Copper-Catalyzed Decarboxylative Alkylation of Terminal Alkynes

Article abstract of DOI:10.1002/adsc.201700798

A copper-catalyzed decarboxylative alkylation of terminal alkynes under mild reaction conditions has been reported. Various alkyl diacyl peroxides were applied as the alkyl source for the formation of C(sp³)?C(sp) bond. A range of terminal alkynes including aryl alkynes and alkyl alkynes delivered the alkylated internal alkynes with good to high performances. Mechanism studies suggested that this reaction involves a free radical pathway. (Figure presented.).

Full text of DOI:10.1002/adsc.201700798

Title: Copper-Catalyzed Decarboxylative Alkylation of Terminal Alkynes
Authors: Changqing Ye, Yajun Li, and Hongli Bao
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700798
Link to VoR: http://dx.doi.org/10.1002/adsc.201700798

10.1002/adsc.201700798
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Copper-Catalyzed Decarboxylative Alkylation of Terminal
Alkynes
Changqing Yea, Yajun Li,a and Hongli Bao*a
a
Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, State Key Laboratory of Structural Chemistry,
Fujian Institute of Research on the Structure of Matter, University of Chinese Academy of Sciences, 155 Yangqiao
Road West, Fuzhou, Fujian, 350002, People's Republic of China
e-mail : hlbao@fjirsm.ac.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Developed by Chatani[13] and Yu[14] et al., the
Abstract. A copper-catalyzed decarboxylative alkylation
of terminal alkynes under mild reaction conditions has
been reported. Various alkyl diacyl peroxides were
applied as the alkyl source for the formation of C(sp3)-
C(sp) bond. A range of terminal alkynes including aryl
alkynes and alkyl alkynes delivered the alkylated internal
alkynes with good to high performances. Mechanism
studies suggested that this reaction involves a free radical
pathway.
alkynylation of C(sp3)-H bonds with alkynyl halides
has been enabled by palladium catalysts. Zhang,[15]
Lei,[16] and Shi[17] demonstrated that without pre-
activation, terminal alkynes would react with
unactivated C(sp3)–H bonds through transition-metal
catalyzed cross-dehydrogenative coupling (CDC)
pathway (Scheme 1c). From the point of enriching
the diversity of alkyl source and catalyst list, the
development of methodologies for the formation of
C(sp3)-C(sp) bond is still urgent.
Keywords: Copper-catalysis; Alkylation; Sonogashira
coupling; Decarboxylation
The formation of C(sp3)-C(sp) bond is a key step for
the construction of internal alkylated alkynes, which
are important fragments widely appeared in natural
products, pharmaceuticals, and molecular organic
materials.[1] Although great success has been
achieved through nucleophilic substitution of
terminal alkynes with alkyl halides or other
electrophiles, with the assistance of stoichiometric
amount of strong base.[2] The catalytic fashion in the
presence of a transition-metal is a more attractive
option.
As is known, alkyl halides are the most frequently
used alkyl source for the formation of C(sp3)-C(sp)
bond. Representatively, the alkyl source involved
Sonogashira reaction,[3] elegantly modified by Fu,[4]
Glorius,[5] Hu,[6] and Liu[7] et al., could directly
employ the non-activated alkyl halides as the alkyl
electrophiles with Pd/Cu/NHC or Ni/Cu as the co-
catalysis system to construct the C(sp3)-C(sp) bonds
(Scheme 1a). [8,9] What's more, alkyl halides could be
used to synthesize the corresponding alkyl metal
reagents such as Grignard reagent,[10] organozinc
reagents,[11] organotin reagents et al.,[12] which are
alternative forms of alkyl source for the formation of
C(sp3)-C(sp) bond with or without the assistance of
external oxidants (Scheme 1b). Direct C-H
alkynylation of alkane represents another effective
method for the formation of C(sp3)-C(sp) bond.
Scheme 1. Transition-metal catalyzed formation of
C(sp3)-C(sp) bonds with different alkyl sources.
Aliphatic carboxylic acids are readily available
materials and exist widely in nature.[18]
Decarboxylation of aliphatic carboxylic acids or their
activated forms will generate alkyl sources, which are
frequently used as the alkylating reagents.[19] This
methodology has been applied for the formation of
C(sp3)-C(sp) bonds with a hetero-atom at the ß–
position of the carbon-carbon triple bond, mainly
owing to its ability to stabilize the produced alkyl
source after decarboxylation.[20] The decarboxylative
alkylation with alkyl diacyl peroxides and terminal
1
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10.1002/adsc.201700798
Advanced Synthesis & Catalysis
when the reaction temperature was decreased to 10 oC
(Table 1, entry 7). Next, the effect of base was
studied and found that Et3N could deliver 3a in 55%
GC yield (Table 1, entries 8-11). Other solvents such
as DMF, DMSO, and THF had poorer performances
than CH3CN (see supporting information). As the
investigation proceeded, we began to screen the
ligands, such as phosphine ligands, amino acid
ligands, and other bidentate nitrogen ligands (Table 1,
entries 12-16 and SI). However, the yield of 3a could
not be further improved with these ligands.
Delightfully, the yield of 3a was increased with a
lower concentration and further improved to 83%
with a higher catalyst loading (Table 1, entries 17 and
18).
alkyne still remain unsolved. Inspired by the employ
of aliphatic carboxylic acids as the alkylating reagent
source,[21] we report herein a copper catalyzed
decarboxylative alkylation of terminal alkynes with
alkyl diacyl peroxides as the alkyl source under very
mild conditions (Scheme 1d).
Table 1. Reaction condition optimization [a, b]
Yield
Entry Copper salt
Ligand
Base
(%)[c]
23
CuI
dtbpy
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
Et3N
1
Table 2. Scope of terminal alkynes[a]
Cu(CH3CN)4PF6 dtbpy
25
2
CuBr·SMe2
CuCl
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
bpy
30
3
40
4
CuCl2
Cu(OTf)2
CuCl
18
5
19
6
7[d]
8[d]
9[d]
10[d]
11[d]
12[d]
13[d]
14[d]
15[d]
16[d]
17[d,e]
46
CuCl
55
pyridine
K2CO3
---
CuCl
42
CuCl
44
CuCl
37
CuCl
Et3N
43
CuCl
L1
Et3N
37
CuCl
L2
Et3N
28
CuCl
PPh3
L3
Et3N
18
CuCl
Et3N
28
CuCl
dtbpy
Et3N
61
83(78)[h]
CuCl[f]
dtbpy[g] Et3N
18[d,e]
[a]
Reaction conditions: 1a (0.50 mmol), LPO (0.60 mmol), base (3
equiv.), copper salt (10 mol%), and ligand (10 mol%) in 2 mL
CH3CN at 50 oC.
[b] For more details please see supporting information.
[c] Yield was determined by GC analysis.
[d] Reaction temperature was 10 oC.
[e] CH3CN (6 mL).
[f] CuCl (20 mol%).
[g] dtbpy (20 mol%).
[h] Isolated yield in parentheses.
L1 = 4,4’-dimethoxy-2,2’-bipyridine. L2 = 1,10-phenanthroline. L3 =
(4-MeOC6H4)3P. dtbpy = 4,4'-di-tert-butyl-2,2'-bipyridine. bpy = 2,2'-
bipyridine. DIPEA = N,N-diisopropylethylamine.
[a]
Reaction conditions: terminal alkyne 1 (0.50 mmol), LPO (0.60
mmol), Et3N (3 equiv.), CuCl (20 mol%), and dtbpy (20 mol%) in
CH3CN (6 mL) at 10 oC.
With the optimal reaction conditions in hand, the
scope and limitation with respect to terminal alkynes
were explored (Table 2). A wide range of terminal
aryl alkynes with various functionalities tolerated the
mild reaction conditions. While electron-donating
substituents on the benzene ring afforded the
corresponding internal alkynes with high yields,
electron-withdrawing substituents on the benzene
ring offered the desired products with a slight low
Initial screening of the reaction employed 4-
ethynyltoluene (1a) and lauroyl peroxide (LPO, 2a)
as the starting material (Table 1). Gratefully, when
CuI was used as the catalyst, the desired product (3a)
was obtained in 23% GC yield (Table 1, entry 1).
After screening other copper salts, it was found that
CuCl offered a better yield as high as 40% (Table 1,
entries 2-6). A slight improvement was achieved
2
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10.1002/adsc.201700798
Advanced Synthesis & Catalysis
yields (Table 2, 3b-3n). It’s worth mentioning that that the reaction might undergo a free radical process.
good yields was achieved when the substituent was To gain further understanding of this free radical
on the ortho-position of the benzene ring (Table 2, 3k, process, a radical-clock substrate 2m was synthesized
3m, and 3n). Aldehyde group, which is supposed to and applied to the reaction (scheme 2b).[25] It was
be sensitive to oxidative conditions, could participate found that the reaction between terminal alkyne 1a
in the reaction with good performacne, and the and 2-cyclopropylacetic peroxide (2m) afforded the
desired product (3n)[22] was isolated in 76 % yield. enyne product 4m in 38%, which was supposed to
Heteroaromatic ring such as thiophene, quinoline and generate from the radical involved ring-open process
benzofuran, could also be compatible with the from the cyclopropane moiety.
reaction condition, affording the corresponding
product with moderate to high yields (Table 2, 3o, 3p,
3q, and 3r). Pyridine substrate was also tried, but
product 3s was obtained only in 9 % yield. Excitingly,
this methodology was not limited to terminal aryl
alkynes, but could be applied to terminal alkyl
alkynes. Functionalities such as chloro, hydroxyl
group, and amine were tolerated and produced the
1,2-dialkyl acetylene with moderate yields (Table 2,
3t, 3u, 3v, and 3w).
Scheme 2. Mechanism study
We next studied the scope of the alkyl diacyl
peroxides (Table 3). As reported by our studies,
According to above mechanistic studies,
a
DCC-mediated dehydrative condensation between
aliphatic acids and hydrogen peroxide will generate
the alkyl diacyl peroxides, which can be used directly
after simple filtration. After screening, the reaction
temperature could be further dropped to -10 oC
without losing activities. Primary aliphatic carboxylic
acids afforded the corresponding products 4a-4i with
40-85% yields. What’s more, secondary aliphatic
carboxylic acids could also be employed to this
reaction and provided the corresponding products
plausible reaction mechanism for the Sonogashira-
type decarboxylative alkylation of terminal alkyne is
proposed as shown in Scheme 3.[26] The CuI species
(A) react with terminal alkyne to form complex (B)
under base condition. Then homolytic cleavage of the
peroxide will generate the CuII species (C) and the
alkyl free radical. Next, two possible pathways might
be involved in the catalytic cycle. Pathway a: the
interaction between CuII species (C) and alkyl radical
through radical homolytic substitution will generate
the coupling product and regenerate the copper
catalyst. Path b: the CuII species (C) proceed single
electron transfer with the alkyl free radical to form
CuIII intermediate (D), which undergoes reductive
elimination to yield the target product and the active
copper salt.
with moderate yields[23]
.
Table 3. Scope of alkyl diacyl peroxides[a]
[a]
Reaction conditions: terminal alkyne 1a (0.50 mmol), 2 (0.75
mmol, synthesized from alkyl carboxylic acids and used directly after
filtration), Et3N (3 equiv.), CuCl (20 mol%), and dtbpy (20 mol%) in
CH3CN (6 mL) at -10 oC.
To explore the mechanism for the reaction, a
radical trapping experiment was conducted with
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as the
radical-trapping reagent.[24] No desired product 3a
was formed, but compound 5 was detected by GC-
MS analysis (scheme 2a). This result demonstrated
Scheme 3. Proposed reaction mechanism
3
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10.1002/adsc.201700798
Advanced Synthesis & Catalysis
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In summary, we have developed an efficient and
mild copper catalyzed decarboxylative cross-coupling
of alkyl diacyl peroxides with terminal alkynes to
establish C(sp3)-C(sp) bond. Various substituted
internal alkynes were synthesized in moderate to
excellent yields with good functionalities tolerance.
Primary and second alkyl diacyl peroxides were
utilized as the alkyl source. Not only terminal aryl
alkynes but also terminal alkyl alkynes could be
applied to this transformations, which greatly broaden
the alkyne substrate scope for this reaction.
Mechanistic studies suggested that this reaction might
involve a free radical pathway. The application of
alkyl diacyl peroxides as an alkylating reagent for the
functionalization of alkynes is current underway in
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A solution of DMAP (0.224 g, 0.3 mmol), 30% hydrogen
peroxide (3.6 mmol) and acid (3 mmol) in DCM (5 mL)
was cooled to -15 C for about 10 min, then DCC (3.6
mmol) was added. Then the mixture was stirred for another
1.5 hours at -15 C. After the reaction finished, the solution
was filtered through a short pad of silica gel. Then washed
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under vacuum at 10~15 C give the diacyl peroxides which
was used directly in following reactions.
To a 25 mL of Schlenk tube equipped with a Teflon-coated
magnetic stir bar was charged with terminal alkyne (0.5
mmol), CuCl (0.1 mmol), dtbpy (0.1 mmol), Et3N (1.5
mmol), and anhydrous CH3CN (2 mL). The resulting
suspension was stirred at -10 C. The corresponding diacyl
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and then added to reaction via a syringe. Upon completion
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concentrated under vacuum. The crude residue was
purified by flash column chromatography on silica gel to
give the product.
Acknowledgements
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We thank NSFC (grant no. 21402200, 21502191, 21672213),
Strategic Priority Research Program of the Chinese Academy of
Sciences (Grant No. XDB20000000), The 100 Talents Program,
“The 1000 Youth Talents Program” for financial support.
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5
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Copper-Catalyzed Decarboxylative Alkylation of
Terminal Alkynes
Adv. Synth. Catal. Year, Volume, Page – Page
Changqing Ye, Yajun Li, and Hongli Bao*
6
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