Trifluoromethanesulfonyl azide as a bifunctional reagent for metal-free azidotrifluoromethylation of unactivated alkenes

Article abstract of DOI:10.1039/d0sc06473d

Vicinal trifluoromethyl azides have widespread applications in organic synthesis and drug development. However, their preparation is generally limited to transition-metal-catalyzed three-component reactions. We report here a simple and metal-free method that rapidly provides these building blocks from abundant alkenes and trifluoromethanesulfonyl azide (N₃SO₂CF₃). This unprecedented two-component reaction employs readily available N₃SO₂CF₃as a bifunctional reagent to concurrently incorporate both CF₃and N₃groups, which avoids the use of their expensive and low atom economic precursors. A wide range of functional groups, including bio-relevant heterocycles and amino acids, were tolerated. Application of this method was further demonstrated by scale-up synthesis (5 mmol), product derivatization to CF₃-containing medicinal chemistry motifs, as well as late-stage modification of natural product and drug derivatives.

Full text of DOI:10.1039/d0sc06473d

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Trifluoromethanesulfonyl azide as a bifunctional
reagent for metal-free azidotrifluoromethylation of
unactivated alkenes†
Cite this: Chem. Sci., 2021, 12, 3210
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have been paid for by the Royal Society
of Chemistry
Hong-Gui Huang, Weishuang Li, Dayou Zhong, Hu-Chong Wang, Jing Zhao
*
and Wen-Bo Liu
Vicinal trifluoromethyl azides have widespread applications in organic synthesis and drug development.
However, their preparation is generally limited to transition-metal-catalyzed three-component reactions.
We report here a simple and metal-free method that rapidly provides these building blocks from
abundant alkenes and trifluoromethanesulfonyl azide (N3SO2CF3). This unprecedented two-component
reaction employs readily available N3SO2CF3 as a bifunctional reagent to concurrently incorporate both
CF3 and N3 groups, which avoids the use of their expensive and low atom economic precursors. A wide
range of functional groups, including bio-relevant heterocycles and amino acids, were tolerated.
Application of this method was further demonstrated by scale-up synthesis (5 mmol), product
derivatization to CF3-containing medicinal chemistry motifs, as well as late-stage modification of natural
product and drug derivatives.
Received 25th November 2020
Accepted 6th January 2021
DOI: 10.1039/d0sc06473d
rsc.li/chemical-science
Togni's reagent II and TMSN3 were employed as the CF3 and N3
sources, respectively. Following this pattern, iron-catalyzed
Introduction
processes with extraordinary substrate scope were later re-
ported by Hartwig7 and Xu8 (Scheme 1a). Other methods
including oxidative radical relay9 and photo–chemical reac-
tions10 were also successfully developed by a prudent choice of
the precursors of CF3 radical and azide. While these advances
are invaluable, from a practical point of view, the requirement
of transition-metal catalysts and expensive CF3 sources (e.g.,
Togni's reagents, Umemoto's reagent) has impeded their
application especially for scale-up syntheses. Additionally, state-
of-the-art approaches rely on the same three-component
framework: addition of a CF3 radical onto an alkene to form
an alkyl radical or cation11 and then trapped by a separate azide
reagent, which generally constitutes a low level of mass
The development of efficient synthetic methods, ideally those
that convert chemical feedstocks into biologically important
and complex structures, has signicantly advanced drug
discovery and development.1 Direct difunctionalization of
abundant alkenes is of fundamental importance to the
synthesis of high-value molecules as evidenced by increasing
research interests of the synthetic community in recent years.2
Selective azidotriuoromethylation of the carbon–carbon
double bond enables to concomitantly assembly of two vicinal
functional groups (N3 and CF3) in a single step. These structures
have high biological signicance due to the unique physical and
electronic properties of the triuoromethyl group.3 On the other
hand, the azide also serves as a handle for downstream deriv-
atization to triuoromethylated amines and heterocycles.4 In
particular, the well-established click chemistry of azides has
empowered chemists to rapidly generate large libraries of CF3-
containing compounds for screening in drug discovery.5
To date, catalytic methods for efficient access to vicinal tri-
uoromethyl azides from alkenes are still limited. In 2014, Liu
and co-workers pioneered the direct azidotriuoromethylation
of alkenes using copper catalysis (Scheme 1a).6 In their report,
Sauvage Center for Molecular Sciences, Engineering Research Center of Organosilicon
Compounds & Materials (Ministry of Education), College of Chemistry and Molecular
Sciences, Wuhan University, 299 Bayi Road, Wuhan 430072, Hubei, China. E-mail:
wenboliu@whu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0sc06473d
Scheme 1 Azidotrifluoromethylation of alkenes.
3210 | Chem. Sci., 2021, 12, 3210–3215
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Table 1 Reaction optimization
Entrya
Deviation
2ab (%)
1
2
3
4
5
6
7
8
9
None
48
Dioxane, MeCN, DMF, or DCE instead of EtOAc
TBHP or DTBP instead of LPO
AIBN instead of LPO
IN-1 instead of LPO
IN-2 instead of LPO
IN-3 instead of LPO
IN-3 instead of LPO and with 2.5 equiv of N3SO2CF3 (standard conditions)
w/o initiator
17–33
Trace
20
52
50
65
86 (87)c
N.R.
a
Reaction conducted with 1a (0.2 mmol), LPO (10 mol%), and N3SO2CF3 (1.5 equiv., 1 M in hexane) in EtOAc (2 mL) at 80 ꢀC for 12 h. b Determined
by 19F NMR of the crude reaction mixture using an internal standard. c Isolated yield. TBHP, tert-butyl hydroperoxide. DTBP, di-tert-butyl peroxide.
AIBN, 2,20-azobis(2-methylpropionitrile). N.R., no reaction.
efficiency by producing stoichiometric side-products (e.g., 2- toward using triuoromethanesulfonyl azide as a nitrene
iodobenzoic acid).12 Therefore, the development of more envi- precursor for iron-catalyzed intermolecular allylic C–H amina-
ronmentally benign and atom-economic alternatives is practi- tion, we unexpectedly observed azidotriuoromethylation of the
cally appealing and conceptually novel.
alkene (Scheme 1c). A control experiment disclosed that the
Triuoromethanesulfonyl azide (N3SO2CF3) is readily reaction occurred without iron catalysts. Encouraged by this
synthesized in situ by reacting triuoromethanesulfonic anhy- serendipity, we commenced the reaction optimization utilizing
dride with sodium azide in hexane (see ESI†). Although alkene 1a as a model substrate as outlined in Table 1 (see Tables
N3SO2CF3 has been commonly used as
a
diazo- or S1–S3 in ESI† for details). The desired product 2a was obtained
triuoromethanesulfonylamino-transfer reagent,13 its ability to in 48% yield using 10 mol% of lauroyl peroxide (LPO) as
act as a radical precursor received little attention. The homolytic a radical initiator and EtOAc as solvent (entry 1). Reactions in
decomposition of N3SO2CF3 to form a triuoromethyl radical other solvents (e.g., DCM, DMF, MeCN, entry 2 and Table S1†)
was rst observed by Kamigata et al. in 1984.14 In 2018, were less effective. Varying the concentration of the reaction did
a signicant work by the Studer group employed N3SO2CF3 as not further improve the productivity. Aer surveying commonly
an azide source for remote radical C–H azidation of amides by used thermal radical initiators, such as organic peroxides and
a fragmentation/1,5-HAT process.15 Very recently, a notable aliphatic azo compounds (entries 3–7), pentauorobenzoyl
radical 1,3-difunctionalization of allylboronic esters using peroxide (IN-3) emerged as the choice resulting in 65% yield of
N3SO2CF3 was also reported by the same group.16 These 2a (entry 7). To our delight, an increased yield (87%) was ob-
remarkable studies exemplied the potential utilization of tained by using 2.5 equivalent of N3SO2CF3 (entry 8). Unsur-
N3SO2CF3 as a bifunctional reagent.17 Encouraged by this prisingly, radical initiators were indispensable and no desired
precedent and our serendipitous discovery (vide infra), we have product was detected in their absence (entry 9). It should be
developed a two-component azidotriuoromethylation reaction noted that N3SO2CF3 should be handled with cautions,
of unactivated alkenes and N3SO2CF3 (Scheme 1b). In this although its stock solution in hexane is fairly stable (see ESI†).
unprecedented strategy, N3SO2CF3 serves as the sole precursor
Next, we set out to explore the substrate scope of this reac-
for both CF3 and N3 by releasing SO2 gas, which therefore tion (Scheme 2). Aliphatic alkenes bearing a broad array of
minimizes waste generation. This radical chain reaction is functional groups, such as ester, ketone, tosylate, phosphate,
initiated by an acyl peroxide, avoids the use of transition-metal and phthalimide, were viable substrates under the standard
catalysts, tolerates a wide range of functional groups, and is conditions, delivering the corresponding products in moderate
amenable to late-stage modication of natural products and to good yields (2a–2k). Heterocycles relevant to medicinal
drug derivatives.
chemistry, including furan, thiophene, and pyridine, were
tolerated as well, albeit in moderate yields (2l–2n). The mild,
base- and transition-metal-free reaction conditions also allowed
the compatibility of chloride- and bromide-containing
substrates with the halides unscathed (2o–2q). Moreover,
abundant unsaturated hydrocarbons were efficiently converted
Results and discussion
Our group has been interested in developing environmentally
benign direct C–H amination methodologies.18 In our efforts
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Scheme 2 Scope of the alkene azidotrifluoromethylation. Reactions conducted on a 0.2 mmol scale (except 0.4 mmol for 2e) under the
standard conditions (entry 8, Table 1).
into high-value bifunctionalized products in 58–62% yields (2r– into the tertiary azides 2u–2w in 71–81% yields. The reaction
2t). This simple process also pertains to sterically hindered with 1,1,2-trisubstituted alkenes delivered the corresponding
alkenes. 1,1-Disubstituted alkenes were selectively transformed bifunctionalized product 2x in moderate yield. This method
Scheme 3 Late-stage modification of drug and natural product derivatives. Reactions conducted on a 0.2 mmol scale under the standard
conditions (entry 8, Table 1).
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also enabled the efficient installation of peruoroalkyl groups
(e.g., C4F9, eqn (1)), given the readily synthesis of the corre-
sponding sulfonyl azides. Surprisingly, further expanding the
scope to cyclic olen 1y and styrene 1z unexpectively delivering
imine 2y and aldehyde 2z in 92% and 45% yield, respectively
(eqn (2) and (3)), which probably formed through a [3 + 2]
cycloaddition pathway.19 The reaction with vinyl ether resulted
in a complex mixture.19b Attempts to employ acrylate or vinyl
amide under the standard conditions failed to give any desired
azidotriuoromethylation product but led to polymerization of
the substrates (Scheme S1 in ESI†).
The absence of transition metals and operationally simple
procedure of this reaction offers a practical tool in late-stage
modication of densely functionalized biologically relevant
molecules(Scheme 3). Functionalization of a series of complex
structures (3a–3g) derived from pharmaceuticals were success-
fully achieved in good yields (4a–4g). The alkenes synthesized
from terpenes, norneol and menthol, reacted smoothly to the
corresponding azidotriuoromethylated products (4j, 4k) in
71% and 72% yields, respectively. Remarkably, derivatives of
amino acid including glycine (4l), tyrosine (4m), valine (4n),
phenylalanine (4o), and aspartic acid (4p), were amenable as
well to the reaction yielding the corresponding difunctionalized
products smoothly. Moreover, a steroid example and a carbo-
hydrate substrate were also included (4q, 4r). Overall, these
results showcased the broad applicability and usefulness of our
chemistry.
Scheme 5 Mechanistic investigation.
azide 2e bearing a pendant ester group proceeded with
concomitant aminolysis to provide g-butyrolactam 9 in 92%
yield. In a two-step protocol, reduction of azide 2q to amine
followed by subsequent cross-coupling delivered tetrahy-
droquinoline 10 in 57% overall yield.
As shown in Scheme 4, the robustness of the reaction was
demonstrated by a gram–scale reaction (5 mmol) under the
standard conditions providing azidotriuoromethylation
product 2j in 88% yield. The azide unit of the products accessed
above can be conveniently converted into privileged medicinal
chemistry motifs such as amines and heterocycles. Reduction of
azide 2j with In/NH4Cl produced a vicinal triuoromethylated
primary amine 5 in 69% yield. Notably, treatment of 2j with
Notably, all the triuoromethyl-containing compounds
accessed through these derivatizations are highly prevalent and
commonly used in pharmaceuticals or as biological probes,
demonstrating the ease of our chemistry in incorporating of
triuoromethyl group into these reminiscent motifs.
To gain mechanistic insight into this reaction, control
experiments were carried out (Scheme 5). Addition of a radical
scavenger TEMPO completely inhibited the reaction (Scheme
5a). Similarly, butylated hydroxytoluene (BHT) as an additive
prevented the formation of the azidotriuoromethylation
product. The detection of triuoromethyl adduct 11 by GC-MS
(MS ¼ 288.1) indicates that the triuoromethyl radical is
triphenylphosphine formed tetrahydro-1,3-diazepine
6 via
a Staudinger reaction/aza-Wittig sequence. Moreover, [3 + 2]
annulation of 2j with ethynylbenzene and benzoyl cyanide
resulted in the formation of CF3-containing triazole 7 and tet-
razole 8, respectively. Additionally, selective reduction of an
Scheme 4 Synthetic application.
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likely involved in this transformation (Scheme 5b). It is known
that 5-hexenyl radical analogues undergo 5-exo-cyclization to
the cyclopentylcarbinyl radical and are commonly employed as
radical clocks.20 The reaction with substituted 1,6-diene 12
delivered a cyclization product 13 which suggests that the
addition of triuoromethyl radical to the alkene forms an alkyl
radical followed by 5-exo-cyclization (Scheme 5c). On the basis
of the above observations and literature reports,21 a putative
radical chain pathway was illustrated as shown in Scheme 5d.
First, thermal decomposition of the radical initiator IN-3
produces an aryl carboxylic radical I or an aryl radical I0,22 which
reacts with triuoromethanesulfonyl azide generating tri-
uoromethanesulfonyl radical II, following by SO2 release to
form the triuoromethyl radical III.23 Subsequent addition of III
onto the alkene provides an alkyl radical IV. Finally, azidation of
IV with N3SO2CF3 yields the desired product 2 along with
regeneration of the chain carrying radical II.
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Conclusions
In summary, we have developed a transition-metal free azido-
triuoromethylation reaction of unactivated alkenes for the
synthesis of vicinal triuoromethyl azides. This unprecedented
difunctionalization process employs triuoromethanesulfonyl
azide as the sole precursor for both CF3 and N3. The application
was demonstrated by late-stage functionalization of complex
bio-important molecules and converting the products into the
corresponding CF3-containing amine, lactam, and other privi-
leged heterocycles. This study provides an atom economic, cost
efficient, and operationally simple method for the synthesis of
valuable vicinal difunctionalized structures from chemical
feedstocks.
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
We are grateful for the nancial support from NSFC (21971198
and 21772148), the Fundamental Research Funds for Central
Universities (2042019kf0208), Large-scale Instrument and
Equipment Sharing Foundation of Wuhan University and the
Natural Science Foundation of Hubei Province (Grant No.
2020CFA036). We also thank Dr Yiyang Liu (Pzer) for helpful
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