Highly efficient synthesis of unsymmetrical 1,3-diynes from organoalane reagents and alkynyl bromides mediated by a nickel catalyst

Article abstract of DOI:10.1039/c7ra02758c

Highly efficient and simple cross-coupling reactions of alkynylbromides with organoalane reagents for the synthesis of unsymmetrical 1,3-diynes derivatives using Ni(OAc)₂ (2-5 mol%)/(o-furyl)₃P (4-10 mol%) as a catalyst are reported. Excellent yields (up to 94%) were obtained for a wide range of substrates at rt or 60 °C for 2-3 h in Et₂O or toluene.

Full text of DOI:10.1039/c7ra02758c

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Highly efficient synthesis of unsymmetrical 1,3-
diynes from organoalane reagents and alkynyl
bromides mediated by a nickel catalyst†
Cite this: RSC Adv., 2017, 7, 27243
*
Song Mo, Xue-Bei Shao, Gang Zhang and Qing-Han Li
Received 7th March 2017
Accepted 18th May 2017
Highly efficient and simple cross-coupling reactions of alkynylbromides with organoalane reagents for the
synthesis of unsymmetrical 1,3-diynes derivatives using Ni(OAc)2 (2–5 mol%)/(o-furyl)3P (4–10 mol%) as
a catalyst are reported. Excellent yields (up to 94%) were obtained for a wide range of substrates at rt or
60 ꢀC for 2–3 h in Et2O or toluene.
DOI: 10.1039/c7ra02758c
rsc.li/rsc-advances
Diynes have been widely used in organic synthesis due to their cocatalysts, high temperature, relatively long reaction times,
signicance as building blocks.1 Amongst them, conjugated 1,3- and the special reaction medium. The development of more
diynes have signicant applications in the preparation of efficient and atom economical approaches for the synthesis of
natural products,2 pharmaceuticals,3 p-conjugated acetylenic conjugated 1,3-diynes remains as desirable work. In addition to
polymers,4 advanced materials5 and heterocyclic compounds,6 the above reagents, alkynylaluminum reagents have been
as well as in electronic and optical materials,7 and elsewhere.8 extensively used as nucleophiles for organic reactions.27
In addition, 1,3-diynes are common structural motifs found in
In recent times, metal-catalyzed cross-coupling reactions of
pharmaceutically and biologically active compounds, which are electrophiles with alkynylmetallic reagents have provided an
known to have antifungal,9 antibacterial,10 anti-inammatory,11 alternative route for the preparation of alkyne compounds.28
anti-HIV,12 and anticancer properties.13 Thus, their synthesis Previous studies show that organoalanes reagents are a highly
and applications has attracted a great amount of attention over efficient nucleophiles for cross-coupling reactions with
the past few decades. Since the discovery of the Glaser homo- aromatic halides29 or benzylic halides,30 and investigations have
coupling reaction of terminal alkynes in 1869,14 a great demonstrated that nickel is a good catalytic metal.31 To
amount of attention has been given to the development of new continue our efforts in developing coupling reactions using
and efficient procedures for the synthesis of conjugated 1,3- reactive organometallic reagents,29,30,32 we report a novel nick-
diyne derivatives. Recently, a variety of processes have been el(II)-catalyzed cross-coupling of alkynylhalides with alkynyla-
ꢀ
developed for the preparation of conjugated 1,3-diynes, typical luminum reagents at ambient temperature or 60 C in a short
synthetic protocols including the cross-coupling reactions of 1- time with good isolated yield for the synthesis of unsymmetrical
haloalkynes with terminal alkynes,15 cross-coupling reactions of conjugated 1,3-diynes (Scheme 1).
terminal alkynes,16 cross-coupling reaction of terminal alkynes
In order to optimize the reaction conditions, effects of
with 1,2-dihaloalkenes,17 cross-coupling of alkynylsilanes with phosphine ligand, nickel source, reaction time, solvent, the
haloalkynes,18 conjugated diynones decarbonylation reaction,19 amount of organoalanes reagents, and the molar ratio of metal
cross-coupling reactions of aryl iodides and alkynyl acid,20 to ligand on the cross-coupling reaction were investigated using
oxidative coupling of alkynyl Grignard reagents,21 oxidative alkynylaluminum and alkynyl bromide as a model system. The
decarboxylative homo-coupling of alkynyl acids,22 oxidative reaction of diethyl(2-phenylethynyl)aluminum (C6H5C^CAlEt2)
homo-coupling reactions of terminal alkynes with the (1a) with 1-(2-bromo-ethynyl)-4-methylbenzene (p-MeC6H5-
oxidants,23 homo-coupling of alkynyltellurides,24 haloalkynes,25 C^CBr) (2a) using a nickel catalyst of phosphine (eqn (1)) was
and alkynylsilanes.26
rst examined. Under the optimized conditions of 2 mol%
Despite that, these efforts have provided alternative methods Ni(OAc)2 and 4 mol% P(o-furyl)3 conducting in diethyl ether at
for the synthesis of conjugated 1,3-dyines. However, these room temperature over 2 h, producing 3aa in 87% isolated yield
reactions still suffer from excess oxidants, excess bases, (ESI: Tables S1 and S2†).
College of Chemistry and Environmental Protection Engineering, Southwest University
for Nationalities, Chengdu 610041, P. R. China. E-mail: lqhchem@163.com; Fax: +86-
28-85524382
† Electronic supplementary information (ESI) available. See DOI:
Scheme
1 Metal-catalyzed coupling reactions of 1-haloalkynes
10.1039/c7ra02758c
derivatives with organoalanes nucleophiles.
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methoxybenzene was being decreased by methoxyl group.
More importantly, 2-(2-bromoethynyl)pyridine and 2-(2-bro-
moethynyl)thiophene reagents were also effective substrates for
this reaction, giving the corresponding coupling products of
unsymmetrical conjugated 1,3-diynes 3am and 3an in 83% and
94% isolated yield, respectively (Table 1, entries 13 and 14).
Unfortunately, cross-coupling reaction of 4-(2-bromoethynyl)-
(1)
Generalities of the catalytic system were examined with
various 1-bromoalkynes derivatives (eqn (2)), and the results
were summarized in Table 1. As shown in Table 1, various 1-
bromoalkynes derivatives bearing electron-withdrawing or
electron-donating substituents were smoothly coupled to give
the corresponding unsymmetrical conjugated 1,3-diynes 3 in
64–94% isolated yield (Table 1, entries 1–14). The cross-
coupling reaction with 1-bromoalkynes bearing an electron-
withdrawing group, such as uoro, chloro, bromo and tri-
uoromethyl groups, or electron-donating group, such as
methyl, ethyl, propyl and tertbutyl groups, produced the
conjugated products 1,3-diyne in good to excellent yield (Table
1, entries 1–12). In particular, the cross-coupling of 1-bromo-4-
(2-bromoethynyl) benzene (2g) with diethyl-(2-phenylethynyl)
aluminum (1a) could also be selectively obtained the unsym-
metrical conjugated 1,3-diynes 3ag in 74% isolated yield (Table
1, entry 7). However, the cross-coupling reaction of diethyl-(2-
phenylethynyl)aluminum (1a) with 1-(2-bromoethynyl)-4-
methoxybenzene (4-MeOC6H5C^CBr) (2f) only gave products
of unsymmetrical conjugated 1,3-diyne 3af in 64% isolated yield
(Table 1, entry 6) potentially due to the 1-(2-bromoethynyl)-4-
benzonitrile
(2o)
(p-CNC6H5C^CBr)
and
diethyl-(2-
phenylethynyl)-aluminum (1a) (C6H5C^CAlEt2) only gave
products of unsymmetrical conjugated 1,3-diyne 3ao in 5%
yield (Table 1, entries 15). While, coupling of methyl 4-(2-bro-
moethynyl)benzoate (2p) (p-CH3OOCC6H5C^CBr) could not
produce the target compound (Table 1, entries 16).
We subsequently investigated cross-coupling reactions of
substituted aryl alkynylaluminum reagents with various 1-bro-
moalkynes. However, the cross-coupling reaction of diethyl(2-
(4-methoxy phenyl)-ethynyl)aluminum (4-MeOC^CAlEt2) (1b)
with 1-(2-bromo-ethynyl)-benzene (2o) employing the catalyst of
2 mol% Ni(OAc)2 and 4 mol% P(o-furyl)3 yielded coupling
products of unsymmetrical conjugated 1,3-diyne 3bo with only
a 62% isolated yield. Therefore, the reaction conditions were re-
tuned, and the best performed catalyst was found to be 5 mol%
Ni(OAc)2 and 10 mol% P(o-furyl)3 while the reaction was con-
ꢀ
ducting in toluene at 60 C for 3 h (ESI Table S3†), furnishing
3bo with 75% isolated yield.
The reaction scope was further explored on substrates of
R0C^CAlEt2 (R0 ¼ 4-MeOPh (1b) or 4-FPh (1c), 4-MePh (1d)) and
various 1-bromoalkynes using 5 mol% Ni(OAc)2 and 10 mol%
P(o-furyl)3 conducting in toluene at 60 ꢀC for 3 h (eqn (3)), and
results are summarized in Table 2. Satisfactory application
scope was demonstrated by this section of experiment. The
cross-coupling reaction can be applied to C(sp)–C(sp) bond
formations, affording the coupling products unsymmetrical
conjugated 1,3-diynes in 51–80% isolated yield (Table 2, 1–15).
Cross-coupling reactions of substituted aryl alkynylaluminum
reagents 1b or 1c or 1d with 1-bromoalkyne reagents containing
different functional groups such as alkyl, halides (2a, 2g, 2h, 2i,
2j, 2k, 2l) could potentially couple, giving the corresponding
unsymmetrical conjugated 1,3-diynes moderate to good iso-
lated yields (Table 2, entries 1–4, 7, 8, 10–14). Furthermore, 2-(2-
bromoethynyl)thiophene (2n) reacted successfully with
Table 1 Nickel-catalyzed cross-coupling reactions of C6H5C^CAlEt2
(1a) with various 1-bromoalkynes (2)a
Product
3
Entry
1-Bromoalkynes 2
Yieldb (%)
diethyl(2-(4-methoxyphenyl)ethynyl)aluminum
(4-
1
2
3
4
5
6
7
8
4-Me-Ph (2a)
4-Et-Ph (2b)
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
3aj
3ak
3al
3am
3an
3ao
3ap
87
85
86
84
81
64
74
82
90
93
86
92
94
83
<5%
0
MeOPhC^C–AlEt2) (1b) to provide unsymmetrical conjugated
1,3-diyne 3bn with 74% isolated yield (Table 2, entry 5). The 2-(4-
p-tolylbuta-1,3-diynyl)pyridine (3dm) resulting from 2-(2-
bromo-ethynyl)pyridine (2m) and diethyl(2-p-tolyl-ethynyl)
aluminum (4-MePhC^CAlEt2) (1d) was obtained with 72%
isolated yield (Table 2, entry 15). While, the cross-coupling
reaction of diethyl(2-(4-uorophenyl)-ethynyl) aluminium (4-
FPhC^CAlEt2) (1c) with 1-bromo-hept-1-yne (2p) gave the cor-
responding unsymmetrical conjugated 1,3-diyne 3cp with
a 51% isolated yield (Table 2, entry 9). Unfortunately, cross-
coupling of 1-(2-bromoethynyl)-4-methylbenzene (2a) (p-CH3-
4-Propyl-Ph (2c)
4-tBu-Ph (2d)
3-Me-Ph (2e)
4-MeO-Ph (2f)
4-Br-Ph (2g)
4-Cl-Ph (2h)
4-F-Ph (2i)
3-F-Ph (2j)
9
10
11
12
13
14
15
16
2-F-Ph (2k)
4-CF3-Ph (2l)
2-Pyridinyl (2m)
2-Thienyl (2n)
4-CN-Ph (2o)
4-CH3OOC-Ph (2p)
C6H5C^CBr)
and
diethyl(2-(4-methylbenzoate)ethynyl)
aluminium (4-MeOOCPhC^CAlEt2) (1e) could not produce the
a
b
coupling product (Table 2, entries 16).
1a/2 ¼ 1.0/0.5 mmol; 2 mL Et2O; room temperature; 2 h. Isolated
yield.
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Table 2 Cross-coupling reactions of substituted aryl organoalanes Table
3
Cross-coupling reactions of organoalanes reagents
reagents ArC^CAlEt2 (1) with various 1-bromoalkynes (2)a
TMSC^CAlEt2 (1f) with various 1-bromoalkynes (2)a
1 R0
2 R
Product
Yieldb (%)
Entry
2 R
Product
Yieldb (%)
Entry
1
2
3
4
5
6
Ph (2o)
3fo
3fd
3ff
3
3
3fq
80
74
76
72
67
74
1
2
3
4
5
6
7
8
4-MeOPh (1b)
4-MeOPh (1b)
4-MeOPh (1b)
4-MeOPh (1b)
4-MeOPh (1b)
4-MeOPh (1b)
4-FPh (1c)
4-MePh (2a)
4-ClPh (2h)
4-FPh (2i)
3-FPh (2j)
2-Thienyl (2n)
Ph (2o)
3ba
3bh
3bi
3bj
3bn
3bo
3cj
3ck
3cp
3dg
3di
3dj
3dk
3dl
3dm
3af
62
72
71
72
74
75
55
53
51
68
62
73
80
75
72
0
4-tBu-Ph (2d)
4-MeOPh (2f)
4-FPh (2i)
4-CF3Ph (2l)
2-Naphthyl (2q)
a
b
3-FPh (2j)
1/2 ¼ 1.0/0.5 mmol; 2 mL DME; room temperature; 3 h. Isolated
4-FPh (1c)
4-FPh (1c)
2-FPh (2k)
n-Pentyl (2p)
4-BrPh (2g)
4-FPh (2i)
yield.
9
10
11
12
13
14
15
16
4-MePh (1d)
4-MePh (1d)
4-MePh (1d)
4-MePh (1d)
4-MePh (1d)
4-MePh (1d)
4-MeOOCPh (1e)
coupling reaction of dimethyl-(2-phenylethynyl)-aluminum
(1g) (C6H5C^CAlMe2) with p-CH3C6H5C^CBr (2a) gave
product of unsymmetrical conjugated 1,3-diyne 3aa in 68%
isolated yield (Table 4, entry 1). The coupling reaction of prop-
argyl iodide (2q) with diethyl-(2-phenylethynyl)-aluminum (1a)
(C6H5C^CAlEt2) gave product of unsymmetrical conjugated
1,3-diyne 3aa in a 75% isolated yield (Table 4, entry 2). While,
coupling of propargyl chloride (2r) gave coupling product 3aa in
only 48% isolated yield (Table 4, entry 3). Gratifyingly, 70%
isolated yield of product 3aa was obtained using tri(2-
phenylethynyl)-aluminum (1h) as nucleophiles (Table 4, entry
4). The results showed that the different alkylhalides and
dummy ligand of organoalane reagents are compatible with this
procedure.
3-FPh (2j)
2-FPh (2k)
4-CF3Ph (2l)
2-Pyridinyl (2m)
4-MePh (2a)
a
1/2 ¼ 1.0/0.5 mmol; 2 mL toluene; 60 ꢀC; 3 h. b Isolated yield.
The cross-coupling reactions of aliphatic alkynylaluminum
reagents with various 1-bromoalkynes was also investigated
under the same catalytic system. The unsymmetrical conju-
gated 1,3-diyne 3fo was obtained with 52% isolated yield when
1-(2-bromoethynyl)-benzene
(2o)
coupled
with
the
TMSC^CAlEt2 (1f) nucleophile. However, the cross-coupling
reaction of 1-(2-bromoethynyl)-benzene (2o) coupled with the
TMSC^CAlEt2 (1f) favoured a formation of the unsymmetrical
conjugated 1,3-diyne 3fo with 80% isolated yield in DME at
room temperature for 3 h (ESI Table S4†). So, the reaction scope
was further explored on substrates of TMSC^CAlEt2 (1f) and
various 1-bromoalkynes using 5 mol% Ni(OAc)2 and 10 mol%
P(o-furyl)3 conducting in DME at room temperature for 3 h (eqn
(4)), and results are summarized in Table 3. As shown in Table
3, various 1-bromoalkynes derivatives bearing electron-
withdrawing or electron-donating substituents were smoothly
coupled to give the corresponding unsymmetrical conjugated
1,3-diynes 3 in 67–80% isolated yield (Table 3, entries 1–6). The
cross-coupling reaction with 1-bromoalkynes bearing an
electron-withdrawing group, such as uoro and triuoromethyl
groups, or electron-donating group, such as methoxyl and tert-
butyl groups, produced the conjugated products 1,3-diyne in
good isolated yield (Table 3, entries 2–5). In particular, the
cross-coupling of 2-(2-bromoethynyl)naphthalene (2q) with
TMSC^CAlEt2 (1f) could also be obtained the conjugated 1,3-
diynes 3fq in 74% isolated yield (Table 3, entry 6).
While a precise understanding of the reaction mechanism
awaits further study, a plausible catalytic pathway is proposed
in Scheme 2. The rst step is the oxidative addition of 1-bro-
moalkyne (1) to Ni(0) phosphine complex (4) (which in turn
from Ni(OAc)2 and R0C^CAlEt2 (2) reagents) that forms the
Table 4 The effect of the alkylhalides and dummy ligand of organo-
alane reagents on the reactiona
Entry
R (1)
X (2)
Product
Yieldb (%)
1
2
3
4
Me (1g)
Et (1a)
Et (1a)
PhC^C– (1h)
Br (2a)
I (2q)
Cl (2r)
Br (2a)
3aa
3aa
3aa
3aa
68
75
48
70
The effect of the alkylhalides and dummy ligand of organo-
alane reagents on the reaction were investigated, and the results
were summarized in Table 4. As shown in Table 4, cross-
a
b
1/2 ¼ 1.0/0.5 mmol; 2 mL Et2O; room temperature; 3 h. Isolated
yield.
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1,3-diynes. Further application of these conjugated 1,3-diynes
in organic synthesis are underway.
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Acknowledgements
The authors are grateful to the Sichuan Provincial Department
of science and technology support program (No. 2015NZ0033),
and the Postgraduate Degree Construction Project of Southwest
University for Nationalities (No. CX2016SZ063) for nancial
support.
´
´
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