A facile copper(i)-catalyzed homocoupling of terminal alkynes to 1,3-diynes with diaziridinone under mild conditions

Article abstract of DOI:10.1039/c3ob41621f

A novel and efficient Cu(i)-catalyzed oxidative homocoupling of terminal alkynes with diaziridinone as an oxidant is described. Various terminal alkynes can be transformed into the corresponding 1,3-diynes in good yields. The reaction process is base-free

Full text of DOI:10.1039/c3ob41621f

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A facile copper(I)-catalyzed homocoupling of terminal
alkynes to 1,3-diynes with diaziridinone under mild
conditions†
Cite this: DOI: 10.1039/c3ob41621f
Received 8th August 2013,
Accepted 11th September 2013
DOI: 10.1039/c3ob41621f
Yingguang Zhu and Yian Shi*
www.rsc.org/obc
A novel and efficient Cu(I)-catalyzed oxidative homocoupling of
terminal alkynes with diaziridinone as an oxidant is described.
Various terminal alkynes can be transformed into the corres-
ponding 1,3-diynes in good yields. The reaction process is base-
free, operationally simple, and amenable to the gram scale.
a
Table 1 Optimization of the reaction conditions
1
1
,3-Diynes are present in natural products and biologically
b
c
2
Entry
1 (X equiv.)
Time (h)
Conv. (%)
Yield (%)
active molecules. They are also important structural motifs
found in various oligomers, polymers, and materials.
3
A
1
2
3
4
0.51
0.75
0.90
1.0
0.90
0.90
0
2
2
2
2
2
24
24
67
89
100
100
100
0
54
74
80
78
80
—
—
variety of processes have been developed for the preparation of
4
–11
1
,3-diynes.
The oxidative coupling of terminal alkynes rep-
resents one of the most straightforward and widely used
approaches to synthesise these types of compounds. Various
effective methods have been developed, including Cu-
d
5
6
e
7
0
1
2
13
14
15
mediated or catalyzed, Co-catalyzed, Pd-catalyzed, and
a
All reactions were carried out with phenylacetylene (3b) (0.60 mmol),
1
6
17
bimetallic Pd/Cu, Ni/Cu catalyzed systems. Although sig- di-tert-butyldiaziridinone (1), and CuBr (0.030 mmol) in CH CN
3
b
(
0.9 mL) at rt unless otherwise stated. The conversion was based on
nificant progress has been made in this area, the development
of a new, mild, and efficient procedure is still desirable and
valuable.
1
phenylacetylene (3b) and determined by the H NMR analysis of the
crude reaction mixture. Isolated yield based on 3b. The reaction was
carried out under a rigorous argon atmosphere. The reaction was
carried out in the absence of CuBr.
c
d
e
In our earlier studies, we have shown that di-tert-butyldi-
1
8,19
aziridinone (1)
is a highly effective nitrogen source for the
2
0
21
Pd(0)- and Cu(I)- catalyzed diamination of olefins. Recently,
2
2
used for the oxidation of alcohols and oxidative coupling of
anilines using Cu(I) as a catalyst. During our studies on the
oxidation of alcohols, 1,6-diphenylhexa-2,4-diyne-1,6-dione
we have shown that di-tert-butyldiaziridinone (1) can also be
2
3
(
4a) instead of 1-phenylpropynone (3a) was obtained in 31%
yield under the reaction conditions when propargyl alcohol 2
was used as a substrate (Scheme 1). This observation prompted
us to further investigate this homocoupling process of term-
inal alkynes. Herein we wish to report our preliminary studies
on this subject.
The alkyne coupling was investigated with phenylacetylene
(
3b) as the test substrate. Treating 3b with 0.51 equiv. of di-
Scheme 1 Cu(I)-catalyzed oxidative homocoupling of 2.
aziridinone 1 and 5 mol% CuBr in CH CN at rt for 2 h gave
3
1
,3-diyne 4b in 54% yield (Table 1, entry 1). A complete conver-
sion was achieved with 0.90 equiv. of 1, giving 4b in 80% yield
Table 1, entry 3). No coupling products were observed without
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523,
USA. E-mail: yian@lamar.colostate.edu; Fax: +(1)-970-4911801;
Tel: +(1)-970-4917424
(
CuBr catalyst or diaziridinone 1 (Table 1, entries 6 and 7).
The generality of this oxidative homocoupling was sub-
sequently investigated. As shown in Table 2, various aromatic
†
Electronic supplementary information (ESI) available: Experimental pro-
cedures, characterization data, and NMR spectra. See DOI: 10.1039/c3ob41621f
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a
Table 2 Oxidative homocoupling of terminal alkynes
b
Entry
Substrate 3
Product 4
Yield (%)
1
2
3
3b, X = H
3c, X = p-Me
3d, X = p- Bu
3e, X = p-CF
3f, X = p-F
3g, X = m-Cl
3h, X = o-Cl
3i, X = o-CF
4b
4c
4d
4e
4f
4g
4h
4i
80
77
79
83
81
80
64
74
Scheme 2 Proposed catalytic cycle for the oxidative homocoupling.
t
c
4
3
5
6
7
8
of 1-ethynyl-4-(trifluoromethyl)benzene (3e), the coupling
method is amenable to the gram scale (Table 2, entry 4).
While a precise understanding of the reaction mechanism
awaits further study, a plausible catalytic cycle is proposed in
Scheme 2. The CuBr first reductively cleaves the N–N bond of
di-tert-butyldiaziridinone (1) to form four-membered Cu(III)
3
9
79
2
1e, f
species A and/or Cu(II) nitrogen radical B,
with terminal alkyne 3 to form dialkynyl Cu(III) species C.
The reductive elimination of intermediate C affords 1,3-diyne
which reacts
1
1
0
1
64
4
a,b
4
and regenerates the CuBr catalyst.
81
In summary, we have developed a mild and efficient
method for the oxidative homocoupling of terminal alkynes
with CuBr as a catalyst and di-tert-butyldiaziridinone (1) as an
oxidant. Various terminal alkynes can be effectively coupled to
give the corresponding 1,3-diynes in good yields. The reaction
is base-free, operationally simple, and also amenable to the
gram scale. The results further demonstrate the synthetic
utility of diaziridinones. Further efforts to develop other coup-
ling processes with this class of compounds are currently
underway.
1
1
2
3
76
71
1
4
79
75
1
5
Acknowledgements
We are grateful for generous financial support from the
General Medical Sciences of the National Institutes of Health
a
All reactions were carried out with alkyne 3 (0.60 mmol), di-tert-
butyldiaziridinone (1) (0.54 mmol), and CuBr (0.030 mmol) in CH
3
CN (GM083944-06).
(0.9 mL) at rt for 2 h unless otherwise stated. For entry 6, the reaction
time was 6 h. For entry 7, 1.0 equiv. of 1 was used, and the reaction
time was 7 h. For entries 12, 13, and 15, the reactions were carried out
with alkynes 3 (0.60 mmol), di-tert-butyldiaziridinone (1) (0.66 mmol),
b
and CuBr (0.060 mmol) in CDCl
3
(0.9 mL) at rt for 5 h. Isolated yield Notes and references
c
based on alkyne 3. The reaction was carried out on the 10 mmol scale
of 3e.
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