Iron nitrate/TEMPO: A superior homogeneous catalyst for oxidation of primary alcohols to nitriles in air

Article abstract of DOI:10.1002/adsc.201400718

A highly practical, one-step, facile synthesis of aromatic, heteroaromatic, allylic and aliphatic nitriles from primary alcohols catalyzed by ferric nitrate [Fe(NO₃)₃·9H₂O] in the presence of TEMPO, aqueous ammonia and air at room temperature is described.

Full text of DOI:10.1002/adsc.201400718

UPDATES
DOI: 10.1002/adsc.201400718
Iron Nitrate/TEMPO: a Superior Homogeneous Catalyst for Oxi-
dation of Primary Alcohols to Nitriles in Air
Shashikant U. Dighe,a Deepan Chowdhury,a,c and Sanjay Batraa,b,*
a
Medicinal and Process Chemistry Division, CSIR-Central Drug Research Institute, BS-10/1, Sector 10, Jankipuram
Extension, Sitapur Road, Lucknow 226031, India
Fax : (+91) 522-2771941; phone: (+91) 522-2772450/2772550 Extn. 4727, 4705; e-mail: batra_san@yahoo.co.uk or
s_batra@cdri.res.in
Academy of Scientific and Innovative Research, New Delhi, India
National Science Academy-Summer Research Trainee (June–July, 2014) from Department of Chemistry, University of
b
c
Hyderabad, Hyderabad, India
Received: July 23, 2014; Revised: August 25, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400718.
Abstract: A highly practical, one-step, facile synthe-
sis of aromatic, heteroaromatic, allylic and aliphatic
nitriles from primary alcohols catalyzed by ferric ni-
trate [Fe(NO3)3·9H2O] in the presence of TEMPO,
aqueous ammonia and air at room temperature is
described.
KOH, MnO2/MgSO4 and NiSO4/K2S2O8/NaOH,[7] but
the use of stoichiometric amounts of expensive oxi-
dants and generation of large amount of wastes, made
these protocols unsuitable for large scale synthesis.
Nevertheless, with the increasing applications of
molecular oxygen as an oxidant which offers remark-
able advantages such as abundance, low cost and
benign by products like H2O, direct transformation of
alcohols to nitriles was accomplished via catalytic
aerobic oxidation,[10] either under the influence of het-
erogeneous or the homogeneous catalysts. Amongst
heterogeneous catalysts, Mizuno and co-workers used
Ru(OH)x/Al2O3,[11] while Ishida et al. and Hou et al.
Keywords: aerobic oxidation; alcohols; homogene-
ous catalyst; iron; nitriles
The nitrile group is considered to be a vital functional employed MnO2 or MnO2-supported on graphite for
group in organic chemistry as it not only forms part conversion of alcohols to nitrile.[12,13] During compila-
of many important fine chemicals, pharmaceuticals, tion of the present work, Beller et al. reported the use
materials and agrochemicals, but also serves as an in- of novel nitrogen-doped graphene-layered cobalt or
termediate in the preparation of several functional iron oxides (nanocatalyst) under oxygen at 1308C for
groups.[1,2] The classical methods of synthesis of ni- transforming diverse alcohols to nitriles.[14] On the
triles include Sandmeyer reaction and halide/CN ex- other hand homogeneous catalysts generally comprise
change or C-H functionalization of arenes,[3] oxidation of Cu/TEMPO systems which have been demonstrat-
of amines,[4] and dehydration of amides and oximes.[5] ed earlier to oxidize alcohols to aldehydes under
However as these strategies utilize stoichiometric aerobic conditions at ambient temperature.[6a–c,15] Tao
amounts of toxic reagents coupled with high tempera- and co-workers employed Cu(NO3)3/TEMPO in
ture and pressure and often lead to generation of DMSO in a sealed tube flushed with oxygen whereas
wastes, more practical methodologies for nitrile syn- Huang et al. used CuI/TEMPO/bipyridine in ethanol
thesis from inexpensive and readily available starting or acetonitrile in the presence of oxygen to prepare
materials are desirable. As a consequence, many new nitriles from alcohols.[16,17] Muldoon et al. reported an
approaches such as use of DMF as the CN source for open flask procedure for this transformation using
C-H functionalization of arenes,[6] oxidative dehydro- Cu(OTf)2/TEMPO/bipyridine in the presence of air
genation of benzylic alcohols,[7] azides,[8] or methylar- instead of pure oxygen (presence of NaOH increased
enes,[9] have been developed. In particular, the direct the rate of reaction).[18] As the reaction was per-
synthesis of nitrile from alcohol and aqueous ammo- formed in open flask the ammonia was added slowly
nia under oxidative conditions is considered to be an though placing an air balloon could obviate this con-
attractive strategy. Initially such oxidative reactions tinuous addition. However for aliphatic alcohols they
were conducted in the presence of reagents such as I2/ switched to [Cu(MeCN)4][OTf] in the absence of
DIH or TBHP, (Bu4N)2S2O8/Cu(HCO2)2/Ni(HCO2)2/ base. Further they also reported when reactors were
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used (with limiting oxygen mixtures) 1 mol% of DMSO in the presence of oxygen efficiently trans-
CuCl2 is sufficient for the transformation. Kim and forms alcohol to nitriles.[19] These methodologies rep-
Stahl also demonstrated that Cu(OTf)2/TEMPO in resent a major advancement in transformation of al-
cohols to nitriles but have their own drawbacks. Reac-
tions with heterogeneous catalysts require high tem-
perature and pressure while the reactions with Cu/
TEMPO system often require ligands and additives,
long reaction time under heating or are substrate se-
lective. We, therefore, believe that there is a need for
developing a catalytic system which can function at
room temperature with broad substrate scope. Like
Cu/TEMPO system, the Fe/TEMPO system has been
Figure 1. Comparison of previous works with this work.
well studied for the aerobic oxidation of alcohols to
Table 1. Results of the optimization[a] with Fe/TEMPO cata-
aldehydes.[20] This homogeneous catalytic system is
lyst system for the synthesis of benzonitrile from benzyl al-
benign with minimal safety concern and avoids the
cohol
use of any ligand and base. Therefore we considered
exploring the iron- catalyzed direct transformation of
alcohols to nitriles. Herein, we disclose the Fe/
TEMPO-catalyzed aerobic oxidative synthesis of ni-
triles from primary alcohols (Figure 1). The protocol
with this catalytic system is practical and superior to
the ones reported to date as it is performed at room
temperature in the presence of air, requires less time,
does not require slow addition of ammonia, and ac-
commodates a broad range of substrates including ali-
phatic, allylic, aromatic and heteroaromatic systems.
Our study commenced with screening the transfor-
mation of benzyl alcohol to benzonitrile in the pres-
ence of different Fe(III) salts and TEMPO using
liquid ammonia and air. As the best performance of
the Fe/TEMPO system is obtained in a weakly coor-
dinating solvent such as dichloroethane (DCE),[10a] we
initially used different Fe(III) salts (10 mol%) with
TEMPO (10 mol%) in DCE (Table 1). In order to
prevent the evaporation of ammonia, the flask was
capped with rubber septum and the air atmosphere
was maintained using a balloon. We observed that all
the three salts successfully catalyzed the conversion,
entry Fe source (mol%)
TEMPO solvent Yield[b]
(mol%)
(%)
1
2
FeCl3·6H2O (10)
Fe(acac)3 (10)
10
10
DCE
DCE
DCE
MeCN
PhMe
EtOH
DMSO
MeCN
MeCN
MeCN
12
24
54
92
62
30
-
93
93
-
3
4
5
6
Fe(NO3)3·9H2O (10) 10
Fe(NO3)3·9H2O (10) 10
Fe(NO3)3·9H2O (10) 10
Fe(NO3)3·9H2O (10) 10
Fe(NO3)3·9H2O (10) 10
7
8
Fe(NO3)3·9H2O (5)
Fe(NO3)3·9H2O (5)
Fe(NO3)3·9H2O (5)
5
5
-
9[c]
10
[a]
All reactions were carried out using 1a (0.2 g,
1.85 mmol), aq. NH3 (25%) 0.157 mL (9.2 mmol), MeCN
(5 mL), air balloon.
Isolated yields after column chromatography.
The reaction was performed using NaCl (10 mol%) as
additive.
[b]
[c]
Table 2. Scope of substituted benzyl alcohols[a] for aerobic double dehydrogenation
entry
R
yield[b]
(%)
entry
R
yield[b]
(%)
1
2
3
4
5
6
7
2b, 2-Cl
91
87
89
84
82
93
98
8
9
2i, 4-NO2
2j, 4-OMe
2k, 4-OCF3
2l, 2,6-Cl2
2m, 3,4-(OMe)2
2n, 3,4,5-(OMe)3
2o, 2,3-(OCH2O)-6-NO2
92
82
90
88
78
71
92
2c, 2-OMe
2d, 3-OMe
2e, 4-Me
2 f, 4-F
2g, 4-Cl
2h, 4-Br
10
11
12
13
14
[a]
All reactions were carried out using substituted benzyl alcohol (0.2 g, 1.0 equiv), Fe(NO3)3·9H2O (0.05 equiv), TEMPO
(0.05 equiv), aq. NH3 (25%, 5.0 equiv), MeCN (5 mL), air balloon.
Isolated yields after column chromatography.
[b]
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Iron Nitrate/TEMPO: a Superior Homogeneous Catalyst
Table 3. Scope of polyaromatic, allylic, aliphatic and hetero-
but the best yield (58%) of benzonitrile was obtained
using Fe(NO3)3·9H2O/TEMPO (entries 1-3). Optimi-
zation of the solvent system revealed that MeCN
gave a superior yield (92%) of the product (entries 4-
7). Optimization of the amount of the catalyst and
TEMPO revealed that 5 mol% of Fe(NO3)3·9H2O
and 5 mol% of TEMPO is sufficient for the transfor-
mation of benzyl alcohol to benzonitrile (entry 8). In
our hands the addition of NaCl (10 mol%) as additive
as reported by Ma et al. did not influence the yield of
the product (entry 9). The reaction was unsuccessful
in the absence of TEMPO (entry 10). Thus the opti-
mized reaction conditions which worked efficiently
for us were benzyl alcohol (1 equiv), 25% NH3 (aq.)
aromatic alcohols for aerobic double dehydrogenation[a]
(5equiv),
Fe(NO3)3·9H2O
(5 mol%),
TEMPO
(5 mol%), MeCN as solvent under an air balloon at
room temperature for 12 h.
Having defined the optimum conditions for the re-
action, we then investigated the generality of the
methodology employing diverse benzyl alcohols. It
was observed that all the substituted benzyl alcohols
gave the respective products (2b-2o) in excellent
yields in 12 h of reaction time (Table 2). The electron-
ic nature of the substituent did not influence the out-
come. Unlike the observation of Huang et al.,[17] the
protocol was found to be efficient for the electron de-
ficient benzyl alcohol, since 4-nitrobenzyl alcohol
gave product 2i in 92% yield (entry 8). Even for 2,3-
methylenedioxy-6-nitro benzyl alcohol the corre-
sponding nitrile 2o was isolated in 92% yield
(entry 14). We observed that only 3,4,5-trimethoxy-
benzyl alcohol furnished the corresponding benzoni-
trile 2n in only 71% yield (entry 13).
Next we investigated the scope with a range of al-
cohols which included polyaromatic, allylic, aliphatic
and heteroaromatic systems. To our delight the cata-
lytic system could transform all alcohols to their cor-
responding nitriles efficiently (Table 3). The reaction
of polyaromatic, allylic and aliphatic alcohols re-
quired 8-12 h for completion whereas the transforma-
tions for heteroaromatics were completed in less than
5 h. The relative stereochemistry of the allylic alco-
hols was maintained in the nitriles (entries 3,4). In
contrast to the Cu(NO3)3/TEMPO which failed to
afford the nitrile with aliphatic alcohols,[16] this cata-
lytic system efficiently transform the different aliphat-
ic alcohols investigated in this study to the respective
nitriles (3g-h) (entries 5-8). The protocol when ap-
[a]
All reactions were carried out using polyaromatic, ali-
phatic or heteroaromatic alcohols (0.2 g, 1.0 equiv),
Fe(NO3)3.9H2O (0.05 equiv), TEMPO (0.05 equiv), aq.
NH3 (25%) (5.0 equiv), MeCN (5 mL), air balloon.
Isolated yields after column chromatography.
Minor amount of amide was observed if the reaction was
[b]
[c]
allowed to proceed for 5 h.
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use of nitrate salts could oxidize the nitroxyl radical
such as TEMPO into an oxoammonium salt, which is
the active oxidant and could also assist the reoxida-
tion of Fe(II) to Fe(III).[20e] We assume that a similar
mechanism operates here to transform the alcohol to
a nitrile.
Scheme 1. Transformation of a heteroaromatic alcohol into
an amide via the nitrile.
In summary, we have demonstrated that Fe/
TEMPO is a very effective homogeneous catalyst
plied to 1,6-hexanediol afforded adiponitrile (3h) in system for direct conversion of alcohols to nitriles
58% yield in 18 h. The low yield and longer reaction using aqueous ammonia and air as oxidant. As this
time for this substrate may be attributed to the use of catalytic system uses iron and air and does not require
only 5 mol% each of Fe(NO3)3·9H2O and TEMPO the use of any ligand or additive or higher tempera-
for oxidizing two hydroxyl groups. For heteroaromat- ture and pressure, it offers a cheaper and more practi-
ics, it was noticed that if the reactions of (5-methyl- cal option than the methods reported to date. As this
thiophen-2-yl)methanol and (3-phenylisoxazol-5-yl)- protocol accommodates a wide range of alcohols and
methanol to the respective nitriles (3i and 3j) were in general requires less time, it offers a superior and
allowed to continue beyond 4 h, polar side products, sustainable option for the synthesis of nitriles via
which were identified to be the amides, were formed aerobic oxidation. This work therefore updates the
in minor quantities (entries 9,10). Such an observation repertoire of the homogeneous catalysts for the oxida-
for heteroaromatic alcohols was also reported by tive transformation of primary alcohols to nitriles.
Muldoon et al.[18] In a model experiment therefore
the progress of oxidation of (3-phenylisoxazol-5-yl)-
methanol in the presence of Fe/TEMPO system was Experimental Section
closely monitored. The transformation of alcohol to
nitrile 3j (84%) was completed in 4 h, but if the reac- General Procedure for the synthesis of nitriles from
tion was prolonged to 8 h, the corresponding amide 4 primary alcohols as exemplified for benzonitrile 2a
was isolated in 92% yield (Scheme 1). Other hetero-
aromatics also gave the products (3k–n) within 5 h
(entries 11-15). Unlike 1,6-hexanediol, the b-carboline
To a stirred solution of benzyl alcohol 1a (0.2 g, 1.85 mmol)
and NH3 (aq, 25–30% w/w, 0.157 mL, 9.2 mmol, 5.0 equiv)
in MeCN (5.0 mL) was added Fe(NO3)3·9H2O (0.037 g,
derivative bearing two hydroxymethyl groups at 1-
and 3-positions was transformed into bisnitrile deriva-
tive 3o in 86% yield in 8 h with the use of 5 mol% of
the catalyst (entry 15).
The plausible mechanism of the formation of nitrile
from Fe/TEMPO-catalytic system is presented in
Figure 2. In the first step the alcohol is transformed
into the aldehyde via an oxidative dehydrogenation.
The aldehyde then forms an imine which also under-
goes oxidative dehydrogenation under the influence
of the catalytic system in step 2. It has been already
suggested by Ma et al. that the NO2 generated by the
0.092 mmol) and TEMPO (0.014 g, 0.092 mmol) at room
temperature. The flask was capped with a septum and air
was introduced via a balloon; the reaction was allowed to
continue for 12 h at the same temperature. After the reac-
tion was complete (as determined by TLC), the solvent was
removed and the residue was purified by column chroma-
tography on silica gel using hexanes/EtOAc (9.5:0.5) as
eluent to afford 2a (92%, 0.175 g) as a colorless oil. Rf =
0.77 (hexanes/EtOAc, 8:2, v/v); 1H NMR (400 MHz,
CDCl3): d (ppm)=7.45–7.49 (m, 2H), 7.58–7.66 (m, 3H).
Acknowledgements
One of the authors (SUD) gratefully acknowledges the finan-
cial assistance in the form of senior research fellowship from
Council of Scientific and Industrial Research, New Delhi.
DC acknowledges the National Science Academy, Bangalore
for financial assistance in the form of a Summer Research
Fellowship. The authors acknowledge the SAIF Division of
CSIR-CDRI for providing the spectroscopic and analytical
data. This work was carried out from the grant to SB under
the CSIR-Network project BSC0114. This is CDRI communi-
cation no 8797.
Figure 2. Plausible mechanism for the formation of nitrile
from primary alcohol.
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Iron Nitrate/TEMPO: a Superior Homogeneous Catalyst
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6 Iron Nitrate/TEMPO: a Superior Homogeneous Catalyst
for Oxidation of Primary Alcohols to Nitriles in Air
Adv. Synth. Catal. 2014, 356, 1 – 6
Shashikant U. Dighe, Deepan Chowdhury, Sanjay Batra*
6
asc.wiley-vch.de
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 0000, 000, 0 – 0
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These are not the final page numbers!