PIFA-Promoted, Solvent-Controlled Selective Functionalization of C(sp2)-H or C(sp3)-H: Nitration via C-N Bond Cleavage of CH3NO2, Cyanation, or Oxygenation in Water

Article abstract of DOI:10.1021/acs.orglett.9b00751

A novel nitration (via C(sp³)-N breaking/C(sp²)-N formation with CH₃NO₂) mediated by [bis(trifluoroacetoxy)iodo]benzene (PIFA) is described. The NO₂ transfer from CH₃NO₂ to the aromatic group of the substrate is possible with careful selection of the solvent, NaX, and oxidant. In addition, the solvent-controlled C(sp²)-H functionalization can shift to an α-C(sp³)-H functionalization (cyanation or oxygenation) of the α-C(sp³)-H of cyclic amines.

Full text of DOI:10.1021/acs.orglett.9b00751

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
PIFA-Promoted, Solvent-Controlled Selective Functionalization of
C(sp2)−H or C(sp3)−H: Nitration via C−N Bond Cleavage of CH3NO2,
Cyanation, or Oxygenation in Water
Chandrashekar Mudithanapelli, Lama Prema Dhorma, and Mi-hyun Kim*
Gachon Institute of Pharmaceutical Science & Department of Pharmacy, College of Pharmacy, Gachon University, 191
Hambakmoeiro, Yeonsu-gu, Incheon 21936, Republic of Korea
S
* Supporting Information
ABSTRACT: A novel nitration (via C(sp3)−N breaking/
C(sp2)−N formation with CH3NO2) mediated by [bis-
(trifluoroacetoxy)iodo]benzene (PIFA) is described. The
NO2 transfer from CH3NO2 to the aromatic group of the
substrate is possible with careful selection of the solvent, NaX,
and oxidant. In addition, the solvent-controlled C(sp2)−H
functionalization can shift to an α-C(sp3)−H functionalization
(cyanation or oxygenation) of the α-C(sp3)−H of cyclic
amines.
he development of efficient methods for selective C(sp2)−
H or C(sp3)−H functionalizations, which involve the
formation of new C−C and C−heteroatom bonds, has recently
attracted substantial attention.1 In particular, nitro and cyano
groups are the most prevalent functional groups in natural
products,2b,d and they are important precursors in chemical
transformations to access other pharmaceutically and bio-
logically active compounds.2
Hypervalent iodine can facilitate metal-free functionalizations
as well as oxidations.9 Among the derivatives of iodine(III),
PIFA has been successfully applied in oxidative intra- and
intermolecular C−C and C−heteroatom bond formations.10,11
Recently, the Nachtsheim group11e obtained a nitro product by
employing NaNO2 under additive-free conditions11 (Scheme 1,
eq 1). In 2015, Shen et al.10d reported the cyanation of amines
using TMSCN as a cyanide source promoted by PIFA/additives
(eq 2), and the oxygenation of cyclic amines could be achieved
T
Nitroarenes are one of the most important motifs in organic
synthesis. Nitric acid in the presence of another strong acid is
frequently and traditionally used for nitration;3 however, a
variety of metal-based methods4 and some metal-free
approaches have also been developed.5 The above protocols
have several drawbacks, such as requiring harsh conditions
(strong acids), limited substrate scope, overnitration, or the
formation of a mixture of isomers. Thus, the development of
new nitrating agents for regioselective mononitrations has
attracted considerable attention. Recently, organo-nitrating
Scheme 1. Iodine(III)-Supported Functionalizations
6
t
agents, such as BuONO, CH3ONO2, and urea nitrate, have
become attractive and emerged as effective nitrating agents to
overcome these difficulties. These are simpler, more readily
available, less toxic, more direct, and more selective nitrating
agents compared to other reagents.6e−m
Generally, CH3NO2 is used as a polar solvent in organic
conversions. Otherwise, it can serve as a nitro-methylating agent
in cross-coupling reactions7a,b and conjugate additions under
basic conditions.7c−f Surprisingly, we observed that the
combination of PIFA with a NaX can facilitate nitration instead
of cross-coupling (see Scheme S1 in the Supporting
Information). To the best of our knowledge, there have been
no reports of nitrations8 with CH3NO2, which encouraged us to
develop CH3NO2 as an organo-nitrating reagent.
Received: February 28, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00751
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Nitration Optimization
b
yield (%)
entry
oxidant
NaX
solvent
CH3NO2
CH3NO2/H2O(9:1)
C2H5NO2/H2O(9:1)
PhNO2/H2O(9:1)
CH3NO2
CH3NO2/H2O(9:1)
CH3NO2/H2O (1:1)
H2O
CH3NO2/H2O(9:1)
CH3NO2/MeOH(9:1)
CH3NO2/MeOH(9:1)
CH3NO2/MeOH(9:1)
CH3NO2/MeOH(9:1)
CH3NO2/MeOH(9:1)
CH3NO2/H2O(9:1)
CH3NO2/H2O(9:1)
CH3NO2
time (h)
2a
nd
3a
1
2
3
4
5
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
PIFA
NaCN
NaCN
NaCN
NaCN
10
3
6
10
10
10
6
6
6
6
6
87
79
nd
nd
61
53
trace
c
6
NaCN
NaCN
NaCN
NaOH
NaOMe
NaOtBu
NaH
NaOPh
NaBH4
NaOAc
NaHCO3
Et3N
NH4OH
NaCN
NaCN
NaCN
NaCN
7
8
9
10
11
12
13
14
32
93
77
57
46
57
23
nd
nd
trace
47
trace
nd
nd
nd
nd
d
d
d
6
6
d
d
10
10
10
6
10
10
10
10
15
16
17
18
19
20
21
22
CH3NO2
(diacetoxyiodo)benzene
2-iodoxybenzoic acid
Dess−Martin periodinane
PhI
CH3NO2/H2O(9:1)
CH3NO2/H2O(9:1)
CH3NO2/H2O(9:1)
CH3NO2/H2O(9:1)
10
a
b
c
Reaction conditions: 1a (1.0 equiv), oxidant (2.0 equiv), NaX (2.0 equiv) at room temperature (rt). Isolated yield. PIFA (1.0 equiv), NaCN
d
(1.0 equiv). MeOH was used as a cosolvent for water-sensitive bases. nd = not detected.
in moderate yields using PhIO in water (eq 3).12e Unfortunately,
the investigations showed limited substrate scopes and poor
regioselectivities. In most cases, while iodine(III)-catalyzed
reactions require additives and fluorinated solvents to activate
the oxidant,10 large-scale syntheses using such methods will
never be environmentally friendly. Herein, we report (1) a
nitration via C(sp2)−H functionalization, (2) a cyanation, and
(3) an oxygenation12 via C(sp3)−H functionalizations under
metal-free PIFA/NaCN conditions (eq 4).
Initially, we chose aromatic ring-bound cyclic tertiary amine
1a as the substrate to investigate the nitration conditions (Table
1). Using 2.0 equiv of PIFA and 2.0 equiv of NaCN in CH3NO2,
no desired product was observed, and the NaCN was poorly
soluble (entry 1). To our delight, when water was used as a
cosolvent along with CH3NO2 (9:1), NaCN was completely
soluble, and the desired C(sp2)−H nitration product 2a was
exclusively obtained in 87% yield within 3 h (entry 2). This
nitration occurred due to the basicity of CN− (see mechanism).
Next, by screening various nitro solvents, nitroethane was also
found to be a suitable solvent but provided the product in a
lower yield (entry 3), and the reaction did not proceed in the
presence of nitrobenzene (entry 4). Removing NaCN led to no
reaction, and only the elimination of PhI was observed (entry 5).
Finally, a lower yield of 2a was observed with lower loadings of
PIFA and NaCN (entry 6). Our further studies focused on
searching for optimal conditions to generate cyano product 3a
efficiently. When the reaction was conducted in CH3NO2/H2O
(1:1), 2a and alternative C(sp3)−H cyanation product 3a were
obtained. In fact, the reaction of 1a under the above conditions
afforded a 3:1 mixture of 2a:3a with a promising 53% yield of 2a
(entry 7). Gratifyingly, the use of water as solvent led to a
dramatic increase in the yield of 3a, which was selectively formed
as the major product (93% yield, entry 8). Additional studies
focused on changing the sodium salt. Notably, nitration was also
possible using NaOH as a NaX (entry 9). In addition, replacing
NaOH with related sodium bases, such as NaOMe, NaOtBu,
NaH, and NaOPh, resulted in no observable improvements
(entries 10−13). NaBH4 (a mildly basic reducing agent) or
NaOAc (less basic than NaCN) did not furnish the desired
products (entries 14 and 15). Finally, the reaction was tested
with NaHCO3, Et3N, and NH4OH (near the pKa of CN−), but
nitration was only observed in the condition of organic base
Et3N (entries 16−18). A subsequent screening of other oxidants
showed they were completely ineffective (entries 19−21),
proving the key role of PIFA as a superior agent for this C−H
functionalization, as it promoted both selective nitration and
cyanation.
Scheme 2 describes the applicability of our method into
versatile amines for the late-stage nitration of functional
molecules. First, a variety of activating and deactivating aromatic
rings of cyclic amines were examined. All transformations were
performed in the presence of PIFA (2.0 equiv) and NaCN (2.0
equiv) in CH3NO2/H2O (9:1). Surprisingly, substrates with
electron-donating and electron-accepting groups at the para-
B
DOI: 10.1021/acs.orglett.9b00751
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Benzyl chloroformate (Cbz)-protected 4-methoxyaniline (1q)
was selected as the first substrate for our nitration, and it
afforded the corresponding nitro product 2q in 83% yield
(Scheme 2). To our delight, substitution at the para-position
substantially improved the reaction outcome. Thus, carbamates
with a donating group at the para-position were smoothly
converted into the corresponding nitro products in excellent
yields (2q−v). When the scale was adjusted from 0.1 to 1 mmol,
the yield slightly decreased due to the reduced reaction rate in
the heterogeneous system (2t). Surprisingly, when 1r (4-OH)
was used as the substrate, complete conversion to desired
product 2r within 30 min (89% yield) was observed without
oxidative dearomatization of the phenol ring. This indicates that
electron-donating groups at the para-position increase the
efficiency of the amine group attack on PIFA and lead to the
rapid formation of the required products. In contrast, when 1za
(3-OH) and 1zb (2-OH) were used for this nitration,
unidentified side reactions including oxidative dearomatization
caused the substrates to decompose within 5 min.13a When 1w
was used as the substrate, regioselective formation of p-nitro 2w
was observed (isolated in 81% yield).14 Next, it is worth
mentioning that when 3-OCH2OCH3 substituted 1x was tested
as the substrate, a mixture of two regioisomers was obtained.
The two products, 2xa and 2xb, were isolated in 63% and 31%
yields, respectively. At this juncture, when the PIFA and NaCN
loadings were decreased from 2.0 to 1.2 equiv, the
regioselectivity increased, but the isolated yield of 2xa was
moderate (43%), and trace amounts of 2xb were obtained after 8
h. In addition, the reactions of 1ya and 1yb were similar to that
of 1x.
Scheme 2. Substrate Scope of the Nitration
Based on the optimal conditions for C(sp3)-cyanation (Table
1, entry 8), we next examined the cyanation of the same
substrates. Surprisingly, when the reaction was conducted with
five-membered or fused aromatic cyclic amines as substrates,
undesired products (i.e., oxidation product) were generated
along with cyano products 3. In particular, our preliminary
experiment with the optimized conditions revealed that the
cyanation reaction of 1g afforded the desired cyano product in
37% along with 59% of the oxidation product (Scheme 3, eq 1).
a
b
Reaction conditions: 1a−x (1.0 equiv) in CH3NO2/H2O (9:1).
1
a
Scheme 3. Cyanation and Oxygenation Selectivity
mmol scale.
position efficiently underwent the reaction and reached
complete conversion within 3 h, giving nitro compounds 2a−c
in excellent yields (76−87%), but when the 4-F-derived
substrate was employed, only trace 2d was obtained. After the
effect of a substituent at the para-position of the aromatic ring
was tested, a meta-Me group on the aromatic ring (1e) was
tested to show the oxidative biaryl coupling product as the major
product (Scheme S2).13 When 1f was used as a substrate, the
formation of 2f was not observed despite a longer reaction time
(10 h). The above results indicate that selective ortho-nitration
requires a substituent at the para-position to achieve satisfactory
results. In addition, we were happy to find that substrates with
azepine and annulen-5-ylidene skeletons (1j and 1k) gave nitro
products 2j and 2k with high yields (ca. 80%).
With the established reaction condition in hand, we could
assess the scope and generality of the present process. First,
nonamine substrates were examined, including compound 5, a
precursor to cyclic amines. Unfortunately, this protocol did not
tolerated any nonamine substrates. Our investigation extended
the substrate scope from cyclic amines to amides (carbamates).
a
Reaction conditions: 1g (1.0 equiv). nd = not detected.
We then investigated the reaction selectivity between the
cyanation (3g) and the oxygenation (4g) to reveal the necessary
ratio of PIFA and NaCN (Scheme 3). In other words, while
equimolar amounts of PIFA and NaCN (same ratio as the first
series of experiments) also show poor selectivity between 3g and
4g with slightly decreased yields, cyanation occurred more
C
DOI: 10.1021/acs.orglett.9b00751
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Organic Letters
Letter
selectively with 1.5 equiv of PIFA (modified ratio of PIFA to
NaCN 1−0.6).
Six-membered cyclic amines 1a−f were compatible with these
conditions and resulted in high yields (Scheme 4). Notably,
Scheme 5. Scope of the Oxygenation
a
Scheme 4. Scope of the Cyanation
Scheme 6. Plausible Mechanism
a
b
Reaction conditions: 1a−o (1.0 equiv). PIFA (1.5 equiv), NaCN
(2.5 equiv).
when tetrahydroisoquinoline derivatives 1l−o were used as
substrates, the reaction proceeded to complete conversion to
cyanation products 3l−o in H2O/CH3NO2 as well as the
optimal condition. Finally, the cyanations of 1g−o proceeded
with excess NaCN (2.5 equiv) in the presence of PIFA (1.5
equiv) to provide cyano products 3g−o in high yields (73−85%,
Scheme 4).
After the demonstration of the cyanation, we should remind
ourselves of the oxygenations described in Scheme 3. When the
reaction was conducted without NaCN, 4g was isolated in 56%
yield after 24 h (Scheme 3, eq 4). To quickly and selectively
obtain oxidation products, higher quantities of oxidant and
NaCN were essential to afford better results. The reaction at a
higher loading of PIFA (2.2 equiv), using a lower loading of
NaCN (1.1 equiv), selectively provided oxidation product 4g in
85% yield along with traces of 3g (Scheme 3, eq 5). After
optimization of the reaction conditions, we tested the scope of
the oxygenation and the reactivity of various cyclic amines
(Scheme 5). All monosubstituted five-membered and fused
aromatic cyclic amines 1g−p underwent the reaction smoothly
and afforded corresponding oxidation products 4g−p in high
yields (73−91%). However, when less reactive six-membered
cyclic amines were tested, none of the oxygenation products
were observed (4a−f).
OH−/Lewis base (:B) to the methyl group in nitromethane can
be assisted or concerted by I−O formation to produce the
intermediate II with ΔG = −38.65 kcal/mol. Even though the
nitro (I−N) or nitrite (I−O) transfer from CH3NO2 can be
considered, I−O intermediate was selected on the basis of the
literature.15a The intermediate II can be converted into the
intermediate III via the four-membered TS. Finally, the
aromaticity can be regenerated by the loss of a proton from
intermediate III, which gives final nitro product 2. The DFT
calculations (see Figure S1 in the Supporting Information) and
previous literature reports11e,15 elucidate the proposed mecha-
nism at the B3LYP-D3(6-31G**/ LAVCP) level of theory in the
Poisson−Boltzmann solvent model (CH3NO2, ε = 36.56). In
order to support the proposed mechanism, a nucleophile
receiving methyl group from CH3NO2 was traced. When NaCN
was replaced with NaOPh due to the detectability, anisole
(PhOMe) could be detected in TLC, crude NMR, and LCMS.
In conclusion, we have reported the first nitration reaction
with CH3NO2 under PIFA/NaCN conditions via a novel
C(sp3)−N breaking/C(sp2)-N formation mechanism. Notably,
the same composition of PIFA/NaCN can dramatically alter the
reaction: (1) from C(sp2)−H to α-C(sp3)−H functionalization
(changing the solvent) and (2) from cyanation to oxygenation of
α-C(sp3)−H bonds of cyclic amines (changing the ratio of PIFA
to NaCN). The use of a low toxic, inexpensive PIFA as the
oxidant for these three functionalizations, which are compatible
The mechanism in the Scheme 6 starts with the coordination
of PIFA to N-aryl cyclic amine to obtain the intermediate I via
halogen−π interactions between benzene and iodine. The slight
energy gap can permit reversible reaction between cyclic amine
−
and the intermediate I. The eliminated ligand (CF3CO2 ) and
NaCN or NaOH reach the equilibrium between ion pairs to
produce the conjugate base (CN− or OH−). The attack of CN−/
D
DOI: 10.1021/acs.orglett.9b00751
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
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̈
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General information; nitration instead of the cross-
coupling reaction; synthesis of substrates; general
procedures for the nitration, cyanation, and oxygenation
reactions; computational methodology (DFT); refer-
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NMR spectra of the products (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kmh0515@gachon.ac.kr.
ORCID
Mi-hyun Kim: 0000-0002-2718-5637
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported by the Basic Science Research
Program of the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science, and Technology
(No.: 2017R1E1A1A01076642). C.M. thanks CSIR, India.
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DOI: 10.1021/acs.orglett.9b00751
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