Use of hypervalent iodine reagents in visible light-promoted α-ketoacylations of sulfoximines with aryl alkynes

Article abstract of DOI:10.1021/acs.orglett.0c03338

In visible light, sulfoximidoyl-containing hypervalent iodine reagents react with aryl alkynes to give N-α-ketoacylated sulfoximines in good to high yields. The process is metal- and base-free, providing the diketonic products without the use of highly ox

Full text of DOI:10.1021/acs.orglett.0c03338

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Letter
Use of Hypervalent Iodine Reagents in Visible Light-Promoted
α‑Ketoacylations of Sulfoximines with Aryl Alkynes
Chenyang Wang, Ding Ma, Yongliang Tu, and Carsten Bolm*
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ABSTRACT: In visible light, sulfoximidoyl-containing hypervalent
iodine reagents react with aryl alkynes to give N-α-ketoacylated
sulfoximines in good to high yields. The process is metal- and base-
free, providing the diketonic products without the use of highly
oxygenated reagents such as peroxides. Results from mechanistic
investigations suggest the intermediacy of radicals and reveal the
importance of molecular oxygen.
In light of these reports and on the basis of our long-lasting
interest in sulfoximine chemistry,10,11 we started wondering
about N-linked α-ketoamide-type structures of type 1. Such
molecules have already been described, but in general
accessing them involves harsh conditions with transition
metal catalysis and strong oxidizing agents for the couplings
and conversions of sp3 carbons to the corresponding carbonyl
fragments (Scheme 1c).12 By a single transformation, we also
demonstrated that N-alkynylated sulfoximines can be applied
as starting materials; there as well, however, a metal complex
{[Ru(p-cymene)Cl2]2} was applied as the catalyst.13 We now
devised a metal-free approach toward 1 by reacting
sulfoximidoyl-containing hypervalent iodine reagents 2 and
arylalkynes 3 under visible light (Scheme 1d).
Hypervalent iodine compounds have gained significant
attention in recent years, and their reactivity has thoroughly
been studied.14 Derivatives with iodine-bound sulfoximidoyl
groups have only recently been described,15 and their
applicability still needs to be investigated. Under photo-
catalysis, they appear to react via nitrogen-centered radicals as
suggested in N-alkylation reactions.15c,d,f Guided by that
experience, the I-tosyl derivative 2a was chosen as the
sulfoximidoyl source in the present study. The first screening
and optimization of the reaction conditions (Table 1) were
done with phenylacetylene (3a) as the coupling partner. Both
reactants were applied in a 1:1 ratio. In the dark at either room
temperature or 60 °C for 12 h in DCE, no reaction occurred
(Table 1, entry 1). However, performing the same experiment
under light generated by an incandescent light bulb furnished
α-Ketoamides are of profound relevance in medicinal
chemistry.1 Both natural products, such as FK 506 or
rapamycine,2 and synthetic molecules, such as Boceprevir
and Narlaprevir,3 or other peptidomimetic ketoamides4 proved
highly important in addressing life-threatening diseases. Most
recently, corona- and enteroviruses were targeted by such
compounds.5 For the preparation of α-ketoamides, various
protocols have been developed, including α-ketoacylations of
amines with carboxylic acids or acyl halides6 and the use of
oxidative couplings starting from glyoxal derivatives7 or other
carbonyl compounds.8 For applications of alkynyl derivatives,9
the following works are representative: Al-Rashid and Hsung
described oxidations of ynamides with dimethyldioxirane
(DMDO, Scheme 1a),9a and Shah used mixtures of iodine
and TMSOTf in DMSO for amidations of terminal aryl alkynes
leading to α-ketoamides (Scheme 1b).9c
Scheme 1. Preparation of α-Ketoamides
Received: October 6, 2020
© XXXX The Authors. Published by
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American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Investigation of the Substrate Scope
b
entry
solvent
yield (%)
c
1
DCE
DCE
d
2
13
61
3
DCE
4
5
6
7
8
9
10
11
12
DMSO
MeOH
CHCl3
acetone
MeCN
THF
toluene
DMF
DCM
DCM
trace
n.d.
89
20
68
trace
n.d.
trace
90
e
f
13
99 (89)
a
Reaction conditionsare as follows: 2a (0.20 mmol), 3a (0.20 mmol),
b
1
and solvent (3 mL) in air for 24 h. As determined by HNMR
spectroscopy with dibromomethane as the internal standard; n.d. =
not determined. In the dark. In light generated by an incandescent
c
d
e
f
light bulb. Used 1.5 equiv of 2a (0.3 mmol). Yield after column
chromatography.
product 1aa in a 13% yield (as determined by NMR
spectroscopy using an internal standard; Table 1, entry 2).
This result could be improved by applying a 24 W LED as the
light source, which gave 1aa in a 61% yield (Table 1, entry 3).
In a solvent screen (Table 1, entries 3−12), DCE, chloroform,
and DCM were identified as superior over other solvents, with
the latter being the best and providing 1aa in a 90% yield
(Table 1, entry 12). Finally, changing the ratio of 2a and 3a
from the previously used 1:1 to 1.5:1 led to 1aa in a 99%
(NMR) yield. After column chromatography, 89% of 1aa was
obtained (Table 1, entry 13).
Scheme 2. First, substrates with S-aryl-S-methyl cores were
applied. All yields of the resulting products 1ba−ga were good
(73−89%), revealing a high structural tolerance of the process.
Neither steric nor electronic effects were apparent. The yields
for products 1ha−ka ranged from 73 to 92%, showing that also
S,S-diaryl and S-alkylated derivatives of 2 could be successfully
applied. Finally, combinations of an aryl alkyne containing 4-
tert-butyl 3f and hypervalent iodine reagents 2m and 2l with 3-
methoxy and 3-methyl substituents, respectively, were tested,
and in those cases products 1mf (85%) and 1lf (89%) were
also isolated in high yields.
To evaluate the generality of the process, various
combinations of hypervalent iodine reagents 2 and aryl alkynes
3 were studied under the previously identified optimal reaction
conditions (Table 1, entry 13). The results are shown in
Scheme 2. First, 2a was reacted with a range of aryl alkynes. In
general, all reactions proceeded well, affording the correspond-
ing products 1aa-ao in yields ranging from 60 to 95%. The
lowest yield in this series was observed for product 1ah with an
electron-withdrawing 4-cyano group, which suggested that
electronic effects induced by substituents on the alkyne arene
were relevant. This assumption was supported by the yields of
1ae and 1af with electron-donating 4-methyl and 4-tert-butyl
groups, respectively, which were both obtained in a 90% yield.
Steric effects appeared to play a minor role, as revealed by a
comparison of the results of 4-, 3-, and 2-bromo-substituted
products 1ad (81%), 1aj (79%), and 1al (78%), respectively,
all of which were obtained in about the same yield.
Additionally, 2-naphthyl- and 2-thiofuryl-substituted alkynes
3n and 3o, respectively, reacted well with 2a, leading to the
corresponding products 1an and 1ao in 95% and 83% yields,
respectively.16
With the goal to elucidate the relevant mechanistic steps of
the newly found process, a number of test reactions were
carried out (Scheme 3). Again, iodine reagent 2a and
phenylacetylene (3a) were used as representative substrates.
First, the presence of 3 equiv of TEMPO (2,2,6,6-
tetramethylpiperidinooxy) or BHT (2,6-di-tert-butyl-4-methyl-
phenol) inhibited the formation of 1aa, suggesting that those
radical scavengers intercepted a pathway involving relevant
radicals.17 Second, also under argon, product 1aa was not
formed, revealing the central role of air in the process. Third,
Scheme 3. Control Experiments
For assessing the influence of the hypervalent iodine reagent
structure, reactions of a range of compounds 2 with phenyl
acetylene (3a) were studied. Those results are shown in
B
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the latter result was confirmed by running the reaction under
dioxygen, which led to 1aa in a 91% yield. Fourth and finally,
as mentioned already, light was essential, and no product
formation occurred in the dark.
ACKNOWLEDGMENTS
■
C.W., D.M., and Y.T. thank the China Scholarship Council for
predoctoral stipends.
Considering those observations and the results from
previous reports,9 a reaction pathway as shown in Scheme 4
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can be proposed. The photoexcitation of the hypervalent
iodine reagent 2 results in homolytic bond cleavage, generating
sulfoximidoyl radicals A and B. The addition of A to aryl
alkyne 2 forms the vinyl radical C, which reacts with dioxygen
to give the superoxo species D. Ring-closure of D followed by
sequential electron transfer and deprotonation by B provide
the strained intermediate F, which ring-opens to give the
observed N-α-ketoacylated sulfoximine 1 as the final product.
In summary, we discovered a light-induced α-ketoacylation
of sulfoximines starting from hypervalent iodine reagents and
aryl alkynes. Compared to other methods, the reaction
conditions are mild, leading to the products in good to high
yields without the need for metal catalysts or strong oxidants.
A wide range of substituents is tolerated.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03338.
Experimental procedures, characterization data, and
NMR spectra for new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Carsten Bolm − Institute of Organic Chemistry, RWTH Aachen
University, D-52074 Aachen, Germany; orcid.org/0000-
0001-9415-9917; Email: Carsten.Bolm@oc.rwth-aachen.de
Authors
Chenyang Wang − Institute of Organic Chemistry, RWTH
Aachen University, D-52074 Aachen, Germany
Ding Ma − Institute of Organic Chemistry, RWTH Aachen
University, D-52074 Aachen, Germany
Yongliang Tu − Institute of Organic Chemistry, RWTH Aachen
University, D-52074 Aachen, Germany
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03338
Notes
The authors declare no competing financial interest.
C
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