A donor-acceptor complex enables the synthesis of: E -olefins from alcohols, amines and carboxylic acids

Article abstract of DOI:10.1039/d1sc01024g

Olefins are prevalent substrates and functionalities. The synthesis of olefins from readily available starting materials such as alcohols, amines and carboxylic acids is of great significance to address the sustainability concerns in organic synthesis. Metallaphotoredox-catalyzed defunctionalizations were reported to achieve such transformations under mild conditions. However, all these valuable strategies require a transition metal catalyst, a ligand or an expensive photocatalyst, with the challenges of controlling the region- and stereoselectivities remaining. Herein, we present a fundamentally distinct strategy enabled by electron donor-acceptor (EDA) complexes, for the selective synthesis of olefins from these simple and easily available starting materials. The conversions took place via photoactivation of the EDA complexes of the activated substrates with alkali salts, followed by hydrogen atom elimination from in situ generated alkyl radicals. This method is operationally simple and straightforward and free of photocatalysts and transition-metals, and shows high regio- and stereoselectivities.

Full text of DOI:10.1039/d1sc01024g

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A donor–acceptor complex enables the synthesis
of E-olefins from alcohols, amines and carboxylic
acids†
Cite this: DOI: 10.1039/d1sc01024g
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have been paid for by the Royal Society
of Chemistry
*
*
Kun-Quan Chen, Jie Shen, Zhi-Xiang Wang
and Xiang-Yu Chen
Olefins are prevalent substrates and functionalities. The synthesis of olefins from readily available starting
materials such as alcohols, amines and carboxylic acids is of great significance to address the
sustainability concerns in organic synthesis. Metallaphotoredox-catalyzed defunctionalizations were
reported to achieve such transformations under mild conditions. However, all these valuable strategies
require a transition metal catalyst, a ligand or an expensive photocatalyst, with the challenges of
controlling the region- and stereoselectivities remaining. Herein, we present a fundamentally distinct
strategy enabled by electron donor–acceptor (EDA) complexes, for the selective synthesis of olefins
from these simple and easily available starting materials. The conversions took place via photoactivation
of the EDA complexes of the activated substrates with alkali salts, followed by hydrogen atom
elimination from in situ generated alkyl radicals. This method is operationally simple and straightforward
and free of photocatalysts and transition-metals, and shows high regio- and stereoselectivities.
Received 21st February 2021
Accepted 1st April 2021
DOI: 10.1039/d1sc01024g
rsc.li/chemical-science
In recent years, the photochemistry of electron donor–
acceptor (EDA) complexes has emerged as a powerful alternative
Introduction
to metal-based photocatalysts and organic dyes for generating
radicals under the excitation of visible light.11 However, limi-
tations exist due to their generation and reaction modes, and
their applications in defunctionalative olen synthesis are
unknown. Generally, to date, there are two reaction modes for
EDA complexes. One typical mode is the cross-coupling of two
substrates (Scheme 1B, mode a). The other mode involves the
reaction of an external radical trap, and the generated radical is
realized by using a sacricial or catalytic donor compound
(Scheme 1B, mode b). We envision that a new reaction mode
would be possible if a p-block radical and an alkyl radical
bearing a weak C–H bond could be generated via an EDA
complex, and the p-block radical could abstract a C–H bond
from the alkyl radical for the olen formation (Scheme 1B,
mode c). If successful, the photoinduced reaction could serve as
a general and powerful method in olen synthesis, using readily
available and abundant alcohols, amines and carboxylic acids
as starting materials under mild and transition-metal-free
conditions. Recently, Shang and Fu realized an EDA complex
enabled decarboxylative alkylation, where both PPh3 and NaI
were required to form the EDA complex.12 We previously
established that the electrostatic effect of N-heterocyclic car-
bene (NHC) facilitates the formation of an EDA complex to
generate alkyl and iodine radicals, and the stabilization effect of
NHC on the iodine radical further ensured the following
radical–radical couplings.13 Herein, we report an alternative
formation of the EDA complex by only using the electrostatic
effect of NaI, which differs from previous NaI-promoted
Olens are the second most common functionalities in natural
products and have found a wide range of applications in
pharmaceuticals, agrochemicals, materials science and the
food industry.1 This has led to the development of quite
a number of strategies for effectively generating carbon–carbon
double bonds.2 Among them, defunctionalizations have
emerged as some of the most valuable transformations for the
generation of olens by using easily available molecules as
starting materials.3 In this context, decarboxylation of carbox-
ylic acids at high temperatures4 or using a stoichiometric
oxidant,5 enzymes,6 or a high energy lamp7 has been heavily
investigated. Recently, the development of metallaphotoredox
catalysis8 has enabled decarboxylative olen synthesis under
mild conditions directly9 or indirectly.10 However, all these
valuable strategies require a transition metal catalyst, a ligand,
or an expensive photocatalyst or lack general, regio- and ster-
eoselective variants (Scheme 1A). In light of the central role
played by olens in organic synthesis and the increasing
demand for sustainable chemistry, inventing new synthetic
methods starting with readily available materials under transi-
tion metal- and photocatalyst-free conditions is highly
desirable.
School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing
100049, China. E-mail: zxwang@ucas.ac.cn; chenxiangyu20@ucas.ac.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1sc01024g
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Scheme 1 Motivation and synthetic strategy.
photoreactions where PPh3/NHC and the interaction of I-PPh3/ (Scheme 3). The method showed a broad substrate scope.
NHC are required. In this process, the iodine radical could Besides enamides 2–5, a variety of ne chemicals and natural
induce the in situ generated alkyl radical to undergo hydrogen food avours – such as anethole 6, isoeugenol 7, isosafrole 8
atom elimination, thus forming a carbon–carbon double bond, and polysporin precursor 9 were obtained from the corre-
instead of radical–radical coupling (Scheme 1C).
sponding acids in 65–96% yield with uniformly high E-selec-
tivity. It is worth noting that asymmetric carboxylic acids
afforded the internal olen 10 with excellent regio- and E-
selectivities. In addition, both electron-donating (3,5-di-MeO, 4-
Me and 4-tBu) and electron-withdrawing (4-Br and 4-Cl) groups
on the phenyl ring were tolerated to afford the desired products
11–15 in good yields with high selectivity. The ortho-substituent
(2-Cl) was also tolerated in the reaction without apparent
change in the yield and selectivity (16). The reaction of 2-(4-
methoxyphenyl)hexanoic acid gave the desired product 17 in
77% yield with a 98 : 2 E/Z selectivity. The method was also
efficient for the synthesis of E-stilbenes (18–20), which widely
appear in pharmaceuticals and natural products and play an
important role in a large variety of biological activities.
Furthermore, the functional groups had no signicant effect on
the yield and E/Z selectivity. For example, ester, ketone, and
alkene substituted carboxylic acids all worked well and gave the
desired products with excellent E/Z selectivity (21–23). The
Results and discussion
Intrigued by this observation, we rst examined the decarbox-
ylative elimination of a-amino acids for the synthesis of
enamides, which has been widely used for the preparation of
lots of biologically and pharmaceutically important
compounds.14 Notably, Tunge and co-workers reported an
elegant dual photoredox/cobalt catalyst system for their
synthesis, where the hydrogen atom transfer was achieved by
cobalt catalysis. However, this valuable protocol afforded the
desired enamides with moderate E/Z ratios (Scheme 2A).9a To
our delight, the reaction of a-amino acid N-hydroxyphthalimide
(NHPI) ester proceeded well in the presence of 2.0 equiv. of NaI
and acetone as the solvent, affording the desired enamide 1 in
68% yield with high E/Z ratios (Scheme 2B). With this promising
result, we further explored the generality of the transformation
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precursors. A variety of a-amino acids as starting materials
worked well, affording the corresponding olens 21 and 48–52
in good yields and high E/Z selectivities (Scheme 4).
In order to examine the generality of the developed new EDA
reaction mode, we next turned our attention to the dehydration
of alcohols for olen synthesis. Although known methods for
such alcohol transformations have been well studied,18 we wish
to provide an alternative protocol for the dehydration and
further show the broad applications of the present strategy.
Notably, there are only two EDA approaches to generate alkyl
radicals from alcohols. One typical entry is the b-scission of an
alkoxy radical, which was generated from the EDA complex of N-
alkoxyphthalimide and Hantzsch ester (Scheme 5A).19 Another
approach involves the formation of the EDA complex with 2-
iodophenylthionocarbonates, Et3N and B2cat2 (Scheme 5B).20
We were pleased to nd that our newly developed dual alkali
cation activation mode could form an EDA complex by using
pre-activated alcohols (Scheme 5C). Various alcohols were
smoothly converted to the corresponding 1,2-disubstituted
olens 6–11 and 53 in moderate to good yields with excellent E/
Z selectivity. With tertiary alcohol, excellent regio- and stereo-
selectivities were achieved and the corresponding tri-
substituted olens 54 and 55 were obtained in 70% and 81%
yield, respectively. 1-(4-Methoxyphenyl)ethan-1-ol was readily
accommodated under both reaction conditions (56). The reac-
tion of cyclic alcohol also worked, providing the corresponding
product 33 in good yield.
To conrm the possible mechanism, we investigated the
involvement of the EDA complex. While the NHPI ester,
Katritzky salt, alcohol derivatives, NaI or LiBF4 individually
showed no signicant UV/vis absorption in the visible region of
the spectra, an obvious red shi of absorption could be
observed in the spectrum of the mixture of NHPI ester and NaI,
Katritzky salt and NaI, and the mixture of alcohol derivatives,
NaI and LiBF4. These experiments demonstrated the generation
of a photoactive EDA complex (Scheme 6A). Although further
studies are required to fully understand the detailed mecha-
nism, one possible mechanism is proposed in Scheme 6B. The
weak interaction between NHPI ester/Katritzky salts/or alcohol
derivatives and NaI allows the formation of an EDA complex.
Upon visible-light excitation, the photoactive EDA complex
undergoes single-electron transfer, generating the alkyl radical
and iodine radical. Subsequently, the iodine radical abstracts
a hydrogen atom from the resultant alkyl radical to form the
corresponding olens. Instead of direct hydrogen atom
abstraction, an alternative that forms iodoalkanes rst, fol-
lowed by the homolysis of iodoalkanes, is also consistent with
our following mechanistic study. According to the mechanism,
the E-selectivity of the reaction could be attributed to the anti-
periplanar hydrogen atom elimination, which is similar to the
classical E2 elimination.
Scheme
chromatography.
2 Initial studies. Yield of isolated product 1 after
reaction scope was further examined by constructing highly
substituted terminal olens, which is another challenge when
traditional methods are used. Various 1,1-disubstituted and
trisubstituted olens were obtained in high yields (24–30).
Utilizing this simple method, we also synthesized cyclic olens
in good yields (31–35).
Remarkably, this strategy could serve as a convenient
procedure for the late-stage structural modication of natural
products and drugs. NHPI esters derived from diverse scaffolds,
such as Naproxen 36, Ibuprofen 37, Oxaprozin 38, Ketoprofen
39, Flurbiprofen 40, Carprofen 41, Loxoprofen 42, Zaltoprofen
43 and Bufemid 44, were all converted into the corresponding
olens in good to high yields under mild conditions. Interest-
ingly, the use of commercially available a-hydroxy acids resulted
in the desired ketones and aldehydes in high yields (45–47).
On the basis of the photoinduced decarboxylation results, we
endeavored to extend this strategy to the deamination of a-
amino acids for olen synthesis. If realized, this protocol would
represent a mild and selective olen synthesis from general
amines via EDA photochemistry. Primary amines are also
abundant natural feedstocks similar to carboxylic acids and
have recently emerged as powerful reagents to generate alkyl
radicals.15 However, to our knowledge, the deaminative olen
synthesis remained a daunting synthetic challenge, and there is
only one example that exists for this type of transformation,
where a high energy UV lamp (800 W) was employed for the
deamination of N-phthaloyl amino acids.16 Delightfully, aer
optimizing the reaction conditions (see Table S2 in the ESI†), we
successfully established the photoinduced deamination of a-
amino acids by employing Katritzky salts17 as C-centered-radical
On the basis of our previous studies which showed that
iodoalkanes could be easily generated between NHPI esters and
NaI under light irradiation,13 a series of mechanistic experi-
ments were carried out to clarify the possible reaction pathway
(Scheme 6C). One possible pathway operating via traditional E2
elimination of iodoalkanes could be ruled out because without
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Scheme 3 Substrate scope for decarboxylation. Yield of isolated product given. The E/Z ratios were determined by 1H NMR spectroscopy. DIC ¼
N,N0-diisopropylcarbodiimide and DCM ¼ dichloromethane.
irradiation the control experiments (entries 1 and 2) with phthK envisioned that a light-mediated homolysis of the C–I bond of
as the proton extractor only afforded a trace amount of olens. iodoalkanes could be involved. Supportively, under blue LED
According to suggestions by Studer and co-workers,21 we irradiation, the corresponding olens were obtained in good
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1
Scheme 4 Substrate scope for deamination. Yield of isolated product given. The E/Z ratios were determined by H NMR spectroscopy.
Scheme 5 Substrate scope for dehydration. Yield of isolated product given. The E/Z ratios were determined by 1H NMR spectroscopy. DIAD ¼
diisopropyl azodicarboxylate, DCC ¼ dicyclohexylcarbodiimide, DMAP ¼ 4-dimethylaminopyridine, and PNP ¼ p-nitro-phenyl. aUsing method a.
bUsing method b.
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Scheme 6 Mechanistic studies.
yields and high E/Z selectivities (entries 3–5). The radical inhi-
bition and interception studies (entries 6 and 7) further sug-
gested the involvement of the alkyl radical in the reaction.
Acknowledgements
We acknowledge the nancial support from the National
Natural Science Foundation of China (21773240 and 22001248),
Fundamental Research Funds for the Central Universities and
the University of the Chinese Academy of Sciences.
Conclusions
In conclusion, we have developed an operationally simple,
robust and general EDA strategy to synthesize olens by using
simple and easily available alcohols, amines and carboxylic
acids as starting materials. These methods are characterized by
high regio- and E/Z selectivities, and cheap alkali metal salts,
and do not involve transition metals, photocatalysts, or strong
bases and acids, and thus offer a promising solution for various
applications including the concise synthesis of industrially
relevant olens and the modications of commercially available
drugs and natural products.
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