CuSO4
Cu(NO3)2
Cu(OTf)2
Cu(OAc)2
CuO
CuI
–
CuI
[a] Reaction conditions: benzyl alcohol 1a (1.0 mmol), copper salt
(5 mol%), TEMPO (5 mol%), NH3·H2O (aq., 25–28% w/w, x mmol), H2O
(3.0 mL), air, reflux, 18 h. [b] Determined by GC–MS. [c] Isolated yield in
parentheses. [d] Using CH3COONH4 in place of NH3·H2O. [e] Using CuI
(3 mol%). [f] Copper salt was omitted. [g] In the absence of TEMPO.
[a] Reaction conditions: alcohol
(5 mol%), NH3 (aq., 25–28% w/w, 7.9 mmol%), H2O (3.0 mL), air, reflux,
18–24 h; determined by GC, isolated yields in parentheses.
1 (1.0 mmol), CuI (5 mol%), TEMPO
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droxymethyl)benzoate and (4-morpholinophenyl)methanol, no
coupling reactions were observed (Table 2, 2u–w). In addition,
alcohols bearing oxidation-sensitive group such as ÀOH, ÀSMe,
and alkenyl functionalities, were selectively converted into the
corresponding aldehydes as the exclusive products (Table 2,
2l–n, 2s and 2t). This copper-based system has also shown
high compatibility with N/O/S-containing heterocyclic func-
tionalities, and the desired heteroaryl aldehydes were obtained
in good yields (Table 2, 2o–q). This remarkable feature further
inspired us to apply our CuI/TEMPO/NH3 oxidative protocol to
the synthesis of pharmacologically active intermediates. As
demonstrated by the oxidation of sterically demanding alcohol
1x, an important intermediate in the synthesis of Rosuvastatin
Calcium,[12] the desired aldehyde was isolated with a syntheti-
cally useful yield (Table 2, 2x).
bly, the derivatives bearing oxidation-sensitive groups (ÀOH, À
SMe and alkenyl functionalities) also reacted smoothly, provid-
ing satisfying yields of the desired nitriles without the genera-
tion other side products (Table 3, 3o, 3p and 3u, 3v). It is
noteworthy that the protocol could also be extended to
the efficient cyanation of hetero-aromatic compounds
(Table 3, 3q–s).
The catalyst CuI has shown catalytic efficiency during the
transformation, whose TONs and TOFs could reach up to 19.2
and 1.04 hÀ1 (see Table S2 and Table S3).Considering that the
use of a large excess of aqueous ammonia is undesirable, fur-
ther experiments were conducted to investigate if it was possi-
ble to recycle the excessive amounts of ammonia (Table 4).
Table 4. Recycling of excess ammonia for cyanation of benzylalcohol de-
rivative 1c.[a*]
With a useful and highly selective synthesis of benzalde-
hydes in hand, we turned our attention to the preparation of
nitrile derivatives from aryl alcohols (Table 3). To our delight,
Table 3. Direct conversion of primary alcohols into nitriles.[a]
Cycle
Aqua ammonia [equiv]
TEMPO [mol%]
conv.[b] [%]
1
2
3
4
5
6
8.0
5
1
1
1
1
1
>99
98
98
97
96
3.0[c]
3.0[c]
3.0[c]
3.0[c]
3.0[c]
96
[a] Reaction conditions: 4-methoxybenzyl alcohol 1c (1.0 mmol), CuI
(5 mol%), TEMPO (1 mol%), NH3 (aq., 25–28% w/w, 3 equiv), H2O
(3.0 mL), in air, refluxing for 24 h. [b] Determined by GC. [c] The amount
of ammonia/TEMPO freshly added after the previous cycle.
Gratifyingly, the employment of water as a green solvent in
our system greatly facilitates the separation of organic com-
pounds (using ethyl acetate as an extraction solvent) from the
reaction system. The recyclability of our catalyst system was
tested using 4-methoxybenzyl alcohol 1c as a model substrate.
The separated aqueous phase was charged with fresh alcohol,
3.0 equiv aqueous ammonia and 1.0 mol% TEMPO in the resid-
ual aqueous solution after each cycle. To our delight, the re-
covered system was still highly effective for the conversion of
alcohol into the desired nitrile compound with excellent yield
and selectivity (Table 4, entries 2–6). The reusability of the cata-
lyst system using excess aqueous ammonia makes this proto-
col more practically valuable.
On the basis of preliminary mechanistic studies[11b,13] and our
previous work,[9k,m] we propose the CuI mediated TEMPO oxi-
dation mechanism shown in Scheme 1. Initially, CuI coordinates
with NH3 and H2O to form CuI intermediate A, which after oxi-
dation with TEMPOH and O2, generates the CuII intermediate B.
Metallization of the OH group of alcohol 1 with CuII intermedi-
ate B leads to CuII–OR intermediate C, which is subsequently
reduction by TEMPO to give the CuI intermediate A meanwhile
the -OR group undergoes a hydrogen abstraction mechanism
to give the corresponding aldehyde 2. The aldehyde proceeds
a rapid conversion to the imine in the presence of 8.0 equiv
[a] Reaction conditions: substrate (1.0 mmol), CuI (5 mol%), TEMPO
(5 mol%), NH3 (aq., 25–28% w/w, 8 equiv), H2O (3 mL), refluxing under air
for 24 h, determined by GC, isolated yields in parentheses.
the copper-based catalytic system showed excellent reactivity,
selectivity and functional groups compatibility. A plethora of
aromatic primary alcohols underwent selective oxidative con-
versions to afford the corresponding nitriles in good to excel-
lent isolated yields. Benzyl alcohol and its derivatives bearing
electron-donating groups such as MeO, Me, and BnO, exhibit-
ed good reactivity with almost quantitative conversions to the
corresponding products (Table 3, 3a--g). Similar reactivities
were also observed with substrates bearing electron-withdraw-
ing groups, including halogen and nitro substituents (Table 3,
3h–n and 3t) which could be further derivatized. Moreover,
substrates bearing p-COOH or p-CO2Me groups, which exhibit
lower activity in other systems, were also effectively trans-
formed into 4-cyanobenzoic acid and methyl 4-cyanobenzoate,
respectively, as the only products (Table 3, 3w and 3x). Nota-
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H2O (3.0 mL) were added to a 150 mL seal tube. The resulting
deep blue solution was vigorously stirred in air atmosphere at
reflux temperature for 24 h. After the reaction, the residue was ex-
tracted with ethyl acetate (3”5.0 mL). Combined organic phase
and the solvent were removed under vacuum to afford the crude
product, which was purified by column chromatography on silica
gel to give the pure product.
The experiments of excess ammonia reused. After the reaction, the
product was separated from reaction system and the organic
phase was detected by GC-MS, using ethyl acetate (3 ”10.0 mL) as
extraction agent. Then, charging fresh 4-methoxybenzyl alcohol
(0.1382 g, 1.0 mmol), TEMPO (0.0016 g, 0.01 mmol), NH3 (aq., 25–
28% w/w, 230 mL, 3.0 mmol), in the residual aqueous solution. The
solution was vigorously stirred in air atmosphere at reflux tempera-
ture for 24 h. The operation was repeated for 5 times.
Scheme 1. The proposed mechanism for the reaction.
Acknowledgements
ammonia. The transient imine intermediate, being a better
ligand than either the aldehyde, forms the favored CuI-imine
intermediate D, which may trigger the second oxidation cycle
leading to the corresponding nitrile 3.
We acknowledge financial support from the National Natural Sci-
ence Foundation of China (No. 20702051), the Natural Science
Foundation of Zhejiang Province (LY13B020017) and the Key In-
novation Team of Science and Technology in Zhejiang Province
(No. 2010R50018).
3. Conclusions
In conclusion, a novel, environmentally benign and efficient
copper-catalyzed selective alcohol oxidation and cyanation has
been developed in neat water under air. The transformation is
effectively facilitated by aqueous ammonia (NH3·H2O) as the
simplest N-containing ligand. Notably, by tuning the amount
of aqueous ammonia in neat water, the exclusive formation of
both aldehydes and aryl nitriles was accomplished with good
to excellent yields. Additionally, the catalytic system exhibits a
high level of functional group tolerance, being suitable for À
OH, ÀNO2, esters and other heteroaryl functionalities. This one-
pot green oxidation protocol provides an important synthetic
route for the selective preparation of either aldehydes or ni-
triles from commercially available alcohols, a copper source
and ammonia.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: alcohol oxidation · aldehydes · aqueous ammonia ·
aryl nitriles · copper-catalyzed reactions
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Experimental Section
General Procedure for Direct Conversion of Primary Alcohols
into Aldehydes
A mixture of alcohol (1.0 mmol), CuI (0.0095 g, 0.05 mmol), TEMPO
(0.0078 g, 0.05 mmol), aqueous ammonia (2.6”10À2 molLÀ1
,
3.0 mL) were added to a 150 mL seal tube. The resulting deep blue
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