Visible-Light-Induced Vicinal Dichlorination of Alkenes through LMCT Excitation of CuCl2

Article abstract of DOI:10.1002/ange.202010801

This work demonstrates photoredox vicinal dichlorination of alkenes, based on the homolysis of CuCl₂ in response to irradiation with visible light. This catalysis proceeds via a ligand to metal charge transfer process and provides an exciting opportunity for the synthesis of 1,2-dichloride compounds using an inexpensive, low-molecular-weight chlorine source. This new process exhibits a wide substrate scope, excellent functional group tolerance, extraordinarily mild conditions and does not require external ligands. Mechanistic studies show that the ready formation of chlorine atom radicals is responsible for the facile formation of C?Cl bonds in this synthetic process.

Full text of DOI:10.1002/ange.202010801

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Zitierweise: Angew. Chem. Int. Ed. 2020, 59, 23603–23608
Internationale Ausgabe: doi.org/10.1002/anie.202010801
Deutsche Ausgabe: doi.org/10.1002/ange.202010801
Chlorination
Visible-Light-Induced Vicinal Dichlorination of Alkenes through
LMCT Excitation of CuCl2
Pengcheng Lian, Wenhao Long, Jingjing Li, Yonggao Zheng, and Xiaobing Wan*
Abstract: This work demonstrates photoredox vicinal dichlori-
nation of alkenes, based on the homolysis of CuCl2 in response
to irradiation with visible light. This catalysis proceeds via
a ligand to metal charge transfer process and provides an
exciting opportunity for the synthesis of 1,2-dichloride com-
pounds using an inexpensive, low-molecular-weight chlorine
source. This new process exhibits a wide substrate scope,
excellent functional group tolerance, extraordinarily mild
conditions and does not require external ligands. Mechanistic
studies show that the ready formation of chlorine atom radicals
mediate (Scheme 1b).[6b] Recently, Linꢀs group developed the
Mn-catalysed radical dichlorination of alkenes using electric-
ity as the primary energy source and supplying chlorine via
MgCl2 (Scheme 1c).[7] Despite such successes, the majority of
existing dichlorination methods have certain disadvantages,
including the use of hazardous oxidants in stoichiometric
quantities, the need for harsh conditions, and narrow sub-
strates scopes. The diversity of naturally ccurring polychlor-
inated compounds,[8] particularly chlorosulfolipids, poses
significant challenges and requires the development of more
practical techniques for the vicinal dichlorination of alkenes.
Specifically, a wider range of substrates and milder reaction
conditions would be beneficial. Our group proposes that
catalysis driven by visible light has promise as an approach to
devising creative and powerful strategies to address these
issues.
Compared to traditional thermal reactions, visible-light-
induced reactions are typically carried out under more
sustainable and milder conditions (typically room temper-
ature).[9] Over the past decade, great progress has been made
in this field, which has become an important complement to
its thermal counterpart. More importantly, it has become
possible to achieve chemical conversions that are either
difficult or impossible to induce by non-photochemical
processes. Reactions promoted by visible light are often
associated with the use of d6 transition-metal complexes,
including those based on RuII and IrIII, which undergo metal-
to-ligand charge transfer (MLCT) such that small molecules
can be activated to construct more complex compounds. In
sharp contrast, the use of ligand-to-metal charge transfer
(LMCT) is underdeveloped and only a few examples of the
À
is responsible for the facile formation of C Cl bonds in this
synthetic process.
Chlorination reactions are among the oldest and most
fundamental organic transformations, and they have been
extensively studied.[1] As an example, the free radical
substitution reaction of methane with molecular chlorine is
commonly taught in undergraduate courses and is a standard
process for the industrial synthesis of chlorinated solvents.[2]
First discovered in 1877,[3] the vicinal dichlorination of
carbon–carbon double bonds has attracted much attention
owing to the importance of the dichlorinated products.
However, the use of molecular chlorine as the chlorination
reagent suffers from inherent drawbacks, including the high
toxicity and corrosiveness of the chlorine and the harsh
conditions and poor selectivity associated with the reactions.
Consequently, a series of alternatives have been developed
based on the ionic dichlorination reactions of alkenes.[4] These
include the Willgerodt reagent (PhICl2),[4c] SO2Cl2,[4d] the
Mioskowski reagent (Et4NCl3),[4w] NCS-PPh3 (the Yoshimitsu
reagent),[4n] and combinations of chloride salts and oxi-
dants,[4l,m,o,x,z] among others (Scheme 1a). These electrophilic
addition reactions typically exhibit anti selectivity as a result
of the formation of a chloriranium ion as the reaction
intermediate. Enantioselective versions of such reactions
have been well-established, and have been reported by the
Borhan,[5a] Snyder,[5e] Hennecke,[5f] Nicolaou,[5d] and
Burns[5b,c,g] groups. In 2015, a pioneering study demonstrating
the catalytic syn-dichlorination of alkenes was performed by
Denmark and co-workers based on a key selenium inter-
[*] P. Lian, W. Long, J. Li, Y. Zheng, Prof. Dr. X. Wan
Key Laboratory of Organic Synthesis of Jiangsu Province
College of Chemistry, Chemical Engineering and Materials Science
Soochow University
199 Ren-Ai Road, Suzhou, Jiangsu 215123 (P. R. China)
E-mail: wanxb@suda.edu.cn
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202010801.
Scheme 1. Vicinal dichlorination of alkenes.
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application of this process in synthetic chemistry have been
reported in recent years.[10] In nature, chlorine typically exists
in the form of inorganic chloride salts, and so it would be ideal
to use Earth-abundant chloride salts as chlorine sources under
photocatalytic conditions. However, this remains a formidable
challenge given the fact that the oxidation potential of the
chloride anion is much higher than the oxidation potentials of
common photocatalysts. We envisioned that chlorine radicals
could be generated under visible light as a result of the LMCT
excitation of chloride salts and could attack alkenes to
Having established the optimised conditions, we set out to
explore the scope of this reaction. As depicted in Scheme 2,
a variety of substituents, both electron-withdrawing (NO2,
SO2, carbonyl, CN, ester, CF3, F, Cl, Br, I) and electron-
donating (phthalimide, N-hydroxyphthalimide, acetal, Me, t-
Bu, ether) were well-tolerated in this vicinal dichlorination
reaction, and the desired products 3–31 could be isolated in
À
moderate to good yields. Even sulphonamides having free N
H groups reacted well to afford the dichlorinated products in
satisfactory yields (7 and 8). Interestingly, the presence of
heteroatoms such as oxygen and sulfur did not significantly
decrease the efficiency of this reaction, such that the desired
dichlorides 17 and 28 were obtained in moderate yields. In
particular, alkenes containing oxidatively labile amine groups
were transformed into the dichloride product 18, providing an
opportunity for further elaboration. The reaction also pro-
ceeded using thioate and pentafluorobenzene substrates to
provide
a novel yet elegant dichlorination technique
(Scheme 1d).
Based on the previously demonstrated facile LMCT
excitation of cerium salts,[10b,e–i,l] we initially examined the
application of CeCl3·7H2O to the dichlorination of the
aliphatic alkene 1. However, none of the desired product 2
was obtained upon irradiation with a 38 W white LED
(Supporting Information, Table S1, entry 1). A variety of
chloride salts, including those based on main group and
transition metals, were subsequently assessed but none
delivered a significant amount of 2 (for details, see Table S1
in the Supporting Information).
In 1962, Kochi demonstrated that an unfiltered mercury
radiation could promote CuCl2 homolysis, then worked with
a variety of substrates to deliver corresponding products.[11a]
More recently, the ability of Cu-based complexes to function
as visible light photocatalysts has been recognized and
extensively investigated, leading to exciting new possibilities
for unprecedented chemical transformations.[12] Encouraged
by these pioneering works, we examined the use CuCl2 to
induce the vicinal dichlorination of alkenes via a photoredox
process. Using this approach, the desired product 2 was
obtained in 82% yield (Supporting Information, Table S1,
entry 11), which appeared to show the feasibility of our
hypothesis. We subsequently reduced the amount of the Cu
salt that was employed and incorporated a low molecular
weight nucleophilic chlorine source to promote the desired
dichlorination reaction. After extensive screening, the opti-
mized conditions were determined to comprise the use of
CuCl2 (20 mol%) as the catalyst and hydrochloric acid
(2.5 equiv) as the chlorine source in 0.5 mL MeCN under
air with irradiation by a 38 W white LED for 72 h. Under
these optimized conditions, 1 was transformed into 2 with
a high 85% yield. Standard LMCT catalysis normally requires
ligands to accelerate the homolysis of complexes.[10] In
contrast, the present CuCl2-catalyzed process successfully
transferred the chlorine atom to the alkene substrate without
requiring exogenous ligands. Blank experiments showed that
dichlorination did not proceed in the absence of light, even
with heating (Supporting Information, Table S1, entries 17
and 18). Switching the solvent to acetone resulted in a slight
decrease in the yield of product 2 (Supporting Information,
Table S1, entry 19). Furthermore, a good yield of 2 was
obtained under oxygen (Supporting Information, Table S1,
entry 20). Notably, this photocatalytic process generated
water as the only side product and proceeded under mild
conditions, both of which are quite different from traditional
electrophilic chlorination methods.
Scheme 2. [a] Reaction conditions: alkenes (0.2 mmol), CuCl2
(0.04 mmol, 0.2 equiv), HCl (0.5 mmol, 2.5 equiv, 37% in water) in
MeCN (0.5 mL) irradiation with 38 W white light LEDs at ambient
temperature for 72 h. [b] 1.0 mL MeCN was used. [c] 1.0 mmol HCl
(37% in water) was used.
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give moderate yields (22 and 23). A diene was also found to
and electrostatic repulsion provided by the previously instal-
be a viable substrate, affording the desired product 19 in
a diminished but synthetically useful yield.
To further highlight the utility of our method, the reaction
was conducted on a larger scale (5 mmol, 1.1 g), and readily
afforded the 1,2-dichloride product 12 in a 63% yield
[Eq. (1)].
led Cl substituent, such that the CuCl2 preferentially attacks
from the opposite side.[13] Various substituted styrenes were
also assessed with regard to this transformation, with the
results shown in Scheme 3. The reaction proved generally
applicable to mono-substituted, 1,2-disubstituted and 1,1-
disubstituted alkenes. Styrenes with electron-rich, electron-
poor or sterically hindered substituents on the aromatic ring
were tolerated, giving dichlorides in moderate to excellent
yields. The chemoselectivity of this protocol is especially
significant, as free carboxyl, ether, fluoro, chloro, bromo,
iodo, ester, nitro, and trifluoromethyl groups were all readily
tolerated. Aliphatic alkenes were also suitable substrates for
this reaction (46).
A variety of preliminary studies were carried out to gain
information regarding the associated reaction mechanism.
Subsequently, chalcone was employed to investigate the
stereochemistry of this vicinal dichlorination reaction. Only
a small amount of the dichloride product was obtained owing
to the generation of undesired by-products. However, the 1,2-
trans-dichloride product 32 was achieved as the single
stereoisomer in 70% yield when CuCl2 was used as the
The
addition
of
2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO) was found to inhibit the formation of 2 (Sche-
me 4a) and electrospray ionization–high-resolution mass
spectrometry showed that both chlorine and aliphatic radicals
were trapped by this compound. Similar result was obtained
when diphenylethylene was used as the trapping reagent.
À
chlorination reagent under
a
nitrogen atmosphere
Notably, benzylic and aliphatic C H bonds could be activated
(Scheme 3). Both (E)-stilbene and indene delivered the
desired dichloride with high stereoselectivity (33, 34), as
anticipated. We attribute this stereoselectivity to the steric
by the chlorine radical via hydrogen atom abstraction, leading
to the corresponding products in moderate yields (Sche-
me 4b). Additional insights were obtained from a radical
clock experiment with a compound having a cyclopropane
moiety (54) that was found to open to generate a linear 1,3-
dichloride (55) in 18% yield (Scheme 4c). Further mecha-
nistic research also proved the difference and superiority of
Scheme 3. Reaction conditions: alkenes (0.2 mmol), CuCl2 (0.8 mmol,
4.0 equiv) in MeCN (5 mL) irradiation with 38 W white light LEDs at
ambient temperature for 72 h.
Scheme 4. Probe for the possible mechanism.
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our method with the previous methods (for details, see
Acknowledgements
Section 4 in the Supporting Information). Based on these
results, we propose that the visible-light-induced homolysis of
CuCl2 and subsequent chlorine atom transfer are responsible
This work was funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions
(PAPD), NSFC (grant numbers 21971175 and 21572148).
We thank Michael Judge, MSc, from Liwen Bianji, Edanz
Editing China (www.liwenbianji.cn/ac), for editing the Eng-
lish text of a draft of this manuscript.
À
for the C Cl bond formation. Harsh conditions are typically
required to generate chlorine radicals, such as the excitation
of molecular chlorine by UV light. Although the homolysis of
high-valence transition metals[10n,o,q,14] and the electrolysis of
manganese salts[7] have been shown to effectively generate
these radicals, the present method is mild, employs visible
light and uses commercially available CuCl2.[15]
Conflict of interest
On the basis of the above, a plausible mechanism was
presented in Scheme 5. Initially, the irradiation of CuCl2 with
visible light produces an excited state (CuCl2*). LMCT then
takes place to generate chlorine atom radical A, which rapidly
adds to the alkene to produce the C-centered radical B.
Finally, the reaction of radical B and CuCl2 forms the
dichloride product and CuCl.
The authors declare no conflict of interest.
Keywords: alkenes · cupric chloride · dichlorination · LMCT ·
photoredox
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In summary, this work achieved the first-ever vicinal
dichlorination of alkenes promoted by visible light. This
process is mechanistically distinct from, and thus comple-
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experiments demonstrated that the LMCTexcitation of CuCl2
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dichloride products in generally good yields. Further inves-
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formations based on the LMCT excitation of CuII complexes
are currently underway in our laboratory.
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[15] During the preparation of our manuscript, the group of Hwang
reported visible light-induced aerobic oxidation of diarylalkynes
catalysed by CuCl2; for details, see Ref. [11i].
Manuscript received: August 6, 2020
Accepted manuscript online: September 11, 2020
Version of record online: October 25, 2020
23814 www.angewandte.de
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Angew. Chem. 2020, 132, 23809 –23814