Diastereoselective Photoredox-Catalyzed [3 + 2] Cycloadditions of N-Sulfonyl Cyclopropylamines with Electron-Deficient Olefins

Article abstract of DOI:10.1021/acs.orglett.1c00711

A highly diastereoselective, visible-light-induced [3 + 2] cycloaddition between N-sulfonyl cyclopropylamines and electron-deficient olefins is reported. The reactions proceed via the oxidation of a sulfonamide aza-anion by an organic photocatalyst to generate a nitrogen-centered radical. Strain-induced ring opening and intermolecular addition to the olefin generate an intermediate carbon-centered radical that is reduced to an anion prior to 5-exo cyclization. This enables a highly diastereoselective construction of trans-cyclopentanes possessing synthetically useful functional groups.

Full text of DOI:10.1021/acs.orglett.1c00711

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Letter
Diastereoselective Photoredox-Catalyzed [3 + 2] Cycloadditions of
N‑Sulfonyl Cyclopropylamines with Electron-Deficient Olefins
Dawn H. White, Adam Noble, Kevin I. Booker-Milburn,* and Varinder K. Aggarwal*
Cite This: Org. Lett. 2021, 23, 3038−3042
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ABSTRACT: A highly diastereoselective, visible-light-induced [3 +
2] cycloaddition between N-sulfonyl cyclopropylamines and
electron-deficient olefins is reported. The reactions proceed via
the oxidation of a sulfonamide aza-anion by an organic photocatalyst
to generate a nitrogen-centered radical. Strain-induced ring opening
and intermolecular addition to the olefin generate an intermediate
carbon-centered radical that is reduced to an anion prior to 5-exo cyclization. This enables a highly diastereoselective construction of
trans-cyclopentanes possessing synthetically useful functional groups.
yclopropanes possess unique reactivity due to their
inherent torsional and angular strain, with the release of
centered radicals (Scheme 1b).11,12 When subjected to violet-
light irradiation, a diradical was formed from the imine, which
initiated the strain-driven homolysis of the cyclopropane and
enabled [3 + 2] cycloadditions with styrenes. However, the
construction of monocyclic cyclopentanes by intermolecular
cycloaddition proceeded with no diastereoselectivity.12 More
recently, the groups of Jiang and Ooi independently published
enantioselective variants of Zheng’s original protocol (Scheme
1c,d).13,14
Whereas the published protocols provide approaches to
access amino-cyclopentanes in both a diastereoselective and
enantioselective fashion, they are predominantly reliant upon
the use of styrene-based alkenes as the radical acceptor.
Additionally, only Ooi and coworkers employed a protecting
group strategy, where the urea directing group could be
cleaved. Both factors greatly restrict the potential for
downstream synthetic modification. To address these issues,
it was our vision to develop a protocol that employed
protected cyclopropylamines and could be applied to a broad
range of non-styrene-based alkenes, therefore allowing the
synthesis of cyclopentanes with functional synthetic handles.
Herein we report the first [3 + 2] cycloaddition of N-sulfonyl
cyclopropylamines with electron-poor olefins to give trans-
substituted cyclopentanes with high diastereoselectivities using
visible-light photoredox catalysis.
C
ring strain being utilized as a driving force for a diverse range of
reactions.1−3 Cyclopropylamines, in particular, demonstrate a
variety of possible reaction pathways depending on the
conditions employed, as the nitrogen serves as an important
handle for activation.4 Donor−acceptor (DA) cyclopropanes
can be subjected to a variety of cycloadditions or ring-opening
reactions, whereas the transition-metal-catalyzed cleavage of
cyclopropylamines can access a range of small- to medium-
sized heterocycles.2−4 Cyclopropylamines can also be
employed in [3 + 2] cycloadditions with alkenes to generate
amino-cyclopentane motifs through a radical mechanism.
Traditionally, this approach has relied on the use of strong
oxidants or stoichiometric photooxidants in the presence of
UV light to activate the nitrogen via radical cation formation,
which initiates β-scission of the strained cyclopropane ring.5−7
Whereas these methodologies have laid the foundations for
future developments, the harsh radical-generating conditions
often result in overoxidation and low product yields. Modern
visible-light photoredox catalysis provides a mild approach to
combat these synthetic challenges and has facilitated many
developments into [3 + 2] annulations.8−14
In 2012, Zheng and coworkers reported the first visible-light
photoredox-catalyzed [3 + 2] cycloaddition of cyclopropyl-
amine derivatives (Scheme 1a).9a The use of N-aryl cyclo-
propylamines permitted direct oxidation to an amine radical
cation, which triggered cyclopropane opening and subsequent
trapping of the resulting carbon-centered radical with alkenes
or alkynes, affording cyclopentane or cyclopentene products,
respectively.9,10 Although modest diastereoselectivity could be
achieved when generating 5,5-fused ring systems, little or no
diastereoselectivity was observed when using styrene deriva-
tives or acrylonitrile in the construction of monocyclic
structures. Stephenson and coworkers developed an alternative
approach, where imines were employed as masked nitrogen-
We began our investigation by considering potential
protecting groups on the cyclopropylamine. One of the main
drawbacks of exchanging the aryl motif on the cyclopropyl-
Received: March 1, 2021
Published: April 6, 2021
https://doi.org/10.1021/acs.orglett.1c00711
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3038−3042
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Letter
a
Scheme 1. Photoinduced Construction of Cyclopentanes
from Cyclopropylamines
Table 1. Optimization Studies
entry
base
solvent
concentration (M)
d.r.
% yield
b
1
2
3
4
5
6
Na3PO4
Na3PO4
Na3PO4
K2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
none
DCM
MeCN
DMSO
DCM
DCM
DCM
DCM
DCM
DCM
DCM
0.1
0.1
0.1
0.1
80:20
67:33
84:16
84:16
84:16
84:16
84:16
52
20
21
25
56
75
0.1
0.025
0.025
0.025
0.025
c
b
7
82
c d
,
8
9
0
0
c e
,
c
10
0.025
trace
a
1
Yields and d.r. were determined by H NMR analysis using 1,4-
b
dinitrobenzene as the internal standard. Isolated yields after column
chromatography. With Na2SO4 (2 equiv). Reaction performed in
the dark. Without 4CzIPN.
c
d
e
utilizing N-aryl cyclopropylamines and styrenes, which
proceeded with low diastereoselectivity, we were delighted
that 2 was formed with high trans selectivity. (See the SI for
details of the analysis.) Control experiments confirmed that a
photoredox mechanism is operative, as no product was
observed in the absence of a light source or photocatalyst
(entries 8 and 9). Additionally, only trace product was formed
in the absence of a base (entry 10), highlighting the
importance of generating the active radical species through
oxidation of the corresponding sulfonamide anion. We also
tested alternative carbamate protecting groups (Boc, Cbz)
under the optimized conditions, but no reaction was observed.
(See the SI for details.) This highlights the importance of
having either a strong electron-withdrawing group (Ts) on
nitrogen, to enable the generation of an aza-anion, or an aryl
group on nitrogen, both of which facilitate subsequent single-
electron oxidation.
Exploration of the scope began by examining the effect of
the olefin substituents on the reaction (Scheme 2). It was
found that changing the alkyl group on the acrylate had no
negative effect on the yield or diastereoselectivity (2−6);
however, bulkier substituents gave enhanced trans selectivity.
Acrylonitrile gave high yields of product 7, but a drop in the
diastereoselective bias was observed, which can be attributed to
the small size of the nitrile group (vide infra). Acrylamides
were also tolerated, providing products 8−10 with excellent
diastereoselectivities. Other electron-withdrawing functional-
ities on the olefin also proved successful, including sulfones
(11), aryl ketones (12), and phosphonate esters (13). Whereas
sulfone 11 and phosphonate 13 were formed with excellent
diastereoselectivity, phenyl vinyl ketone gave 12 with lower
selectivity. We subsequently explored the use of 1,1-
disubstituted alkenes. Pleasingly, the cyclization of various
methacrylates proceeded well, despite increased steric bulk, to
afford more complex trisubstituted cyclopentanes 14−17.
Although modest yields were obtained, superb levels of
diastereoselectivity were observed for the construction of
these quaternary center-containing products. Interestingly,
vinyl methacrylate chemoselectively provided product 17 in
reasonable yield, despite the presence of the less hindered vinyl
group, which highlights the importance of alkene electronics in
amine for a carbamate or sulfonamide is that the oxidation
potential of the amino group dramatically increases (e.g., Ep/2
vs SCE in MeCN is 0.92 V for N-phenyl cyclopropylamine13
and 2.24 V for N-tosyl cyclopropylamine 1), making the
generation of nitrogen radical cations by single-electron
oxidation challenging for these substrates. However, in the
case of sulfonamides, the acidity of the NH allows
deprotonation followed by facile oxidation of the resulting
aza-anion.15,16 Therefore, we envisioned that N-sulfonyl
cyclopropylamines could undergo a stepwise deprotonation
and oxidation event to initiate a [3 + 2] cycloaddition with
alkenes under mild photoredox conditions.
To investigate the feasibility of this proposal, we studied the
reaction of N-tosyl cyclopropylamine 1 with ethyl acrylate
(Table 1). The organic photocatalyst 2,4,5,6-tetrakis(9H-
carbazol-9-yl) isophthalonitrile (4CzIPN) was chosen because
it can be readily synthesized and proves to be more cost
efficient than transition-metal counterparts.17,18 We were
pleased to observe that using sodium phosphate as the base
and DCM as the solvent gave amino-cyclopentane 2 in 52%
yield after irradiation with blue light-emitting diodes (LEDs)
for 18 h (entry 1). The evaluation of various bases and solvents
led to the identification of potassium phosphate and DCM as
the optimal choices (entries 2−5). The reaction concentration
proved to be a critical factor, as changing from 0.1 to 0.025 M
enhanced the yield of 2 significantly to 75% (entry 6). This is
likely a result of the lower local concentration of olefin
minimizing deleterious polymerization pathways. A drying
agent additive of sodium sulfate also enhanced the yield of 2 to
an optimal 82% (entry 7). In contrast with previous reports
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a
a
Scheme 2. Alkene Scope
Scheme 3. Sulfonamide Scope
a
Performed using 0.1 mmol of N-sulfonylcyclopropylamine. Yields are
of isolated products after chromatographic purification. The d.r. was
b
determined by LCMS analysis of crude reaction mixtures. Performed
using 0.1 mmol of N-((1R,2R)-2-butylcyclopropyl)-4-methylbenze-
c
nesulfonamide. 1,4-selectivity was 54:46.
a
Performed using 0.1 mmol of 1. Yields are of isolated products after
chromatographic purification. The d.r. was determined by LCMS
good selectivity, and trifluoromethyl sulfonamide derivative 31
could be accessed with excellent selectivity. Finally, the effect
of substitution on the cyclopropane ring was briefly surveyed.
We were pleased to observe that the reaction of a trans-
disubstituted cyclopropylamine provided 1,2,4-trisubstituted
cyclopentane 32 in moderate yield and with high 1,2-trans
selectivity; however, no diastereoselectivity was observed at the
four-position.
b
analysis of crude reaction mixtures. Performed using 1.0 mmol of 1,
no Na2SO4, and a reaction concentration of 0.05 M. Performed using
0.2 mmol of 1 and a reaction concentration of 0.1 M. Major
diastereomer not assigned.
c
d
determining the rate of intermolecular radical addition of the
nucleophilic alkyl radical intermediate. We next explored the
reactivity of enoates bearing β-substituents. Diethyl ethyl-
idenemalonate provided tetrasubstituted cyclopentane 18 in
good yield with excellent 1,3-trans selectivity, whereas the
methyl crotonate-derived cyclopentane 19 was formed with
low diastereoselectivity. Interestingly, methyl cinnamate
provided cyclopentane 20 with excellent diastereoselectivity
and with the reverse regioselectivity,19 albeit in low yield.
Given the importance of sulfonamides in medicinal
chemistry,20 we investigated the effect of different sulfonyl
substituents on the cyclopropylamine substrate (Scheme 3). A
wide array of functionalities were found to be tolerated,
including ortho, meta and para substitution patterns (21−29).
The transformation was insensitive to the electronics of the
aryl substituent, with both electron-donating and electron-
withdrawing groups providing the products in high yield with
good trans diastereoselectivity. Additionally, ortho-, meta-, and
para-halides could be implemented (24, 28, and 29), providing
useful handles for further derivatizations. A pyridyl sulfona-
mide gave the corresponding product 30 in good yield with
We next turned our attention to the mechanism of the
cycloaddition reaction. All previously reported photoredox-
catalyzed [3 + 2] cycloadditions of styrene derivatives were
proposed to proceed through a radical cyclization of an
intermediate benzylic radical onto an imine.9−14 However,
under our optimal conditions, styrenes proved unsuccessful
(see the SI for details), whereas electron-deficient olefins
excelled. This led us to suspect that the intermediate
electrophilic radical, formed upon radical addition to the
electron-deficient olefin, was first reduced to the corresponding
stabilized anion prior to cyclization. This was investigated
using allylic acetate 33 (Scheme 4a), which would give
cyclopentane 36 if a radical cyclization of intermediate 34
occurred prior to the single-electron reduction of the nitrogen-
centered radical 35 (path A) but would form imine 38 under
an anionic cyclization pathway due to the rapid elimination of
the acetate group from anion 37 (path B). Interestingly,
cyclopentane 36 was observed in only trace amounts, whereas
the major product was allylic sulfonamide 39, formed via the
reaction of p-toluenesulfonamide with 33. This provides
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steric repulsion between the N-sulfonyl imine and the electron-
withdrawing group (Scheme 4c), where conformer 46 suffers
from unfavorable steric and dipole−dipole interactions, thus
cyclization occurs preferentially through conformer 47 to
provide high selectivity for the trans product.22
Scheme 4. Mechanistic Studies and Proposed Mechanism
Finally, we aimed to demonstrate the synthetic utility of the
cyclopentane products (Scheme 5). Reduction of the ester
Scheme 5. Product Derivatizations
functionality in cyclopentane 2 proceeded smoothly to afford
alcohol 51 in 83% yield, with further esterification being
performed to obtain crystalline derivative 52 used for structural
determination. Saponification and Curtius rearrangement gave
trans-diamino cyclopentane 50 with two different protecting
groups for selective modification. The nosyl-protected 23
underwent sulfonyl deprotection under mild conditions to
form primary amine 48 in 75% yield, allowing the installation
of alternative protecting groups, such as carbamate 49.
In summary, we have developed a protocol for visible-light
photocatalytic [3 + 2] cycloadditions of N-sulfonyl cyclo-
propylamines with electron-deficient olefins to afford novel
cyclopentane ring systems in high yields and with excellent
trans diastereoselectivity. The scope of the alkene partner is
broad and can be extended to the construction of challenging
quaternary centers in a diastereoselective fashion. The robust
nature of the process, coupled with mild reaction conditions,
allows entry to a wide variety of cyclopentane products bearing
useful synthetic handles for subsequent downstream mod-
ification.
indirect evidence of the formation of imine 38, which can be
hydrolyzed by adventitious water to produce p-toluenesulfo-
namide. This observation suggests the 5-exo cyclization
predominantly occurs through an anionic mechanism,
although the formation of trace amounts of 36 suggests that
a minor radical cyclization pathway could also occur. On the
basis of these results, we propose the mechanism shown in
Scheme 4b. After the initial deprotonation of sulfonamide 1 by
the phosphate base, the single-electron oxidation of aza-anion
40 (Ep/2 = 1.18 V vs SCE in MeCN) by photoexcited 4CzIPN
(E1/2 = 1.49 V vs SCE in MeCN)17 generates the nitrogen-
centered radical 41.16,21 Strain-induced ring-opening of the
cyclopropane in 41 occurs through β-scission to form alkyl
radical 42. Intermolecular trapping with an electron-deficient
olefin, followed by single-electron reduction of the resulting
radical 43 by the reduced state of the photocatalyst regenerates
ground-state 4CzIPN and provides the stabilized carbanion 44.
Facile 5-exo cyclization of the nucleophilic carbanion onto the
electrophilic N-sulfonyl imine and protonation deliver the
substituted cyclopentane product 45. We hypothesized that
the diastereoselective bias can be attributed to electronic and
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* Supporting Information
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AUTHOR INFORMATION
Corresponding Authors
■
Varinder K. Aggarwal − School of Chemistry, University of
Bristol, Bristol BS8 1TS, United Kingdom; orcid.org/
0000-0003-0344-6430; Email: v.aggarwal@bristol.ac.uk
Kevin I. Booker-Milburn − School of Chemistry, University of
Bristol, Bristol BS8 1TS, United Kingdom; orcid.org/
0000-0001-6789-6882; Email: k.booker-milburn@
bristol.ac.uk
Authors
Dawn H. White − School of Chemistry, University of Bristol,
Bristol BS8 1TS, United Kingdom
Adam Noble − School of Chemistry, University of Bristol,
Bristol BS8 1TS, United Kingdom; orcid.org/0000-
0001-9203-7828
(13) Yin, Y.; Li, Y.; Gonca̧ lves, T. P.; Zhan, Q.; Wang, G.; Zhao, X.;
Qiao, B.; Huang, K.-W.; Jiang, Z. All-Carbon Quaternary Stereo-
centers α to Azaarenes via Radical-Based Asymmetric Olefin
Difunctionalization. J. Am. Chem. Soc. 2020, 142, 19451−19456.
(14) Uraguchi, D.; Kimura, Y.; Ueoka, F.; Ooi, T. Urea as a Redox-
Active Directing Group under Asymmetric Photocatalysis of Iridium-
Chiral Borate Ion Pairs. J. Am. Chem. Soc. 2020, 142, 19462−19467.
(15) Wang, X.; Xia, D.; Qin, W.; Zhou, R.; Zhou, X.; Zhou, Q.; Liu,
W.; Dai, X.; Wang, H.; Wang, S.; Tan, L.; Zhang, D.; Song, H.; Liu,
X.-Y.; Qin, Y. A Radical Cascade Enabling Collective Syntheses of
Natural Products. Chem. 2017, 2, 803−816.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00711
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the EPSRC (DTA award) for funding. We
thank Dr. Natalie Pridmore (University of Bristol) for X-ray
crystallography analysis and Sayad Doobary (University of
Bristol) and Mark Deeprose (University of Bristol) for
assistance with conducting cyclic voltammetry experiments.
(16) Tanaka, H.; Sakai, K.; Kawamura, A.; Oisaki, K.; Kanai, M.
Sulfonamides as New Hydrogen Atom Transfer (HAT) Catalysts for
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Halogens in Donor−Acceptor Cyanoarenes. J. Am. Chem. Soc. 2018,
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