Selectfluor-mediated regioselective nucleophilic functionalization of N-heterocycles under metal- and base-free conditions

Article abstract of DOI:10.1039/c7gc03106h

A practical and environmentally attractive methodology for the direct diversification of N-heterocycles at ambient temperature under open-air conditions was developed. The obvious advantage of the process is that no toxic reagent, transition metal, base or other additive is employed, thus greatly reducing costs, facilitating post-reaction neutralization and purification and minimizing the environmental impact.

Full text of DOI:10.1039/c7gc03106h

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Selectfluor-mediated regioselective nucleophilic
functionalization of N-heterocycles under metal-
and base-free conditions†
Cite this: DOI: 10.1039/c7gc03106h
Long-Yong Xie,a Jie Qu,a Sha Peng,a Kai-Jian Liu,a Zheng Wang,a Man-Hua Ding,a
a,b
Yi Wang,a Zhong Caob and Wei-Min He
*
Received 16th October 2017,
Accepted 31st December 2017
A practical and environmentally attractive methodology for the direct diversification of N-heterocycles at
ambient temperature under open-air conditions was developed. The obvious advantage of the process is
that no toxic reagent, transition metal, base or other additive is employed, thus greatly reducing costs,
facilitating post-reaction neutralization and purification and minimizing the environmental impact.
DOI: 10.1039/c7gc03106h
rsc.li/greenchem
Introduction
The development of an eco-friendly chemical process to
produce diverse value-added chemicals from the nature-abun-
dant and low-cost raw material is a particularly attractive
concept, but represents a considerable challenge in green
organic synthesis. N-Heterocycles and their derivatives are the
extremely important motifs in bioactive natural products and
synthetic pharmaceuticals.1 It is well-known that the hetero-
cycle core and its modified functional group(s) cooperatively
affected the activity of the molecule, not just the heterocycle
itself. Thus, the research of structure–function relationships
requires the heteroaromatic derivatives containing a set of dis-
tinct functional group(s) attached to heterocycle cores.
Generally, structural modification of N-heterocycles can be
divided into two categories according to the reaction mecha-
nism: (1) Minisci-type radical substitution (Scheme 1a),2 and
(2) the nucleophilic reaction of pre-functionalized
N-heterocycle substrates, including substituted N-heterocycles
(Scheme 1b)3 and electrophilically activated N-heterocycles
(Scheme 1c).4 The major drawback in process (1) is that regio-
selectivity is usually difficult to control and regioisomeric mix-
tures were produced. In particular, the regioisomers of
N-heteroaromatic molecules often have contrasting indications
or even opposing biological activities. The approach (2) pre-
sents the following disadvantages:5 (a) the requirement of a
Scheme 1 Direct preparation of functionalized N-heterocycles.
directing group significantly limits their applications,
especially for the late-stage structural modification, because
the installation and removal of such groups require additional
steps; and (b) additional synthesis procedure and isolation
steps of N-activated heterocycles are often unavoidable.
Although various functional groups can be introduced in
N-heterocycles, most of the functionalization processes incor-
porate only one type of functional group. To the best of our
knowledge, only two examples of diverse nucleophilic
aHunan Provincial Engineering Research Center for Ginkgo biloba, Hunan University
of Science and Engineering, Yongzhou 425100, China.
E-mail: weiminhe2016@yeah.net
bHunan Provincial Key Laboratory of Materials Protection for Electric Power and
Transportation, Changsha University of Science and Technology, Changsha, 410114,
China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7gc03106h
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functionalization of N-heterocycles have been reported. In line concentration of 0.1 M did not affect the reaction yield;
2014, Hartwig and colleagues have pioneered the direct struc- however, the higher concentration of 0.25 M (entry 14) was
tural modification of pyridines via AgF2-mediated sequential selected to provide conditions that minimize solvent waste and
C–H fluorination and nucleophilic substitution (Scheme 1d).6 shorten the reaction time (5 h). Elevated reaction temperature
Very recently, Fier has designed and applied N-((methyl- did not improve the reaction outcome (entry 15) (Table 1).
sulfonyl)oxy)acetimidoyl chlorides to achieve diverse
Using the optimal reaction conditions, the scope of the
functionalization of pyridines (Scheme 1e).7 Although these transformation was explored with respect to both
methods exhibited highly advantageous features, from an eco- N-heterocycles and nucleophiles. To our satisfaction, the qui-
friendly and practical point of view,8 both approaches require nolines containing steric-hindered, electron-donating and elec-
two-step operation procedures and superstoichiometric tron-withdrawing substitute groups all underwent coupling
amounts of bases to facilitate the reaction,9 which leads to not reactions smoothly at the 2-position (3aa–3ja). It is noteworthy
only high cost but also environmental problems. In addition, that the halo-substituents (–Cl, Br and F) on the quinoline ring
the difficulties to remove the trace transition-metal contami- of the substrates were well tolerated (3da, 3ea and 3ja), provid-
nation from the final products, particularly for the late-stage ing a handle for further transformations. The current method
modification of medicine, restrict their applications. It is is also applicable for the efficient synthesis of highly functio-
therefore highly desired to develop an efficient, practical and nalized isoquinoline (3ka), pyridine (3la–3ua), pyrrole (3va)
eco-friendly process that can achieve direct diversification of and N-methylpyrrole (3wa). However, when 3-bromopyridine,
N-heterocycles employing abundant and readily available 4-chloropyridine and 4-methoxycarbonylpyridine were used as
materials under metal- and base-free conditions.
substrates, only a trace amount of the desired product could
We were aware that N-fluoropyridiniums\quinoliniums were be detected. In addition, a range of nitrogen-containing het-
employed as the synthetic precursors for the functionalization erocycle coupling partners are tolerated, including 1H-benzo[d]
of N-heteroaromatic substrates, given their direct C–H bond imidazole (3ab), various substituted 1H-benzotriazole (3ac–
activation at the 2-position.10 For example, Strekowsli has 3af), pyrazole (3ag–3ah), triazole (3ai–3aj) and 1H-tetrazole
reported one-pot sequential N-fluorination and alkoxylation of (3ak). Encouraged by the above results, we set out to investi-
quinoline at ultra-low temperature by using F2 as a fluorinat- gate C–H alkoxylation and hydroxylation. The alcohols with
ing reagent.11 However, the high toxicity, explosivity and cor- diverse chain lengths and isomeric structures did not affect
rosion of fluorine gas severely restrict the strategy for general the outcome of the transformation (4aa–4ae). Neither the
applications. In light of the unique pharmacological activities steric hindrance (4bb) nor the electronic effect (4bc–4bh) of
of diverse heteroaromatic derivatives and also in continuation quinolone rings significantly influenced the reaction.
of our efforts in green chemistry,12 herein, we report a direct Moreover, 58–67% conversion of quinolines 1 and near quanti-
synthesis of diverse heteroaromatic derivatives through
Selectfluor-mediated regioselective nucleophilic functionali-
zation of N-heterocycles under metal- and base-free and open
air conditions (Scheme 1f).
Table 1 Optimization of reaction conditionsa
Results and discussion
To test the possibility of our envisioned process, we chose
commercially available Selectfluor as a potential fluorinating
Entry
[F]
Solvent
Temp.
Yieldb
reagent due to its air and moisture stability yet is an extraordi-
nary powerful donor of an electron-deficient fluorine atom to a
wide number of nucleophiles. To our delight, the desired
2-(1H-benzo[d][1,2,3]triazol-1-yl)quinoline (3aa) was obtained
in 82% NMR yield by using quinoline (1a) as the model sub-
strate and 1H-benzotriazole (2a) as the nucleophilic reagent in
MeCN at ambient temperature for 7 h (entry 1). As anticipated,
the property of the fluorinating reagent played a vital role in
the transformation outcome, and other related fluorides did
not promote C–H functionalization (entries 2–6). Significantly,
the employment of a slight excess of Selectfluor (1.3 equiv.)
was found to provide the desirable 3aa in excellent yield
(entries 7 and 8). In screening different solvents, the reaction
in MeNO2 gave the markedly inferior yield of 3a (entry 9); no
desired products were obtained with THF, DCE, DMF, DMSO
or acetone (entries 10–13). Conducting the reaction at a quino-
1
2
3
4
5
6
7
8
Selectfluor (1 equiv.)
NFSI (1 equiv.)
DAST (1 equiv.)
BAST (1 equiv.)
TBAF (1 equiv.)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeNO2
THF
DCE
DMF
Acetone
MeCN
MeCN
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60 °C
82
N.R.
N.R.
N.R.
N.R.
N.R.
93
Et3N·HF (1 equiv.)
Selectfluor (1.3 equiv.)
Selectfluor (1.5 equiv.)
Selectfluor (1.3 equiv.)
Selectfluor (1.3 equiv.)
Selectfluor (1.3 equiv.)
Selectfluor (1.3 equiv.)
Selectfluor (1.3 equiv.)
Selectfluor (1.3 equiv.)
Selectfluor (1.3 equiv.)
93
85
9
10
11
12
13
14c
15
N.R.
N.R.
Trace
Trace
92%
92%
a Unless otherwise specified, the reactions were carried out in a vial in
the presence of 1a (0.1 mmol), 2a (0.15 mmol), a fluoride reagent and
a solvent (1 ml). b Estimated by 1H NMR using diethyl phthalate as an
internal reference. c Molar concentration: 0.25 mmol ml−1
.
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Table 2 Reaction scopea
Scheme 2 Gram-scale synthesis of 3aa.
Scheme 3 Control experiments.
a All reactions were carried out in a vial in the presence of 1
(0.5 mmol), 2 (0.75 mmol), Selectfluor (0.65 mmol) and MeCN (2 ml);
aminations were performed at room temperature. Alkoxylation and
hydroxylation were performed at 60 °C; isolated yields are reported.
b The percentage of quinoline 1 was recovered.
radical process was likely the main reaction pathway. Secondly,
the intermolecular kinetic isotope effect (KIE) was studied
with respect to the heterocyclic C(sp2)–H/D bonds. A clear KIE
value of 1.1 was obtained in a 1 : 1 mixture of pyridine (1l) and
pyridine-d5 (1l-d5) (Scheme 3b). Besides this, K2i/K2j was deter-
mined to be around 1.6 when a mixture of 1H-1,2,3-triazole
tative selectivity for quinolin-2(1H)-ones could be achieved (2i) and 1H-1,2,4-triazole (2j) was subjected to the standard
using water as the nucleophile (5aa–5af). However, no reaction conditions (Scheme 3c). Taken together, these observations
occurred when aniline, benzamide, morpholine, sodium revealed that the scission of the heterocyclic C(sp2)–H bond
cyanide or 1-hexanethiol was used as the nucleophile might not be in the rate-limiting step and the nucleophilic
(Table 2).
attack on heterocycles would be rate-limiting in the overall
The practicability of this transformation was explored by process. Considering that this category of cross-coupling reac-
employing 1a (20 mmol, 2.58 g) with 2a under the standard tions may involve the haloquinoline intermediate, in situ gen-
reaction conditions (Scheme 2). As anticipated, the desired 3aa erated from the halogenation of quinoline, we attempted to
was formed in 85% yield, which suggested that the method- treat 2-fluoroquinoline 1v with 2a under the standard reaction
ology is a practical process for the preparation of various func- conditions. However, no 3aa was observed (Scheme 3d). The
tionalized N-heterocycles.
treatment of 1-fluoropyridinium tetrafluoroborate (1w) or
To gain some insights into the mechanism, several con- 1-fluoropyridinium triflate (1x) with 2a in MeCN at ambient
trolled experiments were conducted (Scheme 3). When the temperature gave the target product 3aa in good yields
reaction was performed in the presence of 2 equiv. of 2,2,6,6- (Scheme 3e and f). Taken together, these results suggest that
tetramethylpiperidine-1-oxyl (TEMPO) as an additive under N-fluoroquinoliniums might be the reaction intermediate.
standard reaction conditions, 3aa was formed in 23% NMR
Although various mechanistic details of these reactions are
yield (Scheme 3a). The poor yield of 3aa suggested that the not fully understood, two tentative mechanisms for the reac-
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Acknowledgements
We are grateful for financial support from the National Natural
Science Foundation of China (No. 21302048 and 21545010),
the Science and Technology Innovative Research Team in
Higher Educational Institutions of Hunan Province (2012-318)
and the Construct Program of the Key Discipline in Hunan
Province.
Scheme 4 Possible mechanisms.
tion are depicted in Scheme 4 according to the above men-
tioned observations and previous literature reports. Initially,
quinoline 1a is easily oxidized by Selectfluor via a single-elec-
tron-transfer process, producing the radical cation IM113
along with a fluorine radical (Scheme 4a). Then the nucleo-
phile first attacks the α-carbon of the intermediate IM1 to
give the intermediate IM2.4a,c Finally, the fluorine radical
adds to the nitrogen atom of IM2 to form the intermediate
IM3, which undergoes deprotonation/aromatization to
deliver the desired product 3aa. Alternatively, another
process with N-fluoroquinoliniums as the reactive intermedi-
ate could not be excluded (Scheme 4b).11 The coupling reac-
tion may follow a mechanism similar to that of the F2-
mediated alkoxylation of quinoline with EtOH as the nucleo-
phile reported by Strekowsli, with the major difference being
Selectfluor as the fluorinating reagent instead of hazardous
F2. First, quinoline 1a is activated by Selectfluor to form the
active N-fluoroquinolinium intermediate IM4. Subsequently,
IM4 is attacked by the nucleophile to generate the intermedi-
ate IM3.
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This work for the first time provides a facile, practical and
environment-friendly methodology for the one-step diversifica-
tion of N-heterocycles. This protocol occurs to provide
N-heteroaryl- and alkoxy-substituted heterocycles as well as
quinolin-2(1H)-ones without any transition-metal or base,
which is inaccessible through conventional methods.
Noteworthy advantages of the present reaction are as follows:
(a) the abundance and accessibility of inexpensive
N-heterocycles, nucleophilic reagents and Selectfluor; (b) oper-
ational simplicity and mild reaction conditions with good
functional-group tolerance; (c) a broad range of N-heterocycle
substrates: quinoline, isoquinoline, and pyridine as well as
pyrrole; and (d) ease of scale-up. Preliminary mechanistic
investigation suggested that a radical process and a nucleo-
philic attack pathway may be involved in the direct C–H
functionalization reaction.
Conflicts of interest
There are no conflicts to declare.
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