N-Methylpyrrolidin-2-one-Promoted Formation of Functional Esters through C–O Bond Cleavage

Article abstract of DOI:10.1002/ejoc.201700514

Trifluoromethanesulfonic acid catalyzed C–O bond cleavage leading to the preparation of functional esters in the presence of N-methylpyrrolidin-2-one (NMP) was accomplished. Various substrates were well tolerated, and a gram-scale experiment was successfully realized. DFT calculations indicated that NMP plays a decisive role in accelerating nucleophilic attack of the functional acid to generate the functional esters in chlorobenzene.

Full text of DOI:10.1002/ejoc.201700514

DOI: 10.1002/ejoc.201700514
Communication
Cleavage Reactions
N-Methylpyrrolidin-2-one-Promoted Formation of Functional
Esters through C–O Bond Cleavage
Jianming Liu,*[a] Yanyan Wang,[a] Yuanyuan Yue,[a] Na Liu,[a] Jian Zhang,[a] Shufang Zhao,[a]
Qinghu Tang,*[a] and Kelei Zhuo[a]
Abstract: Trifluoromethanesulfonic acid catalyzed C–O bond ment was successfully realized. DFT calculations indicated that
cleavage leading to the preparation of functional esters in the NMP plays a decisive role in accelerating nucleophilic attack of
presence of N-methylpyrrolidin-2-one (NMP) was accomplished. the functional acid to generate the functional esters in chloro-
Various substrates were well tolerated, and a gram-scale experi- benzene.
Introduction
the presence of trifluoromethanesulfonic acid (HOTf) in chloro-
benzene to afford the benzylic cation. Then, Friedel–Crafts
alkylation by chlorobenzene to the reactive benzylic cation
would be a key step in the formation of substituted diphenyl-
methanes (Scheme 1, path I). For the purpose of constructing
functional esters, we thought that a suitable additive could be
found to suppress the Friedel–Crafts alkylation process. The key
intermediate, the benzylic cation, could then be trapped by the
functional acid with the aid of an additive to generate the
desired ester (Scheme 1, path II). In previous reports, NMP was
shown to play a key role in thiazolidinethione-promoted asym-
metric aldol reactions.[17] On the basis of this hypothesis, we
envisioned that N-methylpyrrolidin-2-one (NMP) could interact
with the benzylic cation to stabilize the generated carbocation.
Furthermore, it could rapidly and irreversibly undergo nucleo-
philic attack by the functional acid with subsequent transforma-
tion. Therefore, NMP could be an ideal additive to achieve this
strategy. Herein, we demonstrate that NMP suppresses the
Friedel–Crafts alkylation process and promotes nucleophilic at-
tack of functional acids to generate functional esters.
Ester groups have emerged as powerful building blocks for the
preparation of complicated skeletons, and they are also a key
structural motif in pharmaceutical and natural compounds.[1] To
date, the construction of esters has largely relied on the reac-
tion of activated acid derivatives (e.g., acyl chlorides and an-
hydrides) with alcohols.[2] An alternative route involves the tran-
sition-metal-catalyzed direct carbonylation of aryl halides.[3,4]
Further efforts have been devoted to the direct formation of
functional esters from the esterification of alcohols with alde-
hydes and alcohols.[5–8] Recently, the groups of Jiao and Xu
realized C–C and C–N bond-cleavage reactions, respectively, to
form esters.[9] All of the above routes have their individual ad-
vantages, but they also have their drawbacks, including limited
substrate scope, the use of expensive/toxic reagents, and the
use of harsh reaction conditions. It is well known that aryl alkyl
ethers widely exist in nature. From the viewpoint of sustainable
chemistry, the development of new routes for C–O bond cleav-
age would be a significant advancement from the perspectives
of abundance and cost of starting materials and has remained
underexplored until now.
Recently, cleavage of the C–O bond was demonstrated to be
a powerful strategy to construct C–C bonds, C–H bonds, C–N
bonds, and other bonds.[10–14] Whereas the utility of aryl alkyl
ethers and related compounds as important counterparts has
witnessed dramatic progress, cleavage of the C–O bond in aryl
alkyl ethers could potentially open up new perspectives in the
construction of organic compounds.[15,16] We envisioned that
aryl benzyl ethers could undergo cleavage of the C–O bond in
Results and Discussion
To probe the feasibility of our proposed assumption, we started
our investigation by using 2-(benzyloxy)benzaldehyde (1a) and
cinnamic acid (2a) as the model substrates. First, benzyl
cinnamate (3a) was not observed in the presence of HOTf and
the substituted diphenylmethane was detected by GC–MS
(Table 1, entry 1). To our delight, upon using NMP (10 μL) as an
additive to suppress the Friedel–Crafts reaction, a trace amount
of the target product was obtained (Table 1, entry 2). Encour-
aged by this result, different amounts of NMP were investigated.
It was found that 50 μL was a suitable amount for NMP, and 3a
was obtained in the best yield of 95 % (Table 1, entries 3–6).
Moreover, the employment of trifluoromethanesulfonic salts,
aside from AgOTf, allowed the reaction to proceed smoothly,
and the desired product was generated in excellent yields
[a] Collaborative Innovation Center of Henan Province for Green
Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical
Media and Reactions, Ministry of Education, School of Chemistry and
Chemical Engineering, Henan Normal University
Xinxiang, Henan 453007, P. R. China
E-mail: jmliu@htu.cn
qinghutang@sina.com
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.201700514.
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Scheme 1. NMP-promoted construction of functional esters through C–O bond cleavage.
(Table 1, entries 7–11). Finally, screening of the solvent revealed oxy)methyl]benzonitrile was less reactive and gave desired es-
that chlorobenzene was the best choice for this reaction ter 3k in a low yield. We believe that the CN group affected
(Table 1, entries 12–14).
cleavage of the C–O bond and precluded formation of the de-
sired product. In addition, a series of substituted cinnamic acids
bearing different functional groups, such as methyl, methoxy,
and chloro, could be easily converted into corresponding prod-
ucts 3l–r in good yields.
Table 1. Optimization of the reaction conditions.[a]
Encouraged by the above results, we next investigated the
scope of the reaction with respect to 1a and different substi-
tuted acids (Table 3). A wide range of benzoic acid derivatives
including those with methyl, chloro, and fluoro substituents
were all tolerated under the optimized conditions, and they
afforded corresponding products 5a–d in 86–94 % yield. 1-
Naphthoic acid and [1,1′-biphenyl]-4-carboxylic acid were both
suitable substrates, and desired products 5e and 5f were ob-
tained in yields of 82 and 80 %, respectively. Notably, the
heterocyclic acid benzo[b]thiophene-2-carboxylic acid success-
fully underwent the reaction to furnish target product 5g in
69 % yield. In addition, a long-alkyl-chain-substituted acid un-
derwent the reaction smoothly to generate desired ester 5h in
87 % yield.
Entry
Catalyst
Additive (μL)
Solvent[a]
Yield[b] [%]
1
2
3
4
5
6
7
HOTf
HOTf
HOTf
HOTf
–
–
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
HOTf
N.D.
trace
57
79
87
95
79
trace
93
95
NMP (10)
NMP (20)
NMP (30)
NMP (40)
NMP (50)
NMP (50)
NMP (50)
NMP (50)
NMP (50)
NMP (50)
NMP (50)
NMP (50)
–
–
–
8[c]
9[c]
10[c]
11[c]
12
13
14
AgOTf
Al(OTf)3
Fe(OTf)3
Cu(OTf)2
HOTf
HOTf
HOTf
89
40
31
N.R.
HOTf
1,4-dioxane
NMP
To investigate the effect of the leaving group at the position
of 2-(benzyloxy)benzaldehyde, valuable functional groups, such
as chloro and naphthyl substituents, were examined and found
to be compatible with the catalytic reaction, and the substrate
with a naphthyl group furnished a higher yield of the product.
Then, 1-[2-(benzyloxy)phenyl]ethan-1-one derivatives were effi-
ciently converted into the desired products in good yields. No-
tably, (benzyloxy)benzene without any substituents was tested
under the optimized conditions, and the corresponding prod-
uct was obtained in 54 % yield. This result suggested that a 2-
aldehyde-substituted substrate could easily act as a better leav-
ing group owing to its electron-deficient nature. This protocol
was also successfully applied to cleave the C–O bond of 1,2-
bis(benzyloxy)benzene, which resulted in the desired product
[a] Reaction conditions: 1a (0.30 mmol), 2a (0.60 mmol), catalyst (40 mol-%),
solvent (1.0 mL), 130 °C, 24 h. [b] Yield of isolated product; N.D.: desired
product not detected; N.R.: no reaction. [c] Catalyst (20 mol-%).
With the optimized reaction conditions in hand, we explored
the scope of the substrates, and the results are summarized in
Table 2. First, the effect of substituents on the aryl group at-
tached to 2-(benzyloxy)benzaldehyde was investigated in de-
tail. Reactions of substrates bearing an electron-donating group
(e.g., Me) in the ortho or meta position proceeded smoothly to
afford desired products 3b and 3c in yields of 85 and 57 %,
respectively. Furthermore, substrates bearing electron-with-
drawing groups (e.g., Cl, F, and Br) in the ortho, meta, or para
position reacted successfully and afforded desired products in 93 % yield (Table 4). In addition, benzyl ethyl ether afforded
3d–j in yields of 44–94 %. Unfortunately, 4-[(2-formylphen- the desired product in 84 % yield. Unfortunately, di-n-butyl
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Table 2. Substrate scope of benzylic ethers.[a]
[a] Reaction conditions: 1 (0.30 mmol), 2 (0.60 mmol), HOTf (40 mol-%), NMP (50 μL), chlorobenzene (1.0 mL), N2 atmosphere, 130 °C, 24 h. Yields of the
isolated products are given.
ether did not undergo the C–O bond-cleavage reaction to form quently took place. Then, a deuterium-labeling experiment was
the corresponding product (Scheme 2).
performed, and deuterium-substituted salicylaldehyde con-
firmed that the hydrogen atom of benzoic acid was transferred
to salicylaldehyde (Scheme 4b).
To investigate the role of NMP in the reaction further, two
transition states, TSa and TSb (Scheme 5), were located by us-
ing density functional theory (DFT) calculations. As shown in
To our delight, the current protocol could be successfully
extended to a gram-scale reaction, and the product was deliv-
ered in 64 % yield (Scheme 3). This result showed that the ester-
ification reaction could be perfectly scaled by using HOTf and
NMP under the optimized conditions.
To understand the mechanism, substrate 1a was subjected Scheme 5, our computational results indicated that the path-
to the reaction with benzoic acid (2.0 equiv.), but the expected way associated with transition state TSa was more favorable,
product was not observed in the absence of NMP and benzoic and the free-energy barrier was found to be 5.0 kcal mol–1
acid remained unreacted (Scheme 4a). Salicylaldehyde and sub- lower than that of TSb. Thus, in the presence of the NMP addi-
stituted diphenylmethane were also detected by GC–MS. This tive, nucleophilic attack of cinnamic acid to generate desired
result demonstrated that substrate 1a was decomposed by ester 3a could occur preferentially over the Friedel–Crafts alkyl-
HOTf and that the Friedel–Crafts alkylation reaction subse- ation process.
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Table 3. Substrate scope of the acids.[a]
[a] Reaction conditions: 1a (0.30 mmol), 4 (0.60 mmol), HOTf (40 mol-%), NMP (40 μL), chlorobenzene (1.0 mL), N2 atmosphere, 130 °C, 24 h. Yields of the
isolated products are given.
Table 4. Substrate scope of the method.
[a] Reaction conditions: 6 (0.30 mmol), 2a (0.60 mmol), HOTf (40 mol-%), NMP (50 μL), chlorobenzene (1.0 mL), N2 atmosphere, 130 °C, 24 h. Yields of the
isolated products are given.
Scheme 2. Reaction between di-n-butyl ether and cinnamic acid.
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Scheme 3. Gram-scale experiment.
Scheme 4. Control experiments.
On the basis of the above experiments, a proposed reaction After salicylaldehyde leaves, intermediate II is easily afforded.
mechanism is illustrated in Scheme 6. Initially, 1a undergoes NMP interacts with intermediate II to form intermediate III,
protonation with the aid of HOTf to produce intermediate I. which is followed by decomposition to generate intermediate
Scheme 5. Possible pathways to generate 3a and VI. Values in parentheses are the relative free energies [kcal mol–1] with respect to 1a.
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Scheme 6. Proposed mechanism.
IV. In the above process, the interaction of NMP with the
benzylic cation not only stabilizes the generated carbocation
but also suppresses the Friedel–Crafts alkylation process. After
nucleophilic attack of cinnamic acid, desired product 3a is ob-
tained. Without the aid of NMP, Friedel–Crafts alkylation of chloro-
benzene to the reactive benzylic cation affords intermediate V.
Substituted diphenylmethane VI is observed after loss of
hydrogen.
Acknowledgments
We are grateful for financial support from the National Natural
Science Foundation of China (21573057, 21405034), project
funded by China Postdoctoral Science Foundation
(2014M552005 and 2015T80770), and the Program for Innova-
tive Research Team in University of Henan Province
(17IRTSTHN001).
Keywords: Cleavage reactions · Esterification · Ethers ·
Nucleophilic addition · Reaction mechanisms
Conclusions
In summary, we disclosed an elegant and efficient route for the
construction of functional esters in the presence of HOTf and
NMP. In this transformation, NMP plays important roles in sup-
pressing the Friedel–Crafts alkylation process and in promoting
nucleophilic attack of the functional acid to furnish a functional
ester. Various substituted esters were facilely obtained, and a
variety of functional group were tolerated. We also performed
the reaction on a gram scale under the optimized conditions.
It is believed that these results may inspire future efforts to
focus on the development of NMP to suppress the Friedel–
Crafts alkylation reaction to facilitate the preparation of other
functional compounds.
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Experimental Section
General Procedure: In a Schlenk tube, a mixture of 2-(benzyl-
oxy)benzaldehyde (0.30 mmol), cinnamic acid (0.60 mmol), HOTf
(40 mol-%), NMP (50 μL), and chlorobenzene (1.0 mL) was stirred
under a N2 atmosphere at 130 °C for 24 h. After cooling to room
temperature, the residue was purified by flash column chromatog-
raphy (hexane/ethyl acetate) to afford the desired product.
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Received: April 11, 2017
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