PPh3/Selectfluor-Mediated Transformation of Carboxylic Acids into Acid Anhydrides and Acyl Fluorides and Its Application in Amide and Ester Synthesis

Article abstract of DOI:10.1002/ejoc.201901092

By taking the advantage of PPh₃/Selectfluor system, carboxylic acids are efficiently converted into the pivotal intermediates acyloxyphosphonium ions that can selectively react with a second carboxylic acid or fluoride to in situ yield the corresponding acid anhydrides or acyl fluorides. The developed protocol features commercially availabile reagents, no involvement of base, room temperature conditions, and simple experimental procedure. Additionally, various amides or esters are readily achieved, respectively, with the addition of amines or alcohols.

Full text of DOI:10.1002/ejoc.201901092

A Journal of
Accepted Article
Title: PPh3/Selectfluor-Mediated Transformation of Carboxylic Acids
into Acid Anhydrides and Acyl Fluorides and its application in
amide and ester synthesis
Authors: Zhen Yang, Siwei Chen, Fang Yang, Chenxi Zhang, You Dou,
Qiuju Zhou, Yizhe Yan, and Lin Tang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201901092
Link to VoR: http://dx.doi.org/10.1002/ejoc.201901092
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10.1002/ejoc.201901092
European Journal of Organic Chemistry
COMMUNICATION
PPh3/Selectfluor-Mediated Transformation of Carboxylic Acids
into Acid Anhydrides and Acyl Fluorides and its application in
amide and ester synthesis
Zhen Yang,[a] Siwei Chen,[a] Fang Yang,[a] Chenxi Zhang,[a] You Dou,[a] Qiuju Zhou,[a] Yizhe Yan,[b] and
Lin Tang*[a]
Abstract: By taking the advantage of PPh3/Selectfluor system,
carboxylic acids are efficiently converted into the pivotal
intermediates of acyloxyphosphonium ions that can selectively react
with a second carboxylic acid or fluoride to in situ yield the
corresponding acid anhydrides or acyl fluorides. The developed
protocol features commercial availability of reagents, no involvement
of base, room temperature conditions and simple experimental
procedure. Additionally, various amides or esters are readily
achieved, respectively, with the addition of amines or alcohols.
to afford the amides or esters.[18] Recently, PPh3/CCl3CN-
promoted transformation of carboxylic acids was developed by
Kim and Jang as a supplemental method for acid anhydride
synthesis, even though toxic CCl3CN and additional
triethylamine were used.[19] Very recently, Prakash and co-
workers successfully disclosed a protocol for the synthesis of
acyl fluorides from carboxylic acids by use of a PPh3/NBS/Et3N-
3HF system.[20] However, additional NBS and dangerous Et3N-
3HF were required. Encouraged by these achievements and our
interest in green synthesis,[21] especially in organofluorine
chemistry,[22] we envisioned that PPh3 and Selectfluor might
activate carboxylic acids to offer acyloxyphosphonium ions and
F- (F- was generated from F+ in situ). Then, a second carboxylic
acid or F- could attack the acyloxyphosphonium ions, affording
the corresponding acid anhydrides or acyl fluorides (Scheme 1c).
The proposed protocol employed commercially available
reagents PPh3 and Selectfluor at room temperature without the
involvement of base. Upon adding amines or alcohols, the
construction of amides or esters was efficiently realized.
Introduction
Carboxylic acid derivatives, including acid anhydrides and acyl
fluorides, are important and versatile intermediates in synthetic,
medicinal and material chemistry since these structural motifs
afford straightforward access to a variety of high-value amides,
esters and other functional compounds.[1] In this context, the
significant number of strategies have been successfully
achieved for the preparation of the above carboxylic acid
derivatives. For example, acid anhydrides are usually prepared
through reactions of carboxylic acids with dehydrative partners,
such as phosgene,[2] thionyl chloride,[3] isocyanates,[4] 1,3,5-
triazines,[5]
Vilsmeier-Haack
reagent,[6]
carbodiimides,[7]
phosphoranes[8] and pyridazin-3(2H)-ones,[9] and carboxylate
salts with acylating reagents.[10] The reported protocols for acyl
fluoride synthesis directly from carboxylic acids mainly rely on
the use of cyanuric fluoride,[11] HF/pyridine/DCC complex,[12]
SeF4/pyridine,[13] sulfur-based fluorinating reagents[14] and
(Me4N)SCF3.[15] However, the existing protocols remain limited
because of non-commercial availability of starting materials,
toxicity, thermal instability, high reaction temperature and low
functional group compatibility. Therefore, exploring a more
practical and convenient approach to synthesis of acid
anhydrides and acyl fluorides is highly desirable.
Organophosphorus compounds, which serve as efficient
coupling reagents to activate alcohols, have been extensively
applied in Mitsunobu[16] and Appel[17] reactions. In addition,
carboxylic acids could also be activated by phosphorus reagents
Scheme 1. Phosphorus reagents-mediated transformation of carboxylic acids
into acid anhydrides and Acyl Fluorides.
Results and Discussion
[a]
Dr. Z. Yang, S. Chen, F. Yang, C. Zhang, Y. Dou, Dr. Q. Zhou, Prof.
Dr. L. Tang
As depicted in Table 1, 4-methylbenzoic acid (1b) was chosen
as the model substrate to optimize the reaction conditions. The
effect of alkaline additves was initially investigated in the
presence of PPh3/Selectfluor. The corresponding product 2b
was obtained in 78% and 84% yields, when K2CO3 and Et3N
were added to the reaction, respectively (entries 1 and 2).
Interestingly, an excellent yield of 92% was achieved without
any additive (entry 3). Subsequent experiments were carried out
to evaluate various solvents (entries 4-7). CH3CN could afford
2b in 93% yield, and relative low yields were obtained in THF
College of Chemistry and Chemical Engineering
Xinyang Normal University
Xinyang 464000 (P. R. China)
E-mail: lintang@xynu.edu.cn
https://www.researchgate.net/profile/Lin_Tang27
Prof. Dr. Y. Yan
School of Food and Biological Engineering
Zhengzhou University of Light Industry
Zhengzhou 450000 (P. R. China)
[b]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201901092
European Journal of Organic Chemistry
COMMUNICATION
and DMF. However, the construction of 2b was absolutely
suppressed when DMSO was employed as the solvent, which is
probably due to the side reaction of DMSO with Selectfluor. As
contrast, other organophosphorus compounds, such as P(OEt)3
and P(n-Bu)3, were also evaluated, and no higher yield was
achieved (entries 8 and 9). Further study indicates that reducing
the amount of coupling reagents is unfavourable to the reaction
(entry 10). In addition, the yield of 2b was slightly decreased
when the reaction temperature was increased to 50 oC. The
including phenylacetic, cyclohexanic, cyclopentanic, t-butanic
and adamantanic acid, could take place smoothly to give the
target materials 2s-2y in good yields. It is interesting to note that
acyl fluorides were observed after all the reactions by 19F NMR
analyses, although most of them could not be isolated.
above results show that the formation of 3b is difficult.
Table 1. Optimization of reaction conditions.[a]
Entry
Coupling reagent
Additive
Solvent
Yield [%][b]
2b 3b
1
PPh3/Selectfluor
PPh3/Selectfluor
PPh3/Selectfluor
PPh3/Selectfluor
PPh3/Selectfluor
PPh3/Selectfluor
PPh3/Selectfluor
P(OEt)3/Selectfluor
P(n-Bu)3/Selectfluor
PPh3/Selectfluor
PPh3/Selectfluor
K2CO3
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
THF
78 < 5
2
Et3N
--
84 trace
92 n.d.
3
4
--
93 trace
78 trace
n.d. trace
82 trace
59 n.d.
5
--
6
--
DMSO
DMF
7
--
8
--
CH3CN
CH3CN
CH3CN
CH3CN
9
--
88 trace
77 trace
91 trace
10[c]
11[d]
--
--
[a] Reactions conditions: 1b (0.25 mmol), organophosphorus compound (0.30
mmol), Selectfluor (0.30 mmol), additive (0.3 mmol), solvent (1.0 mL), room
temperature, 4 h, N2 atmosphere. [b] Isolated yield; n.d. = not detected. [c]
PPh3 (0.25 mmol), Selectfluor (0.25 mmol). [d] 50 oC.
After confirming the optimized conditions, we attempted to
extend the scope of carboxylic acids (Scheme 2). Excellent
yields (> 89%) were obtained when electron-donating groups
methyl and methoxyl appeared on the benzene ring (2b-2e).
Carboxylic acids bearing weakly electron-withdrawing groups,
such as fluoro and chloro, could afford the corresponding
products 2f-2h in good yields. Interestingly, the desired products
of acyl fluorides 3i and 3j were isolated. Moreover,
polysubstituted carboxylic acids could also afford the products
2k-2n in 86-92% yields, regardless of the large steric hindrance.
Products 2o, 2p and 2r bearing 4-vinyl, 2-phenyl and 2-
phenoxyl, respectively, were readily achieved in moderate to
good yields. However, alkenyl acid 1q represented low
chemoselectivity, and the corresponding 3q was obtained up to
Scheme 2. Reaction conditions: 1 (0.25 mmol), PPh3 (0.30 mmol), Selectfluor
(0.30 mmol), CH3CN (1.0 mL), room temperature, 4 h, N2 atmosphere; isolated
yield.
39% yield.
A lactone 4u were given via intramolecular
dehydration when 2-(2-hydroxyphenyl)acetic acid was used as
the substrate. In addition, reactions of various aliphatic acids,
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COMMUNICATION
A tandem amidation or esterification was readily achieved with
the addition of amines or alcohols to the reaction mixture
(Scheme 3). Aliphatic amines such as n-octylamine, morpholine,
phenylethylamine and benzylamine were successfully employed
as the substrates, affording the corresponding products (4a-4d,
4f and 4g) in good yields. Increasing the reaction temperature
could also offer the desired esters 4h-4k using alcohols as the
substrates. In addition, aniline and phenol were also active
substrates with the assistance of K2CO3. Compared to previous
reports, this protocol avoids the isolation of acid anhydrides and
acyl fluorides.[23,24]
994.4 Hz) and 19F NMR signal at δ -128.7 (d, J
= 992.5 Hz).
PF
When 1b was added to the above reaction mixture, the 31P and
19F NMR signals of fluorophosphonium ion
A
rapidly
disappeared. And a new peak with 31P NMR signal at δ 64.7 (s)
was observed, which were assigned to the pivotal species
acyloxyphosphonium ion E. Another new 19F NMR signal of acyl
fluoride 3b at
δ 15.9 (s) was represented. In addition,
triphenylphosphine oxide and F2PPh3 were generated during the
process with 31P NMR signals at δ 30.2 (s) and 54.1 (t, J
=
PF
655.1 Hz), respectively.
On the basis of the results above and previous reports,[20]
a
plausible reaction mechanism is proposed as shown in Scheme
5. The reaction of PPh3 with Selectfluor generates
fluorophosphonium ion A, which is rapid converted into
acyloxyphosphonium ion E in the presence of carboxylic acid 1.
Another 1 then attacks E to give the major product acid
anhydride
2
along with the generation of HF and
triphenylphosphine oxide. The in-situ formed F- from Selectfluor
can also attack E to afford small amount acyl fluoride 3.
Scheme 5. Proposed reaction mechanism.
Scheme 3. Reaction conditions: 1)
Selectfluor (0.30 mmol), CH3CN (1.0 mL), room temperature,
1
(0.25 mmol), PPh3 (0.30 mmol),
h, N2
4
atmosphere; 2) then amines or alcohols (0.125 mmol), room temperature, 4 h,
N2 atmosphere, isolated yield. [a] 80 oC. [b] K2CO3 (0.125mmol).
Conclusions
In conclusion, PPh3 and Selectfluor can serve as an efficient and
simple reagent system to activate carboxylic acids, affording the
desired acid anhydrides in good to excellent yields, as well as
acyl fluorides in some cases. Mechanistic studies indicate that
the
reaction
proceeds
through
active
species
of
acyloxyphosphonium ions, which can be quickly converted into
the final products in situ by a second carboxylic acid or F-. Thus,
this method represents prominent advantages of stable
commodity chemicals, broad substrate scope, no use of base,
room temperature conditions, scale-up synthesis and
operational simplicity. Upon adding amines or alcohols, the
formation of a wide array of amides or esters is readily realized.
Further exploration of PPh3/Selectfluor system are currently
underway in our laboratory.
Scheme 4. Scale-up experiments under standard conditions.
The scale-up reactions were carried out under standard
conditions to further assess the synthetic utility of the developed
protocol. Pleasingly, 5 Mmol of 1b could give the desired
products 2b and 4c in good yields (Scheme 4).
Experimental Section
General information
To gain insight into the possible mechanism of these
transformations, the corresponding control experiments were
conducted (see the supporting information for details). Mixing
PPh3 with Selectfluor led to the construction of
Proton nuclear magnetic resonance (1H NMR) spectra, carbon nuclear
magnetic resonance (13C NMR) spectra, 19F fluorine spectra (19F NMR)
and 31P phosphorus spectra (31P NMR) were recorded on a JNM-
ECZ600R/S3 (1H NMR 600 MHz, 13C NMR 150 MHz, 19F NMR 564 MHz
fluorophosphonium ion A with 31P NMR signal at δ 95.0 (d, J PF
=
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10.1002/ejoc.201901092
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COMMUNICATION
and 31P NMR 243 MHz). HRMS were recorded on a MicroMass Waters
Xevo G2-XS QTof. A uncorrected Beijing Tech Instrument Co., LTD X-4
melting point apparatus was used to determine melting points. Unless
otherwise indicated, all reagents were purchased commercially without
further purification.
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General reaction procedures for the synthesis of 2 and 3
To an ovened-dried 10 mL schlenk tube was added carboxylic Acid 1
(0.25 mmol), PPh3 (0.30 mmol) and Selectfluor (0.30 mmol). The tube
was evacuated and back-filled with nitrogen, which was repeated three
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General reaction procedures for the synthesis of 4
To an ovened-dried 10 mL schlenk tube was added carboxylic Acid 1
(0.25 mmol), PPh3 (0.30 mmol) and Selectfluor (0.30 mmol). The tube
was evacuated and back-filled with nitrogen, which was repeated three
times. Anhydrous CH3CN (1.0 mL) was then added to the reaction
mixture via syring. The above mixture was stirred at room temperature
for 4 h. After the reaction was finished, amine or alcohol (0.125 mmol)
was added. Then, the aforementioned mixture continued to be stirred at
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4 h. The resulting mixture was
concentrated under reduced pressure. The obtained crude residue was
purified by flash column chromatography on silica gel with hexane and
ethyl acetate (v:v = 8:1 to 3:1) as eluent to give the desired product 4.
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Acknowledgements
This work was supported by no. 21602190 from the National
Natural Science Foundation of China, no. 172102210456 from
the Foundation of Department of Science and Technology of
Henan Province, no. 17A150049 and 18A150049 from the
Henan Key Scientific Research Project, and the Nanhu Scholars
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Keywords: triphenylphosphine • Selectfluor • carboxylic acids •
acid anhydrides • acyl fluorides
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10.1002/ejoc.201901092
European Journal of Organic Chemistry
COMMUNICATION
An efficient synthesis of acid
anhydrides or acyl fluorides
mediated by PPh3/Selectfluor
synstem is reported. Mechanistic
studies show that the reaction
proceeds through active species
of acyloxyphosphonium ions,
which can be quickly converted
into the final products in situ by a
second carboxylic acid or F-.
Carboxylic Acid Derivatives
Z. Yang, S. Chen, F. Yang, C.
Zhang, Y. Dou, Q. Zhou, Y. Yan,
and L. Tang*
Page No. – Page No.
PPh3/Selectfluor-Mediated
Transformation of Carboxylic
Acids into Acid Anhydrides and
Acyl Fluorides and its application
in amide and ester synthesis
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