Decatungstate as Direct Hydrogen Atom Transfer Photocatalyst for SOMOphilic Alkynylation

Article abstract of DOI:10.1021/acs.orglett.1c00381

A versatile approach for the alkynylation of a variety of aliphatic hydrogen donors, including alkanes, is reported. We used tetrabutylammonium decatungstate as photocatalyst to generate organoradicals from C-H/Si-H bonds via hydrogen atom transfer. The l

Full text of DOI:10.1021/acs.orglett.1c00381

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Letter
Decatungstate as Direct Hydrogen Atom Transfer Photocatalyst for
SOMOphilic Alkynylation
Luca Capaldo* and Davide Ravelli
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ABSTRACT: A versatile approach for the alkynylation of a variety of
aliphatic hydrogen donors, including alkanes, is reported. We used
tetrabutylammonium decatungstate as photocatalyst to generate organo-
radicals from C−H/Si−H bonds via hydrogen atom transfer. The latter
intermediates underwent SOMOphilic alkynylation by methanesulfonyl
alkynes to afford internal alkynes upon loss of a sulfonyl radical. The effect
of different radicofugal groups on the reaction outcome was evaluated and
rationalized via a combined experimental and computational approach.
Scheme 1. SOMOphilic Alkynylation via Hydrogen-Atom
Transfer (HAT)
he CC triple bond is a privileged functional group in
Tdifferent fields, spanning from medicinal chemistry to
polymer science. In fact, both internal and terminal alkynes are
frequently used in drugs due to their capability to modulate the
activity of several proteins,1 as well as for the late-stage
manipulation of organic molecules through click chemistry,2a−d
nowadays routinely used for bioconjugation. What is more,
alkynes are precious building blocks that offer direct access to a
plethora of functionalities, including alkenes, alkanes, halides,
and carbonyl compounds, among the others.3−5 Finally, the
CC triple bond plays a key role in materials science as well,
where the use of Mo- and W-based complexes has been
reported to catalyze alkyne metathesis polymerization.6
Accordingly, it comes as no surprise that diverse synthetic
strategies have been devised for the alkynylation of organic
molecules. Even though most reports deal with classical
nucleophilic and electrophilic alkynylation manifolds, a
conceptually different approach is offered by SOMOphilic
alkynylation.7 In this process, an organoradical generated
during the reaction is trapped by a suitable alkynylation
reagent and, after loss of a radicofugal group (e.g., a sulfonyl or
benziodoxolonyl radical), the hoped-for alkyne is obtained.7
The most direct and atom-economical way to form the
needed C-centered radical is through the homolytic cleavage of
a C−H bond via hydrogen atom transfer (HAT). In one
instance, AIBN (azobis(isobutyronitrile), Scheme 1a) was
exploited to trigger a thermal HAT, while the triflyl radical was
used as the radicofugal group. The fragmentation of the latter
intermediate generated an electrophilic trifluoromethyl radical
that sustained a radical chain process. The reaction could be
performed also in the absence of AIBN, due to the probable
presence of peroxides in the substrate, used as the solvent.8
A more controlled activation manifold is offered by
photocatalyzed HAT, which has secured its place as one of
Alkynylation reactions via direct C−H functionalization: thermal
HAT (a), previous examples of photoinduced HAT (b) and this work
(c). AIBN: azobis(isobutyronitrile). W: tetrabutylammonium deca-
tungstate, (Bu4N)4[W10O32]. CFL: Compact Fluorescent Lamp. Ts:
tosyl.
the most convenient and sustainable ways for the generation of
C-centered radicals.9 Thus, the excited state of the photo-
catalyst (PC) generated upon light absorption can be exploited
Received: February 1, 2021
Published: March 3, 2021
© 2021 The Authors. Published by
American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.1c00381
Org. Lett. 2021, 23, 2243−2247
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a
for the desired homolytic C−H bond cleavage to afford the
needed intermediate.10 Only a few precedents of direct
photoinduced C−H to C−C(sp) bond conversion are
available in the literature, employing benzophenone or 4,4′-
dichlorobenzophenone (Scheme 1b).11−14 Often, high loading
of these aromatic ketones is required to obtain reasonable
yields of the desired products and the reported protocols are
mainly restricted to the activation of rather labile α-to-O and
α-to-N C(sp3)−H bonds, H-donors being used in large excess
or even as the reaction medium. Very recently, phenylglyoxylic
acid (PGA) has been used as well to catalyze the alkynylation
of α-to-O C−H bonds under visible light irradiation (Scheme
1b). Interestingly, when S-heterocycles (e.g., tetrahydrothio-
phene, THT) were employed, a concomitant oxidative ring-
opening occurred.15
Scheme 2. Substrate Scope: H-donors
However, a general platform for the alkynylation of strong
aliphatic C(sp3)−H bonds remains an unmet goal. Building
upon our expertise in the use of TBADT (tetrabutylammo-
nium decatungstate, (Bu4N)4[W10O32]) as photocatalyst for
the functionalization of organic substrates via HAT,16,17 we
evaluated the use of this polyoxometalate to promote the
alkynylation of a variety of aliphatic H-donors, including
ethers, acetals, aldehydes, amides, a silane and even hydro-
carbons.
We identified the alkynylation of tetrahydrofuran (1a) by
((methylsulfonyl)ethynyl)benzene (2a) as our model reaction.
After a routine survey of reaction conditions (Table S1 in the
Supporting Information) in terms of solvent, photocatalyst
loading and light source, we found that the irradiation of an
acetonitrile solution containing 2a (0.1 M), 1a (5 equiv) and
TBADT (2 mol %) with a 40 W LED (λ = 390 nm) yielded 2-
(phenylethynyl)tetrahydrofuran 3 in 74% GC yield after 12 h
(Scheme S1a and Figure S1). The reaction proved to be
exceptionally robust, since it could be run under visible light
irradiation as well (λ = 405 nm, 18 W) or with a catalyst
loading as low as 0.5 mol %, with minimal changes in terms of
3 formation (GC yields consistently >60%), albeit 24 h
irradiation was required in the last case. Control experiments
showed that both light and the photocatalyst were needed to
trigger the observed reactivity, while quantum yield measure-
ments safely excluded a chain propagation mechanism (Φ =
0.07; see the Supporting Information). By reacting an
equimolar mixture of 1a and 1a-d8 with 2a, a kinetic isotope
effect (KIE) of 1.5 was observed, indicating the H-atom
transfer as the rate-determining step of the reaction (Scheme
S2 and Figure S2).
Gratifyingly, the model reaction proceeded smoothly on a
preparative scale (0.5 mmol) and allowed to isolate product 3
in 71% yield (67% when doubling the scale to 1 mmol). We
then screened several oxygenated heterocycles and obtained
products 4−9 in good yields, except for oxetane derivative 5
(Scheme 2). Thus, when 1,3-dioxolane (1d) was employed as
the H-donor, the reaction proceeded to deliver a 1:1 mixture of
isomers 6A and 6B (67% overall yield), resulting from the
functionalization at the acetalic and ethereal positions,
respectively. Differently, the selectivity diverted to the methine
C(sp3)−H in the case of 2-methyl-1,3-dioxolane (1e),
affording products 7A and 7B in 75% overall yield with a
3.3:1 ratio. This selectivity pattern has been previously
reported and can be rationalized based on the stability of the
involved radical intermediates.16b,18 When trioxane (1g) was
used in the role of H-Donor, in addition to the expected alkyne
a
Reaction conditions: an Ar-bubbled MeCN solution (5 mL)
containing 1 (5 equiv), 2a (0.5 mmol, 0.1 M), and TBADT
(2 mol %) was irradiated with an LED lamp (for further details,
b
see the Supporting Information). based on 61% consumption of 2a.
c
d
5 mol % TBADT was used. Reaction performed in the presence of 1
e
f
equiv of NaHCO3. 2 equiv of the silane was used. 4 equiv of DMF
was used. 1 equiv of the aldehyde was used. 10 equiv of the nitrile
was used. Brsm: based on remaining starting material.
g
h
9 (45% yield), the hydroalkylation adduct 9′ was isolated as
well in 26% yield.
We also achieved the functionalization of the α-to-S
C(sp3)−H bond of tetrahydrothiophene to give product 10
in 72% isolated yield, while no ring-opening of the sulfur-
heterocycle was observed.15 Silane 1i (see Chart S1 in the
Supporting Information) was likewise a competent substrate in
the reaction, as demonstrated by the isolation of 11 in 62%
yield. When dimethylformamide was subjected to our
conditions, a mixture of products was obtained in 43% overall
yield, with a significant preference for the functionalization of
the C(sp3)−H (12A) over the formyl C(sp2)−H bond (12B).
To study into greater detail the functionalization of formyl
C(sp2)−H bonds, we subjected heptaldehyde (1k) and 3-
phenylpropionaldehyde (1l) to our reaction conditions: while
product 13 was smoothly obtained in 67% yield from 1k, the
presence of weak benzylic C−H bonds in 1l enabled an
additional reaction path. Indeed, a mixture of 14 (43% yield)
and 14′ (32% yield) was obtained: while 14 derives from the
usual reactivity, we propose that 14′ is formed upon an
addition/1,5-HAT/ring-closure sequence (for further details,
see Scheme S4).
Finally, we extended the present synthetic protocol to
nitriles and cycloalkanes, to eventually achieve the alkynylation
of strong, unactivated, aliphatic C−H bonds. Upon increasing
the amount of TBADT to 5 mol %, the corresponding
unsaturated hydrocarbons were obtained in very good yields
(products 15−19 in Scheme 2, see also Scheme S1 and Figure
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a
S1). Interestingly, product 19 was obtained as a single
regioisomer.
Scheme 4. Survey of Different Radicofugal Groups
We next turned our attention to the scope of the
alkynylating reagent (Scheme 3) and reacted 1a with a series
a
Scheme 3. Substrate Scope: Methanesulfonyl Alkynes
a
Reaction conditions in (a) and (b) as indicated in Table S1 (GC
yields reported). Level of theory in (c): UωB97xD/def2tzvp in the
b
c
gas phase. Consumption of 2r: 83%. 31 was isolated running an
experiment on a 0.5 mmol scale: 41% (49% brsm) isolated yield.
(2q) were recovered unreacted, chloroalkyne 2r led to the
formation of product 31 only (41% isolated yield, Z isomer;
Scheme 4b), in accordance with the report by Hashmi.19
Intrigued by this marked difference in reactivity, we undertook
a computational analysis based on density functional theory
(DFT; UωB97xD/def2tzvp level of theory in the gas phase)
a
Reaction conditions: an Ar-bubbled MeCN solution (5 mL)
•
and compared the aptitude of the different RSO2 moieties vs
containing 1a (5 equiv), 2 (0.5 mmol, 0.1 M), and TBADT (2 mol
%) was irradiated with an LED lamp (for further details, see the
Supporting Information). Based on 86% consumption of 2g. Based
on 88% consumption of 2h. NMR yield, based on 46% consumption
Cl• in the role of radicofugal groups (Table S2). In particular,
the equation reported in Scheme 4c compares the tendency of
MeSO2 and Cl to stabilize the vinyl radical adduct with respect
to their intrinsic stability as free radicals. The largely positive
ΔG value is indicative of a more pronounced aptitude of
MeSO2 to act as a radicofugal group.
b
c
d
e
of 2l. Irradiation time: 36 h.
of methanesulfonyl alkynes (2b−l). The substitution pattern of
the aromatic ring was investigated by testing traps 2b−d at
first: the expected alkynes 20−22 were obtained in good yields
(62−79%), the para-derivative 2d offering the highest yield.
Different halogens were then tested and the corresponding
products 23 and 24 were isolated in 65% and 62% yield,
respectively. Interestingly, the electronic nature of the
substituent had a remarkable effect on the reaction: when
either a strongly electron-withdrawing (CF3 in 2g) or electron-
donating (OMe in 2h) group was present on the aromatic ring,
incomplete conversion of the starting sulfones was observed
(products 25 and 26). On the contrary, methanesulfonyl
alkynes containing milder electron-donating alkyl substituents
on the 4-position (2i,j) proved to be more efficient, and
products 27 and 28 were obtained in good yield (68% in both
cases). Biphenyl alkynyl sulfone 2k was a competent substrate
as well, allowing us to isolate product 29, albeit in a modest
yield (42%). Finally, we attempted the alkynylation of 1a with
an aliphatic derivative (2l), however, the yield of 30 did not
exceed 39% (by NMR, compound not isolated; 83% brsm)
upon prolonged irradiation (36 h).
At the time of writing, Hashmi and co-workers reported the
visible light-photocatalyzed hydroalkylation of chloroalkynes.19
Thus, we started wondering about the role of the radicofugal
group in the present methodology and reacted compounds
2m−r (see Chart S1) with 1a under optimized conditions. We
found that only sulfonyl alkynes gave product 3 in good yields
(Scheme 4a; reactivity order: 2a > 2n > 2m), while the well-
known benziodoxolone-based alkynylating reagent (2o, see
Chart S1) proved to be photochemically unstable under our
conditions and 3 was formed only in a poor yield (17%, data
not shown). Turning to halogenated derivatives, while
(iodoethynyl)benzene (2p) and (bromoethynyl)benzene
On the basis of the above, we propose the mechanism
shown in Scheme 5. The excited state of TBADT (W*) is
Scheme 5. Proposed Mechanism
populated upon UV (or visible) light irradiation, triggering the
cleavage of a C−H bond in the H-donor (e.g., 1a). The thus-
formed organoradical is readily trapped by the alkynylating
reagent (2) to deliver I•. From this point, two scenarios can be
envisaged. On one hand, the loss of a good radicofugal group
•
(e.g., RSO2 ) affords the expected alkyne. Thus, the recovery
of the spent photocatalyst (E[W/Wred] = −0.97 V vs SCE)20 is
entrusted to the highly oxidizing sulfonyl radical (e.g.,
21
•
−
E[MeSO2 /MeSO2 ] = +1.24 V vs SCE), albeit a back-
•
HAT step from Wred to RSO2 may be likewise operative. On
the other hand, when 2r is used as alkynylating reagent, the
elimination of the chlorine atom is disfavored (Scheme 4c) and
the hydroalkylation product 31 is formed via back-HAT or
sequential electron transfer/proton transfer (see Schemes
S3).19 This scenario is corroborated by deuterium labeling
experiments reported in Scheme S3.
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I.; Hong, V. P.; Finn, M. G. Copper-Catalyzed Azide−Alkyne Click
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In conclusion, a robust protocol for the radical alkynylation
of Si−H, formyl C(sp2)−H, and strong C(sp3)−H bonds via
photocatalyzed HAT triggered by the decatungstate anion has
been reported. Methanesulfonyl alkynes, most of which have
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convenient and more atom-economical alternative to common
agents for SOMOphilic alkynylations. The relative stability of
the radicofugal group has proved crucial to obtain the expected
alkynylated products.
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with Acetylenic Triflones. J. Am. Chem. Soc. 1996, 118 (18), 4486−
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Versatile Strategy for Substrate Activation in Photocatalyzed Organic
Synthesis. Eur. J. Org. Chem. 2017, 2017 (15), 2056−2071.
(10) Capaldo, L.; Quadri, L. L.; Ravelli, D. Photocatalytic Hydrogen
Atom Transfer: The Philosopher’s Stone for Late-Stage Functional-
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00381.
■
sı
Experimental details about the used materials and
reaction conditions, GC/MS, sample preparation,
product characterization, kinetic analysis, computational
details, DFT optimized geometries, and NMR spectra
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
Luca Capaldo − Flow Chemistry Group, Van’t Hoff Institute
for Molecular Sciences (HIMS), University of Amsterdam,
1098 XH Amsterdam, The Netherlands; PhotoGreen Lab,
Department of Chemistry, University of Pavia, 27100 Pavia,
Italy; orcid.org/0000-0001-7114-267X;
Email: l.capaldo@uva.nl
Author
Davide Ravelli − PhotoGreen Lab, Department of Chemistry,
University of Pavia, 27100 Pavia, Italy; orcid.org/0000-
0003-2201-4828
(12) Nagatomo, M.; Yoshioka, S.; Inoue, M. Enantioselective
Radical Alkynylation of C(sp3)-H Bonds Using Sulfoximine as a
Traceless Chiral Auxiliary. Chem.: Asian J. 2015, 10 (1), 120−123.
(13) Paul, S.; Guin, J. Radical C(sp3)−H Alkenylation, Alkynylation
and Allylation of Ethers and Amides Enabled by Photocatalysis. Green
Chem. 2017, 19 (11), 2530−2534.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00381
(14) Matsumoto, K.; Nakajima, M.; Nemoto, T. Visible Light-
Induced Direct S0 → Tn Transition of Benzophenone Promotes
C(sp3)−H Alkynylation of Ethers and Amides. J. Org. Chem. 2020, 85
(18), 11802−11811.
(15) Voutyritsa, E.; Garreau, M.; Kokotou, M. G.; Triandafillidi, I.;
Waser, J.; Kokotos, C. G. Photochemical Functionalization of
Heterocycles with EBX Reagents: C−H Alkynylation versus
Deconstructive Ring Cleavage. Chem. - Eur. J. 2020, 26, 14453−
14460.
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has received financial support from the MIUR (SIR
Project “Organic Synthesis via Visible Light Photocatalytic
Hydrogen Transfer”; code: RBSI145Y9R). Calculations were
carried out at the CINECA Supercomputer Center (Italy),
with computer time granted by ISCRA projects (code:
HP10C30YAJ). The authors gratefully thank Nicoletta Floridia
(University of Pavia) for fruitful discussion, Prof. Enrico
Monzani and Prof. Mariella Mella (University of Pavia) for
their support in NMR experiments.
(16) For reviews on tetrabutylammonium decatungstate, see:
(a) Ravelli, D.; Protti, S.; Fagnoni, M. Decatungstate Anion for
Photocatalyzed “Window Ledge” Reactions. Acc. Chem. Res. 2016, 49
(10), 2232−2242. (b) Ravelli, D.; Fagnoni, M.; Fukuyama, T.;
Nishikawa, T.; Ryu, I. Site-Selective C−H Functionalization by
Decatungstate Anion Photocatalysis: Synergistic Control by Polar and
Steric Effects Expands the Reaction Scope. ACS Catal. 2018, 8 (1),
701−713.
(17) For representative works on tetrabutylammonium decatung-
state, see: (a) Laudadio, G.; Deng, Y.; van der Wal, K.; Ravelli, D.;
Nun
̃
o, M.; Fagnoni, M.; Guthrie, D.; Sun, Y.; Noël, T. C(sp3)−H
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