Alkylative Dearomatization by Using an Unactivated Aryl Nitro Group as a Leaving Group: Access to Diversified Alkylated Spiro[5.5]trienones

Article abstract of DOI:10.1021/acs.orglett.1c00469

The cleavage of an unactivated aryl nitro group triggered by alkyl radicals enables a dearomative cyclization, affording diversified alkylated spiro[5.5]trienones in good yields. Using readily available compounds (toluene and analogues, alkanes, ethers, ketones, etc.) as alkylating reagents, various alkyls have been implanted into the spirocycles via C(sp3)-H and Ar-NO2 bond activation with high functional group tolerance. This protocol provides a distinct method for the activation of the aryl nitro group.

Full text of DOI:10.1021/acs.orglett.1c00469

pubs.acs.org/OrgLett
Letter
Alkylative Dearomatization by Using an Unactivated Aryl Nitro
Group as a Leaving Group: Access to Diversified Alkylated
Spiro[5.5]trienones
Dong Xia and Xin-Fang Duan*
Cite This: Org. Lett. 2021, 23, 2548−2552
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ABSTRACT: The cleavage of an unactivated aryl nitro group triggered by alkyl radicals enables a dearomative cyclization, affording
diversified alkylated spiro[5.5]trienones in good yields. Using readily available compounds (toluene and analogues, alkanes, ethers,
ketones, etc.) as alkylating reagents, various alkyls have been implanted into the spirocycles via C(sp3)−H and Ar−NO2 bond
activation with high functional group tolerance. This protocol provides a distinct method for the activation of the aryl nitro group.
Scheme 1. Denitrative Reactions of Nitroarenes
itroarenes are very common and versatile compounds in
organic synthesis that can be conveniently prepared by
N
nitration of the parent arenes. Through reduction and
subsequent diazotization, they are the starting point for
many important building blocks such as anilines, aryl halides,
phenols, and so on.1 Aryl halides and phenol-derived esters
(sulfonates, carbamates, sulfamates, etc.) are the most
commonly used electrophiles for transition-metal-catalyzed
coupling reactions in constructing C−C and C−X bonds.2 In
this context, the direct use of nitroarenes as an electrophile in
coupling reactions, which can avoid the multistep conversions
to access halides and phenol esters, is a challenging yet step-
economic task.3
We have previously reported an efficient arylation reaction
of nitropyridine N-oxides in which the nitro group served as a
better leaving group than the well-known Br and Cl groups
(Scheme 1a).4 Nevertheless, that direct substitution of the
nitro group was limited to activated nitros.1,4,5 On the
contrary, Wu, Shinde, Nakao, You, and coworkers reported
various Pd-, Rh-, and Cu-catalyzed direct couplings of activated
and unactivated nitroarenes to form C−C and C−X bonds
(Scheme 1b).6−9 Despite these achievements, an alkylative and
alkenylative reaction using an unactivated aryl nitro group as a
leaving group remains quite elusive. In addition, the selective
substitution of a nitro group over Br or Cl has not been
documented in these transition-metal-catalyzed reactions.
We herein report an alkylative dearomatization enabled by
the cleavage of an unactivated aryl nitro group (Scheme 1c).
Interestingly, in this dearomatization, the nitro group served as
a much better leaving group than the classic Br and I, which
provides an alternative method for the activation of the aryl
Received: February 8, 2021
Published: March 22, 2021
https://doi.org/10.1021/acs.orglett.1c00469
© 2021 American Chemical Society
Org. Lett. 2021, 23, 2548−2552
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Letter
a
nitro group. We expect that these findings will provide
implications for the activation of aryl nitro group and related
reactions.
Table 1. Optimization of Reaction Conditions
On the basis of our continued interest in the cleavage of
unactivated Ar−NO2 bonds, we hypothesized that a radical-
type alkylation or alkenylation might provide a distinct method
for the activation of the aryl nitro group.10 Meanwhile,
spiro[5.5]trienones are important structural motifs because
such cores and their analogues are often found in natural
products and bioactive compounds (Figure 1).11 We assumed
b
entry
X
catalyst
CuBr
oxidant
T (°C) yield (%)
c
1
2
3
4
5
6
7
8
NO2 (1a)
H (1b)
OMe (1c)
Br (1d)
I (1e)
1a
TBHP
120
120
120
120
120
80
86
28
73
38
40
trace
65
84
77
81
78
77
80
78
trace
76
73
64
56
45
13
11
34
40
70
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
100
130
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
9
10
11
12
13
14
15
16
17
18
19
20
21
22
CuCl
CuI
Cu
Figure 1. Natural products with spiro[5.5]trienone core.
CuO
Cu(OAc)2
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
that a radical alkylation of the nitrobiaryl ynone (1a: X = NO2)
might promote the leaving of a nitro group and a dearomative
cyclization to the desired spiro[5.5]enone core in a tandem
reaction manner.12−14 Thus we started our investigation with
the mode reaction between 1 and toluene, and the results are
outlined in Table 1. It can be seen that the leaving group (X),
catalyst, oxidant, reaction temperature, and solvent had a
significant influence on the outcome of the reaction. Under
optimized conditions (entry 1), the nitro group turned out to
be the best leaving group, whereas the classic good leaving
groups Br and I resulted in only low yields (entries 1−5). It is
noteworthy that all of the reported dearomative cyclizations of
biaryl ynone to spiro[5.5]trienones have relied on MeO to
form CO by demethylation in the Ar2 ring (X = OMe;
shown in Scheme 1c).12 Instead, our protocol provided a new
strategy via a nitro cleavage and afforded an even better yield
(entry 1 vs 3). Ordinarily, alkyl halides are used as radical
precursors, whereas the present reaction used only simple
toluene, representing a C(sp3)−H activation. The screening of
temperature (entries 6−8), Cu source (entries 10−14), and
oxidants (entries 16−22 and 25) indicated that the present
reaction proceeded optimally with 5 mol % CuBr and 4 equiv
of TBHP at 120 °C. It should be mentioned that this mode
reaction also occurred without a catalyst in a 77% yield (entry
9); nevertheless, other substrates gave rather low yields during
our later exploration of the substrate scope. We hypothesize
that Cu(I)/(II) plays a catalytic role by promoting the
generation of free radicals such as t-BuOO· and t-BuO·. (For
details, see Scheme 5.) Solvent optimization showed that
toluene can efficiently serve as both the reactant and the
solvent without the need for other solvents (entries 23 and
24), which made the reaction composition simpler.
d
DCP
e
DTBP
CHP
TBPB
H2O2
f
g
h
K2S2O8
(NH4)S2O8
TBHP
TBHP
TBHP
i
23
24
25
1a
1a
1a
j
k
a
Unless otherwise noted, the reactions were carried out with 1 (0.25
mmol), 2a (2 mL), oxidant (1.0 mmol), and 5 mol % of catalyst for
16 h under a N2 atmosphere. Isolated yield. TBHP, t-BuOOH (70%
aq). DCP, dicumyl peroxide. DTBP, di-tert-butyl peroxide. CHP,
cumene hydroperoxide. TBPB, tert-butyl peroxybenzoate. 30% aq.
In a mixture of toluene and THF (1/1 v/v, 2 mL). In a mixture of
toluene and DMF (1/1 v/v, 2 mL). TBHP was used in 0.75 mmol.
b
c
d
e
f
g
h
i
j
k
undergo the reaction with no debromination or dechlorination
being observed (3b−3f). Xylenes and mesitylene can both
afford the desired monobenzylated products in high yields
(3g−3j). The reaction also showed a high functional group
tolerance: CN and COCH3, which were usually sensitive to
strong oxidants, remained untouched under the present
conditions (3k and 3l). It should be noted that the benzylic
C(sp3)−H demonstrated a high selectivity over the methyl
group of COCH3 (3l) and OCH3 (3m), although these
methyls are also susceptible to the radical reactions. (Also see
Scheme 3.)15 The high selectivity between the different methyl
groups of 1 and 2 was also achieved, in which the methyl on
the Ar1 ring (shown in Scheme 1c) remained untouched (3n
and 3o). Likewise, the Me, Et, t-Bu, and OMe groups on the
Ar3 ring (tethered to the alkyne, shown in Scheme 1c) (3t−
3w), whose benzylic C(sp3)−H or C(sp3)−H in the methyl
group can also produce radicals under the present conditions,
did not interfere with the present benzylation. Regretfully, the
benzylations using ethylbenzene, isopropylbenzene and their
analogues failed in our hands. Although a similar effect was
With the optimized conditions in hand, we investigated the
substrate scope with the initial focus on the reaction of the
aromatic methyl groups of toluene and analogues. Although
the benzylic C(sp3)−H bonds performed efficiently in a range
of radical reactions,15,16 their propensity to form alcohols,
aldehydes, acids, or nitrile in the presence of the oxidants
presents a significant challenge to our desired reaction.17
Despite this challenge, our results outlined in Scheme 2 show
that the benzylative/denitrative dearomatization proved to be
robust and general. Various chloro- or bromotoluenes can
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Scheme 2. Benzylative/Denitrative Dearomatizations to
Spiro[5.5]trienones Using Toluene Derivatives
Scheme 3. Alkylative/Denitrative Dearomatizations to
Spiro[5.5]trienones Using Alkanes, Ethers, and Ketones
a
a
a
Reaction conditions: 1a (0.25 mmol), 2 (2 mL), CuBr (5 mol %),
TBHP (4.0 equiv), 120 °C, 16 h, N2.
reacted preferentially to the α-CH3 group of ketones (3l in
Scheme 2), the α-CH3 or CH2 groups of ketones underwent
the spirocyclization facilely in the absence of aromatic methyl
group(s) in high yields (4j−4n). It can be seen that the α-CH2
group of ketone reacted highly selectively over the α-CH3
group (4m), whereas the α-CH3 group of ketone reacted
selectively over the CH3 group in methyl ether (4n). Only
slight selectivity was observed between the methyl group on
aromatic rings and that in methyl ether (4o). The reasons for
the selectivity between two types of α-C−H in 4m as well as
the lack of selectivity between two methyls in 4o need to be
further explored. Dichloromethane can also participate in the
reaction as a representative of halogenated alkanes, affording
the desired product in good yield (4p). It is noteworthy that
the implantation of ether- (4e−g), acetal- (4h and 4i), and
ketone-containing groups (4j−n) into the spiro[5.5]trienones
enables a handle for further structural modifications.
Several control experiments were conducted to gain an
understanding of the mechanistic details, and trapping of the
radical, the isotopic effect, and the source of the oxygen atom
in the product were confirmed, as illustrated in Scheme 4. The
dearomative cyclization did not take place in the absence of the
benzyl radical source (2a), whereas the radical was successfully
trapped by TEMPO and 1,1-diphenylethylene in the reaction
between 1a and 2a under the standard conditions (Scheme
4a,b). These facts clearly indicated that radical intermediates
derived from the C−H radical precursor were involved in the
reaction. Meanwhile, a significant isotopic effect (3a/3a-D7
7:3) was observed when a mixture of equal equivalents of
toluene (2a) and D8-toluene (2a-D8) was used to react with 1a
under the standard reaction conditions (Scheme 4c),
a
Reaction conditions: 1 (0.25 mmol), 2 (2 mL), CuBr (5 mol %),
TBHP (4.0 equiv), 120 °C, 16 h, N2.
reported in a radical cyclization,18 other reasons besides steric
hindrance remain to be discovered.
Next, we explored the alkylative dearomatization using
alkanes, ethers, ketones, and so on (in Scheme 3). The
previously mentioned failure of the alkylation via secondary
benzyl C−H bonds makes us think that the alkylation using
secondary or tertiary alkylating reagents may be challenging.
Despite this fact, the alkylative spirocyclization using
unactivated cycloalkanes15,19 occurred smoothly (4a−4d).
Common ether solvents (THF, 1,4-dioxane) and anisole can
also participate in the alkylation (4e−4g).15,20 Acetals can
undergo the present reaction in the presence of oxidants and
water (4h and 4i), demonstrating the wide scope and good
functional compatibility. Although aromatic methyl group(s)
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Scheme 4. Control Experiments
Scheme 5. Possible Mechanism
TBHP with readily available alkylating reagents, which afforded
diversified alkylated spiro[5.5]trienones in good yields. Where-
as dearomative 5-exo-trig cyclizations to spiro[4.5]-trienones
have been extensively achieved,13,22 the corresponding 6-exo-
trig cyclizations furnishing spiro[5.5]trienones have rarely been
reported.14c,12,23 Moreover, the radical-type dearomative
cyclization of biaryl ynone to spiro[5.5]trienones has only
successfully achieved difluoromethylation, sulfonylation, and
phosphorylation in the literature.12 Thus our transformation
provided the first examples of a general alkylative dearomatiza-
tion to spiro[5.5]trienones. In addition, the reported
dearomatization relied on a MeO of biaryl ynone to form
the CO group in the products via demethylation. For the
first time, we have accessed spiro[5.5]trienones through an
alkylative spirocyclization by using an unactivated aryl nitro as
a leaving group. In sharp contrast with the previous activation
of nitro groups, which mainly involved the ipso substitution of
a nitro group, our protocol provides a unique strategy for the
activation of nitro groups, for it involves not only a nitro’s ipso
substitution but also alkylation/alkenylation at other positions.
Thus we expect that these findings will provide implications for
the activation of aryl nitros and the development of related
new reactions.
suggesting that the formation of the benzyl radical via the
cleavage of C(sp3)−H of toluene might be involved in the rate-
determining step. To figure out how the nitro group left, we
then determined the source of the oxygen atom of the carbonyl
in 3a formed by the cleavage of the nitro group. Because this
reaction can proceed smoothly under anhydrous conditions to
afford 3a in a comparable yield (Scheme 4d), the oxygen in the
newly formed carbonyl group should not come primarily from
water. An 18O-labeling experiment using t-BuO18OH formed
by heating the mixture of t-BuOOH (anhydrous, 5 M in
decane) and H218O afforded the 18O-labeled product (Scheme
4e; for details, see the Supporting Information), which clearly
showed the source of oxygen in the newly formed carbonyl
group. It should be mentioned that the exchange reaction
producing t-BuO18OH (t-BuOOH + H218O = t-BuO18OH +
H2O) has been reported13,21 and confirmed by us. (For details,
see the Supporting Information.)
On the basis of the previously described study, we proposed
a reasonable mechanism for this transformation (Scheme
5).13,21 The reaction was supposed to start with the O−O
homolytic cleavage of TBHP, which generated HO· and t-
BuO· radicals. The t-BuO· abstracted hydrogen from 2a to
afford the benzyl radical, which subsequently added to the
triple bond of 1a to form the vinyl radical A. This radical
underwent a 6-exo-trig cyclization to afford the radical
intermediate B. The t-BuOO· or t-BuO18O· coupled to radical
intermediate B to furnish intermediate C. Alternatively,
intermediate B was oxidized to carbocation B′, which reacted
with t-BuOOH or t-BuO18OH and subsequently afforded
intermediate C after releasing a proton. Through the O−O or
O−18O homolysis, intermediate C formed D, which yielded 3a
or 18O-3a via a denitration. The ·NO2 released during
denitration might be oxidized to NO2 or couple to t-BuO· to
form t-BuONO2. It can be seen in Scheme 5 that the Cu ion
could promote the formation of t-BuOO·, t-BuO·, carbocation
B′, and NO2 and thus played a catalytic role in the whole
reaction.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00469.
Experimental procedures, characterization data, and
NMR spectra for products (PDF)
Accession Codes
CCDC 1901346 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
In conclusion, we developed a novel and simple akylative/
denitrative dearomatization using simple catalyst and aqueous
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(13) For a denitrative dearomatization of a type of activated
nitroarenes, to furnish azaspiro[4.5]decadientriones, see: Wu, J.; Ma,
D.; Tang, G.; Zhao, Y. Org. Lett. 2019, 21, 7674.
AUTHOR INFORMATION
■
Corresponding Author
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Iglesias, M. J.; Fernandez, I.; Andujar Sanchez, C. M.; Ruiz Gomez, G.
Chem. Rev. 2007, 107, 1580. (b) Roche, S. P.; Porco, J. A., Jr. Angew.
Chem., Int. Ed. 2011, 50, 4068. (c) Reddy, C. R.; Prajapti, S. K.;
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Tang, C.; Jiao, N. Chem. Soc. Rev. 2012, 41, 3464. (b) Girard, S. A.;
Knauber, T.; Li, C. J. Angew. Chem., Int. Ed. 2014, 53, 74. (c) Yi, H.;
Zhang, G.; Wang, H.; Huang, Z.; Wang, J.; Singh, A. K.; Lei, A. Chem.
Rev. 2017, 117, 9016.
(16) Correia, C. A.; Li, C. J. Adv. Synth. Catal. 2010, 352, 1446.
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Xin-Fang Duan − College of Chemistry, Beijing Normal
University, Beijing 100875, China; orcid.org/0000-0003-
1480-853X; Email: xinfangduan@vip.163.com
Author
Dong Xia − College of Chemistry, Beijing Normal University,
Beijing 100875, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00469
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the National Science Foundation of China
(21372031 and 21572022). We also thank Prof. Zhan-Bin
Zhang in our college for his helpful advice.
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