Alkene Difunctionalization Triggered by a Stabilized Allenyl Radical: Concomitant Installation of Two Unsaturated C?C Bonds

Article abstract of DOI:10.1002/anie.202106145

Radical-mediated difunctionalization of alkenes provides a promising approach to introduce one alkenyl or alkynyl group to target compounds. However, simultaneous installation of two unsaturated C?C bonds via alkene difunctionalization remains elusive, at

Full text of DOI:10.1002/anie.202106145

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How to cite:
International Edition: doi.org/10.1002/anie.202106145
German Edition: doi.org/10.1002/ange.202106145
Radical Reactions
Alkene Difunctionalization Triggered by a Stabilized Allenyl Radical:
À
Concomitant Installation of Two Unsaturated C C Bonds
Yunlong Wei, Hong Zhang, Xinxin Wu, and Chen Zhu*
Dedicated to the centenary of Xiamen University
Abstract: Radical-mediated difunctionalization of alkenes
provides a promising approach to introduce one alkenyl or
alkynyl group to target compounds. However, simultaneous
ing a new alkyl radical intermediate which is involved in the
ensuing transition-metal catalyzed cross-couplings,[2d,e,k,n,o] or
trapped by radical acceptors,[2g,3] such as the Fuchs reagent
(Scheme 1A, a). Alternatively, the nascent alkyl radical can
undergo an intramolecular alkenyl or alkynyl migration by
harnessing the tertiary allylic/propargylic alcohols as specific
substrates (Scheme 1A, b).[4] Nevertheless, these approaches
are restricted to mounting a single alkenyl or alkynyl group at
alkenes.
À
installation of two unsaturated C C bonds via alkene difunc-
tionalization remains elusive, attributable to the high instability
and transient lifetimes of alkenyl and alkynyl radicals. Herein,
we report the photocatalytic 1,2-alkynylalkenylation and 1,2-
enynylalkenylation of alkenes for the first time, triggered by the
intermolecular addition of a stabilized allenyl radical to an
alkene. A portfolio of strategically designed, easily accessible
dual-function reagents are applied to a radical docking-
migration cascade. The protocol
À
Simultaneous installation of two unsaturated C C bonds
at both sides of an alkene, which efficiently increases the
has broad substrate scope and effi-
ciently increases the degree of unsa-
turation.
À
U
nsaturated carbon–carbon (C
C) bonds, alkenes and alkynes in
particular, are ubiquitous structural
units in natural products, pharma-
ceuticals, materials science, and
also have significant applications
in synthetic chemistry.[1] Over the
past few decades, radical-mediated
alkene difunctionalizations offers
a robust toolkit to obtain numerous
high value-added products.[2] This
process facilitates the incorpora-
tion of an alkenyl or alkynyl into
target compounds, giving rise to
multi-functionalized alkene or
alkyne adducts. The general proto-
col is initiated by the addition of an
external radical to an alkene, form-
À
Scheme 1. Incorporation of unsaturated C C bonds via radical-mediated alkene difunctionalization.
unsaturation degree and complexity of molecules, is highly
challenging. The main obstacle is inducing typically short-
[*] Y. Wei, H. Zhang, X. Wu, C. Zhu
lived alkenyl or alkynyl radical to add intermolecularly to
alkenes. Of note, alkynyl radicals are normally only found in
low density atmospheric and interstellar environments.[5] In
addition, the strong tendency of alkenyl radicals towards
hydrogen atom abstraction (HAA) from any available source
also competes with the desired radical addition.[6] As a result,
despite the great progress made in radical-mediated difunc-
tionalization of alkenes, concomitant installation of two
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 (China)
E-mail: chzhu@suda.edu.cn
C. Zhu
Key Laboratory of Synthetic Chemistry of Natural Substances,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences
À
unsaturated C C bonds by alkene difunctionalization still
345 Lingling Road, Shanghai 200032 (China)
remains unmet.
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.202106145.
Prompted by the applications of functional group migra-
tion in many elusive alkene difunctionalization reactions,[7] we
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conceive of designing a novel, practical dual-function reagent
for the 1,2-alkynylalkenylation (Scheme 1B). These reagents
are easily accessed within a few steps in gram-scale, in which
an alkenyl and a bromoallenyl group are tethered by a sulfone.
In the hypothesis, the allenyl radical I arising from the dual-
function reagent is supposed to be stabilized by the sulfonyl,
thus enabling its addition to an alkene. After a radical
migration cascade, a new allenyl radical II is generated which
might tautomerize to the propargyl radical III without the
stabilization effect of the sulfonyl group. The subsequent
functionalization of III furnishes the desired 1,2-alkynylalke-
nylation product. However, there are still many concerns.
Free allenyl radicals are generally detected by ESR below
100 K,[8] and mostly involved in transition metal-catalyzed
cross couplings,[9] intramolecular cyclization,[10] and hydrogen/
halogen abstraction.[11] There is no evidence that the allenyl
radical could be stabilized by a sulfonyl group. Therefore, the
intermolecular addition of transient allenyl radical to alkene
in this hypothesis is still questionable. The tautomerization
between allenyl radical and propargyl radical is also a vital yet
uncertain issue to the transformation.[12] If the competitive
conversion of I to IVoccurs or the anticipated conversion of II
to III does not occur, the transformation will be terminated.
Moreover, the possible HAA process induced by the sp2
allenyl radical I or II would also inhibit the reaction.
Herein, we disclose the proof-of-principle studies for the
radical-mediated 1,2-alkynylalkenylation and 1,2-enynylalke-
nylation of alkenes. The sulfone in the dual-function reagent
not only acts as a linker, but the stabilizing group for the
allenyl radical which enables the intermolecular addition step.
To the best of our knowledge, this is the first example of
À
concomitant installation of two unsaturated C C bonds
across alkenes.
The reaction of styrene with dual-function reagent 1a was
implemented to optimize reaction conditions. After a system-
atic survey (for details, see SI), the desired 1,5-enyne product
3a was obtained with 72% yield under the photochemical
conditions by using acetonitrile and methanol as mixed
solvents (Figure 1). The ratio (3:1) of methanol to acetonitrile
Figure 2. Scope of alkenes. Reaction conditions: 1a (0.2 mmol), 2
(1.5 equiv), fac-Ir(ppy)3 (1 mol%), and KH2PO4 (2 equiv) in MeCN/
MeOH (0.5/ 1.5 mL) under N2 at rt, irradiated with 5 W blue LEDs.
[a] 2 (3 equiv) and fac-Ir(ppy)3 (2 mol%). [b] 2 (2 equiv) and fac-
Ir(ppy)3 (2 mol%). [c] MeCN/ ROH or MeCN/ H2O (1.5/ 0.3 mL).
reaction, providing a platform for further product elabora-
tion. Internal alkenes, normally regarded as unfavorable
substrates in radical difunctionalization, presented compara-
ble reactivity herein. Notably, products 3r and 3s were
obtained with unique cis-stereoselectivity, attributed to the
formation of five-membered cyclic transition state during the
alkenyl migration process. Heteroaryl-substituted alkenes
were also compatible with the protocol (3v and 3w).
Functionalized alkenes such as enamide, b,g-butenolide, and
phenyl acrylate were suitable to this protocol and afforded the
corresponding products in useful yields (3x–3z), but the
reaction with vinyl ether only resulted in a very low yield
(< 10%). 1,3-Diene and 1,3-enyne served as suitable pre-
cursors, leading to valuable products with high degrees of
unsaturation (3aa and 3ab). In the latter case, the reaction
Figure 1. Optimized conditions for 1,2-alkynylalkenylation of alkenes.
was crucial to the reaction outcome. Insufficient methanol
resulted in a lower yield of 3a, while use of methanol as the
sole solvent entirely impeded the transformation. In addition,
other bases such as Na2CO3, K2CO3, Et3N, and DIPEA were
not effective for the reaction.
With the optimized reaction conditions in hand, we set
about assessing the generality of the protocol (Figure 2).
Firstly, a variety of aromatic alkenes bearing diverse func-
tional groups were tested. The electronic properties and
positional change (para-, meta-, or ortho-) of substituents did
not have much impact on the reaction outcomes. Bromide (3d
and 3n) and pinacol boronate (3i) remained intact in the
2
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regioselectively occurred at the alkenyl part despite the
presence of an alkyne which is also sensitive to radical
transformations. Remarkably, difunctionalization of aliphatic
alkenes including low-boiling-point representatives, such as
isobutene, proceeded smoothly, in which a set of all-carbon
quaternary centers were constructed regardless of steric
congestion (3ac–3ae). Intriguingly, the reaction of b-pinene,
a widely used radical-clock probe, gave the unusual 1,2-
difunctionalized product 3af instead of common ring-opening
adduct, indicating that the intramolecular radical 5-exo-trig
cyclization is faster than the competitive ring-opening pro-
cess. However, performing the radical-clock experiments with
more reactive vinylcyclopropanes (VCPs, ꢀ 10 times to b-
pinene)[13] led to either the ring-opening product or the
difunctionalized product dependent on the electronic prop-
erty of VCPs (for details, see SI), suggesting that the rate of 5-
exo-trig cyclization may be comparable to the ring-opening
rates of VCPs. Similar to 3r and 3s, the reaction of cyclo-
pentene and norbornene also exclusively delivered the cis-
adducts 3ag and 3ah. The method could be applied to the
late-stage modification of complex natural products. For
instance, 3ai and the highly functionalized 3aj were readily
obtained from dihydrocarvone and picrotoxin, respectively,
manifesting that this protocol is of broad functional group
tolerance.
It was anticipated that methanol could be replaced with
other co-solvents. Indeed, cyclohexanol and 5-hexynyl-1-ol
also proved suitable and afforded the corresponding products
with acceptable yields (4a and 4b). Terminal alkynes were
tolerated in the reaction. Notably, synthetically valuable
propargylic alcohols 4c–4 f were obtained by using water as
co-solvent.
Figure 3. Assessment of dual-function reagents. Reaction conditions:
1b–1n (0.2 mmol), 2a (1.5 equiv), fac-Ir(ppy)3 (1 mol%), and KH2PO4
(2 equiv) in MeCN/ MeOH (0.5/ 1.5 mL) under N2 at rt, irradiated
with 5 W blue LEDs.
To further extend the product diversity, a portfolio of
dual-function reagents was prepared and subjected to the
reaction with styrene (Figure 3). The two methyl groups on
allene could be varied to a cycloalkyl or heterocycle, leading
to the corresponding adducts with slightly lower yields (5a–
5c). Altering a methyl to a larger aliphatic group in the
reagent also presented a comparable yield (5d). Replacing
methyl by phenyl appreciably decreased the yields of
products (5e and 5 f), probably due to the negative p-p
conjugate effect from aryl group which suppressed free
tautomerization between the allenyl and propargylic radical
intermediates. Various substitutions on the alkenyl part were
also apt to afford the desired products regardless of steric and
electronic effects (5g–5k). However, sharply increasing the
steric hindrance of the dual-function reagent (e.g. 1m) would
impede the formation of five-membered cyclic transition state
as well as the ensuing alkenyl migration, thus resulting in the
low yield of 5l. Remarkably, the use of dual-function reagent
1n containing a Z,E-diene moiety simultaneously incorpo-
rated an alkynyl and an E,E-diene into product 5m, indicating
a superb stereocontrolled alkenylation process.
Indeed, the 1,2-enynylalkenylation occurred as expected
under slightly modified conditions by using DMF as sole
solvent. Representative examples were listed to illustrate the
generality of the protocol (Figure 4). Various substituents on
1,3-Conjugated enynes represent a class of highly trans-
formable building blocks in synthetic chemistry.[14] Inspired by
the 1,2-alkynylalkenylation pathway, we envisioned that, in
the absence of nucleophilic co-solvent, the transformation
might give rise to 1,3-enynes and lead to 1,2-enynylalkeny-
lation of alkenes through an alternative deprotonation.
Figure 4. Investigation of 1,2-enynylalkenylation of alkenes. Reaction
conditions: 1 (0.2 mmol), 2 (3 equiv), fac-Ir(ppy)3 (2 mol%), and
KH2PO4 (1 equiv) in DMF (2 mL) under N2 at rt, irradiated with 30 W
blue LEDs. [a] 2 (1.5 equiv).
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the alkene or dual-function reagent were well tolerated,
leading to the corresponding products (6a–6d). Notably,
difunctionalization of 1,3-enyne and 1,3-diene furnished the
intricately branched adducts with dense unsaturated groups
that are otherwise hard to obtain (6e and 6 f). The undesired
over-functionalization of the residual terminal alkenyl in
products were clearly observed, which accounted for the
lower yields. The conversion with highly electron-deficient
acrylate also proceeded (6g), albeit in modest yield. Utiliza-
tion of reagent 1b and 1c afforded the internal enyne
products in synthetically useful yields (6h and 6i), as the over-
functionalization of products without terminal enynyl was
entirely avoided. The reaction with reagent 1e resulted in
a mixture of internal and terminal enynyl products (6j). A
linear multi-unsaturated molecule 6k was also readily syn-
thesized with the use of reagent 1n.
The synthetic value of the method was showcased by
transformation of the products into other valuable molecules
(Scheme 2). In the presence of m-CPBA, 3a was converted to
epoxide 7, in which the alkynyl moiety remained intact.
Selective Lindlar reduction of 3a gave rise to Z,E-diene 8,
providing a formal protocol for radical dialkenylation of
alkenes. The propargylic methoxy in 3a could be easily
removed, and the resultant product 9 was stereoselectively
reduced by DIBAL-H to give Z,E-diene 10. Interestingly, the
propargylic methoxy group could also function as a leaving
group, leading to the formal allenylalkenylation adduct 11
with the use of DIBAL-H. Moreover, the transformation of
enyne 6b provided another formal 1,2-allenylalkenylation
approach, furnishing the fully substituted allene 12 via the
copper-catalyzed radical cyanoalkylation.
Scheme 2. Product transformations. Conditions a: m-CPBA (3 equiv),
NaHCO3 (3 equiv), DCM/H2O, 20 h. Conditions b: Lindlar’s catalyst
(5 wt%, 20 mol%), quinoline (0.5 equiv), 1 atm H2, dry EtOAc, rt,
15 min. Conditions c: Cp2ZrCl2 (1.6 equiv), n-BuLi (2.5 M, 3.2 equiv),
THF, À788C for 1 h, then rt for 3 h. Conditions d: DIBAL-H (2 equiv),
toluene, 508C, 12 h. Conditions e: DIBAL-H (4 equiv), toluene, 808C,
21 h. Conditions f: Cu(OAc)2 (5 mol%), 1,10-phenanthroline
(7 mol%), LPO (3 equiv), TMSCN (4 equiv), dry DCM, 508C, 42 h.
zation of IV to a more stable propargylic radical V followed
by single-electron oxidation with the IrIV species gives rise to
the cation intermediate VI, and regenerates the ground-state
IrIII catalyst. The reaction of VI with nucleophilic co-solvents
(e.g. ROH, H2O, etc.) furnishes the adduct 3, 4 and 5.
Alternatively, in the absence of nucleophiles the reaction
leads to the enyne 6 via the deprotonation of VI.
Subsequent efforts were focused on the reaction mecha-
nism investigation. Firstly, the Stern–Volmer study showed
that the excited-state of Ir(ppy)3 was readily oxidatively
quenched by 1a (Figure S1). This was also verified by the
reduction potential of 1a. The value (Ep/2 = À1.55 V vs. SCE)
III
À
illustrated that the C Br bond of 1a could be reduced by Ir *
In summary, we have disclosed the first example of
(E1/2IV/III* = À1.73 V vs. SCE) (Figure S2).[15] The quantum
yield of the reaction between 1a and styrene was 0.14
(Figure S3), suggesting that a photocatalytic cycle might be
involved as the main pathway. The light on-off experiments
also supported this conclusion that the reaction outcome was
not increased without the light
concomitant incorporation of two unsaturated C C bonds
À
into alkenes by the photocatalytic 1,2-alkynylalkenylation
and 1,2-enynylalkenylation of alkenes. A portfolio of strate-
gically designed dual-function reagents are applied to the
transformation. The sulfone in the dual-function reagent
irradiation (Figure S4).
A
plausible mechanism is
depicted in Figure 5. Single-elec-
tron transfer from the excited-state
of Ir(ppy)3 to dual-function
reagent 1a affords the allenyl rad-
ical I, which is verified by radical
trap experiment with TEMPO (for
details, see SI), and IrIV species.
This sulfone-stabilized allenyl rad-
ical I is efficiently trapped by
alkene to generate the intermedi-
ate II. The ensuing cascade of
intramolecular alkenyl migration
and SO2 extrusion results in a new
allenyl radical IV. The tautomeri-
Figure 5. Proposed reaction mechanism.
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radical to alkene. The reaction features a broad scope of
alkenes that both activated and unactivated alkenes are
suitable substrates. Moreover, the protocol is of great
synthetic value, affording a diversity of complex unsaturated
molecules with high unsaturation degree, which are otherwise
difficult to prepare. This ingenious application of docking-
migration strategy opens up a new vista for radical difunc-
tionalization of alkenes.
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We are grateful for the financial support from the National
Natural Science Foundation of China (21971173), the Project
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: alkene difunctionalization · alkyne · allenyl radical ·
radical reactions · rearrangement
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Manuscript received: May 6, 2021
Revised manuscript received: June 12, 2021
Accepted manuscript online: June 20, 2021
Version of record online: && &&, &&&&
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Communications
Radical Reactions
Disclosed herein for the first time is the
photocatalytic 1,2-alkynylalkenylation and
1,2-enynylalkenylation of alkenes, trig-
gered by the intermolecular addition of
a stabilized allenyl radical to an alkene. A
portfolio of strategically designed, easily
accessible dual-function reagents are
applied to a radical docking-migration
cascade. The protocol has broad sub-
strate scope and efficiently increases the
degree of unsaturation.
Y. Wei, H. Zhang, X. Wu,
C. Zhu*
&&&& — &&&&
Alkene Difunctionalization Triggered by
a Stabilized Allenyl Radical: Concomitant
Installation of Two Unsaturated C C
À
Bonds
6
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
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