Visible-Light-Induced Meerwein Fluoroarylation of Styrenes

Article abstract of DOI:10.1021/acs.orglett.1c01249

An unprecedented approach for assembling a broad range of 1,2-diarylethane derivatives with fluorine-containing fully substituted carbon centers was developed. The protocol features straightforward operation, proceeds under metal-free condition, and accommodates a large variety of synthetically useful functionalities. The critical aspect to the success of this novel transformation lies in using aryldiazonium salts as both aryl radical progenitor and also as single electron acceptor which elegantly enables a radical-polar crossover manifold.

Full text of DOI:10.1021/acs.orglett.1c01249

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Letter
Visible-Light-Induced Meerwein Fluoroarylation of Styrenes
Hai-Jun Tang, Bin Zhang, Fei Xue, and Chao Feng*
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ABSTRACT: An unprecedented approach for assembling a broad
range of 1,2-diarylethane derivatives with fluorine-containing fully
substituted carbon centers was developed. The protocol features
straightforward operation, proceeds under metal-free condition,
and accommodates a large variety of synthetically useful
functionalities. The critical aspect to the success of this novel
transformation lies in using aryldiazonium salts as both aryl radical
progenitor and also as single electron acceptor which elegantly
enables a radical-polar crossover manifold.
Scheme 1. Intermolecular Fluoroarylation of Unsaturated π-
Systems
rganofluorine compounds have been finding increasing
Oapplication in multidiscipline research spanning agro-
chemical development, pharmaceutical industry, and material
sciences, mainly due to their unique physicochemical proper-
ties.1 As such, much effort from synthetic chemists has been
directed to elaborating molecular structures of interest by
developing site-specific fluorination-based synthetic trans-
formations.2 Under this circumstance, transition-metal-cata-
lyzed carbofluorination of π-systems, which not only enables a
straightforward incorporation of fluorine atom but also
provides an extra opportunity for concurrent integration of
carbon-based functionalities such as alkyl,3 trifluoromethyl,4
alkynyl,5 aryl,6 alkoxycarbonyl,7 and cyano,8 proves to be
highly appealing and draws much attention from the synthetic
community in recent years. Nonetheless, the progress of
fluoroarylation reaction lags far more behind as compared with
its counterparts. In this respect, seminal contributions from the
groups of Toste,6c Doyle,6b and Gouverneur6a among others
have virtually enriched this type of transfroamtion (Scheme
1a). In 2016, we coined a strategy of “F-nucleophilic addition
induced functionalization of gem-difluoroalkenes”,9 which also
finds applications in our recent progress in the arena of
fluoroarylation.10 Furthermore, by resorting to visible-light-
promoted gold redox catalysis, we also successfully extended
the fluoroarylation senario to allenoates,11 whereas simple
alkene substrates still proved intractable (Scheme 1b).
Notwithstanding the advancement in this field, the continuing
development of efficacious protocols in this direction is still
highly desirable, especially for those that make use of readily
available nucleophilic fluorination reagents or accommodate
reaction substrates unattainable for prior art.
of aryl radical intermediates using progenitors such as aryl
halides,17 arylhydrazines,15a aryldiazonium salts,14b,c,15b or
diaryliodonium salts,14a and after radical addition of alkenes
three plausible scenarios, including radical coupling and
radical-polar crossover through either single-electron reduction
or oxidation, may follow. While much progress has been
Received: April 14, 2021
Published: May 5, 2021
Since the seminal disclosure of copper-catalyzed aryldiazo-
nium engaged arylation of alkenes by Meerwein in 1939,12
many advancements of this type arylation have been achieved,
which comprise a fertile reaction arsenal for diversity-oriented-
syntheses.13−16 These reactions proceed via in situ formation
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© 2021 American Chemical Society
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Letter
achieved for Meerwein carboamination14 and carbooxygena-
tion,15 the development of Meerwein-type fluoroarylation is
comparatively underdeveloped. In 2012, the Tang group
disclosed a silver-catalyzed intermolecular/intramolecular
fluoroarylation of styrenes using the expensive Selectfluor as
the fluorination reagent,6d despite the fact that the assembly of
fluorine-containing fully substituted carbon centers still
remains challenging.18 Later on, Heinrich group reported an
intermolecular fluoroarylation of monosubstituted simple
alkenes without recourse to metal catalyst, albeit with
somewhat depressed reaction efficiency.6d In view of the
state of the art in this territory, further development of efficient
and practical protocols, particularly those that are amenable for
expedient construction of fluorine-containing fully substituted
carbon centers, are highly appealing.
Based on the precedents and our own works, we aimed to
develop a practical protocol for alkenes fluoroarylation by
virtue of the radical-polar crossover manifold using aryldiazo-
nium salt and nucleophilic fluoride as both aryl and fluoro
donors (Scheme 1c). The intrinsic property of aryldiazonium
salt, which could readily produce aryl radical for reaction
initiation under certain conditions14b,15c while also act as
single-electron acceptor for efficient reaction propagation was
rationalized to lay solid foundation for the feasibility of the
proposal. With our long-standing interest in photoredox
catalysis and fluorine chemistry,19 we would like to describe
herein a visible-light-induced Meerwein fluoroarylation of
styrene derivatives. Notable features with regard to the present
protocol include the followings: (i) fluorine-containing fully
substituted carbon centers could be readily assembled; (ii) no
transition-metal catalyst was required; (iii) nucleophilic
fluorination using cost-effective reagent was developed; (iv)
the incorporation of aryl and fluorine substituents was
orchestrated in a highly selective manner.
a
Table 1. Optimization of Reaction Conditions
b
entry
variation
yield (%)
c
1
no
62, trace
24
38
trace
18
NDP
trace
38
2
THF
3
dioxane
4
5
DMF, DMSO
TMAF (10.0 equiv)
6
7
8
9
DMPU·HF (10.0 equiv)
pyridine·HF (10.0 equiv)
2.5 mol % Ru(bpy)3(PF6)2 added
MeCN (0.5 mL)
65
10
11
1a (0.2 mmol), 2a (0.1 mmol)
74
d
MeCN (0.5 mL), 1a (0.2 mmol), 2a (0.1 mmol)
80 (79 )
a
Unless otherwise noted, the reaction was conducted with 1a (0.1
mmol), 2a (0.2 mmol), and Et3N·3HF (3.0 equiv) in MeCN (1.0
b
mL) under 15 W blue LEDs for 12 h. Yield was determined by 19F
NMR with 1-iodo-4-(trifluoromethyl)benzene as the internal stand-
c
d
ard. At air atomosphere. Isolated yield.
a
Scheme 2. Reaction Scope of Styrene Derivatives
Initial attempt was carried out using 1,1-diphenylethylene 1a
and aryldiazonium salt 2a with Et3N·3HF as nucleophilic
fluorination reagent. To our delight, when a mixture of 1a (0.1
mmol), 2a (0.2 mmol), and Et3N·3HF(0.3 mmol) in CH3CN
(1.0 mL) was irradiated with 15 W blue LEDs for 12 h, 62%
yield of the fluoroarylation product 3aa could be obtained
(Table 1, entry 1). To further facilitate this transformation, a
set of solvents such as THF, dioxane, DMF, etc. were
subsequently screened, albeit with no positive results (Table 1,
entries 2−4). Further interrogation of the fluorination reagent
revealed the superiority of Et3N·3HF, with other alternatives
all leading to depressed reaction efficiencies (Table 1, entries
5−7). Notably, the addition of exogenous photoredox catalyst
proved to be unprofitable, with unexpected attenuation of
reaction yield being observed, in contrary (Table 1, entry 8).
When changing the stoichiometry of 2a applied, however, no
further increase in the yield of 3aa could be materialized.20 It is
delightful to find that an obvious enhancement of reaction
turnover was eventually realized when reducing the amount of
solvent while reversing the stoichiometric ratio of 1a and 2a
(Table 1, entries 9−11). Control experiments further
underscored the importance of reaction time, nitrogen
atmosphere protection, as well as conducive effect of visible
light irradiation, beyond simply acting as a heat source, on the
present transformation.20
a
Unless otherwise noted, the reaction was conducted with 1 (0.2
mmol), 2a (0.1 mmol), and Et3N·3HF (3.0 equiv) in MeCN (0.5
b
mL) under irradiation of 15 W blue LEDs for 12 h. 0.3 mL DCE was
c
added. Yield was determined by 19F NMR with 1-iodo-4-
With the optimal reaction conditions in hand, the reaction
scope with respect to styrene derivative was subsequently
investigated (Scheme 2). Delightfully, halogen substituents
such as F, Cl, and Br, which provide the possibility for further
(trifluoromethyl)benzene as the internal standard.
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derivatization through traditional cross-coupling protocols,
proved to be well tolerated, thus affording the desired products
in good yields (3ba, 3ca, 3ea). Pleasingly, α-Me-styrene
derivatives containing various substitutes were also compatible
to this transformation, delivering the desired products in
moderate to good yields (3ga−3na). It is worth noting that, in
the case of substrates containing extra π-systems, the
fluoroarylation occurred selectively on a more electron-rich
position as showcased in examples of 3ka and 3la. To further
probe the influence of substituents on the α-position of styrene
derivatives, substrates decorated with n-Bu, Bn, Cy, cyclo-
pentyl, and functionalized alkyl substituents containing remote
unsaturated π-system or chlorine atom were examined and
afforded 3oa−3ta in good yields. To further showcase the
generality of this protocol, cyclic styrene derivatives with
exocyclic double bonds were subjected to the standard reaction
conditions, which enabled a smooth assembly of structural
motifs with cyclic fluorine-containing fully substituted carbon
centers (3ua−3wa). Of note, the extension of this reaction to
α,β-disubstituted styrene derivatives was also feasible, which
delivered the corresponding products in moderate yields but
with low diastereocontrol (3xa−3za, 3aaa).
quinolone and benzothiazole engaged in this reaction readily
to furnish the desired producst 3gs and 3mt in 50% and 60%
yields, respectively. To further exemplify the applicability of
this protocol, aryldiazonium salts derived from coumarin, L-
menthol, and vitamin E were then tested, which delivered
respective products without compromising reaction yields
(3gu−3gw).
To gain insight into the reaction mechanism of the present
Meerwein fluoroarylation process, a panel of control experi-
ments were implemented. Initially, when styrenes 1a′ and 1a″
devoid of α-substituent were subjected to the standard reaction
conditions, no fluoroarylation products could be acquired, thus
indicating that single electron transfer (SET) between
secondary benzylic radical intermediates, as involved in these
cases, with an aryldiazonium salt tend to be an inefficient
process (Scheme 4a). It is worth pointing out that
Scheme 4. Control Experiments
Subsequently, the reaction generality with regard to
aryldiazonium salts was surveyed (Scheme 3). Halogen
a
Scheme 3. Reaction Scope of Aryldiazonium Salts
a
Unless otherwise noted, the reaction was conducted with 1 (0.2
mmol), 2 (0.1 mmol), and Et3N·3HF (3.0 equiv) in 0.5 mL MeCN
fluoroarylation product of 1a' could be obtained, albeit in
11% yield, via the Heinrich protocol by leveraging the radical
type fluorine atom transfer with the electrophilic fluorination
reagent.6d We assume that the key to the amenability of
monosubstituted styrene derivative resides in disclosing an
efficient radical-polar crossover manifold regarding the
secondary radical intermediate. Therefore, the photocatalyst
was further examined in the reaction of 1a′. Pleasingly, the
fluoroarylation product 3a′a could be obtained in 22% yield
when Ir(ppy)3 was used as the catalyst, and this modification
also enabled the formation of product 3a″a in 30% yield for
substrate 1a″ (Scheme 4a). To further challenge the generality
of the present reaction, aliphatic alkene 1ab was interrogated
under standard reaction conditions; however, no desired
product could be obtained (Scheme 4b). Furthermore, when
cyclopropyl-decorated styrene substrate 1ac was employed, the
fluoroarylation proceeded uneventfully to afford product 3aca
in 69% yield, with the cyclopropyl group being unscathed
(Scheme 4c). Meanwhile, when Ph-substituted analogue 1ad
b
under irradiation of 15 W blue LEDs for 12 h. Yield was determined
by 19F NMR with 1-iodo-4-(trifluoromethyl)benzene as the internal
c
standard. 1.0 mL MeCN and 0.5 mL DCE was used as the solvent.
d
0.3 mL DCE was added.
atoms such as F, Cl, and Br were well compatible in this
transformation, delivering products 3mb−3md in moderate
yields. Aryldiazonium salts bearing synthetically useful groups
such as ester, carbonyl, CF3, CN, and NO2 successfully
participated in this reaction and gave the desired products in
up to 84% yield (3ge−3gn). Notably, aryldiazonium salts with
electro-donating/neutral substituents were also amenable
despite the efficiency being somewhat attenuated, thus
providing products in moderate yields (3go−3mq). Moreover,
ethynyl-decorated substrate 2r was also competent and
afforded product 3mr in 41% yield, with the alkyne group
remaining intact throughout the reaction. Additionally,
heteroarene-based substrates such as those derived from
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was employed, the anticipated radical ring-opening fluorination
product 3ada could be obtained in 19% yield in the presence
of photoredox catalyst Ir(ppy)3(Scheme 4d). In order to
further prove the radical nature of this reaction, the model
reaction was tested in the presence of TEMPO, which resulted
in a dramatic impediment of reaction turnover, thus suggesting
the involvement of radical species in this transformation
(Scheme 4e). Additionally, we also inspected the reaction
between 1a and 2a but with the absence of Et3N·3HF, which
delivered Meerwein arylation product 4aa in 30% yield.
However, further submitting 4aa to the standard reaction
conditions gave no formation of desired product 3aa, which
firmly precluded the involvement of 4aa as the key
intermediate in the present transformation (Scheme 4f).
Finally, to demonstrate the practicality of the present
fluoroarylation protocol, a scale-up reaction at 1.0 mmol was
also conducted, which led to the formation of 3aa in 70%
yield.20
Optimization, experimental procedures; compound
characterization (1H NMR, 13C NMR, and HRMS);
1H NMR and 13C NMR reprints (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Chao Feng − Technical Institute of Fluorochemistry (TIF),
Institute of Advanced Synthesis, School of Chemistry and
Molecular Engineering, Nanjing Tech University, Nanjing
211816, P. R. China; orcid.org/0000-0003-4494-6845;
Email: iamcfeng@njtech.edu.cn
Authors
Hai-Jun Tang − Technical Institute of Fluorochemistry (TIF),
Institute of Advanced Synthesis, School of Chemistry and
Molecular Engineering, Nanjing Tech University, Nanjing
211816, P. R. China
Bin Zhang − Technical Institute of Fluorochemistry (TIF),
Institute of Advanced Synthesis, School of Chemistry and
Molecular Engineering, Nanjing Tech University, Nanjing
211816, P. R. China
Given all experimental investigations above, we tentatively
proposed a possible reaction mechanism (Scheme 5). The
Scheme 5. Proposed Mechanism
Fei Xue − Institute of Material Physics & Chemistry, College of
Science, Nanjing Forestry University, Nanjing 210037, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01249
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support of the
National Natural Science Foundation of China (21871138)
and the Natural Science Foundation of Jiangsu Province
(BK20170984), the “Thousand Talents Plan” Youth Program,
the “Jiangsu Specially-Appointed Professor Plan”, and the
“Innovation & Entrepreneurship Talents Plan”.
reaction initiation was based on the generation of aryl radical I
by aryldiazonium salt 2, which is a well-known process that is
promoted by light irradiation.15d Then the nascent aryl radical
I further underwent regioselective homolytic addition to
styrene derivatives to afford tertiary benzyl radical II. The
following SET between benzyl radical II and aryldiazonium salt
2 would readily produce the carbonium intermediate III
accompanied by the regeneration of aryl radical I, which could
effectively propagate the turnover by reentering the productive
cycle. Meanwhile, the nucleophilic fluorination of species III
by Et3N·3HF smoothly provided the final product 3.
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