Visible-Light Decatungstate/Disulfide Dual Catalysis for the Hydro-Functionalization of Styrenes

Article abstract of DOI:10.1021/acs.orglett.1c00189

We describe an efficient photoredox system, relying on decatungstate/disulfide catalysts, for the hydrofunctionalization of styrenes. In this methodology the use of disulfide as a cocatalyst was shown to be crucial for the reaction efficiency. This photoredox system was employed for the hydro-carbamoylation, -acylation, -alkylation, and -silylation of styrenes, giving access to a large variety of useful building blocks and high-value molecules such as amides and unsymmetrical ketones from simple starting materials.

Full text of DOI:10.1021/acs.orglett.1c00189

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Letter
Visible-Light Decatungstate/Disulfide Dual Catalysis for the Hydro-
Functionalization of Styrenes
Alexis Prieto* and Marc Taillefer*
Cite This: Org. Lett. 2021, 23, 1484−1488
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ABSTRACT: We describe an efficient photoredox system, relying
on decatungstate/disulfide catalysts, for the hydrofunctionalization
of styrenes. In this methodology the use of disulfide as a cocatalyst
was shown to be crucial for the reaction efficiency. This
photoredox system was employed for the hydro-carbamoylation,
-acylation, -alkylation, and -silylation of styrenes, giving access to a
large variety of useful building blocks and high-value molecules
such as amides and unsymmetrical ketones from simple starting
materials.
nactivated alkenes are ubiquitous reagents in organic
chemistry as they are inexpensive, stable, and readily
catalyst ([W10O32]5−H) and a radical species (•R2). The latter
would then react with alkenes, forming the radical intermediate
(I). Finally, the catalytic cycle would be closed through the
consecutive single electron transfer/proton transfer (SET/PT)
steps from the reduced decatungstate ([W10O32]5−H) to the
intermediate I, delivering the hydro-functionalized product
(Figure 1). Considering the mechanism, we wondered why
unactivated alkenes reacted so differently from their activated
counterparts. The most likely explanation is the difference of
the reduction potential between the intermediates Ia (activated
alkene) and Ib (unactivated alkene). Indeed, SET reduction
U
available. As a consequence, they have been intensively used as
starting materials in a plethora of organic transformations.1
Among them, the hydrofunctionalization of unactivated
alkenes has been recognized as a valuable method to construct
complex molecules but, above all, to introduce functional
groups with full atom efficiency.2,3 This transformation is still
at the center of modern research as illustrated, for instance, by
the recent developments in the photocatalyzed hydroamina-
tions4,5 and hydroalkoxylations.4,6 In this field, the photo-
catalytic hydrocarbonation of unactivated alkenes has been to
date underexplored. On the other hand, CH-functionalization
has become a powerful strategy for the construction of high-
value molecules from readily available starting materials.7 The
hydrogen atom transfer (HAT) reactions promoted by a
photocatalyst have appeared as a promising alternative to the
traditional transition-metal-catalyzed CH-activation which
usually required the use of directing groups. For instance,
the decatungstate photocatalyst,8 which has been democratized
by Fagnoni/Albini8a−d and recently used by the MacMillan9e,f
red
between the reduced decatungstate [W10O32]5−H (E1/2
([W10O32]4−/[W10O32]5−H] = −0.96 V vs Ag/Ag+ in
acetonitrile)11 is thermodynamically favored with the inter-
red
mediates Ia (E1/2 = −0.66 V vs SCE in acetonitrile, for
•CH2(CH3)CO2Et/−CH2(CH3)CO2Et),12 while the latter is
disfavored with the intermediates Ib (E1/2red = −1.43 V vs SCE
in acetonitrile, for •CH2Ph/−CH2Ph).13 To overcome this
limitation, we envisaged developing a process that would
unlock such reactivity by using disulfides as cocatalysts. Indeed,
given that organic disulfides form the corresponding thiyl
and Noel9g groups, proved to be effective for the generation of
radical species under light irradiation,14 and that such radicals
̈
various radical species through direct HAT.9h The radical
intermediate can then be involved in various transformations
such as the hydrocarbonation of alkenes. While the
decatungstate photocatalyzed hydrocarbonation was found to
be efficient with activated alkenes (Michael acceptor),9a−d the
reaction mainly afforded poor conversions or led to product
mixtures in the presence of unactivated alkenes (Figure 1).10
The typical mechanism8 for the photocatalyzed hydro-
carbonation using decatungstate photocatalyst would start
with the photoexcitation of the decatungstate ([W10O32]4−)
(Figure 1). The excited complex (*[W10O32]4−) would
abstract a hydrogen atom from the substrate (R2H) leading
to the generation of the reduced form of the decatungstate
red
can be easily reduced (E1/2
= +0.16 V vs SCE, for
•SPh/‑SPh),15 we considered using them as cocatalysts acting
as both electron mediators and hydrogen donors, through the
in situ formation of thiols (Figure 1).4 In situ formed thiols
might also be advantageous, as they can act as a polarity-
Received: January 19, 2021
Published: February 3, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00189
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1484−1488
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Table 1. Optimization of the Hydrocarbamoylation
a
Reaction
b
Entry
Deviation from standard conditions
Yield 3a (%)
1
2
3
4
5
6
7
8
w/o (MeOpPhS)2
N.R.
57
42
75 (59)
N.R.
N.R.
70
(PhS)2 instead of (MeOpPhS)2
c
d
PhSH instead of (MeOpPhS)2
e
none
w/o light
w/o TBADT
365 nm instead of 405 nm
3 mol % TBADT
1a (2 equiv), 2a (1 equiv)
70
49
9
a
Reaction conditions: 1a (0.3 mmol), 2a (0.6 mmol) in ACN (0.6
b
mL). Yields were determined by 1H NMR using trichloroethylene as
an internal standard. 20 mol %. Around 10% thiol−ene product was
detected. Isolated yield. TBADT = Tetra-n-butylammonium
c
d
e
decatungstate. N.R. = no reaction, the starting materials were fully
recovered.
showed that the photocatalyst and the light irradiation are both
required for the reaction efficiency (Table 1, entries 5 and 6).18
Finally, the change of light source, the increase of the catalytic
loading, or the change of stoichiometric ratio did not lead to
further improvement (Table 1, entries 7−9). It is important to
mention that a larger quantity of the thiol−ene reaction
product was observed with the use of thiol species. The use of
disulfides seems to prevent the thiol−ene side reaction, likely
by maintaining a lower concentration of thiyl radical in the
media than when thiols are employed.
With the optimized conditions in hand, we then explored
the scope of the TBADT/disulfide photocatalyzed hydro-
carbomoylation of styrene derivatives (Scheme 1). The
reaction furnished the primary amide derivatives 3 in moderate
to good yields, regardless of the electronic nature and the
Figure 1. Decatungstate-photocatalyzed hydrofunctionalization of
alkenes.
reversal catalyst, enabling radical-chain propagation by
suppressing polarity-mismatching in the HAT processes.
Seminal work from Roberts and co-workers demonstrated
that the radical hydroacylation of aliphatic alkenes with
aldehydes is sluggish in the absence of thiol, while the reaction
in the presence of a catalytic amount of thiol is impressively
effective.3a Herein, we describe preliminary results demonstrat-
ing that the use of disulfides is crucial for achieving the
decatungstate-mediated hydrofunctionalization of styrenes
under light irradiation.
a
Scheme 1. Scope of the Hydrocarbamoylation of Alkenes
Our starting investigations focused on the hydrocarbamoy-
lation of styrene 1a with formamide 2a catalyzed by TBADT
photocatalyst (tetra-n-butylammonium decatungstate) (Table
1).16 The first experiment was conducted in acetonitrile
without disulfide, 1 mol % of TBADT, and as expected no
reaction was observed (Table 1, entry 1). Remarkably, the
addition of 10 mol % of phenyl disulfide to the previous
reaction conditions switched on the reactivity, allowing the
formation of the product 3a in moderate yield (Table 1, entry
2). The use of thiophenol instead of the phenyl disulfide
allowed the formation of 3a, albeit in lower yield (Table 1,
entry 3). Best results upon optimization were obtained with
(MeOpPhS)2 affording the product 3a in 75% NMR yield and
59% isolated yield (Table 1, entry 4).17 Control experiments
a
Reaction conditions: 1 (0.5 mmol), 2 (1 mmol), TBADT (1 mol %),
(MeOpPhS)2 (10 mol %) in ACN (1 mL). Yields refer to isolated
products. 5 mmol scale, 48 h.
b
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Letter
position of the aromatic substituents (3a−3h). Naphthyl and
pyridinyl19 aromatic patterns (3i−j), proved to be suitable
substrates for the transformation. Attempts for the hydro-
carbamoylation of aliphatic vinyl compounds failed. Then, we
investigated the reaction with formamide derivatives bearing
N- or N,N-substituents.20 The reaction proceeded efficiently
with N-alkyl and -aryl formamides affording the corresponding
amides (3k−3n) in moderate to good yields. The reaction
proved to be suitable for scale-up experimentation (5 mmol),
delivering the product 3k in a similar yield compared to the
small scale. However, the reaction was ineffective with the
N,N-diphenylformamide (2o), and no reaction occurred.
Finally, the hydrocarbamoylation reaction tolerates a wide
range of functional groups, including alkyl, ether, naphthyl,
pyridinyl, halide (F, Cl, Br), thus offering opportunities for
further diversification.
ketones 5 were obtained in moderate to good yields, and a
large variety of functional groups was tolerated including
halides, esters, and nitriles (Scheme 2). Aliphatic aldehydes
were also suitable reagents in the reaction, leading to the
formation of the corresponding dialkyl ketones (5j−5l) in
moderate to good yields (Scheme 2). To demonstrate the
utility of our methodology, the late-stage functionalization of
the ( )-citronellal with the styrene 1a was carried out.
Pleasingly, the reaction afforded the product 5m in moderate
yield (Scheme 2). Then, the hydroalkylation and the
hydrosilylation of styrene 1a were explored, respectively with
1,3-benzodioxole (4n) and triphenylsilane (4o), leading to the
formation of the expected products (5n−5o), albeit in
moderate yields (Scheme 2).22
Finally, we turned our attention to the elucidation of the
reaction mechanism. The control experiments carried out
previously (Table 1 and Scheme 2) allowed us to rule out
potential mechanistic routes. Indeed, the reactions performed
without the disulfide catalyst were ineffective. This result tends
to prove that the direct hydrogen back-donation from
[W10O32]5−H to the intermediate Ib is not possible (Figure
1). Meanwhile, the result seems to indicate that the TBADT
acts as a catalyst and not as an initiator. Therefore, a radical
chain propagation might be excluded. When the reaction was
performed only with disulfide catalyst, the reactivity was
completely shut down, suggesting that the success of the
reaction is unlikely due to the polarity-reversal catalysis, in
which the thiyl radical (•SAr) would perform HAT on starting
materials 2 or 4.3,18 To gain additional insights, experiments
were carried out without the TBADT catalyst in the presence
of a radical initiator ((BzO)2) (see Supporting Information
(SI) for more details). These reactions did not lead to the
formation of 5a, confirming that the thiyl radical is not a
polarity reversal catalyst in our reaction. These results are in
accordance with previous work,3a which have demonstrated
that aryl thiyl radicals (•SAr) were not suitable to abstract
hydrogen from aldehydes. Finally, to gain further insight into
the operating mechanism, experiments in the presence of
radical scavengers and deuterium labeling experiments were
performed (Scheme 3). The hydroacylation reaction of 1a with
the benzaldehyde 4a was thus repeated in the presence of the
radical scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO; 1.5 equiv). Interestingly, formation of coupling
product 5a was completely inhibited. However, no TEMPO-
Next, we studied without further optimization other
hydrocarbonations with a particular focus on the hydro-
acylation process (Scheme 2), as photomediated hydro-
a
Scheme 2. Other Hydrofunctionalizations of Alkenes
a
Scheme 3. Deuterium Labeling Experiments
a
Reaction conditions: 1 (0.5 mmol), 4 (0.75 mmol), TBADT (1 mol
%), (MeOpPhS)2 (10 mol %) in ACN (1 mL). Yields refer to isolated
b
c
products. 5 mmol scale, 48 h. 1H NMR yield is given as the product
was inseparable from the disulfide catalyst.
acylations are extremely scarce in the literature.21 We were
pleased to observe that the reaction proceeded well with
styrene 1a and benzaldehyde 4a, affording the ketone 5a in
moderate yield. With these substrates, the control experiments
demonstrated that the disulfide and the TBADT catalysts18
were crucial to the reaction efficiency (Scheme 2). We also
demonstrated the robustness of our method by performing the
reaction on a semipreparative 5 mmol scale, which pleasingly,
delivered the product 5a with similar yields.
a
1
Then, we explored the scope of the hydroacylation reactions
with various alkenes and aldehydes. All the unsymmetrical
Yields were determined by H NMR using trichloroethylene as an
internal standard.
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C. Additions to non-activated alkenes: Recent advances. Arabian J.
Chem. 2020, 13, 799−834.
adduct was observed (see SI for more details). The reaction
was also conducted in the presence of galvinoxyl (1.5 equiv),
which again switched off the reactivity (see SI for more
details).
(2) (a) Pirnot, M. T.; Wang, Y.-M.; Buchwald, S. L. Copper Hydride
Catalyzed Hydroamination of Alkenes and Alkynes. Angew. Chem., Int.
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2017, 22, 1901−1929.
Then, we performed deuterium labeling experiments
(Scheme 3). First, the reaction was carried out in the presence
of the compound 4a-d1 under standard reaction conditions
(Scheme 3a). We were pleased to observe 43% of deuterium
incorporation at benzylic position, suggesting that the disulfide
species is involved in the deuterium transfer from the
benzaldehyde 4a-d1 to the compound 5a-d1, likely through
the in situ generation of thiol (see SI for more details). To
confirm the hypothesis, we considered performing the reaction
with the benzaldehyde 4a and replacing the disulfide species by
its deuterated thiol counterpart (4-MeOPhSD; 20 mol %).
Under these conditions, we observed 15% of deuterium
incorporation, demonstrating that the thiol species is a putative
intermediate in the mechanism (Scheme 3b and Figure 1).
Those experiments are in accordance with the dual role of the
disulfide catalyst as both an electron mediator and a hydrogen
donor (Figure 1).
In summary, we have demonstrated that the use of the
disulfide/TBADT dual catalysis can unlock various hydro-
functionalization reactions of unactivated alkenes. This
catalytic system allowed the formation of high-value molecules
such as amides and unsymmetrical ketones from abundant
starting materials. Preliminary mechanistic studies tend to
prove that the disulfide catalyst acts as an electron mediator
and a hydrogen donor. We are conducting ongoing studies to
further understand and exploit this dual catalytic system in
other radical processes.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00189.
(8) For reviews on Decatungstate photocatalyst: (a) Ravelli, D.;
Fagnoni, M.; Fukuyama, T.; Nishikawa, T.; Ryu, I. Site-Selective C−
H Functionalization by Decatungstate Anion Photocatalysis: Syner-
gistic Control by Polar and Steric Effects Expands the Reaction Scope.
ACS Catal. 2018, 8, 701−713. (b) Ravelli, D.; Protti, S.; Fagnoni, M.
Decatungstate Anion for Photocatalyzed “Window Ledge” Reactions.
Acc. Chem. Res. 2016, 49, 2232−2242. (c) Crespi, S.; Fagnoni, M.
Generation of Alkyl Radicals: From the Tyranny of Tin to the Photon
Democracy. Chem. Rev. 2020, 120, 9790−9833. For prior studies,
see: (d) Hill, C. L. Introduction of Functionality into Unactivated
Carbon-Hydrogen Bonds. Catalytic Generation and Nonconventional
Utilization of Organic Radicals. Synlett 1995, 1995, 127−132.
(e) Renneke, R. F.; Pasquali, M.; Hill, C. L. Polyoxometalate systems
for the catalytic selective production of nonthermodynamic alkenes
from alkanes. Nature of excited-state deactivation processes and
control of subsequent thermal processes in polyoxometalate photo-
redox chemistry. J. Am. Chem. Soc. 1990, 112, 6585−6594.
(9) (a) Qrareya, H.; Ravelli, D.; Fagnoni, M.; Albini, A.
Decatungstate Photocatalyzed Benzylation of Alkenes with Alkylar-
omatics. Adv. Synth. Catal. 2013, 355, 2891−2899. (b) Dondi, D.;
Fagnoni, M.; Albini, A. Tetrabutylammonium Decatungstate-Photo-
sensitized Alkylation of Electrophilic Alkenes: Convenient Function-
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4163. (c) Protti, S.; Ravelli, D.; Fagnoni, M.; Albini, A. Solar light-
driven photocatalyzed alkylations. Chemistry on the window ledge.
Chem. Commun. 2009, 7351−7353. (d) Dondi, D.; Ravelli, D.;
Fagnoni, M.; Mella, M.; Molinari, A.; Maldotti, A.; Albini, A. Regio-
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Detailed experimental procedures and characterization
data for compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Alexis Prieto − ICGM, Univ Montpellier, CNRS, ENSCM,
34000 Montpellier, France; orcid.org/0000-0002-2922-
4652; Email: alexis.prieto@enscm.fr
Marc Taillefer − ICGM, Univ Montpellier, CNRS, ENSCM,
34000 Montpellier, France; orcid.org/0000-0003-2991-
5418; Email: marc.taillefer@enscm.fr
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00189
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully thank Dr. Florian Jaroschik (CNRS) for the
proofreading and helpful discussions, and Pascale Guiffrey
(CNRS) for her help with the GC-MS.
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