Phosphine-Mediated Dimerization of Conjugated Ene-Yne Ketones: Stereoselective Construction of Dihydrobenzofurans

Article abstract of DOI:10.1002/adsc.201700031

A new strategy for the phosphine-mediated dimerization of conjugated ene-yne ketones to produce functionalized dihydrobenzofurans has been developed, affording diversified 4,5-dihydrobenzofurans in moderate to excellent yields with high diastereoselectivities under mild conditions. This new synthetic method can tolerate a variety of functional groups and can be performed on a gram scale and in an asymmetric variant using the chiral phosphine Xyl-BINAP to give the desired products with up to 94% ee. (Figure presented.).

Full text of DOI:10.1002/adsc.201700031

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DOI: 10.1002/adsc.201700031
Very Important Publication
Phosphine-Mediated Dimerization of Conjugated Ene-Yne
Ketones: Stereoselective Construction of Dihydrobenzofurans
Cheng-Zhi Zhu,a Yao-Liang Sun,a Yin Wei,c and Min Shia,b,c,
*
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering,
East China University of Science and Technology, 130 Mei Long Road, Shanghai 200237, Peopleꢀs Republic of China
State Key Laboratory and Institute of Elemento-organic Chemistry, Nankai University, Tianjin 300071, Peopleꢀs Republic
of China
b
c
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 354 Fenglin Lu, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-6416-6128; e-mail: mshi@mail.sioc.ac.cn
Received: January 10, 2017; Revised: January 21, 2017; Published online: && &&, 0000
Supporting information for this article can be found under: http://dx.doi.org/10.1002/adsc.201700031.
Abstract: A new strategy for the phosphine-mediat-
ed dimerization of conjugated ene-yne ketones to
produce functionalized dihydrobenzofurans has
been developed, affording diversified 4,5-dihydro-
benzofurans in moderate to excellent yields with
high diastereoselectivities under mild conditions.
This new synthetic method can tolerate a variety of
functional groups and can be performed on a gram
scale and in an asymmetric variant using the chiral
phosphine Xyl-BINAP to give the desired products
with up to 94% ee.
yne ketones via the phosphonium enolate B1. In this
reaction, the cyclization of B1 gave phosphonium
ylide C1, affording a vinylfuran via a Wittig-type ole-
fination (Scheme 1, 2).[5] It could be realized that, in
all the cases, the in situ generated active species, a vi-
nylphosphonium ylide, sets the stage for the elimina-
tion of the phosphine along with one or more proton
transferring processes to accomplish the isomerization
or cyclization reactions.
Thus far, successful examples for the construction
of functionalized furans by isomerization of carbonyl
group-containing ene-ynes have been extensively ex-
plored.[6] On the basis of these outstanding precursory
examples, we envisaged that EWG-activated ene-yne
ketones would go through a similar process as Kuro-
daꢀs one to generate a phosphonium ylide B2 and
a resonance zwitterionic intermediate C2 via inter-
mediate A2. Since the existing additional EWG can
Keywords: dimerization; ene-yne ketones; function-
alized furans; phosphines; Wittig olefination
In the past two decades, nucleophilic phosphine orga- stabilize the anionic intermediate, the resonance
nocatalysis has been developed to a powerful synthet- structure may more lean to the zwitterionic inter-
ic tool for the construction of a wide variety of novel mediate C2, which could be more easily captured by
molecules.[1] Among these synthetic protocols, phos- another ene-yne ketone molecule, thus affording a dif-
phine-mediated/catalyzed annulations of activated ferent reaction outcome (Scheme 1, 3). To our delight,
ene-ynes, allenes or Morita–Baylis–Hillman (MBH) we found that the dimerization of ene-yne ketones
carbonates have played pivotal roles for the synthesis indeed took place via zwitterionic intermediate C2
of functionalized carbo- and heterocyclic skeletons.[2] through a Michael addition and a subsequent Wittig
A phosphonium dienolate as a reactive species can be olefination to produce dihydrobenzofurans.
easily generated from activated alkynes upon phos-
Furan derivatives are of considerable interest in
phine catalysis to conduct g-umpolung addition or cy- both synthetic and medicinal chemistry due to the nu-
cloaddition reactions as explored by Trost, Lu and merous natural products and pharmaceuticals incor-
Krische (Scheme 1, 1).[3] In the phosphine-catalyzed porating this key heterocyclic framework.[7] However,
activation of ene-yne ketones, the isomerization of synthetic methods for preparing dihydrobenzofurans
carbon-carbon multiple bonds to the conjugated tri- from alkenes or alkynes have been less explored until
enes has been reported by Lu also via the in situ gen- now. To the best of our knowledge, only one example
erated phosphonium dienolate A1 (Scheme 1, 2).[4] has been reported, affording 3,6-dimethyl-4,5-dihydro-
Moreover, Kuroda and co-workers reported another benzofuran as a minor product.[8] Our phosphine-
example of the phosphine-initiated reaction of ene- mediated dimerization reaction of conjugated ene-yne
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Scheme 1. Previous work and our working hypothesis.
ketones described above can efficiently deliver func- moiety of the conjugated ene-yne ketones 1 (Table 1).
tionalized 4,5-dihydrobenzofurans under mild condi- As illustrated, when R1 was a substituted phenyl
tions from readily available starting materials. This in- group with different electronic properties (e.g., elec-
teresting finding encouraged us to further explore this tron-neutral, electron-rich, or electron-deficient) such
novel synthetic method under phosphine mediation.
as methyl, methoxy, halogen-substituted phenyl
Initial studies using ene-yne ketone 1a as the sub- group, the reactions proceeded very well, affording
strate were aimed at determining the reaction out- the corresponding products 2a–2g in high yields. A
come and subsequently optimizing the reaction condi- heterocyclic aromatic ring was also tolerated to give
tions. The results are summarized in Table S1 in the the respective product 2h in 90% yield. An interesting
Supporting Information and we finally identified that consequence was that when R1 was an ortho-substitut-
the optimal conditions comprise carrying out the reac- ed aromatic group, the products 2i–2k were produced
tion in CH2Cl2 at room temperature and using a half as two diastereometric mixtures, probably due to the
molar equivalent of PPh3 as the promoter to give 2a fact that steric hindrance blocked the free rotation,
in 98% yield (Scheme 2). When a larger amount of giving two diastereomeric rotamers. Unfortunately,
PPh3 was used, the yield of 2a could not be improved a sterically encumbered substrate, the 1-naphthyl
further. The structure of 2a has been unambiguously group-substituted ene-yne ketone 1l, did not give the
assigned by X-ray diffraction and the CIF data are corresponding product 2l under the standard condi-
shown in the Supporting Information.[9]
tions. When R1 was an aliphatic group, the reaction
Having established the optimal reaction conditions, also worked smoothly after increasing the concentra-
we next investigated the substrate scope with respect tion of the employed substrates 1m–1o, giving the de-
to a wide range of substituents (R1) on the alkyne sired products 2m–2o in reasonable yields ranging
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Scheme 3. The phosphine-mediated reaction of 1x.
Scheme 2. The optimal conditions for the PPh3-mediated di-
merization of 1a.
group was essential because dimethyl malonate as
substrate 1x did not go through the same reaction
from 40% to 62%. The reason why these products under the standard conditions, but afforded 1,3-diene
were formed in lower yields could be that the corre- 3a in 65% yield upon increasing the amount of PPh3
sponding vinylphosphonium ylide bearing an aliphatic to 1.0 equiv. and using water (1.0 equiv.) as an extra
group produced upon phosphine attack was not as additive (Scheme 3). It was believed that 3a was the
stable as those of the aromatic ones (see intermediate consequence of H2O addition to the vinylphosphoni-
G4 in Scheme 5). This straightforward protocol allows um intermediate A3 generated by phosphine addi-
for the gram-scale synthesis, as shown for 2a tion,[10] which could be evidence for a 1,6-addition of
(Table 1). In all the cases, the corresponding products phosphine to the alkynyl moiety rather than the al-
were formed exclusively in the trans-configuration.
kenyl moiety. The structure of 3a has been identified
Subsequently, we evaluated the substrate scope by X-ray diffraction and the related information has
with respect to various electron-withdrawing groups been presented in the Supporting Information.[9]
substituted carbonyl moiety and the results are shown When the EWG was a benzenesulfonyl group, the
in Table 2. When the acetyl group was replaced by corresponding benzofurans 2ta and 2tb were obtained
propionyl, the reaction proceeded smoothly to afford in 44% and 45% yields, respectively, suggesting that
the corresponding product 2p in 63% yield. As for a subsequent a-elimination of the benzenesulfonyl
substrate 1q bearing a sterically hindered isopropyl group took place to give the aromatization product.[11]
group, no reaction occurred, presumably due to the The structure of 2ta has been also determined by X-
steric effect. Furthermore, benzoyl group-containing ray diffraction and the CIF data are given in the Sup-
substrates 1ra–1rf could also give the corresponding porting Information.[9] The NO2-containing substrate
products 2ra–2rf in 60–85% yields. As summarized in 1u produced the corresponding product 2u in 50%
Table 2, when R1 group was a para-methoxyphenyl, yield. The geometric configurations of these sub-
meta-methylphenyl, para-chlorophenyl, 3,4-dimethyl- strates did not affect the reaction outcome. However,
phenyl and 3-thienyl group, all of the reactions pro- for substrates 1v and 1w in which the EWG is CN or
ceeded smoothly, indicating that the electronic prop- CF3, the reactions only gave complex product mix-
erties of the aromatic group did not have a significant tures.
impact on the reaction outcome. When the EWG was
To further clarify the pathway of this phosphine-
an ester group, substrate 1s could undergo the reac- mediated dimerization of conjugated ene-yne ketone
tion very well to give the desired product 2s in 60% reaction, 31P-NMR spectroscopic tracing experiments
yield. However, it should be noted that one carbonyl have been conducted. As indicated in Scheme 4,
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Table 1. Substrate scope of substituents (R1) on the alkynyl moieties.[a]
[a]
Unless otherwise stated, all reactions were carried out with 1 (1.0 mmol), PPh3 (0.5 mmol) in CH2Cl2 (3.0 mL) at room
temperature.
[b]
Reactions were carried out with 1 (3.0 mmol), PPh3 (1.5 mmol) in CH2Cl2 (3.0 mL).
under the standard conditions, the reaction of 1a was molecular Wittig reaction to afford dihydrobenzofur-
carried out in a NMR tube. After a reaction time of an 2a and O=PPh3.[12]
3 min, the peak B of O=PPh3 appeared (peak A is
In a preliminary attempt, the asymmetric variant of
PPh3) and a peak C, corresponding to a reaction inter- 1a for the production of enantiomerically enriched 2a
mediate, were clearly present. As the reaction pro- was evaluated with several chiral phosphine (see
ceeded, the signal of PPh3 decreased and the signal of Table S2 in the Supporting Information for the de-
O=PPh3 increased. After 60 min, PPh3 was trans- tails). We found that chiral xyl-BINAP worked very
formed to O=PPh3 completely (Scheme 4).
well, enabling the isolation of product 2a in 50%
According to the above experimental results and yield along with 90% ee (Table 3). As a consequence,
further NMR spectroscopic tracings, a plausible reac- substrates 1b, 1c, 1e, 1g and 1h were utilized in this
tion mechanism for the phosphine-mediated dimeriza- newly established asymmetric dimerization, affording
tion of conjugated ene-yne ketones to afford dihydro- the desired products 2a–2f in moderate yields with
benzofurans has been proposed as shown in Scheme 5 75–90% ee values (Table 3). This is the first example
by using 1a as a model substrate. The reaction is initi- of the asymmetric synthesis of dihydrobenzofuran de-
ated by a 1,6-addition of phosphine to 1a, followed by rivatives under phosphine mediation.
cyclization to yield the key zwitterionic intermediate
In conclusion, we have developed a novel phos-
D4, stabilized by EWG (acyl group) via intermediates phine-mediated dimerization reaction of ene-yne ke-
A4, B4 and C4, which undergoes Michael addition tones to stereoselectively produce functionalized di-
with another molecule of 1a to give intermediate E4. hydrobenzofurans in good yields. A mechanistic in-
After an isomerization and a subsequent 1,5-proton vestigation involving the NMR spectroscopic tracing
shift, ylide G4 is generated which undergoes an intra- experiments revealed that this transformation pro-
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Table 2. Substrate scope for the formation of dihydrobenzofurans or benzofurans by the EWG activated ene-yne ketones.[a]
[a]
Unless otherwise stated, all reactions were carried out with 1 (1.0 mmol), PPh3 (0.5 mmol) in CH2Cl2 (3.0 mL) at room
temperature.
Scheme 4. 31P NMR spectroscopic tracing experiments.
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ceeds through an isomerization/Michael addition/
Wittig reaction cascade. A variety of functional
groups can be tolerated and a gram-scale synthesis is
possible. Furthermore, the asymmetric variant can be
achieved, giving the desired dihydrobenzofurans in
good ee values. This is a new reaction mode in phos-
phine-catalyzed/mediated chemical transformations.
Efforts are underway to discern the reaction mecha-
nism and apply this new reaction to the synthesis of
biologically active substances.
Experimental Section
General Procedure for Synthesis of Benzofurans 2
Under an argon atmosphere, PPh3 (0.5 mmol, 0.5 equiv.) was
added to
a solution of conjugated ene-yne ketone
1 (1.0 mmol, 1.0 equiv.) and dry dichloromethane (3.0 mL)
in a Schlenk tube. The reaction mixture was stirred for 12 h
at room temperature until 1 was completely consumed by
TLC monitoring. Then the solvent was removed under re-
duced pressure and the residue was purified by a silica gel
flash column chromatography (eluent: petroleum ether/
EtOAc, 10/1) to give the product 2.
Scheme 5. A plausible mechanism for the formation of 2a.
Table 3. Xyl-BINAP-mediated asymmetric dimerization.[a,b]
[a]
All reactions were carried out on a 0.2 mmol scale in 3.0 mL of solvent.
Isolated yields.
Determined by chiral HPLC.
[b]
[c]
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Detailed descriptions of experimental procedures and their
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We are grateful for the financial support from the National
Basic Research Program of China [(973)-2015CB856603],
the Strategic Priority Research Program of the Chinese Acad-
emy of Sciences (Grant No. XDB20000000), and the Nation-
al Natural Science Foundation of China (20472096,
21372241, 21572052, 20672127, 21421091, 21372250,
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Phosphine-Mediated Dimerization of Conjugated Ene-Yne
Ketones: Stereoselective Construction of
Dihydrobenzofurans
9
Adv. Synth. Catal. 2017, 359, 1 – 9
Cheng-Zhi Zhu, Yao-Liang Sun, Yin Wei, Min Shi*
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