Carbostannylation of Alkynes Catalyzed by an Iminophosphine-Palladium Complex

Full text of DOI:10.1021/ja974206k

J. Am. Chem. Soc. 1998, 120, 2975-2976
2975
Scheme 1
Carbostannylation of Alkynes Catalyzed by an
Iminophosphine-Palladium Complex
Eiji Shirakawa,* Hiroto Yoshida, Takuya Kurahashi,
Yoshiaki Nakao, and Tamejiro Hiyama
Department of Material Chemistry
Graduate School of Engineering
Kyoto UniVersity, Sakyo-ku, Kyoto 606-8501, Japan
Scheme 2
ReceiVed December 12, 1997
Carbometalation of alkynes generates cis-substituted alkenyl-
metals and is one of the most useful reactions for stereoselective
olefin synthesis, since the resulting alkenylmetals can be trans-
formed further to variously substituted ethylenes.1 In particular,
carbocupration,2 zirconium-catalyzed carboalumination3 and nickel-
catalyzed carbozincation4 have high synthetic potential due to wide
applicability. Although alkenylstannanes are useful synthetic
precursors for various olefinic targets,5 no report has been
published on the transition-metal-catalyzed carbostannylation of
alkynes.6-8 Here, we report the palladium-catalyzed carbostan-
nylation of alkynes using alkynylstannanes.
Table 1. Carbostannylation of Alkynes Catalyzed by
Iminophosphine (1)-Palladiuma
We have already reported that tributyl(phenylethynyl)tin (2a)
adds oxidatively to a palladium(0) complex coordinated by N-(2-
(diphenylphosphino)benzylidene)-2-phenylethylamine (1) and that
the resulting oxidative adduct (3) is involved in the catalytic cycle
of the palladium-catalyzed coupling of 2a with aryl iodides
(Scheme 1).9 We envisaged that palladium complex 3 should
react with alkynes to give carbostannylation products. This turned
out to be the case.
Treatment of 2a with a 1:2 mixture of [PdCl(π-C3H5)]2-1 (5
mol % of Pd) under an acetylene atmosphere (1 atm) in THF at
50 °C for 2 h gave tributyl[(Z)-2-(phenylethynyl)ethenyl]tin (5a)10
in 81% yield11 as a single isomer through an exclusive syn-
addition (Scheme 2). The use of triphenylphosphine (2 equiv to
palladium) in place of 1 gave only 48% yield of 5a in a prolonged
period (43 h). A Pd(0)-1,3-bis(diphenylphosphino)propane
temp time yield
entry R1
R2
R3
(°C) (h) (%)b prod(s)
5/6c
1
2
3
4
5
6
7
8
9
Ph (2a)
H
H
H
(4a)d 50
(4b) 50
2
3
90
4
21
44
5
4
16
29
81 5a
Ph (2a) CO2Et
Ph (2a) CO2Et
Ph (2a) Ac
78 5b, 6b
57 5c, 6c
76 5d, 6d
81 5e, 6e
82 5f, 6f
52 5g, 6g >99/1
66 5h
72 5i, 6i
80 5j, 6j
20/80
1/>99
15/85
92/8
Me (4c)e 90
H
H
H
H
H
H
H
(4d) 50
Ph (2a) Ph
(4e)
(4f)
50
50
Ph (2a) 4-CH3C6H4
Ph (2a) EtO
Bu (2b) H
91/9
(4g)f 50
(4a)d 50
(4b) 50
Bu (2b) CO2Et
12/88
92/8
10 Bu (2b) Ph
(4e)
50
a The reaction was carried out in THF (5 mL) using an alkynylstan-
nane (0.459 mmol) and an alkyne (1.38 mmol) in the presence of
iminophosphine 1 (0.022 mmol) and [PdCl(π-C3H5)]2 (0.011 mmol).
b Isolated yield based on the alkynylstannane is given. c Determined
by 1H or 119Sn NMR. d The reaction was carried out under an acetylene
atmosphere (1 atm). e Solvent ) dioxane. f Ethoxyacetylene (0.459
mmol) was used.
(1) Knochel, P. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming,
I., Semmelhack, M. F., Eds.; Pergamon Press: New York, 1991; Vol. 4,
Chapter 4.4, pp 865-911.
(2) Normant, J. F.; Alexakis, A. Synthesis 1981, 841-870.
(3) (a) Negishi, E.; Takahashi, T. Synthesis 1988, 1-19. (b) van Horn, D.
E.; Negishi, E. Pure Appl. Chem. 1981, 53, 2333-2356.
(4) Stu¨demann, T.; Knochel, P. Angew. Chem., Int. Ed. Engl. 1997, 36,
93-95.
complex, which also was shown to be added oxidatively by 2a,9
was much less effective to give 5a in 28% yield even after 22 h.
The carbostannylation of various alkynes catalyzed by the
Pd-1 catalyst was next examined (Scheme 2, Table 1). The
reaction of 2a with ethyl propiolate (4b) gave carbostannylation
products consisting of regioisomers 5b and 6b in a 20/80 ratio
(entry 2). The carbostannylation of ethyl 2-butynoate (4c) resulted
in higher regioselectivity, though higher temperature and longer
reaction time were required (entry 3). A ketonic acetylene,
1-butyn-3-one (4d), reacted with 2a smoothly with a regioselec-
tivity similar to 4b (entry 4). The reaction of arylacetylenes 4e
and 4f with 2a was relatively slow to give the corresponding
carbostannylation products in high yields (entries 5 and 6) but
with a reversed regioselectivity. The reaction of ethoxyacetylene
(4g) also proceeded with high regioselectivity with the preference
for 5g over 6g (entry 7). Tributyl(1-hexyn-1-yl)tin (2b) also
reacted with alkyne 4a, 4b, or 4e, giving the corresponding
alkenylstannanes with the regioselectivity similar to 2a (entries
8-10).12
(5) Pereyre, M.; Quintard, J.-P.; Rahm, A. Tin in Organic Synthesis; Butter-
worth: London, 1987.
(6) Carbostannylation of alkynes using a particular combination of
substrates, (stannylethynyl)amines and dimethyl acetylenedicarboxylate, is
reported. Himbert, G. J. Chem. Res. S 1979, 88-89.
(7) Alkenylation of ketones or phenols using terminal alkynes and SnCl4
is considered to proceed through carbostannylation of stannylacetylenes. (a)
Hayashi, A.; Yamaguchi, M.; Hirama, M.; Kabuto, C.; Ueno, M. Chem. Lett.
1993, 1881-1884. (b) Yamaguchi, M.; Hayashi, A.; Hirama, M. J. Am. Chem.
Soc. 1993, 115, 3362-3363. (c) Yamaguchi, M.; Hayashi, A.; Hirama, M. J.
Am. Chem. Soc. 1995, 117, 1151-1152.
(8) Carbostannylation may alternatively be accomplished by carbocupration
of alkynes followed by quenching with tin halides/triflate. Westmijze, H.;
Meijer, J.; Vermeer, P. Recl. TraV. Chim. Pays-Bas 1977, 96, 194-196.
(9) Shirakawa, E.; Yoshida, H.; Hiyama, T. Tetrahedron Lett. 1997, 38,
5177-5180.
(10) Configuration of alkenylstannane 5a could not be determined directly,
1
because the olefinic protons of 5a had the same chemical shift in H NMR.
The (Z)-configuration of 5a was confirmed after the transformation to the
corresponding alkenyl iodide (10) by iodolysis (Scheme 4). The coupling
constant between olefinic protons of 10 was 8.3 Hz, which is typical to a
cis-disubstituted ethylene. Iodolysis of R- or â-(alkynyl)alkenylstannanes to
the corresponding (alkynyl)alkenyl iodides proceeds with retention of con-
figuration: Stracker, E. C.; Zweifel, G. Tetrahedron Lett. 1991, 32, 3329-
3332.
(12) No isomer other than 5 and 6 was obtained out of the four possible
isomers in use of alkynes 4b-g. The hydrolysis of the alkenylstannanes to
the corresponding alkenes revealed that the stannyl group in 5 had a proton
at the geminal position, whereas that in 6 had an R2 group. Syn-addition in
use of acetylene led us to the conclusion that both 5 and 6 are also syn.
(11) The yield based on alkynylstannane should be lower than 95%, because
5% of alkynylstannane would be consumed for the reduction of Pd(II) to Pd-
(0).
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Published on Web 03/12/1998

2976 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Communications to the Editor
Scheme 3
Scheme 4
be explained by the followings. In the cases of arylacetylenes,
carbopalladation giving alkenylpalladiums would prefer 7a to 7b
by steric reason. Accordingly, alkenylstannanes 5 are afforded
as main products. In contrast, electron-deficient alkynes 4b, 4c,
and 4d are likely to suffer the addition of the alkynyl group in a
Michael fashion giving 6 predominantly through 8b.
Finally, we confirmed the utility of the carbostannylation
products by the transformation of carbostannylation product 5a
to various compounds (Scheme 4). The Pd-tri(2-furyl)phosphine-
catalyzed coupling14 of 5a with 4-nitroiodobenzene gave enyne
9 in 85% yield.15 Iodolysis or hydrolysis of 5a afforded the
corresponding alkenyl iodide (10) or enyne (11) in good yield,
respectively.
In conclusion, we have demonstrated that the carbostannylation
of alkynes takes place with alkynylstannanes to give conjugated
(stannyl)enynes in a stereoselective manner. Studies on the details
of the mechanism as well as synthetic applications to various
unsaturated substrates and organostannanes are in progress in our
laboratories.
The catalytic cycle should first involve the oxidative addition
of an alkynylstannane to the Pd(0) complex as discussed before.9
Successive insertion of an alkyne to the C-Pd bond (carbopal-
ladation)13 of 3 followed by reductive elimination is likely to
afford the carbostannylation product and regenerate the Pd(0)
complex (Scheme 3). The opposite regioselectivity observed in
use of arylacetylenes and alkynes having a carbonyl group can
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JA974206K
(14) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113, 9585-9595.
(15) A [PdCl(π-C3H5)]2-1 complex (5 mol % of Pd, Pd/1 ) 1) gave 10
only in 50% yield along with the corresponding (E)-isomer (10%). For the
coupling of organostannanes with aryl halides catalyzed by Pd-1, see:
Shirakawa, E.; Yoshida, H.; Takaya, H. Tetrahedron Lett. 1997, 38, 3759-
3762.
(13) Some reaction may take place through the stannylpalladation of an
alkyne. A catalytic cycle of the palladium-catalyzed hydrostannylation-
cyclization of 1,6-diynes is discussed to proceed in similar dual pathways.
Lautens, M.; Smith, N. D.; Ostrovsky, D. J. Org. Chem. 1997, 62, 8970-
8971.