Photoredox Catalytic Activation of Sulfur Hexafluoride for Pentafluorosulfanylation of α-Methyl- and α-Phenyl Styrene

Article abstract of DOI:10.1002/cctc.201800501

Sulfur hexafluoride is inert, non-toxic, and cannot simply be applied as pentafluorosulfanylation reagent. We present the first photoredox catalytic way to convert it into pentafluorosulfanylated α-methyl and α-phenyl styrenes simply by using light. The work tackles the challenges of precise activation of sulfur hexafluoride by a photoredox catalyst with designed consecutive electron transfer cycles in a fashion that styrenes trap the generated pentafluorosulfanyl radical. The method overcomes the highly problematic access to vinylic and allylic pentafluorosulfanyl styrenes and combines it with the disposal of the most potent greenhouse gas. Together with the use of light as energy source, an exceptionally high level of sustainability is gained.

Full text of DOI:10.1002/cctc.201800501

Accepted Article
Title: Photoredox catalytic activation of sulfur hexafluoride for
pentafluorosulfanylation of α-methyl and α-phenyl styrene
Authors: David Rombach and Hans-Achim Wagenknecht
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To be cited as: ChemCatChem 10.1002/cctc.201800501
Link to VoR: http://dx.doi.org/10.1002/cctc.201800501
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10.1002/cctc.201800501
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Photoredox catalytic activation of sulfur hexafluoride for
pentafluorosulfanylation of α-methyl and α-phenyl styrene
David Rombach,[a] and Hans-Achim Wagenknecht*[a]
Abstract Sulfur hexafluoride is inert, non-toxic, and cannot simply be
applied as pentasulfanylation reagent. We present the first
photoredox catalytic way to convert it into pentafluorosulfanylated -
methyl and -phenyl styrenes simply by using light. The work tackles
the challenges of precise activation of sulfur hexafluoride by a
photoredox catalyst with designed consecutive electron transfer
cycles that styrenes trap the generated pentafluorosulfanyl radical.
The method overcomes the highly problematic access to vinylic and
allylic pentafluorosulfanyl styrenes and combines it with the disposal
of the most potent greenhouse gas. Together with the use of light as
energy source, an exceptionally high level of sustainability is gained.
usage in a standard research laboratory as well as broad
industrial use of these methodologies nearly impossible. In recent
work, there was progress by the chlorofluorination of dibenzyl
sulfides to SF4Cl-compounds. However, these reactions are also
highly dangerous to handle due to the use of chlorine gas and
subsequent fluorination by HF or ZnF2[12,13] and do not transfer the
final functional group but require the preinstallation of a
thiofunction in earlier synthetic steps. The accessibility of SF5-
alkyl compounds is even more restricted due to the lackage of any
methodology which is not based on the use of mixed or low sulfur
fluorides today.[7] In contrast to the high reactivity and toxicity of
the mixed sulfur fluorides there is the notorious inertness of sulfur
hexafluoride (SF6) which renders this inexpensive gas as a
promising pentafluorosulfanylation reagent for organic synthesis.
However, SF6 cannot yet simply be applied as chemical
pentasulfanylation reagent for organic compounds. Due to its
susceptibility to infrared light excitation SF6 is the strongest
greenhouse gas known to humankind today. It displays a 22,800-
fold higher greenhouse potential than carbon dioxide and has a
mean lifetime in the atmosphere of about 3,200 years.[14] SF6 is
still indispensable in many applications, especially in the context
of high voltage switchgears, and needs finally to be destroyed
after usage, but should be better reused for chemical
transformations in order to significantly gain more sustainability.
We follow this idea and present herein a completely new
photocatalytic method that applies SF6 as substrate, precisely
activates it by LED light at 365 nm for chemical transformations,
and transfers it to organic compounds modified with SF5 groups.
By this new photochemical method, a new access to potentially
valuable SF5-modified -methyl and -phenyl styrenes for further
transformations is gained while the highly potent greenhouse gas
SF6 is destroyed. The method is designed as a highly clean
reaction regarding the formation of fluorinated side products.
Introduction
Fluorinated compounds play not only an important role in
pharmaceutical chemistry,[1-3] but also in agrochemicals,[4] and in
materials for e.g. optoelectronics, because fluorine helps to
design unique properties by its significant electronic influence.
The most common fluorinated substituent is the trifluoromethyl
(CF3) group.[5] However, the search for even more effective and
more stable fluorinated groups is an important task. In contrast to
the extensively applied CF3 group, the pentafluorosulfanyl (SF5)
group is a relatively new fluorinated substituent and one of the
most underexplored ones; therefore often designated as
“forgotten functional group”.[6] This is surprising with respect to the
proposed, highly beneficial properties of the SF5 group, especially
as bioisosteric replacement in pharmaceutically active
compounds,[7] but also for the design of polymerization
catalysts.[8] In contrast to the weakness of the CF3 group, namely
its sensitivity towards hydrolytic activation, the SF5 group is both
thermally and chemically stable and not prone to hydrolysis under
physiological conditions, is highly electronegative, and is
lipophilic.[9] This renders the SF5-compounds as prospective
alternatives to common CF3-compounds in drugs, and does not
simply represent a more expensive perfluorinated group. So far,
the exploration and the use of the SF5 group are dramatically
limited by its very difficult synthetic accessibility[10,11] due to the
extraordinary toxicity and availability of the reagents that the very
few available methods are based on, such as the dangerous
mixed sulfur fluorides SF5Cl and SF5Br as well as the highly toxic
S2F10. The extraordinary toxicity of these compounds makes the
Results and Discussion
The conventional photochemical activation of SF6 requires highly
energetic UV light (185 nm≙650 kJ/mol) in the presence of
styrene and yields only SF4 and sulfur as products.[15] The main
challenge in photoactivation of SF6 is to establish a single electron
reduction step and stabilize the resulting reactive SF5 radical after
dissociation of the fluoride anion in order to form a carbon-SF5
bond. This is a very difficult task due to the energetics of the bond
enthalpies of the consecutively reducible S-F-bonds in SF6, which
means that the subsequent reduction step yields the stable
[a]
Dipl.-Chem. David Rombach, Prof. Dr. Hans-Achim Wagenknecht
Institute of Organic Chemistry
Karlsruhe Institute of Technology (KIT)
Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
E-mail: Wagenknecht@kit.edu
-
molecule SF4 after fragmentation of the resulting SF5 anion.[16]
However, it was also shown by nanocalorimetric studies that the
·-
fragmentation channel of the radical anion SF6 is highly
Supporting information for this article is given via a link at the end of
the document.
dependent on the excess energy that is brought into by the
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10.1002/cctc.201800501
ChemCatChem
FULL PAPER
reducing electron.[17] This is the most critical issue for SF6
activation by photoredox catalysis. In general, photoredox
catalysis emerged over the last few years and applies visible light
as energy source for organic reactions.[18-24] Recently, Jamison et
al. developed a method to activate SF6 by the widely used and
standard Ir(ppy)2(dtbppy)PF6 using highly reducing conditions by
addition of a sacrificial reductant. They used it for allylic
deoxyfluorinations.[25] This approach, however, does not allow the
introduction of SF5 groups to organic compounds. This is not
surprising since at such low electron excess energies, the
irradation with second LED (522 nm, vide infra). The fluoride of
product can be eliminated by treatment with the Lewis acid BF3 in
order to form 4 that carries the SF5 group in the vinylic position in
more than 95% yield. This type of photoreaction works also well
with -methyl styrene (0.05 M) (5) to the addition product 6 (see
SI). BF3-induced eliminations of 3 and 6 yield 4 and 7, respectively,
that carry the SF5 group (see SI) in the vinylic or allylic position
and thereby nicely complements this photoredox catalytic
approach (vide infra).
·-
fragmentation of photoactivated radical anion SF6 yields only
reactive fluoride radicals, lower sulfur fluorides and non-reactive
SF5 anions.[17] Photoredox catalysis is, however, a tool to
precisely control electron transfer processes by both the intensity
of the irradiation power and the excess energy of the transferred
electron that is tuned by the photophysical properties of the
excited state of the chosen photoredox catalyst. Thus, we focused
our work on a photoredox catalytic approach that avoids an
excess of reducing agents and carefully controls the local
“reductivity” of the reaction medium. Since early reports showed
that alkali metals in the presence of polyarenes are suited to
overcome kinetic barriers to reduce SF6 to sulfide and fluoride[26]
we anticipated to cut off the kinetically favoured channel of
consecutive reductions by a carefully designed photoredox
catalytic cycle in combination with a two-photon absorption.[27]
Accordingly, a photocatalyst with a strongly reducing excited state
is needed. N-Phenylphenothiazines are some of the most strongly
reducing photoredox catalysts known today[28] because the
excited state potentials of -2.1-2.5 V are getting close to the
reduction potential of -2.7 V of solid sodium.
Figure 1. Photoredox catalytic activation of SF6 using N-phenyl-phenothiazine
(1) as photoredox catalyst and pentafluorosulfanylation of 1,1-diphenylethylene
(2) and -methyl styrene (5) to 3 and 6, and subsequently the products with
vinylic SF5 group 4 and allylic SF5 group 7. The inset shows the different
fragmentation channels of the photoredox catalytically formed SF6·- depending
on the excess energy that is high in case of 1 and low in case of standard
Ru(bpy)3Cl2. The image shows the irradiation setup using LEDs (see SI).
Our photoredox catalytic approach consisted of 5 mol% N-
phenyl-phenothiazine (1, for electrochemical charaterization see
Supplementary Information (SI)) as photoredox catalyst, 1,1-
diphenylethylene (2) as substrate (0.1 M) in acetonitrile as solvent
due to its large electrochemical window avoiding undesired
reductive side reactions (Figure 1). The irradiation was performed
by 365 nm LEDs (max=368 nm) and 525 nm LEDs (max=512 nm,
for LED spectra see SI), additionally (vide infra). The successful
and selective activation of SF6 was indicated by the formation of
a SF5-modified carbon species, probably the solvent acetonitrile
according to NMR spectroscopy. This issue was related to the
high reactivity and hydrogen abstraction ability of the generated
SF5 radical. This makes the SF5-radical too short-lived for the
desired selective chemical reaction with 2. An increase of the
substrate concentration was not successful due to undesired side
reactions. Copper(II) salts are known to stabilize the similarly
behaving CF3 radical and manage to bind to both generated
radicals and mediate the bond formation.[29,30] Thereby, the
coordination of copper(II) to radicals can drastically enhance the
lifetime of short-lived radicals and enable selective reactions
using even very short-lived transient radicals. In our chemistry,
Cu(acac)2 showed the best performance of enabling addition of
SF6 to substrate 2. In fact, the selectivity for the desired product 3
was dramatically enhanced to the almost complete suppression
of dimerization using a low substrate concentration of 0.05 M as
well as 30 mol% of catalyst in the reaction mixture however the
yield dropped significantly due to overreduction of the generated
radical and the resulting quenching of to the key intermediates.
Finally we found optimized reaction conditions to get the product
in up to 63% by addition of a low amount (10 mol%) of Cu(acac)2
together with 5 mol% 1 in the reaction mixture (Figure 2) and
3
1.4
9
1.2
1.0
0.8
0.6
0.4
0.2
0.0
)
)
)
)
1
1
1
%
%
%
ol
ol
Cu(II
Cu(II
Cu(II
ol
Cu(II
1,
1,
1,
1,
5 m
%
%
%
20 m
%
10 m
2,
ol
ol
ol
2,
ol
2,
M
M
M
5 m
10 m
20 m
0.1
30 m
2,
0.1
0.1
2,
2,
2,
M
M
M
M
0.1
0.1
0.1
Figure 2. Relative ratio of pentafluorosulfanylation product 3 and substrate
dimerization product 9 after irradiation of substrate 2 at 365 nm in the presence
of different amounts of photoredox catalyst 1 and 10 mol% Cu(acac)2 according
to GC/MS-EI analysis.
In order to gain mechanistic insights (Figure 3), detailed optical-
spectroscopic quenching studies gave important hints. The
photoredox catalyst 1 was stable under irradiation in absence of
a quencher molecule. After addition of SF6 no reaction was
observed at all in the dark. After irradiation of the sample solution
for 1 min at 365 nm a red color was observed, which was shown
to be persistent in the absence of light. The intermediate species
that results from charge separation between photoexcited 1 and
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SF6 (in the presence of 2) was identified by
spectroelectrochemistry (for spectrum see SI), especially by its
absorbance bands at 274 nm and 514 nm, as the radical cation
1·+ of the photoredox catalyst. This initial electron transfer yields
the radical anion SF6·- as transient species. The radical cation 1·+
is not able to oxidize 2 and regenerate the photoredox catalyst
into the ground state, because this back electron transfer is about
100 kJ/mol endergonic which was also shown in theoretical
calculations of reduction potentials by DFT. Only the irradiation of
1·+ at 365 nm or 530 nm reduced its concentration due to
oxidation of 2. This second electron transfer is the key activation
step for 2. According to the Rehm-Weller equation (without the
Coulomb energy), the driving force for this step is estimated by
product and that the yield is increased from 32% to 49% under
comparable conditions by irradiation both at 368 nm where 1
absorbs and 522 nm where 1·+ absorbs, our studies imply the
following extended photoredox catalytic cycle (Figure 3). After
excitation of 1, the excited electron is selectively transferred to
SF6 that dissociates into the reactive SF5 radical and the fluoride
anion. An interesting feature of the usage of 1 is that both the
photocatalyst itself and its radical cation 1·+ absorbs at light at 365
nm (for UV/Vis absorption spectrum see SI). That means that the
second excitation of 1·+ in the presence of substrate 2 yields the
substrate radical cation 2·+ after a second electron transfer that
regenerates 1 and closes the extended photoredox catalytic cycle.
The radical cation 2·+ allows for addition of the SF5 radical
generating the SF5-substituted 1,1-diphenylethyl cation 8, which
is consecutively trapped by the generated fluoride ion into the
product 3. In this mechanistic scenario, it can be assumed that
copper(I) stabilizes the SF5 radical and 2·+ enabling their reaction
to product cation 829. This is the second key chemical step in the
whole mechanism since it forms the C-SF5 bond. Therefore, the
global electrophilicity index of the SF5 radical was calculated by
means of DFT using the method developed by Parr[35] correlating
the chemical hardness and the chemical potential, which are both
easily accessible by DFT calculations (for details see SI). The
calculated value of 3.7 for the reactivity parameter gives highly
electrophilic character which is expected for such a highly electron
deficient radical. The regioselective addition of the SF5 radical to
the less substituted position of the electron deficient radical 2·+
was assumed based on the stabilized character of the generated
cation 8 and further supported by the calculation of spin densities
that showed substantial character of localisation at C-1 (Figure 3
right). This explains nicely the radical attack of the SF5 radical at
this position. Finally, we isolated the products 3 and 6 and verified
their structure by means of NMR spectroscopy. A detailed
structure analysis was carried out using 13C-1H as well as 19F-1H
and 19F-13C correlation spectroscopy which proved the
regioselective addition of the SF5 radical and fluoride anion in
products 3 and 6 (for NMR spectra see SI).
G=Eox(2·+/2)-Ered(1·+/1)-E00.
Using
Eox(2·+/2)=1.7 V,[31]
Ered(1·+/1)=0.7-0.8 V (for cyclic voltagrams see SI)[32] and
E00=2.4 eV (514 nm), this electron transfer step is clearly
exergonic (G=-1.4--1.5 eV). To study this reaction more
precisely, the radical cation 1·+ was chemically synthesized by
oxidation of 1 with NOPF6.[33] The red solid could be isolated and
is stable at room temperature under inert conditions. The radical
cation 1·+ reacted very slowly with 2 in the dark under inert
conditions by the loss of <2% absorbance at 711 nm over 20 h
(Figure 3 left. This result was further supported by 1H NMR studies
that showed that the concentration of 2 in the reaction mixture
stayed constant over a period of 11.5 h in the dark, even in
presence of the chemically formed radical cation 1·+ (for 1H
spectra see SI). These results ruled out a significant reactivity of
1·+ with 2 which is in agreement with the results of Moutet.[34] In
contrast, the radical cation 1·+ reacts by irradiation at 525 nm in
the absence of 2 and the spectroscopic signature at 711 nm
vanishes over 5 h (Figure 3 left). The careful analysis of these
UV/Vis absorption spectra indicated a complex between 2 and 1·+
that enables the second excitation and electron transfer under
non-diffusion controlled conditions.
The addition of the SF5 radical, subsequent back electron transfer
to radical cation 1·+ and finally the reaction with the fluoride anion
would represent a simpler alternative mechanism. However, we
observe a substrate dimerization product 9 that can only be
formed by the sustrate radical cation 2·+. Taken together with the
observations that we do not find any H-abstraction SF5-alkyl
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1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
711 nm
600
650
700
 / nm
750
800
0.8
0.6
0.4
0.2
0.0
in the dark
irradiation
at 525 nm
0
10 20 30 40 50 60 70 80
t / min
Figure 3. Mechanistic proposal for the photoredox catalytic cycle of 1 (box) that converts the substrate 2 into product 3. The spectra (left) show the spectroscopic
signature of the intermediate radical cation 1·+ after the first photoinduced electron transfer from the photoredox catalyst 1 to SF6 (top) and its conversion by the
second photoinduced electron transfer and oxidation of the substrate 2 to its radical cation 2·+ (bottom). The calculated spin density of the substrate radical cation
2·+ (right) rationalizes the regioselectivity of the subsequent reactions to product 3.
The detailed reaction conditions were further developed to
support not only the proposed reaction mechanism but to improve
the yield. If it is assumed that the quenching reaction of the
excited photocatalyst 1* by SF6 is a bimolecular reaction it should
be dependent on the concentration of SF6 and 1. Increasing the
SF6 pressure from 2 up to 6 atm (for determination of the SF6
pressure see SI) has not a reasonable impact on the yield of 3 (ca.
10%). Although the solubility of SF6 in acetonitrile as solvent
should be describable by Henry’s law suggesting a linear increase
in concentration with pressure, the sensitivity of this parameter is
quite low. This concludes that the formation of SF6·- is not the rate
limiting step and the first electron transfer is probably fast. The
raise of catalyst concentration from 5 mol% to 10 mol% reduces
the yield due to unproductive overreduction of the generated SF5
radical. The experimental results show a strong dependence of
product formation on the two operating catalytic cycles which
have to be connected precisely by controlling the rate of forming
the SF5 radical and the photoredox catalyst cation 1·+. Slowed
down trapping of 1·+ decreases the amount of formed desired SF5-
modified product 2 and favours the formation of the substrate
dimer (Figure 2).
deprotonation. The similar procedure using 5 delivers 7. There is
an interesting side result by this reaction: While CF3 groups are
sensitive towards abstraction of fluoride anion, the SF5 group
stayed completely stable during the attack by this strong Lewis
acid. This is an important feature of the SF5 group that it makes it
very attractive as stated in the introduction.
Conclusions
We found the first selective activation of SF6, which generates the
transient SF5 radical that reacts with carbon to form exclusively
SF5-substituted
organic
products,
in
particular
pentafluorosulfanylated -methyl and -styrene products. The
formation of the SF5 radical instead of the fluoride radical is likely
due to the precisely matching excess electron energy of the
excited electron. Based on the newly gained knowledge of the
solution behaviour of the initially formed radical anion SF6·- which
was so far only studied in the gas phase, our results confirm the
strong dependence of the fragmentation channel on the electron
excess energy of the transferred electron in solution. Since
-
electron excess energies of less than 2 eV yield the anion SF5
(that decomposes to SF4 and fluoride anions) N-phenyl
phenothiazine 1 is the right choice to turn SF6 into a precious
pentafluorosulfanylation reagent by means of photoredox
catalysis. This chromophore provides electrons with a sufficiently
For the elimination of the vicinal fluorine substituent in 3 to
generate exclusively SF5-substituted alkene 4 as final product all
attempts to abstract the acidified proton in -position of the
strongly electronegative SF5 group by Et3N or carbonates failed.
This is likely because the resonance energy that is liberated by
the elimination of HF does not compensate for the loss in energy
due to the cleavage of the strong C-F bond. However, we could
show that the addition of 38 eq. BF3·Et2O as Lewis acid yielded
the elimination product 4 after stirring the reaction mixture under
air for 3 h at room temperature in almost quantitative yield. The
enthalpy gain using the Lewis acid assisted elimination seems to
overcompensate for the C-F bond enthalpy and therefore allows
for clean elimination of the fluoride anion, followed by
·-
high electron excess to allow fragmentation of SF6 into the
desired reactive SF5 radical and fluoride anions. It is important to
note that further increase of electron energies is not productive
-
-
because it yields the lower fluorine species SF4 , SF3 , SF2 and
-
·-
F2 .[1-3] The electron affinity of SF6 generating SF6 is still under
debate[17] but the threshold for electron attachment generating
-
SF6 was determined to be about 0 V in various experimental
studies.[36] This is in very good agreement with our results since
the estimated excited state potential of 1 is Ered(1·+/1*)=-2.1V and
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the electron attachement energy of 0 V yields an electron excess
energy of about 2.1 V which corresponds to the fragmentation
channel into the SF5 radical. Common Ru(bpy)3Cl2 as photoredox
catalyst has an estimated excited state reduction potential of
Ered(Ru3+/Ru2+*)=-0.88V that gives an excess electron energy of
(3 x 100 mL). The combined organic phases were dried over
Na2SO4. The solvent was evaporated and the crude product was
purified by column chormatography (SiO2, Cyclohexane, Rf = 0.3).
The product was gotten as colorless solid (1.082 g, 3.93 mmol,
97 %). 1H NMR (400 MHz, Acetonitrile-d3) δ 7.70 – 7.60 (m, 2H),
7.60 – 7.46 (m, 1H), 7.45 – 7.30 (m, 2H), 7.05 (dd, J = 7.5, 1.7 Hz,
2H), 6.91 (ddd, J = 8.1, 7.3, 1.7 Hz, 2H), 6.84 (td, J = 7.4, 1.4 Hz,
2H), 6.22 (dd, J = 8.1, 1.4 Hz, 2H). 13C{1H} -NMR (126 MHz,
Acetonitrile-d3) δ 145.1, 141.9, 131.9, 131.4, 129.3, 128.1, 127.6,
123.6, 120.9, 117.17. HR-EI-MS m/z (calcd.) = 275.0769 [M•+];
m/z (found) = 275.0767 [M•+].
General procedure of adding SF6 to styrenes. Under inert gas
atmosphere in a Young-type Schlenk tube 0.1 mmol of the
substrate as well as 5 mol% of 1 and 10 mol% Cu(acac)2. was
dissolved in 1 mL MeCN. The reaction was subjected to three
freeze-pump-thaw cycles. Then the reaction mixture was frozen
to -196°C, the vessel was evacuated and the atmosphere was
exchanged against SF6 (70 mL) using a gas dosage glass
apparatus. After complete transfer of the gas volume to the
reaction vessel the vessel was sealed and SF6 was resublimed to
the gas phase while letting come the reaction mixture to room
temperature. The reaction was irradiated at 365 respective
525 nm at 20°C for 21 h carried out by 19F-NMR spectroscopy
after addition of standard to the crude reaction mixture and
dilution with 300 µL CDCl3.
-
about 0.8 V, which serves the SF5 anion generating channel,
which is in good agreement with the observed lower sulfur
intermediates.[25] In our approach, the SF5 radical can be trapped
by activated styrenes, represented by 2 and 5, as substrates
yielding SF5-substituted organic products in yields of up to 63%.
Furthermore, we developed a method to abstract the vicinal
fluoride anion and prepare the vinyl- and allylpentafluorosulfanyl
compounds 4 and 7, respectively. These are valuable precursors
for further chemical transformations comparable to the
trifluoromethylthiolated styrenes by Glorius et al.,[37] the styrene
addition products by Nicewicz et al.,[38] and as polymerization
precursors.[39] Furthermore, vinylic and allylic SF5 compounds
allow for a wide variety of functionalization to prepare small SF5-
containing building blocks. Our approach is not yet a routinely
applicable protocol for the pentafluorosulfanylation of any organic
compound but we could show for the first time, that one can turn
SF6 in a precious pentafluorosulfanylation agent by addressing
the correct fragmentation channel in solution. We believe that this
is a milestone in understanding the chemical properties of the
reduction of SF6 which opens up a new era in organofluorine
chemistry. The impact of SF5 substituents in pharmacology,
agrochemistry, modern optoelectronic functional materials and
other fields will be speeded up by this facile and synthetically
useful access of SF5-modified organic compounds. Our
methodology avoids the highly toxic and often not even
commercially available precursor molecules SF5Cl, SF5Br and
S2F10 to SF5-alkenes[11] and uses instead the inert and non-toxic
SF6 as valuable precursor molecule for such transformations by
comparable yields. As side effect, the usage of SF6 as a chemical
resource reduces its climate change and greenhouse effect. Our
vision is not to only destroy SF6[40] after its usage in industrial
applications but to connect SF6 with chemical synthesis to
generate a symbiosis and facilitate the disposal of SF6 by
generation of valuable molecules. We showed that the
combination of photoredox catalysis and SF6 chemistry is a
powerful tool in the synthesis of SF5-containing -methyl and -
phenyl styrenes Together with the use of light as energy source,
an exceptionally high level of sustainability is gained.
Pentafluoro-(2-fluoro-2,2-diphenylethyl)-6-sulfane (3). The
compound was prepared according to the general procedure
using 17.6 µL (18.0 mg, 0.100 mmol) 2, 1.4 mg 1 and 2.6 mg
Cu(acac)2 in metal organic grade solvent using two wavelength
irradiation for 21 h at 20°C. The product was determined by 19F-
NMR (64% yield). Purification of the material was carried out by
removing the solvent under reduced pressure and column
chromatography (SiO2, hexanes) using a micro column (Rf = 0.2).
1H NMR (400 MHz, Chloroform-d) δ 7.45 – 7.29 (m, 10H), 4.49
(dp, JHF = 21.4, 7.9 Hz, 2H). 13C{1H} NMR (101 MHz, CDCl3) δ
140.65, 140.42, 128.78, 128.63, 125.11, 125.03, 119.12, 115.78,
97.53, 95.66, 76.55, 76.43, 76.32, 76.20, 76.08. 19F-NMR (471
MHz, Chloroform-d) δ 86.48 – 78.62 (m), 70.95 (ddt, J = 147.9,
9.2, 8.7 Hz), -155.43 (tp, J = 21.9, 11.8 Hz). HR-EI-MS m/z
(calcd.)
C14H12F632S.
= =
326.0564 [M•+]; m/z (found) 326.0565 [M•+]
Pentafluoro(2-fluoro-2-phenylpropyl)-6-sulfane (6). The
compound was prepared using the general procedure using
6.50 µL (5.92 mg, 0.050 mmol) 5, 0.70 mg 1 and 1.30 mg
Cu(acac)2 using filtered Sureseal solvent (Aldrich). The reaction
mixture was subjected to irradiation for 21h at 20°C using a
365 nm LED. The product yield was determined by 19F-NMR
(27%). There was a product distribution that also yielded the
elimation product yielding molecule in significant yield (15%).
Purification of the adduct was carried out using column
chromatography (SiO2, n-pentane) using a pipet column. The
product is highly volatile and could not be seen after spotting on
TLC at room temperature. The TLC therefore was g precooled
before spottinby dipping it into liquid nitrogen for some seconds.
Immediate staining with KMnO4 staining reagent indicated the
product as yellowish spot0 (Rf = 0.3). The solvent could not be
completely removed for characterization due to the higher boiling
point of pentanes compared to the product. Complete evaporation
of the solvent was tried but yielded residual pentanes in NMR.
Structural characterisation was therefore carried out in the
.
Experimental Section
Supplementary Information gives the complete experimental
methods.
Synthesis of N-Phenylphenothiazine (1). N-Phenylpheno-
thiazine was synthesized similar to the reported procedure.[41]
810 mg Phenothiazine (4.06 mmol, 1.00 eq.) were dissolved in
8.0 mL anhydrous toluene. Then 780 mg (520 µL, 4.98 mmol,
1.23 eq.) bromobenzene, 587 mg (5.23 mmol, 1.29 eq.) KOtBu
as well as 71 mg (0.489 mmol, 12 mol%) (t-Bu)3PHBF4 was
added. Then 112 mg Pd2dba3 (0.122 mmol, 24 mol% Pd) added.
The reaction mixture was degasses using three freeze-pump-
thaw cycles and was finally refluxed for The reaction mixture was
let come to room temperature and 100 mL EtOAc and 50 mL
water was added. The reaction mixture was extracted with EtOAc
1
presence of pentanes. H NMR (300 MHz, Chloroform-d) δ 7.48
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10.1002/cctc.201800501
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– 7.32 (m, 5H), 4.06 (ddp, J = 29.9, 22.7, 7.3 Hz, 2H), 1.86 (d, J
= 21.6 Hz, 3H). 13C NMR (101 MHz, Chloroform-d) δ 142.01,
128.88, 128.87, 128.48, 124.20, 124.11, 119.30, 115.96, 78.02 -
79.00 (m), 27.94, 27.08. 19F NMR (471 MHz, Chloroform-d) δ
84.10 – 81.99 (m), 70.59 (ddt, J = 147.5, 9.2 Hz), -152.43 – -
152.71 (m). HR-EI-MS m/z (calcd.) = 264.0407 [M•+]; m/z (found)
= 264.0408 [M•+].
let come to room temperature and then 2 mL hexanes was added.
A red solid precipitated. The red solid was washed with hexanes
three times to remove excess of PTA. Then the solid was dried
under reduced pressure and characterized by absorption
spectroscopy.
General procedure for elimination of the vicinal fluoride.
Under air the vicinal fluorinated sulfur pentafluoride (12.6 mM)
was dissolved in CDCl3. Then BF3∙Et2O (473 mM, 37.6 eq.) was
added and the mixture immediately turned slightly redish. The
mixture was stirred for 3 h at room temperature. Then the
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft
(grant Wa 1386/16-1) and KIT is gratefully acknowledged. D.R.
thanks the Landesgraduiertenstiftung Baden-Württemberg for a
doctoral fellowship and the GRK 1626 “Chemical photocatalysis”
for participation in their qualification program.
1
progress was checked by H-NMR. No other SF5 species was
formed during reaction. The mixture was quenched by addition of
1 mL NaHCO3 solution and was extracted with CHCl3 (3 x 5 mL)
and the combined organic phases were dried over Na2SO4. The
solvent was removed carefully under reduced pressure and was
subjected to column chromatography.
Keywords: photochemistry • addition • photocatalysis • electron
transfer • phenothiazine
(2,2-Diphenylvinyl)-pentafluoro-6-sulfane (4). In a screw-cap
vial 4.11 mg 3 were dissolved in 1 mL of CDCl3 and BF3∙Et2O was
added and the mixture was stirred at room temperature for 3h.
The starting material was cleanly converted to the product. The
yield was determined by 1H-NMR spectroscopy (>95%). The
crude oily product was purified by column chromatography (SiO2;
2% acetone in hexanes). The product was gotten as highly
volatile oil. Therefore to characterize the compound the solution
was not completely taken to dryness but characterized in
presence of some pentanes. 1H NMR (300 MHz, Chloroform-d) δ
7.43 – 7.28 (m, 6H), 7.25 – 7.19 (m, 4H), 6.87 (p, J = 8.4 Hz, 1H).
13C{1H} NMR (101 MHz, Chloroform-d) δ 147.64 (q, J = 5.7 Hz),
139.03, 137.55 (q, J = 18.3 Hz), 137.32, 128.58 (q, J = 1.8 Hz),
128.31, 128.14, 128.06. 19F NMR (471 MHz, Chloroform-d) δ
86.19 – 81.57 (m), 68.57 (dd, J = 152.4, 8.5 Hz). HR-ESI-MS m/z
(calc.) = 306.0500 [M•+]; m/z (found) = 306.0502 [M•+].
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Entry for the Table of Contents
Layout 1:
COMMUNICATION
The green and sustainable way:
Using designed photoredox catalysis it
is possible to transfer the SF5 group
from inert SF6 to organic substrate.
D. Rombach, H.-A. Wagenknecht*
Page No. – Page No.
Photoredox catalytic activation of
sulfur hexafluoride for
pentafluorosulfanylation of styrenes
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