Decarboxylative Alkynylation and Carbonylative Alkynylation of Carboxylic Acids Enabled by Visible-Light Photoredox Catalysis

Article abstract of DOI:10.1002/anie.201504559

Visible-light-induced photocatalytic decarboxylative alkynylations of carboxylic acids have been developed for the first time. The reaction features extremely mild conditions, broad substrate scope, and avoids additional oxidants. Importantly, a decarboxy

Full text of DOI:10.1002/anie.201504559

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International Edition: DOI: 10.1002/anie.201504559
German Edition: DOI: 10.1002/ange.201504559
Cross-Coupling Reaction
Decarboxylative Alkynylation and Carbonylative Alkynylation of
Carboxylic Acids Enabled by Visible-Light Photoredox Catalysis**
Quan-Quan Zhou, Wei Guo, Wei Ding, Xiong Wu, Xi Chen, Liang-Qiu Lu,* and Wen-
Jing Xiao*
Abstract: Visible-light-induced photocatalytic decarboxylative
alkynylations of carboxylic acids have been developed for the
first time. The reaction features extremely mild conditions,
broad substrate scope, and avoids additional oxidants. Impor-
tantly, a decarboxylative carbonylative alkynylation has also
been carried out in the presence of carbon monoxide (CO)
under photocatalytic conditions, which affords valuable
ynones in high yields at room temperature.
C
arboxylic acids are fundamental feedstocks which are
manufactured in large amounts. They are often employed as
robust and versatile precursors in the synthesis of pharma-
ceuticals, agro-, and fine chemicals, among which decarbox-
ylative cross-coupling reactions of carboxylic acids have
received increasing interests from academic and industrial
settings in the past decade.[1] Despite advances, an important
challenge that has not been fully addressed in this field is how
to activate carboxylic acids for further transformations under
environmentally friendly and sustainable conditions. Very
recently, visible-light-induced photoredox catalysis[2] has been
identified as an ideal approach to generate radicals from
carboxylic acids.[3] Therefore, a wealth of radical transforma-
tions have been achieved using this strategy at room temper-
ature (i.e., decarboxylative reduction,[4a] arylation,[3d,4b] vinyl-
ation,[4c,d] alkylation,[4e,f] oxidative amidation,[3c] and fluorina-
tion[4g–i]). However, to the best of our knowledge, no direct
radical decarboxylative alkynylations or their related carbon-
ylative alkynylations of carboxylic acids through visible-light
photoredox catalysis have been reported.
Scheme 1. Decarboxylative alkynylation/carbonylative alkynylation reac-
tions of carboxylic acids. BI=benziodoxolone; X=Br or Cl.
alkynylation of carboxylic acids with ethynylbenziodoxolones
(EBX) as the alkynylating agent in the presence of stoichio-
metric oxidants (Scheme 1a). Shortly after, the Xu group
developed a copper-catalyzed decarboxylative alkynylation
reaction of quaternary a-cyano acetate salts using an alkynyl
halogen (X = Br, Cl) as the starting material at 1308C in
DMAc (N,N-dimethylacetamide; Scheme 1b).[6b] To further
improve the radical decarboxylative alkynylation reaction of
carboxylic acids and exploit new transformations of these
chemicals, we disclosed a decarboxylative alkynylation and
carbonylative alkynylation of carboxylic acids through visi-
ble-light photoredox catalysis for the first time (Scheme 1c).
These reactions feature very mild conditions (i.e., room
temperature and low-energy visible-light irradiation), no
additional oxidant, and a broad substrate scope.
Initially, the decarboxylative alkynylation reaction was
examined with cyclohexyl carboxylic acid (1a) and phenyl-
EBX[6c–e] (2a) as model substrates in the presence of 3 mol%
Ir[dF(CF3)ppy]2(dtbbpy)PF6 (PC1, dF(CF3)ppy = 2-(2,4-
difluorophenyl)-5-(trifluoromethyl)pyridine, dtbbpy = 4,4-di-
tert-butyl-2,2’-bipyridine) as a photocatalyst and 1.5 equiv of
K2HPO4 as a base.[7] To our delight, this reaction does indeed
proceed under the irradiation of a bulb of 7 W blue LEDs, to
afford the desired alkynylation product 3aa in modest yield as
determined by gas chromatography (GC; Table S1, entry 1:
29% yield). Then, various metal or organic photocatalysts,
bases, solvents, additives, and the ratio of 1a to 2a were tested
to improve the reaction efficiency (Tables S1–S2). Finally, we
could isolate the desired product 3aa in 81% yield under the
optimal conditions (see the footnote in Table 1). Control
experiments indicated that both photocatalyst and visible
Transition-metal-catalyzed decarboxylative alkynylation
of carboxylic acids is among the most direct conversions for
the preparation of alkynes.[1b,f,5] However, only a handful of
works on radical decarboxylative alkynylations of carboxylic
acids have been disclosed. In this regard, Li and co-workers[6a]
in 2012 reported a silver-catalyzed, radical decarboxylative
[*] Q.-Q. Zhou,[+] W. Guo,[+] W. Ding, X. Wu, X. Chen, Prof. Dr. L.-Q. Lu,
Prof. Dr. W.-J. Xiao
Key Laboratory of Pesticide & Chemical Biology
College of Chemistry, Central China Normal University (CCNU)
152 Luoyu Road, Wuhan, Hubei 430079 (China)
E-mail: luliangqiu@mail.ccnu.edu.cn
wxiao@mail.ccnu.edu.cn
[+] These authors contributed equally to this work.
[**] We are grateful to the National Science Foundation of China
(21232003, 21202053, 21272087, and 21372266), FANEDD
(201422), and Central China Normal University (CCNU15A02007)
for support of this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504559.
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Table 1: Scope of carboxylic acids in the decarboxylative alkynylation
Table 2: Scope of EBX reagents in the decarboxylative alkynylation
reaction.[a]
reaction.[a]
Entry
2: R’
Product
Yield [%][b]
1
2
3
4
5
6
7
8
9
2a: phenyl
3ha
3hb
3hc
3hd
3he
3hf
3hg
3hh
3hi
98
97
99
99
96
94
87
82
70
2b: 4-methylphenyl
2c: 4-fluorophenyl
2d: 4-chlorophenyl
2e: 4-phenyl phenyl
2 f: 3,5-dimethylphenyl
2g: 2-thiophenyl
2h: tert-butyl
2i: n-pentyl
[a] Please see the footnote in Table 1.
heteroaryl-substituted EBX reagent 2g was applicable as
well, affording the desired product in good yield (Table 2,
entry 8: 3hg, 87% yield). To extend the reaction scope, alkyl-
substituted EBX reagents were investigated. It has been
found that, for example, EBX reagents with bulky tert-butyl
(2h) and linear n-pentyl (2i) groups can be readily employed
in the reaction, providing the corresponding products in good
yields (Table 2, entry 8: 3hh, 82% yield; entry 9: 3hi, 70%
yield).
[a] Standard condition A: 1 (0.40 mmol), 2 (0.60 mmol), Cs2CO3
(0.60 mmol), PC1 (0.012 mmol), and 40 mg 4 Š MS in DCM (6 mL) at
room temperature under the irradiation of a bulb of 7 W blue LEDs for
4 h. Yield of the isolated products.
light were essential in this decarboxylative alkynylation
reaction.
It is well known that ynones serve as important structural
motifs in bioactive molecules, as well as useful building blocks
in heterocycle synthesis.[8] To access such chemicals, palla-
dium-catalyzed carbonylation reactions of alkynylating
reagents, carbon monoxide (CO), and organic iodides[8a,c] or
aryl diazonium salts[7b] have been recently developed. At the
beginning of this year, the Wangelin group[9a] and our group[9b]
independently discovered a multicomponent coupling reac-
tion of aryldiazonium salts, CO,and alcohols, namely radical
alkoxycarbonylation reactions, through visible-light photo-
redox catalysis. Based on the above-mentioned results, we
have successfully carried out the decarboxylative carbon-
ylative alkynylation of carboxylic acids with CO and EBX
reagents. After briefly screening the reaction conditions,[10]
ynone 4a was isolated in high yield under our optimal
conditions (Table 3, 4a: 80% yields). As highlighted in
Table 3, a variety of cyclic carboxylic acids exhibit good
reactivity in this radical carbonylative alkynylation reaction
(4a–e: 50–90% yield). Importantly, triisopropylsilyl- and tert-
butyl-substituted EBX reagents were well tolerant with this
reaction, providing the corresponding ynone products 4 f and
4g in 56% and 27% yield, respectively. Note that the silicon
group on the product (4 f) can be easily removed or converted
into other groups. Furthermore, this photocatalytic carbon-
ylative alkynylation is quite general with respect to acyclic
carboxylic acids. It is feasible to convert primary, secondary,
and tertiary acyclic carboxylic acids into linear or branched
ynone products, generally in moderate to good yields (4h–4k:
42–77% yield). A limitation of this transformation is that the
carbonylative alkynylation reactions of a-amino- or a-
oxygen-substituted carboxylic acids cannot occur. Reactions
Next, the generality of this alkynylation reaction was
investigated under optimal conditions. As summarized in
Table 1, a broad range of carboxylic acids was determined to
be applicable to this transformation. The decarboxylative
alkynylation reaction of various carbocyclic carboxylic acids
proceeded smoothly, affording the corresponding alkynyla-
tion products in high yields (3aa–3ea: 53–81% yield).
Significantly, this reaction tolerates a broad range of oxa-
heterocyclic carboxylic acids. For example, carboxylic acids
with a- and g-oxygen atoms are excellent reaction partners,
providing the decarboxylative alkynylation products in high
yields (3 fa–ha: 83–98% yield). Perhaps more importantly,
primary carboxylic acids and acyclic a-amino acids can
readily undergo this transformation under our standard
conditions, thus resulting in the formation of structurally
diverse alkynes (3ia: 56% yield) and a-alkynyl amines in
good to excellent yields (3ja–oa: 76–98% yield). Notably, this
photocatalytic strategy was also tolerant to a-ketonic carbox-
ylic acids. When aryl- or alkyl-substituted a-ketonic acids
were employed, the desired ynone products were obtained in
good yields (3pa: 77% yield; 3qa: 53% yield).
Experiments that probe the scope of EBX reagents were
performed for this visible-light photocatalytic carbonylative
alkynylation reaction. As highlighted in Table 2, various aryl-
substituted EBXs works well in this reaction. The electronic
and steric properties of substituents on the benzene ring did
not significantly influence the reaction efficiency, and the
corresponding coupling products were isolated in excellent
yields (Table 2, entries 1–6: 94–99% yield). Remarkably, the
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acid (1r),[12] was successful (Scheme 2, Eq. (1): 3ar, 52%
yield), leaving the hydroxy and alkene group untouched.
Moreover, a gram-scale decarboxylative alkynylation reac-
tion of substrates 1h and 2a was carried out under standard
conditions. This reaction proceeded well and afforded the
desired coupling product 3ha in an excellent yield (Scheme 2,
Eq. (2): 95% yield). Significantly, the decarboxylative alky-
nylation reaction can be successfully carried out under
irradiation with natural solar light, affording the desired
product 3ha in a shortened reaction time (Scheme 2, Eq. (2):
97% yield). In addition, one of the reaction products, N-Boc-
protected propargylamine (3ja), can be elegantly converted
to oxazolidinone 5 through a AuI-catalyzed annulation
reaction (Scheme 2, Eq. (3): 81% yield).[13] When 2-cyclo-
propylacetic acid was used as the substrate, a ring-opened
product 7 was afforded in a moderate yield (Scheme 2,
Eq. (4): 52% yield). This result not only supported the radical
mechanism for the carbonylative alkynylation reaction, but
also provided an important precursor (7), which can be used
in the preparation of the fused-ring molecule 8.[14]
Table 3: Representative results of decarboxylative carbonylative alkyny-
lation reactions.[a]
[a] Standard condition B: 1 (0.20 mmol), 2 (0.30 mmol), Cs2CO3
(0.15 mmol), PC1 (0.004 mmol) in DCM (4 mL) at room temperature
under the atmosphere of CO (60 bar) and the irradiation of two bulbs of
8 W blue LEDs for 4 h. Yield of the isolated products.
To understand these two transformations, plausible reac-
tion pathways were proposed with carboxylic acid 1a and
phenyl-substituted EBX 2a as the model substrates. As shown
in Scheme 3, an initial single-electron oxidation of carboxylic
of these substrates usually produce the alkynylation products
as the major product rather than the expected carbonylation
products.[11]
Demonstrations of the synthetic utility of our decarbox-
ylative alkynylation as well as carbonylative alkynylation
processes have been performed. Very delightfully, this visible-
light photocatalytic protocol allows the decarboxylative
alkynylation of complex molecules under extremely mild
reaction conditions. As presented in the top of Scheme 2,
decarboxylative alkynylation of a natural terpenoid, ursolic
Scheme 3. Possible reaction mechanism.
acid 1a by the excited state of the iridium (III) photocatalyst
delivers the cyclohexyl radical (A) while the photocatalyst is
reduced to its low-valence state [IrII].[15] The radical addition
of A to the alkynylating reagent 2a generates the intermedi-
ate B, which undergoes a subsequent radical elimination
reaction to yield the desired product 3aa, with the generation
of benziodoxolonyl radical (BIC).[6a,16] Finally, the [IrII] species
was oxidized by BIC to [IrIII] to complete the photocatalytic
cycle. In the case of the decarboxylative carbonylative
alkynylation reaction in the presence of carbon monoxide,
radical A can be trapped by CO to produce an acyl radical C.
This newly generated radical reacts with 2a, followed by the
generation of BIC, to provide the ynone 4a.[8a,e]
In conclusion, we have successfully developed photo-
Scheme 2. Synthetic utility of methodology.
catalytic direct radical decarboxylative alkynylation and
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Angew. Chem. Int. Ed. 2015, 54, 1881 – 1884; Angew. Chem.
2015, 127, 1901 – 1904.
carbonylative alkynylation reactions of carboxylic acids for
the first time. These reactions allow the preparation of various
alkynes and ynones in good to excellent yields under very
mild conditions. Unique from other transition-metal-cata-
lyzed protocols, the current carbonylative alkynylation reac-
tions can directly use readily available carboxylic acids as the
starting materials. More importantly, the utilization of these
two reactions has been demonstrated by the derivatization of
naturally occurring ursolic acid and the preparation of
oxazolidinones.
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Keywords: alkynylation · carbonylation · decarboxylation ·
photocatalysis · visible light
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Angew. Chem. 2015, 127, 11348–11351
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Received: May 22, 2015
Revised: June 17, 2015
Published online: July 6, 2015
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