A Mild TEMPO-Catalyzed Aerobic Oxidative Conversion of Aldehydes into Nitriles

Article abstract of DOI:10.1002/adsc.201501130

An efficient method to prepare nitriles from aldehydes using hexamethyldisilazane (HMDS) as the nitrogen source has been developed. The reactions were performed with 2,2,6,6-tetramethylpiperidine l-oxyl (TEMPO) as the catalyst, NaNO₂ or TBN as the co-catalyst, and molecular oxygen as the terminal oxidant under mild conditions. A variety of aromatic, heteroaromatic, aliphatic and allylic aldehydes could be converted into their corresponding nitriles in good to excellent yields.

Full text of DOI:10.1002/adsc.201501130

UPDATES
DOI: 10.1002/adsc.201501130
A Mild TEMPO-Catalyzed Aerobic Oxidative Conversion of
Aldehydes into Nitriles
Chaojie Fang,a Meichao Li,a Xinquan Hu,a,b Weimin Mo,a Baoxiang Hu,a Nan Sun,a
Liqun Jin,a and Zhenlu Shena,*
a
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, Peopleꢀs Republic of China
Fax: (+86)-571-8832-0103; phone: (+86)-571-8832-0416; e-mail: zhenlushen@zjut.edu.cn
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese
b
Academy of Sciences, Lanzhou 730000, Peopleꢀs Republic of China
Received: December 18, 2015; Revised: January 18, 2016; Published online: March 17, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501130.
Abstract: An efficient method to prepare nitriles
from aldehydes using hexamethyldisilazane
Because of the ready availability of aldehydes, they
have been utilized as attractive starting materials for
synthesis of nitriles, and several methods have been
reported with different compounds as nitrogen sour-
ces.[16] However, these reported methods all have
some drawbacks: (i) stoichiometric oxidants, like
TBHP and NaICl2, were needed; (ii) limited sub-
strates could be used; (iii) transition metal catalysts
were essential in most cases. Thus, developing an eco-
nomical, environmentally friendly and efficient
method for the direct synthesis of nitriles from alde-
hydes still remains a great challenge. Recently, Bailey,
Leadbeater and co-workers have provided a new
metal-free route to prepare an array of nitriles from
aldehydes mediated by an oxoammonium salt (4-acet-
amido-2,2,6,6-tetramethylpiperidine-1-oxoammonium
(HMDS) as the nitrogen source has been devel-
oped. The reactions were performed with 2,2,6,6-
tetramethylpiperidine l-oxyl (TEMPO) as the cata-
lyst, NaNO2 or TBN as the co-catalyst, and molecu-
lar oxygen as the terminal oxidant under mild con-
ditions. A variety of aromatic, heteroaromatic, ali-
phatic and allylic aldehydes could be converted into
their corresponding nitriles in good to excellent
yields.
Keywords:
aldehydes;
hexamethyldisilazane
(HMDS); nitriles; oxidation; oxygen; 2,2,6,6-tetra-
methylpiperidine 1-oxyl (TEMPO)
À
tetrafluoroborate, AcNH-TEMPO+ BF4 ) with hexa-
methyldisilazane (HMDS) as the nitrogen source.[17]
This oxidation system is quite simple and can be oper-
As a useful intermediate,[1] the nitrile moiety is one of ated under mild conditions. Nevertheless, 2.5 equiv. of
the most important functional groups[2] and can be the oxoammonium salt must be employed as the oxi-
easily converted into corresponding amines,[3] dant and 1.1 equiv. of pyridine was used to absorb the
amides,[4] ketones,[5] carboxylic acids[6] and esters.[7] highly toxic by-product BF3.
Furthermore, it is also a key motif in numerous useful
It is well known that molecular oxygen is an ideal
compounds such as bioactive natural products,[8] phar- terminal oxidant due to its remarkable advantages, in-
maceuticals,[9] and functional materials.[10] The Sand- cluding cleanness, great abundance, inexpensiveness
meyer reaction was used as the classical synthetic and high atom efficiency. Previously, our group re-
method for aromatic nitriles, while toxic metal cya- ported
a
2,2,6,6-tetramethylpiperidine
1-oxyl
nides were always required to undergo a cross-cou- (TEMPO)/tert-butyl nitrite (TBN)/O2 catalytic oxida-
pling with aryl halides.[11] Later on, a large number of tion system for the aerobic oxidation of alcohols, in
new methods such as ammoxidation of alcohols,[12] ox- which TEMPO was employed as the catalyst, TBN as
idation of amines,[13] and dehydration of amides[14] or the co-catalyst, and molecular oxygen as the terminal
aldoximes[15] have been developed as alternative path- oxidant.[18] In this metal-free system, the key point is
ways for the synthesis of nitriles. These methods offer that TBN served as the NO equivalent to activate
great improvements, but always need high tempera- molecular oxygen. In fact, this system was expanded
ture or pressure, stoichiometric or excess amount of from our previous catalytic oxidation systems, such as
highly reactive oxidants, transition metal catalysts or TEMPO/Br2/NaNO2/O2,[19] TEMPO/HBr/TBN/O2,[20]
metal complex catalysts.
TEMPO/1,3-dibromo-5,5-dimethylhydantoin/NaNO2/
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O2,[21] and extended to the TEMPO/DDQ/TBN/O2
system[22]
.
Inspired by our previous work, we attempted to re-
place tÀhe stoichiometric oxidant AcNH-TEM-
PO+ BF4 by the TEMPO/NaNO2 (or TBN)/O2
system. During the compilation of this present work,
Kim et al. reported an aerobic oxidative conversion
of aldehydes into nitriles catalyzed by a nitroxyl radi-
cal/NOx system.[23] Unfortunately, aliphatic aldehydes
were not suitable for use in their reaction system.
Herein we report an efficient methodology for the
synthesis of nitriles from aldehydes with HMDS as
the nitrogen source using TEMPO as the catalyst,
NaNO2 or TBN as the co-catalyst and O2 as the ter-
minal oxidant under mild conditions (Scheme 1).
Initially, we started to optimize the reaction condi-
tions with benzaldehyde (1a) as the model substrate
(Table 1).The reaction was carried out with 10 mol%
of TEMPO, 10 mol% of NaNO2, 10 mol% of NaBF4,
2.5 equiv. of HMDS in the solution of acetic acid and
acetonitrile with an oxygen balloon at 508C. Benzoni-
trile (1b) could be obtained with 58% yield as deter-
Scheme 1. The new methodology for the synthesis of nitriles
from aldehydes
mined by GC using an internal standard method in tive, NaBF4, was removed from the catalytic system,
8 h (entry 1). When the reaction temperature was de- but the GC yield of 1b dropped into 65% (entry 4).
creased from 508C to 308C, the GC yield of 1b in- This phenomenon suggested that NaBF4 had played
creased dramatically to 81% (entry 3). Then the addi- an important role in this reaction and we just won-
Table 1. Optimization of the direct synthesis of benzonitrile from benzaldehyde.[a]
Entry
HMDS [equiv.]
TEMPO [mol%]
NaNO2 [mol%]
X (mol%)
Solvent
Temp. [8C]
Yield [%][b]
1
2
3
4
5
6
7
8
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.0
1.5
1.0
2.5
2.5
2.5
2.5
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
NaBF4 (10)
NaBF4 (10)
NaBF4 (10)
–
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
PhCH3
PhCl
50
40
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
38
52
81
65
67
95
20
trace
trace
trace
trace
trace
23
37
80
72
50
NaCl (10)
KPF6 (10)
KSCN (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (10)
KPF6 (5)
9
10
11
12
13
14
15
16
17
18
19
20
21[c]
THF
EtOAc
DCM
DMF
DMSO
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
77
69
60
95
5
5
10
5
10
KPF6 (10)
[a]
Reactions conditions: 1a (4 mmol, 0. 424 g), 1 mL of AcOH, 5 mL of solvent, oxygen balloon, 8 h.
Yields were determined by GC using an internal standard method.
Reaction time was 4.5 h.
[b]
[c]
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A Mild TEMPO-Catalyzed Aerobic Oxidative Conversion of Aldehydes
Table 2. Oxidative conversion of aldehydes to nitriles.[a]
dered whether other salts have the same or much
better effects. Thereby several salts, such as NaCl,
KPF6, and KSCN were tested (entries 5–7). We ob-
served that the best GC yield of 1b moved up to 95%
on using KPF6 (entry 6), while KSCN had a negative
effect (entry 7). The effects of additives on the reac-
tion might be relevant to the formal redox potential
of TEMPO+/TEMPO in the presence of different
anions.[24] Several other solvents were also examined
and the results showed a crucial effect of the solvent.
When some strong polar solvents, like DMF and
DMSO, were employed, 1b was obtained in 23% and
27% GC yield, respectively (entries 12 and 13),
whereas other less polar solvents, such as PhCH3,
PhCl, THF, EtOAc and dichloromethane, produced
a negligible amount of nitrile (entries 8–12).
Encouraged by the above results, the loadings of
TEMPO, NaNO2, KPF6 and HMDS were reduced.
When the loading of TEMPO was reduced to
5 mol%, the GC yield of 1b went down to 77% ac-
cordingly (entry 18). In addition, reducing the loading
of NaNO2 or KPF6 could also lead to a lower yield of
1b (entries 19 and 20). When the load of HMDS was
reduced to 2.0, 1.5, and 1.0 equiv., the GC yield of 1b
was decreased to 80%, 72%, and 50%, respectively
(entries 15–17). In addition, it was found that the full
conversion of 1a could be achieved with a 95% GC
yield of 1b in 4.5 h with 10 mol% of TEMPO,
10 mol% of NaNO2, 10 mol% of KPF6, 2.5 equiv. of
HMDS in the mixture of acetic acid and acetonitrile
at 308C (entry 21).
After getting the optimal reaction conditions, the
substrate scope of this methodology was investigated
and the results are summarized in Table 2. Initially,
a series of benzaldehydes with a wide variety of sub-
stituent groups were tested. Results indicated that
substrates with electron-withdrawing groups, most of
which could be converted into their corresponding
benzonitriles in 5 h (3b–8b), were more active than
those with electron-donating ones in this transforma-
tion (9b–15b). Almost all of the substrates with elec-
tron-donating groups needed a longer reaction time
or some promotions of the reaction conditions. For
example, it was impossible to drive the conversion of
[a]
Reaction conditions: aldehyde (4 mmol), HMDS
(10 mmol), TEMPO (0.4 mmol), NaNO2 (0.4 mmol),
KPF6 (0.4 mmol), 1 mL of AcOH, 5 mL of CH3CN, O2-
balloon, 308C. Y=yield of the isolated product.
[b]
Values in parentheses were determined by GC using an
internal standard method.
NaNO2 (15 mol%), KPF6 (15 mol%).
TEMPO (15 mol%), NaNO2 (15 mol%), and KPF6
[c]
[d]
(15 mol%), 408C.
TBN (15 mol%) without NaNO2 and AcOH, 50 mL of
[e]
CH3CN.
[f]
NaNO2 (20 mol%), KPF6 (20 mol%).
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4-methylbenzaldehyde (9a) to be finished essentially (25b–27b). The reactions of allylic aldehydes were
under the optimized conditions until the reaction time also studied. Aerobic oxidation converison of cinna-
was prolonged to 5.5 h. More than that, extending re- maldehyde (28a) to cinnamonitrile (28b) could be
action time was an invalid means for some benzalde- successfully performed with the TEMPO/KPF6/TBN
hydes, such as 4-methoxybenzaldehyde (12a) and 3,4- catalytic system, while (E)-2-methyl-3-phenylacrylal-
dimethoxybenzaldehyde (13a). Both of them needed dehyde (29a) could be oxidized to (E)-2-methyl-3-
higher loadings of catalysts to complete their conver- phenylacrylonitrile (29b) with the TEMPO/KPF6/
sions. Besides, the steric hindrance of substituent NaNO2 catalytic system.
groups also had an effect on reaction rate. The ortho-
Compound 30b displays appreciable anti-inflamma-
position substituted substrates accomplished their tory and selective COX-2 inhibitory activity.[26] Gener-
transformation more difficultly than those with meta- ally, there used to be two common routes for the syn-
or para-positioned groups, which was evidenced by thesis of 30b (Scheme 2). In one route, aldehyde 30a
the comparison of 9a, 10a and 11a. In order to know was oxidized by an excess amount of iodine with am-
the applicability of the protocol for acid labile groups, monia as the nitrogen source in THF to afford com-
two acid-sensitive substrates (16a, 17a) were tested. pound 30b directly.[27] The other route included two
To our delight, 16b and 17b were obtained in satisfy- steps, formation of the intermediate 30c and then
ing yields.
a
subsequent dehydration process with POCl3/
Next, the feasibility for the nitrile formation of TEA.[28] To further expand our newly developed
polycyclic and heterocyclic systems was examined. A method, compound 30b was subjected to our oxida-
representative polycyclic substrate tolerated our cata- tion system, the reaction was smoothly performed and
lytic system giving the product 18b in excellent yield. a 95% isolated yield of 30c was obtained.
Analogously, some heterocyclic examples containing
As mentioned in the above discussion, electron-
sulfur, oxygen and nitrogen atoms were also verified withdrawing groups had a positive effect on the trans-
to give high yields of nitriles (19b–22b). However, the formation of benzaldehydes to benzonitriles. It was
S-containing substrate 19a and N-containing substrate assumed that the formation rate of 4-nitrobenzonitrile
21a required a longer reaction time and extra 5% of (2b) must be fast, but the reaction could not reach
NaNO2 and KPF6 for full conversions.
completion unless the loadings of NaNO2 and KPF6
Adamantane-1-carbaldehyde (23a), an a-saturated were increased to 15 mol% and the reaction time pro-
aliphatic aldehyde, gave the desired nitrile 23b in longed to 10 h (Table 2). Meanwhile, a great deal of
95% yield. Aliphatic aldehydes with a-H atoms could precipitate was generated shortly after the beginning
undergo a self-aldol condensation easily in the pres- of the reaction and disappeared very slowly. Hence,
ence of silylated amines.[25] This result was proved by we speculated that the reaction might involve an in-
the representative aliphatic aldehyde, 3-phenylpropa- termediate which would be transformed to the final
nal (24a), which did not afford the target product product. The poor solubility of the intermediate in
under our optimized reaction conditions. According the oxidation conversion of 4-nitrobenzaldehyde led
to Kellyꢀs work,[17] adding HMDS slowly to the reac- to the lower rate of generation of 2b.
tion solution was an effective way to avoid this un-
Nishiyamaꢀs group has synthesized a series of N’,N-
wanted side reaction. But unfortunately, it did not disubstitued methanediamine derivatives from HMDS
work in our reaction system, and the same result was and aldehydes catalyzed by ZnCl2,[29] N,N’-dibenzyli-
observed on adding aldehyde 24a slowly into the reac-
tion solution. However, to our surprise, when NaNO2
was replaced by the equal amount of TBN and the
acetic acid was removed from the reaction mixture,
an about 20% yield of 24b was obtained. With this en-
couraging finding in hand, the final yield of 24b
reached up to 62% by adding aldehyde slowly into
the reaction mixture with a ten-fold amount of sol-
vent. The above results demonstrated that there was
a competitive relationship between the oxidation re-
action and the self-aldol condensation of 24a under
alkaline conditions. When in acidic solution, the rate
of self-aldol condensation remained overwhelmingly
superior to the desired oxidation reaction. Much to
our delight, some other a-unsaturated aldehydes, such
as undecanal, helional and N-Boc-piperidin-4-ylfor-
maldehyde also could be converted into the desired
nitriles with satisfying yields in this promoted method system.
Scheme 2. Application of the TEMPO/KPF6/NaNO2/O2
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dene-1-phenylmethanediamine (1c) was one of exam- the only side product was benzaldehyde. Furthermore,
ples. We successfully synthesized and separated com- when the reaction mixture of the oxidative conversion
1
pound 1c with benzaldehyde, HMDS and AcOH in 1a to 1b was directly subjected to H NMR analysis,
CH3CN. Then 1c was submitted to our TEMPO/ 1c could also be detected after reaction time of 1 h
KPF6/NaNO2/O2 oxidation system; to our delight, the (Scheme 3). Thus we speculated that 1c was the cru-
expected product benzonitrile (1b) was detected and cial intermediate for the current synthesis of 1b.
Scheme 3. 1H NMR spectral evidence for the mechanism: (1) 1a; (2) 1b; (3) 1c; (4) the reaction mixture of oxidative conver-
sion 1a after a reaction time of 1 h.
Scheme 4. A proposed overall reaction mechanism.
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The overall reaction mechanism for the present General Procedure for the Synthesis of Nitriles from
aerobic oxidative conversion of benzaldehyde to ben- a-Unsaturated Aldehydes as Exemplified for
zonitrile is described in Scheme 4. Under the acidic Undecanedinitrile (25b)
conditions, NaNO2 can release NO, which will be
easily oxidized into NO2 by O2. Benzaldehyde reacts
with a magnetic stirring bar and a thermometer, were added
To a 100-mL, two-necked, round-bottom flask equipped
with HMDS to give N,N’-dibenzylidene-1-phenylme-
thanediamine (1c), which can be oxidized by
TEMPO+ to give benzaldehyde and benzonitrile in
the presence of H2O. At the same time TEMPO+ is
reduced to TEMPOH. TEMPOH is subsequently re-
generated to TEMPO+ by NO2, which turns into NO
immediately.
In conclusion, we have successfully developed an
efficient methodology for the aerobic oxidative con-
version of aldehydes to their corresponding nitriles
with HMDS as the nitrogen source using TEMPO as
the catalyst, NaNO2 or TBN as the co-catalyst. The
homogeneous catalytic system is practical, environ-
mentally friendly as it is performed at low tempera-
ture under atmospheric pressure without any transi-
tion metal catalysts and accommodates a broad range
of substrates including aromatic, heteroaromatic, ali-
phatic and allylic aldehydes with good to excellent
yields. This protocol has been successfully applied in
the synthesis of a biological compound. In addition,
an overall reaction mechanism has also been pro-
posed.
50 mL of CH3CN, 1.62 g (10 mmol) of HMDS, 0.0624 g
(0.4 mmol) of TEMPO and 0.1106 g (0.6 mmol) of KPF6.
Then the flask was sealed by a rubber plug after being
charged with oxygen to replace the air in it. The flask was
placed into a preheated water bath (308C). Meanwhile,
72 mL (0.6 mmol) of TBN were injected to the solution fol-
lowed by stirring for 5 min under a dioxygen atmosphere
(balloon). After that, 0.681 g (4 mmol) of undecanal was in-
jected into the solution slowly (25 times, every 10 min once).
The work-up procedure was as same as that for the synthesis
of 1b, and 25b was obtained as a colorless liquid; yield:
0.380 g (62%).
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21376224, 21206147) and
Hangzhou Qianjiang Distinguished Experts Project.
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