Metal-Free Visible-Light Synthesis of Arylsulfonyl Fluorides: Scope and Mechanism

Article abstract of DOI:10.1002/chem.202101056

The first metal-free procedure for the synthesis of arylsulfonyl fluorides is reported. Under organo-photoredox conditions, aryl diazonium salts react with a readily available SO₂ source (DABSO) to afford the desired product through simple nucleophilic fluorination. The reaction tolerates the presence of both electron-rich and -poor aryls and demonstrated a broad functional group tolerance. To shed the light on the reaction mechanism, several experimental techniques were combined, including fluorescence, NMR, and EPR spectroscopy as well as DFT calculations.

Full text of DOI:10.1002/chem.202101056

Communication
doi.org/10.1002/chem.202101056
Chemistry—A European Journal
Metal-Free Visible-Light Synthesis of Arylsulfonyl Fluorides:
Scope and Mechanism
Dan Louvel,[a] Aida Chelagha,[a] Jean Rouillon,[b] Pierre-Adrien Payard,[a] Lhoussain Khrouz,[b]
Cyrille Monnereau,[b] and Anis Tlili*[a]
Abstract: The first metal-free procedure for the synthesis of
arylsulfonyl fluorides is reported. Under organo-photoredox
conditions, aryl diazonium salts react with
a readily
available SO2 source (DABSO) to afford the desired product
through simple nucleophilic fluorination. The reaction
tolerates the presence of both electron-rich and -poor aryls
and demonstrated a broad functional group tolerance. To
shed the light on the reaction mechanism, several exper-
imental techniques were combined, including fluorescence,
NMR, and EPR spectroscopy as well as DFT calculations.
Arylsulfonyl fluorides (ArSO2F) are ubiquitous building blocks
that find a plethora of applications in life science technologies.[1]
In particular, ArSO2F are routinely used as covalent protein
inhibitors and biological probes.[2] In comparison to other
arylsulfonyl halides, ArSO2F present numerous advantages such
as their high resistance towards reduction and high thermody-
namic stability towards nucleophilic substitutions including
hydrolysis. In addition, arylsulfonyl fluorides are substrate of
choice for fluoride exchange (SuFEx) for click chemistry leading
to a variety of aryl sulfones derivatives.[3] In spite of the general
interest of ArSO2F, their direct synthesis is scarcely reported in
the literature. Regarding the state of the art, the most often
used methods rely on halogen exchange starting from the
unstable arenesulfonyl chloride. Other indirect methods have
been also disclosed in the literature (Figure 1),[4] yet the direct
construction of carbon-SO2F constitute a highly attractive
option. Within this framework, the group of Bagley, Willis and
co-workers reported a one-pot two-step procedure for the
synthesis of ArSO2F starting from (heteroaryl)aryl iodides or
Figure 1. Introduction to arylsulfonyl fluorides.
bromides in conjunction with commercially available and easy
to handle 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) ad-
duct (DABSO)[5] for the in situ generation of sulfinate under
palladium catalysis, followed by an addition of N-fluorobenze-
nesulfonimide (NFSI) as an electrophilic fluorine source.[6] The
same approach was also extended to alkenyl triflates.[7] A similar
strategy was disclosed by the group of Ball where Selectfluor
was used as an electrophilic fluorine source.[8] Finally, the group
of Liu and Chen recently reported a copper catalysed approach
to forge in situ aryl sulfonyl chloride starting with arene
diazonium salt and DABSO followed by the use of an external
fluoride source to obtain the desired product.[9] It should be
mentioned that a copper free procedure has been also
[a] D. Louvel, A. Chelagha, Dr. P.-A. Payard, Dr. A. Tlili
Institute of Chemistry and Biochemistry (ICBMS-UMR CNRS 5246)
Univ Lyon, Université Lyon 1,
CNRS, CPE-Lyon, INSA
43 Bd du 11 Novembre 1918, 69622
Villeurbanne (France)
developed using Na2S2O5 in the presence of electrophilic
[4c]
°
fluorine source (Selectfluor) at 70 C (Figure 1). In spite of
those recent successes, the development of metal free, mild
and straightforward procedures towards C-SO2F bond formation
remain highly desirable especially from a late-stage functional-
isation perspective. In this context, organo-photoredox catalysis
constitutes an attractive option as it will leverage the orthogon-
ality of transition metal catalysis through SET processes.[10]
With our continuous interest for the development of new
organo-photoredox processes we report herein an unprecedent
E-mail: anis.tlili@univ-lyon1.fr
[b] Dr. J. Rouillon, L. Khrouz, Dr. C. Monnereau
Univ Lyon, Ens de Lyon,
CNRS UMR 5182
Université Lyon 1
Laboratoire de Chimie,
69342, Lyon (France)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202101056
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strategy for the direct access to arylsulfonyl fluorides under
metal-free visible-light conditions.[11]
Changing the organo-photocatalyst to other cyanoarenes
organo-photocatalyst (PC3 and PC4, Table 1, entries 7–8) was
beneficial in improving the yield up to 60%. Eosin Y (PC5,
Table 1, entry 9) as well as Fukuzumi catalyst (PC6, Table 1,
entry 9) were not effective under our conditions providing
yields that did not exceed 10%. To further enhance the reaction
outcome, the impact of additives was studied. When using KHF2
(5 equiv.) the desired product was obtained in 70% yield
(Table 1, entry 11). Other fluoride sources such as LiBF4 and
NaPF6 were tested but only provided marginal yields (Table 1,
entries 12 and 13). It is important to note that no product
formation was observed either without blue LED irradiation or
in the absence of the photocatalyst (Table 1, entries 14–16).
With the optimised conditions in hand, we turned our focus
to studying the scope of the reaction (Scheme 1). Initial
investigations were carried out with arene substituted with
electron donating groups. Herein, good to excellent yields were
The optimisation started with 4-methylphenyldiazonium
salts 1a as a model substrate in the presence of 1 equivalent of
DABSO, 4CzIPN (PC1) as an organo-photocatalyst in MeCN
under blue LED irradiation at room temperature for 16 h.
Conversion of reagent 1a to the desired product 2a was
observed albeit in a low yield of 10% (Table 1, entry 1).
Encouraged by this first result we decided to investigate the
impact of the solvent and photocatalyst on the reaction
outcome. Using PC1, only traces of the desired product were
observed in butyl acetate (5%, Table 1 entry 2). Screening other
solvents including DMSO, toluene, MeOH as well as Et2O turned
out to be deleterious to the reaction outcome (Table 1, entries
3–6).
Table 1. Reaction optimisation.[a]
PC
PC1
Solvent
MeCN
Additive
À
Yield [%][b]
10
1
2
3
4
5
6
7
8
PC1
PC1
PC1
PC1
PC1
PC3
PC4
PC5
PC6
PC3
PC3
PC3
PC3
À
butyl acetate
DMSO
PhMe
MeOH
Et2O
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
À
5
0
0
0
À
À
À
À
0
À
60
60
5
10
70
10
<5
0
À
9[c]
10
11
12
13
14[d]
15[d]
16
À
À
KHF2
LiBF4
NaPF6
KHF2
KHF2
KHF2
0
0
À
[a]
Reactions were performed with diazonium salt 1 (0.3 mmol, 1 equiv.),
Scheme 1. Reaction scope. Reactions were performed with diazonium salt 1
(0.3 mmol, 1 equiv.), DABSO (0.15 mmol, 1 equiv.), KHF2 (1.5 mmol, 5 equiv.),
PC3 (2 mol%) and MeCN (2 mL). The reaction mixture was stirred at room
temperature for 16 h under blue LED irradiation and inert conditions. Yields
shown are those of isolated products; yields determined by 19F NMR
spectroscopy with PhCF3 as internal standard are shown in parentheses.
DABSO (0.15 mmol, 1 equiv.), (1.5 mmol, 5 equiv.), PC (2 mol%) and MeCN
(2 mL), unless otherwise noted. The reaction mixture was stirred at room
[b]
temperature for 16 h under blue LED irradiation and inert conditions.
Yields determined by 19F NMR spectroscopy with PhCF3 as internal
[c]
[d]
standard. Under green LED irradiation. Without light irradiation.
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obtained in the presence of methyl or methoxy groups (2a–e).
Interestingly steric hindrance was not deleterious to the
reaction outcome, since up to 80% yield were reached for ortho
substituted methoxyphenyl (2c). Furthermore, we investigated
the presence of halogen substituents on the arene ring. Taking
advantage of the metal-free catalytic procedures depicted
herein, we demonstrated that fluoro-, chloro-, bromo- and
iodoarenes could be converted to the desired sulfonyl fluoride
derivatives with synthetically useful yields (2g–j). Afterwards,
we decided to expand the reaction scope to electron poor
substituted arenes starting material. In particular, we demon-
strated that our reaction conditions tolerate the presence of
several electron withdrawing groups including, ester,
trifluoromethyl, cyano as well as nitro motifs. Herein also, ortho,
meta or para position were smoothly converted to the
corresponding products (2k–q). Interestingly, heterocyclic com-
pounds could also be converted to their corresponding sulfonyl
fluorides in moderate yields (2r–s). Finally, we demonstrated
that our protocol could be extended to complex molecular
architecture and compound 2t was obtained in a good yield of
50% demonstrating the potential application for late-stage
transformation.
Having illustrated the broad scope and the high functional
group tolerance of the developed metal-free procedure we
started investigating the reaction mechanism. Fluorescence
studies, EPR spectroscopy as well as DFT calculations were
combined to understand the role of the photocatalyst, and its
efficiency was assessed by measuring of the photoconversion
quantum efficiency. Assuming a photoinduced single electron
transfer (P-SET) as the initial stage of the reaction sequence, the
hypothetical catalytic cycle may either start by oxidation of
DABSO (or in situ generated DABCO) or by reduction of the
diazonium salt. In order to evaluate the occurrence of
(potentially concurrent) electron transfers between the photo-
catalyst and the different species, and to quantify the associated
kinetics, PC3 was involved in successive Stern-Volmer experi-
ments with the diazonium salt 1a, DABSO and DABCO (Figure 2,
I). In agreement with the facile reduction of diazonium salt, it
turns out that the luminescence of the exited photocatalyst is
efficiently quenched in his presence, with a Stern-Volmer
constant KSV =256 MÀ 1. We also confirmed efficient quenching
of the luminescence of the exited photocatalyst in the presence
of DABCO (KSV =229 MÀ 1), but also to a slightly lesser extent
with DABSO (KSV =164 MÀ 1). With a measured luminescence
lifetime τf =4.2 ns for PC3, it was possible to derive for the later
a quenching rate coefficient kq of 6.1×1010, 5.45×1010 and 3.9×
1010 MÀ 1sÀ 1 with 1a, DABCO and DABSO, respectively. These
three values appear in quantitative agreement with the
hypothesis of a dynamic, diffusion limited quenching process,
and suggest that multiple SET pathways may occur in solution.
It is however important to underline that, while quantita-
tively similar for DABCO and DABSO, the SET process results in
a markedly different evolution of the system. Thus, while PC3
remains relatively photostable for moderate irradiation times
(ca. 10 min) in the presence of DABCO and DABSO, fast
photobleaching occurs in the presence of 1a with a kinetic rate
that correlates well with the amount of added diazonium salt
(Figure 2, b). This is indicative of the formation of a reactive
radical intermediate which, in the condition used in our
spectroscopic measurements, evolves by addition onto PC3 and
ultimately clearance of the later. Such a behaviour agrees well
with the expected characteristics of a tolyl radical.
To further investigate the nature of the formed radical in
the reaction media we decided to undertake EPR experiments
under visible light irradiation (Figure 2, II). Attempts to perform
such experiments with mixtures either devoid of PC3 or [PhN2]
led to complex signatures with overall low intensities, suggest-
ing dead-end mechanisms. For the sake of clarity, description of
these spectra will not be discussed in the manuscript but is
nevertheless available in SI. Irradiation of a mixture of PC3 with
diazonium salt 1a with blue LED (λ=455 nm) in the presence
of α-phenyl-N-tert-butylnitrone (PBN) as radical trap results in
the clean formation of a single spin adduct A with a signature
typical of a C-centred tolyl radical (g=2.006, aN =14.8 G and
aH =2.8 G, Figure 2, II, top),[12] as already observed in some of
our previous works.[11]
When mixing PC3, DABSO and diazonium salt 1a, in the
presence of PBN, the obtained signature comprises, in addition
to a signal obtained in the previous irradiation experiment with
PC3 and 1a only and attributed to a tolyl adduct (g=2.006,
aN =14.8 G and aH =2.5 G), a secondary signal radical which
characteristic values (g=2.006, aN =13.6 G and aH =1.6 G) are
compatible with a O-centred radical species,[13] presumably on a
tosyl radical. Complementary experiments using DMPO as a
radical trap reveal, besides the presence of the abovementioned
tolyl (g=2.006, aN =14.5 G and aH =21.4 G) and O-centred tosyl
B (g=2.006, aN =12.8 G and aH =13.8 G) radical adducts a third
species compatible with a S-centred tosyl adduct (g=2.006,
aN =13.9 G and aH =15.0 G).[14] In the experiment with PBN, the
latter may overlap with that of the tolyl adduct. Note that the
formation of the tolyl radical was also observed by mixing
DABSO and 1a in the absence of photocatalyst (see the
Supporting Information for the complete description of these
experiments and associated DFT calculations). This radical is
formed due to SET between in situ generated DABCO and the
diazonium. However, under these conditions no tosyl radical
species could be detected and the formation of the tolyl radical
do not lead to the product.
Afterwards, the photochemical quantum yield of the
reaction was investigated (Figure S2). In this study, reactant 1g
was preferred to 1a, in order to facilitate monitoring of the
reaction through 19F NMR. In the optimised concentration
conditions (entry 7 in Table 1), following our previously re-
ported NMR actinometric protocol[15] (depicted in full in the
Supporting Information) and monitoring the real-time disap-
pearance of 1g, we were able to determine a photoconversion
quantum yield of the diazonium reactant Φ=0.43; this
indicated
a stepwise mechanism. Interestingly, while the
disappearance of 1g could be nicely fitted with a first order
kinetic model, a typical sigmoidal curve, with an induction
period of about 10 minutes was observed for the appearance of
2g, characteristic of a mechanism operating by successive
reactions. Note also that similar experiment performed in the
absence of DABSO but otherwise identical concentration
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Figure 2. Mechanistic investigation: I) luminescence studies: a) Stern Volmer plots of luminescence evolution of PC3 upon incremental addition of 1a (green),
DABCO (red) and DABSO (orange); b) real<-time monitoring of PC3 photobleaching in the presence of various amounts of 1a. II) EPR spectroscopy: EPR spin-
trapping spectra upon irradiation (λ =455 nm) in the presence of BPN in MeCN at room temperature. Blue=experimental, red =simulated. III) DFT
calculation: Free energy [kcal molÀ 1] pathway for the reaction of a photogenerated phenyl radical with DABSO calculated at the DFT level. a) Barrierless
process in electronic energy, see energy scan in the Supporting Information. IV) Proposed mechanism.
conditions resulted in rapid photodegradation of the catalyst
and therefore premature stalling of photoinduced disappear-
ance of 1g, in line with spectrofluorimetric experiments.
In order to gain more insight on the reaction mechanism,
DFT calculations were undertaken (Figure 2, III). Reaction of Ph.
tion of DABCO by fluorine anion on the salt is easily accessible
and yield the final product PhSO2F.
In light of the obtained results, we propose the following
mechanism. Photoinduced reduction of the diazonium salt
occurs through SET from the excited PC3, yielding arene radical
*
.
with DABSO to yield PhSO2 and DABCO is favoured
and radical cation photocatalyst (PC3 +). Subsequently the aryl
(À 14.8 kcal mol-1, Figure 2, III). Given the nature of the stepwise
mechanism, the generated DABCO may reduce PC3.+ formed
upon reduction of the diazonium salt to regenerate the
photocatalyst PC3 and DABCO.+. The latter may combine with
radical collapses with SO2 to form a new radical tosyl species.
The released DABCO is able to regenerate the photocatalyst
*
PC3 in its ground state and DABCO +. This latter may combine
*
with PhSO2 to generate the sulfonium salt intermediate C’.
.
PhSO2 to generate the sulfonium salt intermediate C (Figure 2,
Afterwards, a nucleophilic fluorine attack is taking place to form
the desired arylsulfonyl fluoride and regenerate DABCO (Fig-
ure 2, IV).
III, ΔG=À 10.4 kcalmolÀ 1).[16] Subsequent nucleophilic substitu-
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Wang, M. Shabab, S. Kumari, J. G. Villamor, C. J. McLaughlin, E.
Weerapana, M. Kaiser, B. F. Cravatt, R. A. L. van der Hoorn, Chem. Biol.
2013, 20, 541.
In summary, the first metal-free visible-light-induced proto-
col for the synthesis of arylsulfonyl fluorides is described. The
key to success was the combination of aryl diazonium salts,
DABSO and the cyanoarene PC3 under blue LED irradiation. The
mild conditions afforded a broad scope and high functional
group tolerance. Furthermore, mechanistic investigations were
performed including luminescence, EPR spectroscopy, 19F NMR
spectroscopy as well as DFT calculations. Together, those
studies allowed us to identify several key intermediates, based
on which we propose a plausible mechanism for the reaction.
Further investigations exploiting the use of cyanoarenes as
photocatalysts are under investigation in our laboratory for the
selective formation of new radical as well as the synthesis of
valuable building blocks.
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Conflict of Interest
The authors declare no conflict of interest.
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Keywords: arylsulfonyl fluorides
investigations · metal-free synthesis · organo-photoredox
· fluorine · mechanistic
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Manuscript received: March 23, 2021
Accepted manuscript online: April 7, 2021
Version of record online: ■■■, ■■■■
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COMMUNICATION
D. Louvel, A. Chelagha, Dr. J. Rouillon,
Dr. P.-A. Payard, L. Khrouz, Dr. C.
Monnereau, Dr. A. Tlili*
1 – 6
Metal-Free Visible-Light Synthesis of
Arylsulfonyl Fluorides: Scope and
Mechanism
The key to success in the new metal-
free visible-light-mediated synthesis
of arylsulfonyl fluorides was using cya-
noarene PC3 as organo-photocatalyst.
A variety of arene diazonium slats
were smoothly converted to the cor-
responding product. Mechanistic in-
vestigations including luminescence,
EPR spectroscopy as well as DFT cal-
culation allowed us to identify several
key intermediates and propose a
plausible mechanism.