Copper-catalyzed distannylation of alkynes

Article abstract of DOI:10.1039/c3cc47173j

A copper(i)-phosphine complex has been found to facilely promote the distannylation of alkynes with the aid of a base, where a catalytically generated Cu-Sn species serves as a key intermediate. The broad versatility of the copper catalysis has been demonstrated by the facile distannylation of unfunctionalized aliphatic alkynes, being hardly achievable with the conventional palladium catalysts.

Full text of DOI:10.1039/c3cc47173j

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Copper-catalyzed distannylation of alkynes†
Hiroto Yoshida,* Ayako Shinke and Ken Takaki
Cite this: Chem. Commun., 2013,
49, 11671
Received 19th September 2013,
Accepted 21st October 2013
DOI: 10.1039/c3cc47173j
www.rsc.org/chemcomm
A copper(I)–phosphine complex has been found to facilely promote the
We first carried out the reaction of 1-octyne (1a) with
distannylation of alkynes with the aid of a base, where a catalytically hexamethyldistannane (2) in NMP in the presence of
14
generated Cu–Sn species serves as a key intermediate. The broad Cu(OAc)(PPh3)3 and Cs2CO3, and found that distannylation
versatility of the copper catalysis has been demonstrated by the smoothly took place to give a vic-distannylalkene (3a) in 60%
facile distannylation of unfunctionalized aliphatic alkynes, being yield, whose stereochemistry (cis addition) was unambiguously
3
hardly achievable with the conventional palladium catalysts.
determined by the JSn–H value of the vinyl proton (entry 1,
Table 1).15 Addition of Cs2CO3 was found to be indispensable
In view of the demand for organostannanes in their utilization for for the distannylation to proceed (entry 2), and DMF was the
chemoselective C–C bond formation through Migita–Kosugi–Stille best among solvents surveyed (entries 3–6). A catalytic amount
reaction etc.,1 development of convenient and potent methods for of the base sufficed for the facile distannylation (entries 7 and
forming C–Sn bonds, especially in a catalytic process, has been an 8), and the product was not produced at all in the absence of
important subject in modern synthetic organic chemistry.2 Recently, the copper catalyst (entry 9).
much attention has been directed toward catalytic C–B3 and C–Si4
Under the optimized reaction conditions (entry 7, Table 1),
bond formation reactions under copper catalysis, where key Cu–B5 1-hexyne (1b), 1-decyne (1c) and branched aliphatic terminal
and Cu–Si6 species arise from activation of a diboron (for Cu–B), a alkynes bearing i-amyl (1d), i-butyl (1e) or cyclopentyl (1f) were
silylborane4a–g and a disilane4h,i (for Cu–Si). Although a similar convertible into the respective distannylalkenes (3b–3f) in high
strategy should be applicable to the formation of C–Sn bonds, yields (entries 1–5, Table 2). The prominent copper catalysis
the lack of practical approaches to the catalytic generation of toward the electronically non-biased aliphatic terminal alkynes
Cu–Sn species7,8 has discouraged the widespread utilization of should be worth noting, because the reaction using a conven-
this potential methodology.
tional palladium catalyst became sluggish, leading to low
Among the various approaches to forming C–Sn bonds, yields of the products (Scheme 1).9f,16 In contrast, the reaction
synchronous installation of two stannyl moieties into C–C triple of t-butylacetylene (1g) only gave a 10% yield of 3g, probably
bonds, namely distannylation,9–11 is of high synthetic value, owing to the steric congestion around the triple bond (entry 6).
because the resulting vic-distannylalkenes are convertible into
multisubstituted alkenes via chemoselective transformation of
both the stannyl moieties.12 Although the distannylation has
hitherto been carried out under palladium catalysis by using
distannanes,9 further improvements in catalytic systems, which
enhance versatility of this synthetic method, would be highly
desirable.13 We report herein unprecedented activation of a
distannane with a copper(I) complex, which leads to the efficacious
generation of Cu–Sn species, and its synthetic application to
the first copper-catalyzed distannylation of alkynes.
Table 1 Optimization of reaction conditions for distannylation
Entry
Solvent
x
y
Time (h)
Yielda (%)
1
2
3
NMP
NMP
Toluene
Acetonitrile
DCM
DMF
DMF
DMF
DMF
10
10
10
10
10
10
2
200
0
200
200
200
200
20
11
46
24
24
24
17
13.5
24
24
60
0
86
75
40
88
80d
72
0
4
5b
6
Department of Applied Chemistry, Graduate School of Engineering,
Hiroshima University, Higashi-Hiroshima 739-8527, Japan.
7c
8
1
0
10
10
E-mail: yhiroto@hiroshima-u.ac.jp; Fax: +81-82-424-5494; Tel: +81-82-424-7724
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization data. See DOI: 10.1039/c3cc47173j
9e
a
b
c
d
e
Determined by NMR. 40 1C. 60 1C. Isolated yield. 100 1C.
c
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Table 2 Cu-catalyzed distannylation of alkynesa
Entry
R
Time (h)
Yieldb (%)
Product
1
2
3
4
5
6
7
8
nBu (1b)
nOct (1c)
iAmyl (1d)
iBu (1e)
Cyp (1f)
5
11
3.5
3
80
90
85
62
78
10
76
51
54
54
41
64
58
13
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3
tBu (1g)
60
7.5
12.5
16
20.5
51
24
24
24
(CH2)3Cl (1h)
(CH2)3CN (1i)
CH2NMe2 (1j)
CH2NEt2 (1k)
CH2OMe (1l)
Ph (1m)
9
10
11
12c
13d
14
CH2OTHP (1n)
(CH2)4OH (1o)
Scheme 2 Reaction using 1,7-octadiyne.
a
General procedure: 1 (0.45 mmol, 1.5 equiv.), 2 (0.30 mmol, 1 equiv.),
b
Cu(OAc)(PPh3)3 (2 mol%), Cs2CO3 (20 mol%), DMF (0.2 mL). Yields of
isolated 3. KOtBu instead of Cs2CO3. LiOtBu instead of Cs2CO3.
c
d
Cu–B and Cu–Si species, a key intermediate should be a Cu–Sn
species (5) derived from a CuOR0 complex and a base-activated
distannane (6) (Scheme 3). Such activation has already been
reported in the base-catalyzed disproportionation of unsymmetrical
hexaalkyldistannanes.18,19 Subsequent addition of 5 to a C–C triple
bond of an alkyne affords b-stannylalkenyl copper species 7,20
which is then captured with Me3SnOR0 8 to give the distannylation
product 3 with regeneration of the CuOR0 complex.21 The
preferential formation of the hydrostannylation products
(4o and 4o0) in the reaction of 1o is ascribable to fast proto-
nolysis of 7 by the hydroxy moiety in 1o.22
Scheme 1 Pd-catalyzed distannylation of 1-octyne.
The distannylation was also applicable to 5-chloro-1-pentyne (1h)
and 5-hexynenitrile (1i) to afford the products (3h and 3i) in 76%
and 51% yield, leaving these functional groups (cyano and chloro
groups) intact (entries 7 and 8). The course of the distannylation
was little affected by coordinating groups in propargylic amine
(1j and 1k) and ether (1l), although the yields became
moderate (entries 9–11). In addition, phenylacetylene (1m) and
THP-protected propargylic alcohol (1n) could facilely be coupled
with 2, providing 64% and 58% yield of the products (3m and 3n)
by use of a strong base (KOtBu or LiOtBu) instead of Cs2CO3
(entries 12 and 13).17 When an alkyne bearing a hydroxy moiety (1o)
was employed as a substrate (entry 14), hydrostannylation products
(4o and 4o0) were predominantly produced (Fig. 1).
In conclusion, we have demonstrated that the treatment of a
distannane with a copper(I) complex and a base led to facile
generation of a Cu–Sn species via activation of a Sn–Sn bond,
which served as a key intermediate of the catalytic distannylation
The salient catalytic activity of the present copper system has
also been demonstrated by the reaction of 1,7-octadiyne (1p),
both of whose triple bonds underwent the distannylation to
give the tetrastannylation product 3p in 66% yield, being in
marked contrast to the results obtained with the previously
reported palladium catalyst, where only a 12% yield of the
distannylation product 3p0 was obtained (Scheme 2).
Similarly to the copper-catalyzed borylation and silylation of
C–C multiple bonds that proceed through the formation of the
Fig. 1 Hydrostannylation products in the reaction of 1o.
11672 Chem. Commun., 2013, 49, 11671--11673
Scheme 3 A plausible catalytic cycle for distannylation.
c
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 11671--11673 11673