Organophosphane-Promoted Synthesis of Functionalized α,β-Unsaturated Alkenes and Furanones via Direct β-Acylation

Article abstract of DOI:10.1021/acs.orglett.9b03116

We report a phosphine-mediated direct β-acylation of α,β-unsaturated 1,3-diketones with acyl chlorides and a base. Functionalized furanones were also prepared by the reaction of cinnamic acid and acyl chloride according to our protocol via β-acylation. Our studies revealed that α,β-unsaturated 1,3-diketones with an electron-donating group at the second position favor the formation of β-acylated products, whereas those with oxygen, such as anhydrides, favor furanones via an unprecedented C-acylation/cyclization sequence.

Full text of DOI:10.1021/acs.orglett.9b03116

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Organophosphane-Promoted Synthesis of Functionalized α,β-
Unsaturated Alkenes and Furanones via Direct β‑Acylation
Yan-Cheng Liou, Yin-Hsiang Su, Kuan-Chun Ku, Athukuri Edukondalu, Chun-Kai Lin,
You-Syuan Ke, Praneeth Karanam, Chia-Jui Lee, and Wenwei Lin*
Department of Chemistry, National Taiwan Normal University, 88, Sec. 4, Tingchow Road, Taipei 11677, Taiwan, R.O.C.
S
* Supporting Information
ABSTRACT: We report a phosphine-mediated direct β-
acylation of α,β-unsaturated 1,3-diketones with acyl chlorides
and a base. Functionalized furanones were also prepared by
the reaction of cinnamic acid and acyl chloride according to
our protocol via β-acylation. Our studies revealed that α,β-
unsaturated 1,3-diketones with an electron-donating group at
the second position favor the formation of β-acylated
products, whereas those with oxygen, such as anhydrides, favor furanones via an unprecedented C-acylation/cyclization
sequence.
Scheme 1. (a) Synthesis of Furans and (b) β-Acylation of
α,β-Unsaturated 1,3-Diketones and Anhydrides
hosphine-mediated reactions of electron-deficient alkenes
Phave emerged as a powerful tool in organic synthesis to
construct biologically active and medicinally useful com-
pounds.1 Owing to the divergent mechanistic roles played by
the phosphonium species, their study greatly benefits the
development of a new type of organophosphane-mediated
reaction.2 On the contrary, the direct transformation of
electron-deficient alkene C−H bonds into C−C bonds is a
fundamental and important challenge and has substantial
benefits in organic synthesis.3 Such methods are mostly
developed by utilizing organometallic reagents and transition-
metal catalysts.4 Considering the prominence of this trans-
formation in organic synthesis, the development of efficient
methods is highly desirable due to their potential to construct
complex molecules.
Previously, we demonstrated a novel method for the
synthesis of furocoumarins and cyano-substituted furans from
cinnamoyl-4-hydroxy-chromenones and 2-cinnamoyl-cyanoa-
cetophenones by using PBu3 and acyl chloride via the C-
acylation/cyclization sequence (Scheme 1a).5 As part of our
efforts in the exploration of organophosphane chemistry,6 we
were interested in the extension of this reaction by designing
appropriate α,β-unsaturated carbonyl compounds. Here we
report a new method for the synthesis of functionalized α,β-
unsaturated 1,3-diketones and furanones from respective α,β-
unsaturated 1,3-diketones and acids/anhydrides via direct β-
acylation (Scheme 1b). This result was quite surprising
because it resulted in the β-acylation adduct, as opposed to
our hypothesis that the presence of acyclic compounds might
facilitate the betaine formation and result in a Wittig product.
Initially, we treated dibenzylated α,β-unsaturated 1,3-
diketone (1a) with Bu3P and benzoyl chloride (2a) in the
presence of Et3N in CH2Cl2 under Wittig reaction conditions
(Table 1). Two products, 3aa and 4aa, were obtained in 87%
yield and a trace amount of yield, respectively, within 1 h
(entry 1). To our surprise, the obtained product 3aa was
further characterized as a β-acylation adduct. In such an
instance, we have realized that the carbon center between two
ketone groups plays a very crucial role to control the reaction
pathways to provide different products. Previously, we have
also reported the first β-acylation of 2-arylidene-1,3-indan-
diones with acyl chlorides and a base catalyzed by organo-
phosphane.7
The examination of several other phosphines could not
enhance the yield of the 3aa because most of the phosphines
were found to be ineffective, except for PEt2Ph and PMe2Ph
(entries 1−4). The evaluation of other factors such as the
reaction media and the base (entries 5−8) established CH2Cl2
as a better solvent and Et3N as an optimal base. (See the SI.)
Next, the effect of the stoichiometry of PBu3 was also tested,
and the results indicated that the β-acylation product 3aa
Received: September 3, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03116
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
products 3fa and 3ia in good yield (78 and 74%) within 2 h,
whereas 1j bearing an ortho-chloro substitution furnished the
product 3ja in low yield (34%), even after 43 h, as a
consequence of steric hindrance.
Table 1. Optimization of the Reaction Conditions
Furthermore, different acyl chlorides 2b−m were also tested
for the reaction with 1a. The acyl chlorides 2b and 2g with a
para-chloro and meta-chloro substitution furnished the
products 3ab and 3ag in 76−80% yield, whereas 2k with an
ortho-chloro substitution gave the product 3ak in only 43%
yield. Surprisingly, acyl chloride 1f, having a strongly electron-
withdrawing nitro group, resulted in the product 3af in only
24% yield. In addition, heteroaryl-substituted acyl chlorides,
such as 2l and 2m (R3 = 2-thienyl and 2-furyl) also worked
well with 1a to furnish the products 3al and 3am in up to 60%
yield within 1 h.
Next, the influence of geminal disubstitution on the course
of the reaction was investigated. Replacing the sterically bulky
benzyl groups with methyl groups also successfully furnished
the desired β-acylated product 6 within 1 h in 60% yield
(Scheme 3a). However, when a substrate bearing a mono
b
entry
PR3
PBu3
PPh3
PMe2Ph
PEt2Ph
PBu3
PBu3
PBu3
PBu3
PBu3
solvent
base
t (h)
3aa (%)
c
1
2
3
4
5
6
7
8
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
CH3CN
C2H4Cl2
CH2Cl2
CH2Cl2
Et3N
Et3N
Et3N
Et3N
DIPEA
Et3N
Et3N
Et3N
Et3N
Et3N
1
1
1
1
2
1
1
1
18
1
87 (trace)
c
c
c
c
c
53 (5)
78 (1)
80 (2)
65 (3)
75 (3)
79
19
28
d
9
e
10
PBu3
a
Reactions were carried out with compound 1a (0.1 mmol), PR3 (1.2
equiv), PhCOCl (2a) (1.1 equiv), and base (1.2 equiv) in dry solvent
(0.5 mL) under argon at 30 °C. Yield of the product 3aa was
determined by NMR analysis of the crude reaction mixture using
b
c
d
Ph3CH as an internal standard. NMR yield of the product 4aa. PBu3
e
(0.2 equiv). PBu3 (0.5 equiv).
Scheme 3. Examination of Geminal Disubstitution
could be obtained in good yield only when a stoichiometric
amount of PBu3 was employed (entries 9 and 10). Finally, the
most suitable conditions for the reaction were determined as
listed in entry 1.
With the optimal conditions in hand, we next investigated
the substrate scope, and the results are shown in Scheme 2.
a b
,
Scheme 2. Synthesis of Compound 3 via β-Acylation
substitution 7 was tested, surprisingly, the 3-furanone product
8 was obtained (Scheme 3b). It could be understood that the
β-acylation adduct generated would still exist in the enolic
form and facilitate the intramolecular oxa-Michael addition,
resulting in the 3-furanone product 8. The structure of 3-
furanone 8 was further confirmed by X-ray diffraction analysis.
(See the SI.) Furthermore, with an unsubstituted diketone 9,
the reaction did not proceed, and the starting materials could
be recovered (Scheme 3c). This could be due to the existence
of diketone 9, predominantly in the enolic form, that renders
inertness toward the phospha-Michael addition.
Furthermore, we wished to introduce a little diversity to the
substrates by incorporating ester and nitrile groups. Under the
optimized conditions, ester-bearing substrate 10 afforded the
desired β-acylated product 11 in 47% yield whereas the nitrile
substrate 12 surprisingly resulted in the Wittig product 13 in
69% yield besides the β-acylated product 14 in only 20% yield
(Scheme 4). This gives a vital hint that the 1,3-diketone motif
is very crucial for the predominant formation of β-acylated
products.
Afterward, the reason for the formation of furan 4aa in our
optimization studies was also investigated. It was speculated
that the furan 4aa might be obtained via the β-acylated product
3aa. To test our hypothesis, the β-acylated adduct 3aa was
treated with PBu3 (1.1 equiv) in CH2Cl2. After 1 h, 4aa was
obtained in 26% yield. Similarly, 5 afforded furan 15 in 70%
yield via the β-acylated product 6 (Scheme 3a) after the
reaction time was prolonged to 72 h (Scheme 5). It is evident
that the reaction of β-acylated product formation is catalyzed
a
Reactions were carried out with 1 (0.1 mmol), PBu3 (1.2 equiv),
R3COCl (1.1 equiv), and Et3N (1.2 equiv) in dry CH2Cl2 (0.5 mL)
b
c
under argon at 30 °C. Isolated yield. NMR yield of the product 4.
d
Reaction was performed on 2 mmol scale.
Various substrates 1a−e reacted with PhCOCl (2a) to
generate the corresponding β-acylated products 3aa−ea in
moderate to good yield. Substrates 1a and 1d bearing electron-
withdrawing R2 substitutions provided greater yields of the
products 3aa and 3da as compared with the substrates 1b and
1c with electron-donating R2 substitutions, irrespective of their
position. Notably, substrate 1e bearing a heteroaryl sub-
stitution (R2 = 2-furyl) also afforded the desired product 3ea in
78% yield. Next, the effect of R1 substitution was examined
with substrates 1f−j. It was found that the electronic and steric
properties of substituents have a significant impact on the
reaction outcome. The diketones 1f and 1i bearing chloro
group at the para and meta positions, respectively, gave desired
B
DOI: 10.1021/acs.orglett.9b03116
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Organic Letters
Letter
furanones were obtained. It was delightful to find that the
cinnamic acid derivative 16b was more suitable for the reaction
because it provided a good to high yield of the corresponding
furanones (up to 78%).
Scheme 4. Examination of Other Compounds
To investigate the mechanism for the formation of furanone
17, several control experiments have been done to confirm the
intermediates like ketene, the β-acylated adduct, and the
reactivity of anhydride and imide.9 Unfortunately, the reaction
of α,β-unsaturated imide 23 with PBu3, benzoyl chloride, and
Et3N was unsuccessful in furnishing the desired product, and
only 24 and 25 were obtained under the standard reaction
conditions.10 To our delight, the reaction of furanone 17bg
with PBu3 and NH4Cl afforded the β-acylated cinnamic acid
18 in 90% yield (Scheme 7). It is evident that the furanone 17
further reacts with PBu3 to undergo the reversible reaction.
by PBu3, and a second phospha-Michael addition to 6 leads to
the Wittig product 15.
Scheme 5. Generation of Furans 4aa/15 from 3aa/5
Scheme 7. Control Experiment for β-Acylated Compound of
18
Next, we turned our attention to test the reactivity of other
1,3-dicarbonyl compounds such as anhydrides. Accordingly, we
have carried out the reaction with cinnamic acid 16a, 4-
BrPhCOCl (2c, 2.2 equiv), Et3N (2.4 equiv), and PBu3 (1.2
equiv) following our optimized protocol. Surprisingly, instead
of β-acylated product, 2-furanone derivative 17ac was obtained
in 51% yield after 3 h. It could be reasoned that the β-acylation
adducts generated were reactive enough to undergo a
nucleophilic addition with phosphine and generate furanone
17ac. Owing to the diverse biological applications of
furanones,8 we wished to pursue this study a little further.
After a brief screening of phosphines, different bases, solvents,
and various other factors, the optimal results were obtained.
(See the SI.)
With the optimized reaction conditions established, the
scope of the other substrates was investigated (Scheme 6).
Various acyl chlorides 2a−j were tested for the reaction with
different acids such as 16a, 16b, and 16c. The results indicate
that the acyl chlorides bearing electron-withdrawing groups
were more effective for the reaction than the ones that bear
electron-donating groups. Moreover, in the case of acids 16a
and 16c, only moderate yields (up to 51%) of desired
On the basis of these results, a plausible reaction mechanism
is proposed in Scheme 8. The reaction is initiated by the
Michael addition of PBu3 to 1, giving rise to the zwitterion I
that undergoes O-acylation with acyl chloride 2 to result in
phosphonium salt II. The protonation by Et3N generates ylide
III, which, upon subsequent intramolecular cyclization,
generates the betaine intermediate IV. The carbonyl group
of ketone functionality then presumably facilitates the cleavage
of the C−O bond and the expulsion of PBu3 to afford the
corresponding β-acylated product 3 via intermediate V.
Notably, upon prolonging the reaction time, the regenerated
PBu3 triggers a second Michael addition onto product 3,
leading to oxaphosphetane XI via zwitterionic species X. The
subsequent elimination of phosphine oxide from XI results in
the formation of furan product 4. This could be the reason for
the utilization of a stoichiometric amount of phosphine.
On the contrary, the addition of PBu3 to the α,β-unsaturated
anhydride 19 (formed from acid 16 and acyl chloride 2)
generates a ketene-bearing phosphonium salt VI, the presence
of which was confirmed by HRMS analysis.9 The deprotona-
tion of ketene VI by Et3N leads to ylide VII, which undergoes
C-acylation with mixed anhydride to generate intermediate
VIII. Furthermore, the addition of carboxylate onto the
carbonyl of the ketene species VIII leads to IX, which upon
losing PBu3 gives β-acylated intermediate 20. The Michael
addition of PBu3 to 20 and the subsequent cyclization results
in XII via intermediate X. The addition of a carboxylate anion
to XII results in the expulsion of PBu3 to afford furanones 17.
In summary, we have demonstrated the novel metal-free
synthesis of β-acylated α,β-unsaturated 1,3-diketones and 2-
furanones by utilizing the α,β-unsaturated 1,3-diketones and
α,β-unsaturated acids, respectively, in the presence of PBu3,
acyl chloride, and a base. We have also developed the novel
concept of β-acylation of α,β-unsaturated carbonyl compounds
utilizing organophosphane, illustrated by using two conceptual
prerequisite substrates (1,3-diketone and anhydride), to
provide corresponding products in moderate to good yield.
a b
,
Scheme 6. Synthesis of 2-Furanones 17 via β-Acylation
a
Reactions were carried out with compound 16 (0.3 mmol), R3COCl
(2.2 equiv), Et3N (2.4 equiv), and PBu3 (1.2 equiv) in dry CH2Cl2
b
(1.5 mL) under argon at 30 °C. Isolated yield of the product 17.
c
Reaction was performed on a 1 mmol scale.
C
DOI: 10.1021/acs.orglett.9b03116
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Organic Letters
Letter
Scheme 8. Plausible Mechanism for the β-Acylation
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03116.
■
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Optimization data, experimental procedures, character-
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Accession Codes
CCDC 1484673, 1561009−1561010, 1845180, and 1947032
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wenweilin@ntnu.edu.tw.
ORCID
Yan-Cheng Liou: 0000-0002-1013-0808
Athukuri Edukondalu: 0000-0003-4593-3843
Praneeth Karanam: 0000-0003-3982-3525
Wenwei Lin: 0000-0002-1121-072X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology of the
Republic of China (MOST 107-2628-M-003-001-MY3) for
financial support.
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(9) Several control experiments have been done. The intermediates
22−26 were characterized by NMR, and the ketene phosphonium salt
VI was confirmed by the HRMS analysis. For the experimental details,
see the Supporting Information.
(10) Imide derivative 23 (nitrogen between both of the ketone
groups) has been examined to get desired furanones, but only 24 and
25 were obtained; see the Supporting Information.
E
DOI: 10.1021/acs.orglett.9b03116
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