Visible-Light-Mediated Regioselective Chlorosulfonylation of Alkenes and Alkynes: Introducing the Cu(II) Complex [Cu(dap)Cl2] to Photochemical ATRA Reactions

Article abstract of DOI:10.1021/acscatal.8b04188

A visible-light-mediated photocatalyzed protocol utilizing copper-phenanthroline-based catalysts has been developed that can convert a large number of olefins into their chlorosulfonylated products. Besides the Cu(I) complex [Cu(dap)₂]Cl, now well-established in photo-ATRA processes, the corresponding Cu(II) complex [Cu(dap)Cl₂] proved to be often even more efficient in the title reaction, being advantageous from an economic point of view but also opening up new avenues for photoredox catalysis. Moreover, the copper complexes outperformed commonly used ruthenium, iridium, or organic dye based photocatalysts, owing to their ability to stabilize or interact with transient radicals by inner sphere mechanisms. The use of stoichiometric Na₂CO₃ in combination with the copper photocatalysts was found to be essential to convert unactivated olefins to the desired products, in contrast to activated olefins for which no additive was required. As suggested by appropriate control experiments, the role of Na₂CO₃ is attributed to prevention of poisoning of the catalyst.

Full text of DOI:10.1021/acscatal.8b04188

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Article
Visible-Light-Mediated Regioselective Chlorosulfonylation
of Alkenes and Alkynes: Introducing the Cu(II) complex
[Cu(dap)Cl2] to photochemical ATRA reactions
Asik Hossain, Sebastian Engl, Eugen Lutsker, and Oliver Reiser
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04188 • Publication Date (Web): 21 Dec 2018
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ACS Catalysis
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Visible-Light-Mediated Regioselective Chlorosulfonylation of Alkenes
and Alkynes: Introducing the Cu(II) complex [Cu(dap)Cl2] to photo-
chemical ATRA reactions
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Asik Hossain‡, Sebastian Engl‡, Eugen Lutsker‡, and Oliver Reiser*
Institut für Organische Chemie, Universität Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany.
ABSTRACT: A visible light mediated photocatalyzed protocol utilizing copper-phenanthroline based catalysts has been developed, which can
convert a large number of olefins into their chlorosulfonylated products. Besides the Cu(I)-complex [Cu(dap)2]Cl, being by now well-estab-
lished in photo-ATRA processes, the corresponding Cu(II)-complex [Cu(dap)Cl2] proved to be often even more efficient in the title reaction,
being advantageous from an economic point of view, but also opening up new avenues for photoredox catalysis. Moreover, the copper com-
plexes outperformed commonly used ruthenium, iridium or organic dye based photocatalysts, owing to their ability to stabilize or interact with
transient radicals by inner sphere mechanisms. The use of stoichiometric Na2CO3 in combination with the copper photocatalysts was found to
be essential to convert unactivated olefins to the desired products contrasting activated olefins for which no additive was required. As suggested
by appropriate control experiments, the role of Na2CO3 is attributed to prevent poisoning of the catalyst.
KEYWORDS: photocatalysis, copper, ATRA reaction, hetero-difunctionalization, sulfone.
In synthetic organic chemistry, carbon-carbon and carbon-heteroa-
tom bond-forming reactions are fundamental to achieve the synthe-
sis of a desired product through consecutive increase in molecular
complexity. As a result, the development of new catalytic methodol-
ogies by employing inexpensive catalysts at low loading are highly
desirable in terms of sustainable chemistry. In this regard, visible
light photocatalysis1–9 mediated by metal complexes has become
useful to achieve unique transformations under mild reaction condi-
tions.10 Recently, this strategy has been utilized for the difunctional-
ization of carbon-carbon multiple bonds11 by various scientific
groups12–19 including our own,20–23 which offers unique opportunities
to diversify alkenes or alkynes for further transformations.
Scheme 1: Switchable bond formation using different sulfonyl chlo-
rides as ATRA reagent using copper photocatalysts.
Stephenson and co-workers53 describes this idea with two examples,
i.e. with the [Ru(bpy)3]Cl2 photocatalyzed addition of para-toluene-
sulfonylchloride (1a) to styrene (2a) or norbornene, while pioneer-
ing studies using CuCl or CuCl2 for the title reaction under thermal
conditions (≥ 100 °C) were reported as well.51,52
Sulfones are important motifs which can be found in many drugs
and natural products24–27. Many efforts28–31 have been made by vari-
ous scientific groups32–43 for the direct incorporation of a sulfone
functionality into organic molecules. Recently44,45 our group has dis-
covered that CF3SO2Cl can be used as an Atom Transfer Radical Ad-
dition (ATRA) reagent46,47 under photochemical condition using
[Cu(dap)2]Cl as a photocatalyst without extrusion of SO2. In this
case, copper(I)-complexes served as singular catalysts because of
their ability to interact with substrates either via direct ligand trans-
fer, or via a rebound mechanism involving organocopper(III)-
species.48 Intrigued by this unique transformation, we questioned
about the reactivity of other sulfonyl chlorides as ATRA reagents. A
single electron transfer (SET) from the excited state of a photocata-
lyst to an aryl or alkyl sulfonyl chloride may form a S-centered radi-
cal49–50 which can add to olefins and ultimately give rise to hetero-
difunctionalized products (Scheme 1). In fact, seminal work by
Thus, we also started our investigation with para-toluenesul-
fonylchloride (1a) (Ered = –0.94 V vs SCE) and styrene (2a) (1
equiv) in the presence of 1 mol % [Cu(dap)2]Cl54,55 (ECu(II)/Cu(I)* = –
1.43 V vs. SCE; dap = 2,9-bis(p-anisyl)-1,10-phenanthroline) as a
photocatalyst under visible light irradiation (λmax = 530 nm). We
were pleased to observe formation of the desired product 4a in an
excellent yield (Table 1, Entry 1) after 24 h. Unexpectedly, the anal-
ogous copper(II) complex [Cu(dap)Cl2]20 also produced the de-
sired product 4a in 95% yield (Table 1, Entry 2). Instead, with
[Ru(bpy)3]Cl2 (ERu(III)/Ru(II)* = –0.81 V vs. SCE; bpy = 2,2’-bipyri-
dine), highly reducing fac-[Ir(ppy)3] (EIr(IV)/Ir(III)* = –1.73 V vs. SCE;
ppy
=
2-phenylpyridine) or
[Ir{dF(CF3)ppy}2(dtbbpy)]PF6
(EIr(IV)/Ir(III)* = –0.89 V vs. SCE; dF(CF3)ppy = 2-(2,4-difluoro-
phenyl)-5-trifluoromethylpyridine, dtbbpy = 4,4’ - di-tert-butyl-2,2’
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Under the best conditions established for styrene (2a, Table 1,
Page 2 of 8
Table 1. Reaction Optimizationa
1
Entry 1), employing the unactivated olefin 3a gave only low
amounts of the desired ATRA product 5a (Table 1, Entry 7). How-
ever, when this reaction was performed in the presence of 1 equiva-
lent of K2HPO4, a drastic increase in product yield (60%) was ob-
served (Table 1, Entry 8). The use of stoichiometric amounts of
K2CO3 lowers the product yield to 18% (Table 1, Entry 9), while the
use of 1 equivalent Na2CO3 instead of K2CO3 increases the isolated
product yield to 92% (Table 1, Entry 10). Using 1 equivalent NaCl
as an additive instead again drastically decreases the yield of 5a to 8%
(Table 1, Entry 11), making it unlikely that the process is dependent
on Na+-cocatalysis. Given that the overall reaction is a net addition,
substoichiometric amounts of base should be sufficient, i.e. to scav-
enge traces of HCl that could form in the process. Nevertheless, re-
ducing the amount of Na2CO3 to30 mol % is accompanied by a de-
crease in product formation (55%, Table 1, Entry 12). Moreover,
when other photocatalysts were employed such as [Ru(bpy)3]Cl2,
fac-[Ir(ppy)3] or Na2-Eosin Y, poor yields were observed and nota-
bly, irrespective if Na2CO3 is used as an additive or not (Table 1, En-
tries 13–17). Again, the use of the copper(II)-catalyst [Cu(dap)Cl2]
was also possible, producing 5a in 72% isolated yield (Table 1, Entry
20) after 72 h irradiation. Control experiments proved the necessity
of both catalyst and light (Table 1, Entries 18-19) as well as the im-
portance of dap ligand in combination with copper salts for this
transformation (Entries 21-24, also see SI). Thus, the conditions es-
tablished in entry 1 and 2 (for activated olefins 2) and entry 10 and
20 (for unactivated olefins 3) were found to be best and were subse-
quently applied to explore the scope for this reaction. It should be
noted that the possibility of using [Cu(dap)Cl2] offers a considera-
ble cost advantage, given that only half the amount of dap ligand has
to be employed.
2
3
4
5
6
7
8
9
en-
try
1c
photocatalyst
(1 mol %)
additive
(equiv)
olefin
(equiv)
yieldb
[Cu(dap)2]Cl
[Cu(dap)Cl2]
No
No
2a (1)
2a (1)
2a (1)
2a (1)
2a (1)
2a (1)
96(96)
95
2
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3
[Ru(bpy)3]Cl2 No
80
4
fac-[Ir(ppy)3]
[Ir-F]
No
45
5
No
07
6
Na2-Eosin Y
[Cu(dap)2]Cl
[Cu(dap)2]Cl
[Cu(dap)2]Cl
[Cu(dap)2]Cl
[Cu(dap)2]Cl
[Cu(dap)2]Cl
No
NR
7
No
3a (1 or 2) 05 or 09
8
K2HPO4 (1)
K2CO3 (1)
Na2CO3 (1)
NaCl (1)
3a (2)
3a (2)
3a (2)
3a (2)
60
9
18
10c
11
12
13
14
15
16
17
18
19d
20c,e
21f,g
22f,g
97(92)
08
Na2CO3 (0.3) 3a (2)
55
[Ru(bpy)3]Cl2 Na2CO3 (1)
[Ru(bpy)3]Cl2 No
3a (2)
3a (2)
3a (2)
3a (2)
3a (2)
3a (2)
3a (2)
3a (2)
2a (1)
2a (1)
2a (1)
2a (1)
20
55
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
Na2-Eosin Y
No
Na2CO3 (1)
No
26
30
Na2CO3 (1)
Na2CO3 (1)
Na2CO3 (1)
Na2CO3 (1)
03
NR
NR
77(72)
NR
NR
NR
NR
For a wide variety of vinyl arene substrates 2, both Cu(I)- and
Cu(II)-dap catalysts could be successfully employed in the title re-
action ( Schemes 2a and 2b). We were pleased to observe that both
electron rich and electron poor sulfonyl chlorides 4a–4j underwent
the coupling with styrene 2a in high yields (80-98%, Scheme 2a).
Considering thiophene derived substrates a potential poison for the
copper(I)-photocatalyst, they nevertheless also provided the ATRA
products 4k–4m in excellent yields (>90%). Likewise, alkyl sulfonyl
chlorides and gave the desired ATRA products 4n–4p in high yields
(˃80%). Noteworthy, halogenated sulfonyl chlorides as well as al-
kylsulfonyl chlorides have been reported to be incompatible for such
ATRA reactions using [Ru(bpy)3]Cl2.54 Different styrene derivatives
in the coupling with tosyl chloride 1a as an ATRA reagent (Scheme
2b) were broadly applicable (4q–4u, 4w–4y, 84 – 97% yield) with
few limitations (4v). Alpha or beta alkyl substitution on the styrene
was also found to be suitable for this reaction (4z, 4aa, 4ac), while
stilbene showed poor reactivity giving only 7% yield of 4ab. Remark-
ably, when two new stereocenters are formed (4aa, 4ab, 4ac, 4ae,
5w), only one diastereomer was formed. Interestingly, both trans
and cis-beta methyl styrene provided a single diastereomer57 of 4aa
which upon treatment with triethylamine gave the corresponding E-
vinyl sulfone exclusively (Scheme 2d). Benzylic or allylic chlorides
showed no cross reactivity, allowing the isolation of 4ad in 97% and
of 4ae in 86%, yield, respectively. Notably, the Cu(II) catalyst
[Cu(dap)Cl2] performed in general at least as well but in a number
of cases significantly better (4d, 4e, 4f, 4p) compared to the Cu(I)
catalyst [Cu(dap)2]Cl.
[Cu(dap)2]Cl
[Cu(dap)Cl2]
CuCl or CuCl2 No
dap
No
No
No
23g,h CuCl/phen
24g,h CuCl2/phen
Reaction conditions: a1a (0.50 mmol, 1 equiv), photocatalyst (1 mol
%), in CH3CN (0.25 M) under N2 at room temperature (25–30 °C).
Reaction times were 24 h for 2a and 48 h for 3a. LEDs have been used
for irradiation (see SI). For [Cu] and Na2-Eosin Y, Green LED (λmax
=
530 nm) and for other photocatalysts Blue LED (λmax = 455 nm). b1H
NMR yields. cIsolated yields are in parenthesis. dReaction was performed
in the dark. 72 h reaction time. f5 mol% catalyst was employed. traces
e
g
(<6%) of product formation was observed when 3a was used in combi-
h
nation with Na2CO3 (see SI). 5 mol% copper salt with 10 mol% phen
was used. NR = No Reaction. phen = 1,10-phenanthroline.
-dipyridyl, [Ir-F]) or Na2-Eosin Y (EEY * = –1.11 V vs SCE) under
.+
/EY
irradiation the yield of the desired product 4a was found to be signif-
icantly lower (Table 1, entries 3 - 6) which is consistent with the re-
port by Stephenson and co-workers53 but surprisingly not consistent
with a more recent report,56 according to which 1a does not result in
any product formation in the title reaction. It should be noted that
the reduction potential of all catalysts is sufficient to generate the tol-
uylsulfonyl radical upon PET to TsCl (1a).
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Scheme 2: Scope of the reactiona.
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aGeneral reaction conditions: Sulfonyl chloride 1 (0.50 mmol, 1 equiv), [Cu(dap)2]Cl (1 mol %) or [Cu(dap)Cl2] (1 mol %) in CH3CN. Condition
A: Activated olefin 2 (0.50 mmol, 1 equiv), reaction time was 24 h unless otherwise noted. Condition B: Unactivated olefin 3 (1.00 mmol, 2 equiv),
Na2CO3 (0.50 mmol, 1 equiv). b48 h reaction time. csingle diastereomer, tentatively assigned analogously to syn-4aa. d72 h reaction time. e4 equiv olefin
was used. All reactions were performed under N2 atmosphere at room temperature (25–30 °C) with green LED (λmax = 530 nm). Isolated yields are
given.
Scheme 3: Unsuccessful olefins and reaction scale up.
Further screening of a number of substrates (see SI, Scheme S2)
confirmed the results detailed in Table 1, in which the copper pho-
tocatalysts outperform ruthenium and iridium-based ones as well as
organic dyes.
Moving to unactivated olefins 3a (Scheme 2c), the coupling with
a variety of arylsufonyl chlorides proceeded well, provided that the
reaction now is performed in the presence of 1 equiv of Na2CO3 (5a-
g). Various potentially sensitive functionalities like hydroxyl, car-
bonyl, N-Boc-amine, primary bromine etc. were tolerated well giv-
ing rise to 5g–5j, 5q–5t in 65%–96% isolated yields. A phenolic OH
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Scheme 4: Sequential functionalization of two different olefins.
Scheme 6: Further Transformations of ATRA products.
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Scheme 5: Scope for alkynesa.
isolated in good to excellent yields as a single regioisomer (8a-8d, up
to 97% isolated yields as a separable mixture of E and Z isomers). In
case of disubstituted alkyne 7e, the ATRA product was still formed,
however, the yield was moderate in this case (45-46%). However,
unactivated alkynes such as 1-hexyne were found to be inactive to-
wards this reaction, which is consistent with a recent report.61
To show the synthetic utility of this transformation, we tested
some of the ATRA products in their reactivity with bases. Upon
treatment of 5v with potassium tert-butoxide62 a twofold elimination
occurred giving rise to 9 (Scheme 6a), while weak bases (NEt3) re-
sulted in the formation of vinylsulfones24 10 in high yield (Scheme
6b). Vinyl sulfones have been proven to be valuable synthons in
asymmetric reactions63,64, cycloadditions65,66, and metalations,67 but
can also be transformed to alkynes,62 thus offering overall a route for
the dehydrogenation of alkenes.
aReaction condition: 1a (0.50 mmol, 1 equiv), alkyne 7 (0.50 mmol,
1 equiv) and copper photocatalyst (1 mol %) in CH3CN under N2 at-
mosphere at room temperature with green LED (λmax = 530 nm). Iso-
lated yields are given. E and Z ratios are based on the isolated products.
A plausible mechanism considering the unique role of the copper
catalysts in the title transformation is outlined in Scheme 7. When
the reaction was performed with [Cu(dap)Cl2] as a catalyst, we as-
sume that nevertheless a reduction to Cu(I) occurs being the cata-
lytically active species, for example via a visible-light-induced
group under the basic conditions is deprotonated and as a result we
could not obtain 5m, but the corresponding methyl or Boc-pro-
tected substrates cleanly gave rise to 5n and 5o. In general, ortho-
substitution decreases the reaction rates, as it was also observed in
the synthesis of 5p. Thiophene derived sulfonyl chlorides were again
found to be compatible with the reaction condition but required a
longer reaction time, allowing the isolation of 5u in 83% yield after
84 h irradiation. The limitation for unactivated olefins 3 is found for
the coupling with alkyl sulfonyl chlorides, which did not yield any
product. Strongly activated olefins (2q-2s) and cyclohexene 3q
yielded mostly polymerized products58 (Scheme 3a). Scale up with
either catalyst was demonstrated with the synthesis of 4a on gram
scale in excellent yields (>90%) without the need to prolong the re-
action time or the catalyst loading (Scheme 3b).
Scheme 7: Proposed reaction mechanism
Given the necessity for employing Na2CO3 as an additive for al-
kenes 3 for the chlorosulfonylation, we seized the opportunity to
perform the sequential functionalization of 6 (Scheme 4). Indeed,
6a could be obtained in the first step in 80% yield using TsCl (Con-
dition A) followed by a second ATRA reaction with PhSO2Cl (Con-
dition B) to give rise to 6b in 69% isolated yield. An even better over-
all yield (65%) was obtained when a single flask reaction was per-
formed in which 1 mol% [Cu(dap)2]Cl was found to be sufficient for
the sequential functionalization of the two different double bonds.
Next, we evaluated alkynes59,60,61 for the ATRA reaction with sul-
fonylchlorides (Scheme 5). Gratifyingly, phenylacetylene derived
substrates 7 were suitable substrates and the ATRA products were
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ATRA reactions, being beneficial from an economic point of view by
Scheme 8: Catalyst stability testa
1
2
3
4
5
6
7
8
requiring only half the amount of dap ligand compared to
[Cu(dap)2]Cl.
i) [Cu(dap)2]Cl (1 mol%)
without Na2CO3
24 h, rt, 530 nm
+
5a
4a
TsCl
+
+
ASSOCIATED CONTENT
Ph
ii) then 2a (1 equiv)
24 h, rt, 530 nm
(47%)
(7%)
3a (1 equiv)
1a
Supporting Information. Experimental details, further optimization,
product yield comparisons with different photocatalysts, characteriza-
tion data, mechanistic experiments and copies of NMR spectra of new
compounds and X-Ray of compound 5a. This material is available free
of charge via the Internet at http://pubs.acs.org.
i) [Cu(dap)2]Cl (1 mol%)
without Na2CO3
24 h, rt, 530 nm
5a
+
4a
TsCl
Ph
9
ii) then 2a (1 equiv)
and Na2CO3 (1 equiv)
24 h, rt, 530 nm
(13%)
3a (1 equiv)
(82%)
1a
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AUTHOR INFORMATION
Corresponding Author
*E-mail: oliver.reiser@chemie.uni-regensburg.de
a) 1H NMR yields with diphenylmethane as internal standard.
homolytic cleavage of the Cu(II)-Cl bond68,20 (Scheme 7a) forming
a Cu(I) intermediate I. I might get coordinated either by another
dap ligand with chloride as a counter anion (which results in the for-
mation of [Cu(dap)2]Cl) or by the solvent69. I in its excited state I*
can form sulfonyl radicals IV upon one electron reduction of sul-
fonyl chloride 1 (Scheme 7b). In turn, a Cu(II) species of type II
such as [Cu(dap)Cl2] is formed, which has been independently syn-
thesized from CuCl2 and dap20 being found to be a capable photo-
catalyst for the title reaction in this study as well. The radical IV can
add to the olefin forming a C-centered radical V which takes back
chlorine from II concurrent with the regeneration of catalyst I (path
a, black arrow). Intermediate V could also bind to the Cu(II) species
II (path b, pink arrow), being a persistent radical, to give rise to a
Cu(III) intermediate70,71 VI. Reductive elimination from VI leads to
the formation of the desired product and the active catalyst I is re-
generated by trapping the free ligand.
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by GRK 1626 (Chemical Photocatalysis), the
Studienstiftung des Deutschen Volkes and the Fonds der Chemischen
Industrie (fellowship for E.L.). We thank Ms. B. Hischa and Dr. U.
Chakraborty (both UR) for the X-Ray analysis of 5a.
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We surmised that the distinct requirement for employing hetero-
geneous inorganic bases, in particular Na2CO3, which has also been
shown to be beneficial in other photoredox reactions,44,46,47 might
also prevent the poisoning of the copper catalysts. When a mixture
of 1a and 3a was irradiated in absence of Na2CO3 for 24 h (Scheme
8c) followed by addition of styrene (2a) with continuing irradiation
for another 24 h, the formation of only 47% (1H NMR yield) of 4a
was observed along with 7% 5a. When the experiment was repeated
but Na2CO3 was added together with styrene, the respective yields
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heterogeneous base for the copper catalyst. The more reactive vi-
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process, apparently prevent catalyst deactivation as well, since these
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chain mechanism less likely72,73 and supports the rebound mecha-
nism.
In conclusion, we have developed a highly efficient, first row tran-
sition metal based photocatalytic protocol to convert a large variety
of olefins to their corresponding vicinal chlorosulfonylated adducts.
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tion reaction which produced vinyl sulfones in high yields. On the
other hand, double elimination can produce alkynes, thus, the sul-
fonylchloride mimics Br2 traditionally used for the conversion of al-
kenes to corresponding alkynes. Evidence that the role of inorganic
bases, especially Na2CO3, that are often employed as additives in
photoredox catalyzed processes prevent catalyst poisoning is pro-
vided. Last but not least, the bench stable Cu(II)-complex
[Cu(dap)Cl2] is shown for the first time to promote efficiently
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