Copper-Assisted Direct Nitration of Cyclic Ketones with Ceric Ammonium Nitrate for the Synthesis of Tertiary α-Nitro-α-substituted Scaffolds

Article abstract of DOI:10.1021/acs.orglett.7b00040

An efficient and direct Cu-assisted nitrating approach to create synthetically valuable and challenging tertiary α-nitro-α-substituted moieties has been developed using ceric ammonium nitrate as a nitrating reagent, oxidant, and Lewis acid. Notably, the commonly used clinical drug ketamine was smoothly synthesized in four steps.

Full text of DOI:10.1021/acs.orglett.7b00040

Letter
pubs.acs.org/OrgLett
Copper-Assisted Direct Nitration of Cyclic Ketones with Ceric
Ammonium Nitrate for the Synthesis of Tertiary α‑Nitro-α-
substituted Scaffolds
Zhi-Qiang Zhang, Tao Chen, and Fu-Min Zhang*
State Key Laboratory of Applied Organic Chemistry and Department of Chemistry, Lanzhou University, Lanzhou 730000, P.R. China
*
S Supporting Information
ABSTRACT: An efficient and direct Cu-assisted nitrating approach to create synthetically valuable and challenging tertiary α-
nitro-α-substituted moieties has been developed using ceric ammonium nitrate as a nitrating reagent, oxidant, and Lewis acid.
Notably, the commonly used clinical drug ketamine was smoothly synthesized in four steps.
itration of aliphatic C−H bonds has been a challenging
our knowledge, only two approaches have been developed to
prepare challenging α-nitro α-aryl ketone moieties. One is the
arylation of α-nitro ketone using tributylphenylstannane
1
N
topic in modern organic synthesis, especially nitration of
C−H bond at the α-position of the carbonyl group.
Consequently, many synthetic methods have been developed to
efficiently prepare α-nitro ketones and simultaneously meet the
requirements for green chemistry, such as preformation under
mild reaction conditions, use of relatively low-toxicity nitrating
8a
(Scheme 1a). The other is the nitration of highly active 1,3-
Scheme 1. Outline of Preparation of α-Aryl α-NO Cyclic
2
Ketones
2
reagents, and simple manipulation. However, the nitration of the
C−H bond at the α-position of the carbonyl group for the
construction of aza-quaternary carbon centers containing NO
groups remains an unexploited avenue.
2
3
Tertiary α-nitro, α-substituted ketones are unique moieties in
organic synthesis, pharmaceutical molecules, and materials
science. These scaffolds not only exist in bioactive compounds
usedasherbicidesbutalsoserveasintermediatesforthesyntheses
of natural products and functional molecules based on diverse
4
transformations of the nitro and carbonyl groups. For example,
the selective reduction of the nitro group provides the tertiary
5
amine, which is a common moiety of bioactive molecules
6
7
including erythraline alkaloids and the drug ketamine. The
simultaneous reduction of nitro and carbonyl groups could
generate the privileged 1,2-amino alcohol ligands for asymmetric
catalysts (Figure 1). Owing to their difficult construction using
conventional nitrating methods, the methods for their prepara-
tion to date have been rarely reported, especially for the
preparation of α-nitro α-aryl ketone moieties. To the best of
8
b
dicarbonyl compounds (Scheme 1b). However, from green and
step-economic points of view, there are some disadvantages inthe
developed methods, such as the relative toxicity of the arylating
stannane, Hg(OAc) and Pb(OAc) reagents, the extra chemical
2 4
transformation for the preparation of suitable precursors, and the
narrow substrate scope. Therefore, the development of a direct,
efficient, andgeneralmethodforthepreparation ofthesemoieties
is essential.
Ceric ammonium nitrate (CAN) has been widely applied in
organic synthesis, especially as a special single-electron-transfer
9
oxidant. Due to its low toxicity, low cost, safety, air and moisture
Figure 1. Selective functional molecules related to tertiary α-nitro α-aryl
ketone moieties.
Received: January 15, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00040
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
stability, and ease of handling, it has received increasing attention
in recent decades. Notably, either combined with excess of
NaNO as a NO source or alone, CAN can promote the nitration
Table 1. Optimization of the Reaction Conditions
2
2
2
of the C (sp )−H bond for the preparation of nitroalkenes,
1
0
nitroamides, or nitroarenes. However, CAN used as a NO
2
3
source has been rarely reported for the nitration of C(sp )−H
bonds, especially the preparation of tertiary α-nitro ketones.
Continuing our research interests in “three birds with one stone”
b
temp
(°C)
yield
(%)
entry
reagent (equiv)
CAN (2.0)
additive
CuO
solvent
1
2
3
4
5
6
7
8
9
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
80
80
80
80
80
80
80
80
80
43
53
46
45
50
60
55
50
41
11
chemicalreagents, wespeculated thatCANcouldserve asLewis
acid, oxidant, and nitrating reagent for the nitration of a C−H
bond at the α-position of a cyclic ketone, resulting in the
construction of the challenging tertiary α-nitro-α-substituted
scaffolds under suitable conditions. Here, we report our research
on this topic based on the multifunctional properties of CAN
CAN (2.0)
CAN (2.0)
CAN (2.0)
CAN (2.0)
CAN (2.0)
CAN (2.0)
CAN (2.0)
Bi(NO ) ·5H O
CuSO4
CuCl2
c
Cu(acac)2
Cu(OAc)2
d
Cu(OTf)2
CuCl
(
Scheme 1).
Initially, we selected commercially available 2-phenylcyclohex-
Cu(OAc)2
3
3
2
(
2.0)
anone as a model substrate to test our designed nitration using
CAN as a multifunctional reagent. However, competitive side
reactions needed to be suppressed, such as the nitration of the
1
0
Fe(NO ) ·9H O
Cu(OAc)2
DCE
DCE
80
80
45
43
3
3
2
(
2.0)
Cu(NO
2.0)
2
12
11
)
3
3
·3H O
2
Cu(OAc)
2
C(sp )−H bond on the phenyl ring, the formation of benzyl
(
1
3
nitrates, the cleavage of the C−C bond at the ketone α-
position, and the self-coupling of 2-phenylcyclohexanone.
1
1
1
1
2
3
4
5
AgNO (2.0)
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
DCE
DCE
DCE
DCE
80
80
80
80
27
27
48
49
3
1
4
15
AgNO (2.0)
2
After some attempts, to our delight, the desired product 2a was
isolated in 43% yield when the reaction was performed in 1,2-
dichloroethane(DCE)at80°Cfor12h(entry1). Inspiredbythis
t
e
BuONO (2.0)
isopentyl nitrite
e
(2.0)
16
initialresult, further screening ofother additiveswas conducted.
16
17
fuming HNO
NaNO (20.0)
3
(2.0)
Cu(OAc)
Cu(OAc)
2
DCE
DCE
80
80
50
0
The results showed that both Cu(I) and Cu(II) salts gave
2
2
a
moderate yields (entries 2−8), and the highest yield (60%) of
Reactions were performed using 2-phenylcyclohexanone (0.2 mmol)
in 2.0 mL of solvent at the noted temperature under an argon
atmosphere in a 15 mL sealed tube. isolated yield. acac = acetyl
product 2a was obtained using Cu(OAc) as the additive (entry
2
b
c
6
). The equivalents of CAN, the reaction temperature, and the
16
d e
solvents were varied, and no better results could be obtained.
acetonate. Tf = trifluoromethanesulfonyl. Under an oxygen
atmosphere.
Other nitro sources were then investigated. When Bi(NO ) ·
3
3
5
H O, Fe(NO ) ·9H O, or Cu(NO ) ·3H O was applied, a
2 3 3 2 3 3 2
a
Scheme 2. Scope of Substituted Cyclohexanones
slightly decreasing yield was obtained because the hydroxylation
side product at the α-position was observed (entries 9−11); using
AgNO or AgNO to replace CAN led to obviously decreasing
3
2
yields (entries 12 and 13). Interestingly, tert-butyl nitrite and
isopentyl nitrite, two commonly used organic nitrating reagents,
were applied, and product 2a was isolated in almost identical
yields (entries 14 and 15). Additionally, fuming HNO afforded
3
the desired product 2a in 50% yield (entry 16), whereas NaNO2
proved to be a completely ineffective reagent (entry 17).
Therefore, the combinations listed in entry 6 in Table 1 were
selected as the optimal reaction conditions.
With the optimal reaction conditions in hand, we investigated
the generality of this novel nitration. Substrates bearing different
substituents on the aromatic ring were subjected to the optimal
conditions, and the results are shown in Scheme 2. The tested
substrates reacted smoothly, affording the corresponding
products 2b−j in moderate to good yields, and the structure of
17
product 2g was further confirmed by X-ray crystallography. For
example, the p-F, o-F, m-F products (2b−d) can be isolated in
a
Reaction conditions: 1 (0.2 mmol), additive (0.04 mmol), CAN (0.4
b
mol) in DCE (2.0 mL). The reaction was performed on a 1 g scale.
5
3%, 51%, and51%yields, respectively, and thesubstratesbearing
the phenyl group at the para-position, the electron-donating
group (OMe) at the meta-position or the electron-withdrawing
Next, a series of benzyl ketone derivatives were subjected to the
optimal reaction conditions; the results are shown in Scheme 3.
Both cyclopentanone and cycloheptanone derivatives reacted
smoothly, affording the corresponding products 2n and 2o. Both
3-phenylchroman-4-one and 1-methyl-3,4-dihydronaphthalen-
2(1H)-one were ideal substrates, and their expected products 2p
and 2q were isolated in moderate yields. Since 3-alkyl-3-amino-2-
oxindoles are a key skeleton in bioactive alkaloids and clinical
group (NO ) at the para-position provided the desired products
2
2h−j, respectively. These results demonstrated that the positions
or electronic properties of substituents on the aromatic ring had
slighteffectsonthistransformation. The2-naphthalenederivative
was an ideal substrate, giving the expected product 2k in 50%
yield. Benzocyclic ketones reacted well, furnishing the desired
product 2l in 70% yield; its analogue bearing a OMe group on the
benzocyclic ring was also obtained in an acceptable yield.
18
drugs, 3-methylindolin-2-ones bearing different N-protecting
B
DOI: 10.1021/acs.orglett.7b00040
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
7d
Scheme 3. Scope of Benzyl Ketones
using the modified methods to provide the target molecule in
6% yield (Scheme 5). Notably, the current approach for the
5
preparation of ketamine is forward and efficient.
Scheme 5. Short Synthesis of Ketamine
Finally, control experiments were conducted to elucidate a
possible mechanism (Scheme 6). Replacement of CAN with
a
Scheme 6. Some Control Experiments
Reaction conditions: substrate 1 (0.2 mmol), additive (0.04 mmol),
CAN (0.4 mmol) in 2.0 mL of DCE.
groups were tested. The nitrating reactions proceeded smoothly
under the optimal conditions, and the desired products 2r−u
were obtained in 53−67% yield. These results indicated that
diverse protecting groups at the N-1 position of 3-methylindolin-
2
-one could be tolerated. Hence, these transformations can
provide various choices in further chemical transformations of
these important indolin-2-one skeletons.
ammonium cerium(IV) sulfate could not promote the desired
transformation, and this result reveals that the NO group is
derived from a nitrate anion other than the ammonium ion in
2
Other cyclic ketone substrates were also tested, and the
reaction results are enumerated in Scheme 4. To our pleasure, the
CAN(eq1). InthepresenceorabsenceofCe(SO ) ·4H O, using
4
2
2
NO BF to replace CAN did not provide the desired product 2a,
a
2
4
Scheme 4. Scope of Alkyl-Substituted Substrates
eliminating the possibility that the electrophilic NO cation was
2
involved in the nitrating process (eq 2). A slight effect of
electronic factors of the substitutes on the aryl ring was observed
(
Scheme 2), partly ruling out the possibility of a carbocation
intermediate. Therefore, when (2,2,6,6-tetramethylpiperdin-1-
yl)oxyl (TEMPO) was introduced under the optimal reaction
conditions, the desired nitration was obviously suppressed,
resulting in isolated compound 4 in 38% yield along with trace 2a
a
Reaction conditions: substrate 1 (0.2 mmol), additive (0.04 mmol),
(
eq 3). This result indicates that NO may be a reactive species in
b
2
CAN (0.4 mmol) in 2.0 mL of DCE. Reaction at 80 °C.
the current transformation. Further support for this speculation
was derived from the evolution of brown gas observed in some
desired products 2v−y were obtained. The 2-methyl-, 2-butyl-, or
nitrating cases. The phenomenon is consistent with NO formed
2
2
-cyclopropyl-substituted cyclohexanones reacted well and led to
through the decomposition of CAN above a certain temper-
19
the expected products in relatively low yield, albeit with longer
reaction times at 90 °C. Notably, 2-methyl-1-tetralone proved to
be an ideal substrate, providing the desired product 2y in 59%
yield. Unfortunately, the alkenyl-substituted substrates failed to
afford the corresponding products 2z and 2aa, resulting in
complex mixtures.
ature.
Based on the above-mentioned experimental results and the
2
0−22
reported literature,
a possible reaction mechanism was
9,21
proposed (Scheme 7). First, the CAN served as a Lewis acid to
Scheme 7. Proposed Reaction Mechanism
In these novel transformations, versatile functional groups,
such as halogens, NO , and OMe on an aryl ring, could be further
2
reacted with other partners to increase the diversity and
complexity of products, a universal strategy for medical chemistry
andmaterials science. Tofurther demonstrate thesynthetic utility
of these transformations, the model reaction was carried out in
gram scale and afforded the expected product 2a in slightly lower
5
2% yield (Scheme 2). This result suggests that the current
protocol can be used to synthesize α-nitro α-aryl cyclic ketones
derivatives on a large scale.
After expansion of the scale synthesis of this transformation, we
turned our attention to the synthetic application. Ketamine is a
widely used nonopioid anesthetic drug that has attracted the
interest of medical and synthetic chemists since the 1950s due to
its simple chemical scaffold with diverse clinical values. Starting
from the product 2f, selective reduction afforded the primary
amine 3 in 89% yield, which was subsequently monomethylated
promote the formation of the enolized intermediate I, which was
7
20c
easily oxidized by CAN to generate radical II. The resulting
19
radical II either was directly trapped by NO derived from CAN
2
without Cu species or reacted through the possible intermediate
20e
IV stabilized by Cu species to afford the desired product 2a.
22
Certainly, becauseNO isahighlyreactivespecie, otherreaction
2
pathways cannot yet be excluded.
C
DOI: 10.1021/acs.orglett.7b00040
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
In summary, an efficient and direct Cu-assisted nitrating
approach toward the synthetically challenging tertiary α-nitro, α-
substituted cyclic ketones scaffolds has been developed using
CAN as a Lewis acid, oxidant, and nitro source. Ketamine, which
has been used as an anesthetic and analgesic in the clinic, was
efficiently synthesized by this novel nitration. Additionally, the
mechanistic studies proposed a possible radical process.
Expansion of this new reaction system for nitration of other
compounds is underway in our group and will be reported in the
near future.
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ASSOCIATED CONTENT
Supporting Information
■
*
S
(
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b00040.
́
(
4
Experimental procedures, spectroscopic data, and NMR
spectra of compounds (PDF)
X-ray crystallographic data of compound 2g (CIF)
(
(
d) Jayakanthan, K.; Madhusudanan, K. P.; Vankar, Y. D. Tetrahedron
AUTHOR INFORMATION
Corresponding Author
■
*
2004, 60, 397. For CAN used as NO source, see: (e) Dinct rk, S. D.;
Ridd, J. H. J. Chem. Soc., Perkin Trans. 2 1982, 965. (f) Itoh, K.-i.;
Horiuchi, C. A. Tetrahedron 2004, 60, 1671.
̧ u
̈
2
E-mail: zhangfm@lzu.edu.cn.
(
11) (a) Lu, H.; Zhang, F.-M.; Pan, J.-L.; Chen, T.;Li, Y.-F. J. Org. Chem.
014, 79, 546. (b) Chen, T.; Peng, R.; Hu, W.; Zhang, F.-M. Org. Biomol.
Chem. 2016, 14, 9859.
12) For selected examples, see: (a) Chawla, H. M.; Mittal, R. S.
Synthesis 1985, 1985, 70. (b) Majetich, G.; Hicks, R. Radiat. Phys. Chem.
ORCID
2
Fu-Min Zhang: 0000-0001-5578-1148
Notes
(
1
1
2
995, 45, 567. (c) Strauss, C. R.; Trainor, R. W. Aust. J. Chem. 1995, 48,
665. (d) Grenier, J.-L.; Catteau, J. P.; Cotelle, P. Synth. Commun. 1999,
9, 1201. (e) Sathunuru, R.; Biehl, E. ARKIVOC 2004, 89.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(
13) (a) Baciocchi, E.; Mandolini, L.; Rol, C. J. Am. Chem. Soc. 1980,
We gratefully acknowledge financial support from the NSFC
1
02, 7597. (b) Paolobelli, A. B.; Gioacchinia, F.; Ruzziconi, R.
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Nos. 21272097 and 21290181) and PCSIRT of MOE (No. IRT-
Tetrahedron Lett. 1993, 34, 6333. (c) Graham, T. H.; Jones, C. M.; Jui,
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DOI: 10.1021/acs.orglett.7b00040
Org. Lett. XXXX, XXX, XXX−XXX