Copper-Catalyzed Decarboxylative Oxyalkylation of Alkynyl Carboxylic Acids: Synthesis of ?-Diketones and ?-Ketonitriles

Article abstract of DOI:10.1021/acs.orglett.9b00520

A novel copper-catalyzed decarboxylative oxyalkylation of alkynyl carboxylic acids with ketones and alkylnitriles via direct C(sp³)-H bond functionalization to construct new C-C bonds and C-O double bonds was developed. This transformation is featured by wide functional group compatibility and the use of readily available reagents, thus affording a general approach to ?-diketones and ?-ketonitriles. A possible mechanism is proposed.

Full text of DOI:10.1021/acs.orglett.9b00520

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Decarboxylative Oxyalkylation of Alkynyl
Carboxylic Acids: Synthesis of γ‑Diketones and γ‑Ketonitriles
Yi Li,† Jia-Qi Shang,† Xiang-Xiang Wang,† Wen-Jin Xia,† Tao Yang,† Yangchun Xin,‡ and Ya-Min Li*,†
†Faculty of Life Science and Technology, Kunming University of Science and Technology, Kunming 650500, P.R. China
‡Katzin Diagnostic & Research PET/MR Center, Nemours/Alfred I. DuPont Hospital for Children, Wilmington, Delaware 19803,
United States
S
* Supporting Information
ABSTRACT: A novel copper-catalyzed decarboxylative oxyalkylation of alkynyl carboxylic acids with ketones and alkylnitriles
via direct C(sp3)−H bond functionalization to construct new C−C bonds and C−O double bonds was developed. This
transformation is featured by wide functional group compatibility and the use of readily available reagents, thus affording a
general approach to γ-diketones and γ-ketonitriles. A possible mechanism is proposed.
γ-Diketones1 and γ-ketonitriles2 are important synthetic
building blocks for many biologically significant compounds
and pharmaceuticals. γ-Diketones are also well-known versatile
precursors for the synthesis of five-membered heteroarenes
(Paal−Knorr reaction). Thus, numerous methods have been
developed for the construction of γ-diketones and γ-
ketonitriles,3,4 and among them, introducing the carbonyl or
cyanomethyl group through a direct C(sp3)−H functionaliza-
tion of ketones or alkylnitriles has attracted considerable
attention owing to the avoidance of prefunctionalization,
exclusion of strong bases, and high atom economy.5−7 Wang et
al. reported that two ketones underwent an oxidative coupling
at their α-position to build the γ-diketones in the presence of
copper or silver salt.5a,b Carbo-carbonylation of arylalkenes
with ketones via a SOMO-enamine was also developed to
construct γ-diketones by Huang and Xie.5c Wang, Xing, and
co-workers reported the oxidative coupling of ketones with
alkenes via C(sp3)−H bond functionalization by a copper/
manganese catalytic system to form γ-diketones.5d The direct
oxidative coupling of alkenes with alkyl nitriles has also been
developed to construct γ-ketonitriles.6a Very recently, Wu,
Kim, and co-workers reported a 1,4-refunctionalization of β-
diketones with alkyl nitriles by iron catalysis for the synthesis
of γ-ketonitriles.6b Despite these advances, developing an
efficient and general method to construct γ-diketones and γ-
ketonitriles is still in need.
been made to construct C−C,9 C−N,10 C−P,11 C−B,12 C−
Si,13 and C−S14 bonds by the decarboxylative coupling
reactions of alkynyl carboxylic acids. The decarboxylative
difunctionalization of alkynyl carboxylic acids, such as
oxytrifluoromethylation,9c iodofluoroalkylation,9f nitroaminox-
ylation,10c oxyphosphination,11c hydroxysulfonylation,14e,f and
disulfonylation,15b were also achieved. However, the decar-
boxylative oxyalkylation of alkynyl carboxylic acids remained
underexplored. As part of our interest in C(sp3)−H bond
functionalization and decarboxylative coupling,15 herein we
present a novel and general decarboxylative oxyalkylation of
alkynyl carboxylic acids with ketones and alkylnitriles to
construct γ-diketones and γ-ketonitriles, respectively (Scheme
1).
Scheme 1. Decarboxylative Oxyalkylation of Alkynyl
Carboxylic Acids
Initially, the reaction of phenylpropiolic acid 1a with acetone
2a (both as reactant and solvent) was carried out as the model
for the optimization of reaction conditions. When phenyl-
propiolic acid was treated with acetone in the presence of
Cu(ClO4)2·6H2O (10 mol %) and K2S2O8 (3.0 equiv) with
H2O as cosolvent at 80 °C, the desired γ-diketone 3a was
Decarboxylative-coupling reactions is one of the most
powerful synthetic strategies for the formation of C−C and
C−heteroatom bonds.8 Compared with terminal alkynes,
alkynyl carboxylic acids are more convenient to handle and
synthesize, and their use as surrogates for terminal alkynes has
thus drawn much attention in recent years. Much effort has
Received: February 9, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00520
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Letter
isolated in 16% yield (see Table S1 in the Supporting
Information). In order to improve the reaction efficiency, other
oxidants were tested, and benzoyl peroxide (BPO) gave the
best results (see Table S1). Evaluation of a series of metal salts
demonstrated that Cu(ClO4)2·6H2O was superior to others
(see Table S2). Screening different ratios of acetone to H2O
revealed that the mixture of acetone to H2O in a 3:1 ratio was
the most efficient solvent for this decarboxylative oxyalkylation
(see Table S3). The addition of base could influence the
reaction efficiency. Various bases were tested, and NaHCO3
was found to be optimal (see Table S4). Interestingly, an
improvement in yield was achieved when Mn(OAc)3·2H2O
was employed as an additive, while the addition of other
additives did not allow a further improvement (see Tables S5
and S6). Further optimization of the reactant ratio revealed
that the best result was achieved when 1a, Cu(ClO4)2·6H2O,
BPO, NaHCO3, and Mn(OAc)3·2H2O in a molar ratio of
1.0:0.1:3.0:3.0:0.3 were stirred in acetone/H2O [3/1 (v/v)] at
80 °C for 6 h, furnishing 3a in 76% yield (see Table S7, entry
3).
drawing groups (Me, MeO, AcNH, F, Cl, Br, NO2, CN, Ac,
and CO2Me) on the aromatic ring reacted well with acetone to
afford the γ-diketones in moderate to good yields (3a−s).
What’s more, ortho-substituted phenylpropiolic acids also
exhibited a high reactivity (3b−g). 2-Naphthyl-substituted
propiolic acid and heterocyclic thienylpropiolic acid were also
suitable substrates for the transformation, affording the
products 3t and 3u in 33% and 34% yield, respectively.
However, alkylpropiolic acids, hex-2-ynoic acid and 3-cyclo-
propylpropiolic acid, failed to give the corresponding γ-
diketones probably due to their low reactivity (3v and 3w).
Furthermore, the scope of ketones was examined. Not only
primary acetone but also secondary ketones are amenable to
the present transformation. Pentan-3-one provided the product
3aa in 66% yield. Cyclic ketones including cyclobutanone,
cyclopentanone, and cyclohexanone also showed good
efficiency, affording the γ-diketones in moderate yields
(3ab−ad). However, the tertiary 2,4-dimethylpentan-3-one
was inert under the standard conditions (3ae). Asymmetric
ketones, butan-2-one and methyl 3-oxobutanoate, selectively
provided the γ-diketones 3af and 3ag in 62% and 33% yields,
respectively. This result implied that the stability of the α-
carbonyl C-centered radical is the major factor that influences
the regioselectivity. It was pleasant to find that ethyl acetate
was compatible with this catalytic system, albeit in a low yield
(3ah). When the decarboxylative oxyalkylation of phenyl-
propiolic acid with acetone was carried out on a gram scale
(9.0 mmol), the product 3a was obtained in 72% yield.
The direct C(sp3)−H functionalization of alkylnitriles is one
of the most powerful methods for introducing the cyanomethyl
group. We postulated that the decarboxylative oxyalkylation of
alkynyl carboxylic acids might also be achieved if ketones were
replaced by alkylnitriles. Gratifyingly, acetonitrile successfully
underwent the decarboxylative oxyalkylation reaction with
phenylpropiolic acid under the optimized conditions, furnish-
ing the desired γ-ketonitrile 5a in 41% yield (Scheme 2).
Because γ-ketonitriles are important synthetic building blocks,
developing new reaction conditions for the construction of
such compounds is necessary. We next optimized the reaction
conditions, and the results revealed that the optimum
conditions are Cu(OAc)2 (5 mol %), Mn(OAc)3·2H2O (0.5
equiv), BPO (2.0 equiv), and NaHCO3 (2.0 equiv) in a
CH3CN/H2O [5/1 (v/v)] solvent at 90 °C for 6 h under an
air atmosphere. Under these conditions, 5a was isolated in 73%
yield (for details, see Table S9).
With the optimal conditions in hand, the scope of the
decarboxylative oxyalkylation was investigated (Scheme 2).
Initially, alkynyl carboxylic acids were examined. The
arylpropiolic acids with both electron-donating and -with-
Scheme 2. Substrate Scope for the Decarboxylative
Oxyalkylation of Alkynyl Carboxylic Acids with Ketones
a
The scope of this decarboxylative oxyalkylation of alkynyl
carboxylic acids with alkylnitriles is shown in Scheme 3. Both
electron-rich and -poor phenylpropiolic acids could be
transferred to the γ-ketonitriles 5a−s in moderate to good
yields, and a variety of functional groups including alkyl,
alkoxy, amide, halides, acetyl, ester, nitrile, and nitro were well
tolerated. Ortho-substituted phenylpropiolic acids also showed
good efficiency. However, 2-naphthyl-substituted propiolic
acid and heterocyclic thienylpropiolic acid produced the
desired products 5t and 5u in only 33% and 35% yields,
respectively. Alkylpropiolic acid, hex-2-ynoic acid, was inert
under the standard conditions (5v). Aside from the
acetonitrile, methoxyacetonitrile was a suitable substrate,
although the corresponding γ-ketonitrile 5w was obtained in
a low yield. Like the construction of γ-diketones, the large-scale
preparation of γ-ketonitriles was also achieved under these
condition. When 12.0 mmol of phenylpropiolic acid was
a
Reaction conditions: 1a (0.60 mmol), Cu(ClO4)2·6H2O (10 mol
%), Mn(OAc)3·2H2O (0.3 equiv), BPO (3.0 equiv), and NaHCO3
(3.0 equiv) in 6 mL of ketone/H2O [3/1 (v/v)] at 80 °C for 6 h.
b
c
Isolated yield. The reaction was performed in the presence of 9.0
d
mmol of 1a. Reaction conditions: 1a (0.60 mmol), Cu(ClO4)2·
6H2O (10 mol %), Mn(OAc)3·2H2O (0.3 equiv), BPO (3.0 equiv),
and NaHCO3 (2.0 equiv) in 6 mL of ketone/H2O [3/1 (v/v)] at 100
e
°C for 6 h. Reaction conditions: 1a (0.60 mmol), Cu(ClO4)2·6H2O
(10 mol %), Mn(OAc)3·2H2O (0.3 equiv), BPO (3.0 equiv), and
NaHCO3 (3.0 equiv) in 6 mL of CH3CN/H2O [3/1 (v/v)] at 80 °C
for 6 h.
B
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S1). These results revealed that alkynyl copper should be
involved in the process of decarboxylative oxyalkylation. When
the reaction of phenylpropiolic acid with acetonitrile was
performed under Ar atmosphere, 5a was obtained in 68% yield,
which suggests that the oxygen atom in 5a might originate
from H2O rather than O2 (see Table S9). To further confirm
the oxygen donor, the isotopic-labeling experiment was carried
out in the reaction between phenylpropiolic acid and
acetonitrile (Scheme 4c). When H218O (with 98% abundance
of 18O) was used as a cosolvent instead of normal H216O, the
18O-labeled γ-ketonitrile 5a′ (with 87% abundance of 18O) was
obtained in 63% yield. This result illustrates that water is the
oxygen donor. Finally, a large primary kinetic isotope effect
(KIE, 4.7) was observed in the reaction of phenylpropiolic acid
with acetone/acetone-d6, thus suggest that the rate-determin-
ing step might be the C(sp3)−H bond cleavage of acetone
(Scheme 4d). A primary KIE value (2.2) was also observed in
the decarboxylative oxyalkylation of phenylpropiolic acid with
CH3CN/CD3CN (see the SI).
Scheme 3. Substrate Scope for the Decarboxylative
Oxyalkylation of Alkynyl Carboxylic Acids with
Alkylnitriles
a
a
Reaction conditions: 1a (0.60 mmol), Cu(OAc)2 (5 mol %),
Based on the relevant literature7,9f,14e,15b and the afore-
mentioned experimental results, a plausible mechanism for this
decarboxylative oxyalkylation is proposed in Scheme 5.
Mn(OAc)3·2H2O (0.5 equiv), BPO (2.0 equiv), and NaHCO3 (2.0
equiv) in 9 mL of alkyl nitrile/H2O [5/1 (v/v)] at 90 °C for 6 h.
b
c
Isolated yield. The reaction was performed in the presence of 12.0
d
mmol of 1a. Reaction conditions: 1a (0.60 mmol), Cu(OAc)2 (5
mol %), Mn(OAc)3·2H2O (0.5 equiv), BPO (2.0 equiv), and
NaHCO3 (1.0 equiv) in 9 mL of CH3CN/H2O [5/1 (v/v)] at 90
°C for 6 h.
Scheme 5. Proposed Mechanism
treated with acetonitrile, the γ-ketonitrile 5a was obtained in
65% yield.
To gain mechanistic insights into this decarboxylative
oxyalkylation of alkynyl carboxylic acids, we designed several
control experiments. Initially, the decarboxylative oxyalkylation
process was remarkably suppressed when the radical inhibitors,
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and 2,6-di-tert-
butyl-4-methylphenol (BHT) were added, and TEMPO/BHT-
captured products were detected by HRMS (see the SI). These
results indicate that a radical pathway is probably involved in
this transformation. Furthermore, when alkynyl copper 6 was
employed as reactant, γ-ketonitrile 5a was isolated in 24%
yield, along with trace amounts of diyne 7 (Scheme 4a). The
reaction of acetonitrile with ethynylbenzene 8 was also
performed under the standard conditions, and 5a was obtained
in 12% yield. Diyne 7 was not detected in this reaction
(Scheme 4b). A similar yield was achieved when ethynylben-
zene was added slowly to the reaction solution (see Scheme
Initially, BPO undergoes the thermal hemolytic cleavage to
afford the benzoate radical. The hydrogen atom abstraction
from acetone/acetonitrile by benzoate radical generates the α-
carbonyl or-cyano C-centered radical. Meanwhile, the alkynyl
copper 6 was produced from the decarboxylation of alkynyl
carboxylic acid in the presence of copper salt. Subsequently,
regioselective addition of α-carbonyl or -cyano C-centered
radical to the intermediate 6 leads to vinyl radical A, which
could be oxidized by BPO to generate the cation intermediate
B. Compound B is trapped by H2O to form the enolate C.
Finally, keto−enol tautomerism and quench with H2O of
enolate C lead to the final product 3a or 5a. Mn(OAc)3·2H2O
might act as a co-oxidant.
Scheme 4. Mechanistic Studies
In conclusion, we have demonstrated the first decarbox-
ylative oxyalkylation of alkynyl carboxylic acids with simple
ketones and alkylnitriles via direct C(sp3)−H functionalization
following C−C bond formation. The mechanistic study
suggested that the reaction proceeds through a radical pathway
and water is involved in the transformation as an oxygen
donor. The present decarboxylative oxyalkylation provides a
new and general protocol for the efficient preparation of γ-
diketones and γ-ketonitriles from readily available alkynyl
carboxylic acids.
C
DOI: 10.1021/acs.orglett.9b00520
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Wang, C.; Zhang, J.; Huang, M.; Kim, J. K.; Wu, Y. Org. Chem. Front.
2018, 5, 2496.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00520.
■
S
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Experimental procedures, characterization data of all
1
products, H, and 13C NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: liym@kmust.edu.cn.
ORCID
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Ya-Min Li: 0000-0003-4233-5274
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project is supported by the National Natural Science
Foundation of China (Nos. 21662021 and 21871114) and the
Applied Basic Research Foundation of Yunnan Province
(2017FB016).
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