Direct Cyclopropanation of α-Cyano β-Aryl Alkanes by Light-Mediated Single Electron Transfer Between Donor–Acceptor Pairs

Article abstract of DOI:10.1002/chem.202100341

Cyclopropanes are traditionally prepared by the formal [2+1] addition of carbene or radical based C1 units to alkenes. In contrast, the one-pot intermolecular cyclopropanation of alkanes by redox active C1 units has remained unrealised. Herein, we achieve

Full text of DOI:10.1002/chem.202100341

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Direct Cyclopropanation of a-Cyano b-Aryl Alkanes by Light-
Mediated Single Electron Transfer Between Donor–Acceptor Pairs
Jing Li,[a] Martin J. Lear,[b] and Yujiro Hayashi*[a]
Abstract: Cyclopropanes are traditionally prepared by the
formal [2+1] addition of carbene or radical based C1 units
to alkenes. In contrast, the one-pot intermolecular cyclo-
propanation of alkanes by redox active C1 units has re-
mained unrealised. Herein, we achieved this process
simply by exposing b-aryl propionitriles and C1 radical
precursors (N-oxy esters) to base and blue light. The over-
all process is redox-neutral and a photocatalyst, whether
metal- or organic-based, is not required. Our findings sup-
port that single electron transfer (SET) from the a-cyano
carbanion of the propionitrile to the N-oxy ester is facili-
tated by blue-light via their electron donor–acceptor
(EDA) complex. The a-cyano carbon radical thus formed
can then lose a b-proton to form a p-resonance stabilised
radical anion that preferentially couples at the benzylic b-
position with a decarboxylated C1 radical unit. This new
transition metal-free chemistry tolerates both electron rich
and electron deficient (hetero)aryl systems, even sulfide or
alkene functionality, to afford a range of cis-aryl/cyano cy-
Figure 1. (a) Cyclopropanation of alkene feedstocks; (b) Cyclopropanation of
alkane feedstocks. EWG=electron withdrawing group. Lg=leaving group.
clopropanes bearing congested tetrasubstituted quaterna-
ry carbons.
adopt addition-elimination processes (Figure 1a, right).[8,9]
representative example is the reaction of sulfur ylides with
enones (the Corey–Chaykovsky reaction).[8] Recently, the addi-
tion of radicals to alkenes followed by intramolecular polar dis-
placement, as catalysed by photoredox additives, provides a
new entry for the preparation of cyclopropanes.[9] To date,
however, direct intermolecular cyclopropanation methods rely
heavily on alkenes as the starting materials (Figure 1a) or
resort to the intramolecular transition metal mediated CÀH ac-
tivation of alkanes.[10] To the best of our knowledge, there are
no reports for the direct intermolecular cyclopropanation of al-
kanes with C1 components in an overall redox neutral manner
without transition metal catalysts. Moreover, it is a significant
challenge to prepare sterically congested cyclopropanes
flanked by contiguous quaternary stereocenters.[8b] In this con-
text, we disclose a direct method to access cis-aryl/cyano cy-
clopropanes from readily available alkanes with redox active
esters as C1 components (Figure 1b).
A
Cycloalkanes feature in a wide variety of bioactive com-
pounds[1] and natural products,[2] and are regarded as highly
versatile intermediates in synthesis.[3] Indeed, the development
of new methods to access 3-membered cyclopropyl rings con-
tinues to test organic chemists on the bench today.[4] The most
traditional and robust methods to prepare cyclopropanes in-
volve reacting alkenes with free carbenes or carbenoids (C1
components) in a formal [2+1] cycloaddition (Figure 1a, left).
Representative examples include the generation of carbenoids
from diethylzinc and halomethanes (the Simmon–Smith reac-
tion)[5] or by using transition metals with diazo compounds,[6]
hyperiodo ylides[7] and so forth.[8] Other reliable approaches for
the cyclopropanation of aryl and electron-deficient alkenes
[a] Dr. J. Li, Prof. Y. Hayashi
Department of Chemistry, Graduate School of Science
Tohoku University, Aza-Aoba, Aoba-ku, Sendai 980-8578 (Japan)
E-mail: yujiro.hayashi.b7@tohoku.ac.jp
This investigation was inspired by our recent oxidative stud-
ies of a-substituted malononitriles using O2 and neutral nucle-
ophiles (Figure 2a).[11] Detailed mechanistic studies demonstrat-
ed that a single electron transfer (SET) process occurs between
a-anionic malononitriles and molecular oxygen to produce a-
carbon radicals en route to the formation of acyl cyanides.[11a]
[b] Dr. M. J. Lear
School of Chemistry, University of Lincoln
Brayford Pool, Lincoln LN6 7TS (UK)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202100341.
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Table 1. Optimisation of the reaction conditions.[a]
Entry
2
Base
Solvents
Yield (%)[b]
1
2
3
4
5[c]
6
7
8
2a
2b
2c
2d
2a
2a
2a
2a
2a
2a
2a
2a
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
no base
NaOtBu
K2CO3
KOtBu
KOtBu
KOtBu
KOtBu
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
61
<5
51
<5
<5
<5
57
55
54
10
<5
73
9
10
11
12[d]
CH3CN
THF
DMSO
Figure 2. (a) Oxidative amidation/esterification using O2; (b) Unexpected cy-
clopropanation. NPhth=N-phthalimide. DMSO=dimethyl sulfoxide.
[a] Reaction was carried out with 1a (0.2 mmol), 2a–d (1.3 equiv), base
(1.1 equiv) in DMSO (2 mL) at 358C for 20 h. [b] Isolated yields. [c] Reac-
tion was carried in the dark. [d] 2a (1.5 equiv) and KOtBu (1.2 equiv) were
used.
The acyl cyanide can then react with different nucleophiles
such as amines, alcohols and thiols.[11b] Given a suitable elec-
tron acceptor, under the right conditions, we reasoned that
the a-anion of benzyl malononitrile 1a would undergo a SET
to KOtBu (entry 7 and 8). Solvents such as DMF, CH3CN and
event to afford the corresponding carbon-centred radical 1a’ THF resulted in low efficiency in this system (entry 9–11). Final-
(Figure 2b). As a suitable partner for this SET event, we select-
ed the N-phthalimido-N-oxy ester 2a as the electron acceptor.
Here, the radical anion of 2a would undergo an entropy fav-
oured decarboxylative expulsion of an N-phthalimide anion to
release the O-stabilised radical 2a’.[12] The complementary radi-
cals 1a’ and 2a’ were then expected to couple homogenically
at the a-position to afford the sterically congested adduct 3. In
practice, however, treatment of benzyl malononitrile 1a and
the N-oxy ester 2a with KOtBu in DMSO gave no desired
adduct 3 or reaction. Given recent observations that visual
light can facilitate SET processes for electron donor–acceptor
(EDA) charge-transfer complexes,[13] 1a and 2a were exposed
to blue light in the presence of KOtBu. Although no trace of
adduct 3 was observed, the cyclopropane 4a was isolated in
moderate yield. This discovery revealed, for the first time, an
unprecedented transition metal-free procedure to form cyclo-
propanes in one-pot by the redox coupling of matched cya-
noalkane donors and C1 acceptors.
ly, the yield of the desired product 4a was improved to 73%
when the amount of KOtBu and 2a was increased to 1.2 and
1.5 equivalents, respectively (entry 12).
With an optimal method in hand, we investigated the scope
of this new cyclopropanation method. As shown in Figure 3a,
the redox-active ester 2a can react with both a-benzyl malo-
nonitrile and a-benzyl cyanoethanoate to afford the desired
cyclopropanes 4a and 4d in good yields, respectively. In addi-
tion, this process exhibited excellent chemoselectivity for elec-
tron rich (NHAc, 4e; -OMe, 4 f) and electron poor (-CF3, 4g;
-CN, 4h) para-substituted phenyl groups, providing cyclopro-
pane products in good yields (60–89%) as single geometric
isomers.[8b] Aryl halogens (F, Cl, Br; 4i–4k) also displayed excel-
lent tolerance to these reaction conditions. Similar functional
group compatibilities were exhibited among aromatic sub-
strates with substituents at the meta- and ortho-positions (4l,
4m). Fused aromatics (4n), pyridine (4o) and thiophene (4p)
moieties were tolerated by this method in good yield. Besides
aromatic N- and S-functionality, this cyclopropanation method
was also found compatible with non-activated alkenes as dem-
onstrated with the O-allylic ester (4r).
While arylmethyl malononitriles successfully gave cyclopro-
panes, presumably due to aryl stabilisation of intermediates,
alkyl malononitriles lacking b-aryl groups gave no reaction and
unsubstituted or a-monosubstituted derivatives of 2 resulted
in low product yields, due to competing coupling between the
liberated C1 radicals and the liberated N-phthalimide by-prod-
uct (see supporting information for details). Nevertheless, ex-
pansion of this method to a,a-disubstituted redox-active
esters 2 was found to be possible (Figure 3b). Cyclic systems
performed particularly well and afforded a range of spirocyclo-
To capitalise on this discovery in a more general context, we
selected a-benzylmalononitrile 1a as the model substrate to
optimise the formation of cyclopropane 4a (Table 1). We found
that the reaction depended strongly on the type of leaving
group (Lg) on compounds 2. Whereas no product was ob-
served with 2b (Lg=OCOCF3) and 2d (Lg=Br), 2c (Lg=OBz)
provided the desired product 4a in a lower yield than 2a
(Lg=OAc) (cf. entry 1 and 3). We thus decided to employ 1a
and 2a for further optimisation studies. When the reaction was
carried out in the dark (entry 5) or in the absence of a base
(entry 6), no desired product 4a was observed. Next, we
turned our attention to screening the bases and solvents. Sev-
eral bases such as NaOtBu and K2CO3 gave lower yields relative
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Figure 3. Scope of the photoinduced decarboxylative cyclopropanation of alkanes. For 5g–i, dr ratios were determined by H NMR analysis of crude material.
propanes in excellent yields (5a–5d). The spirocyclopropane
product 5e is notable because competing reactions involving
sulfur were not observed under the transition metal-free condi-
tions, which prevails in metal carbenoid chemistry.[14] In addi-
tion, the a-allylic redox ester (2) bearing a competing alkene
functionality gave the cyclopropane 5g in moderate yield.
Overall, excellent chemoselectivity and functional group toler-
ance were observed, although difficult-to-remove minor
amounts of unreactive alkene side-products occurred in some
instances, giving trace impurities in the spectroscopic data (see
Supporting Information).
Second, when the reaction was carried out at 58C instead of
358C with the preformed potassium salt of 1a, the desired cy-
clopropanation product 4a was formed in 10% yield together
with 41% of the b-alkylated acetate 6a (Figure 4, Eq. (1)).
Third, when acetate 6a was exposed to KOtBu without light,
the desired cyclopropane 3a was generated smoothly in excel-
lent yield (Eq. (2)). Fourth, reaction of the 1a salt with the all-
carbon a-substituted version 7 of 2a gave the b-alkylated pro-
duct 8a in high yield (Eq. (3)). These observations and control
reactions strongly indicated the acetate 6a to be a key inter-
mediate and that both light and base were essential for its
generation. Further control reactions were carried out to sup-
port the formation of b-alkylated intermediates like 6a and
products like 8a (Figure 4, Eq. (4)). Radical trapping experi-
ments were conducted by using salt 1a and the redox ester of
citronellic acid 9. Under standard conditions, the reaction af-
forded the b-substituted products 8b and 8c in 83% and
<5% yields, respectively. This result indicated the intermediacy
of a carbon-centred radical 10 and its cyclisation to a more
stable tertiary radical 11[15] before coupling to the b-position of
1a. Alkene-based or Knoevenagel-like substrates remain un-
We next turned our attention to the reaction mechanism
(Figure 4). First, in the absence of blue light (465 nm) to acti-
vate an EDA complex or a base to form an EDA complex, the
decarboxylative cyclopropanation of 1a and 2a was complete-
ly suppressed (Table 1, cf. entry 5 and 6). Indeed, formation of
the possible EDA complex between the active ester 7 and the
K+ salt of 1a was confirmed in CH3CN by UV/Vis absorption
spectroscopy, whereby the dramatic colour change correlated
to a large bathochromic shift in the absorption spectrum (Fig-
ure 4a; see Supporting Information for more details).[13a]
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Figure 5. Proposed photoinduced radical-anion crossover events between a-
anion of 1 and electron-acceptor 2 to afford cyclopropane 4 or 5.[12–18]
photoinduced single electron transfer (SET) event, when paired
as its electron-donor/acceptor (EDA) complex.[13,16] This SET
event consequently generates the b-acidic a-radical 1’ and the
radical anion of 2, which fragments to the C1 a-radical 2’ via
release of CO2 and N-phthalimide anions.[12] Loss of the b-
proton from the a-radical 1’ then affords the a/b-radical anion
of 1’.[17,18] Next, intermolecular homogenic coupling between
the radical anion 1’ and the O-stabilised radical 2’ forges a
new Csp3ÀCsp3 bond at the b-position and the resulting a-anion
of 6 cyclises to furnish the corresponding cyclopropane 4 or 5.
In summary, we disclose a new photosynthetic method to
directly couple cyano alkanes (1) with electron accepting C1
carbon units (2) to yield functionalised cyclopropanes (4/5) in
a redox-neutral and highly chemoselective manner. The inter-
molecular reaction does not necessitate transition metals,
photo-catalysts or alkene substrates or intermediates (just ex-
posure to a base and light). Mechanistic evidence supports the
reaction to involve a photoinduced single electron transfer
(SET) event by virtue of an electron-donor/acceptor (EDA) com-
plex being formed between the anion of the a-substituted ma-
lononitrile or cyanoethanoate (1) and the redox ester (2). This
SET event from the a-anion of 1 induces a decarboxylative
fragmentation of 2 to form stabilised radical species of 1’ and
2’ en route to b-carbon homogenic coupling and intramolecu-
lar SN displacement of the acetate from 6. In this way, a new in-
termolecular entry for the preparation of aryl cyclopropanes 4/
5 flanked by two adjacent quaternary stereocenters is realised,
which circumvented the need for rare transition-metals to b-
functionalise carbonyl compounds via CÀH bond activation
mechanisms.[19] Given the ubiquity and intermediacy of aryl cy-
clopropanes in bioactive natural products and drugs,[1–3] we an-
Figure 4. Evidence demonstrating electron donor–acceptor (EDA) pairing
and initial radical coupling at the benzylic b-position for a-anionic salts of
malononitrile 1a.
reactive under the conditions studied and are considered un-
likely intermediates in the mechanism.
Based on the above control experiments, a plausible mecha-
nism for this new cyclopropanation reaction is proposed in
Figure 5. First, the potassium salt of 1 is generated in situ via
deprotonation with KOtBu in DMSO. The generated a-anion of
1 can then react with the N-hydroxyphthalimide ester 2 via a
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Acknowledgements
The authors are grateful to Prof. Shen Ye and Dr. Ken-ichi
Inoue (Tohoku University) for the invaluable studies of UV/Vis
absorption spectroscopy. This work was supported by JSPS KA-
KENHI Grant Number JP20H04801 in Hybrid Catalysis for Ena-
bling Molecular Synthesis on Demand, and JP19H05630.
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The authors declare no conflict of interest.
Keywords: alkane · cyclopropanation · decarboxylation
radical · single electron transfer
·
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Manuscript received: January 30, 2021
Revised manuscript received: February 8, 2021
Accepted manuscript online: February 9, 2021
Version of record online: March 5, 2021
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