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Couplings with Monoorganotin Compounds A УRadicalФ Twist from the Original Stille Reaction.

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Cross-Coupling Reactions
Couplings with Monoorganotin Compounds:
A “Radical” Twist from the Original Stille Reaction
Antonio M. Echavarren*
cross-coupling · nickel · palladium ·
reaction mechanisms · stannanes
The Stille cross-coupling reaction of
tetraorganostannanes R4Sn' with organic electrophiles is one of the most
reliable methods for the selective formation of carbon–carbon bonds,[1] in
particular Csp2–Csp2 bonds.[2–4] However,
this method suffers the disadvantage
that only one of the four groups at tin
is transferred to the organic electrophile. Also, the triorganotin halide
R3SnX that forms stoichiometrically as
a byproduct is toxic and, on occasions,
its separation from the desired product
can be rather difficult.
Recent developments in cross-coupling methodology allow Stille reactions
to be performed under milder conditions.[2, 3] This is more clearly seen by
comparing the best current methods for
the coupling of aryl bromides with
Stilles original procedure published in
1987 (Table 1).[5] Fu and co-workers
discovered that palladium complexes
[Pd2(dba)3]/PtBu3 or [Pd(PtBu3)2], that
bear bulky phosphane groups, are excellent catalysts for the coupling of
aryl bromides at room temperature.[6]
Verkade and co-workers developed related conditions for the Stille coupling
reaction by using proazaphosphatranes
as P ligands.[7] Concurrently, Baldwin
and co-workers found a synergetic effect
of fluoride anion and CuI ions that
allowed the coupling of aryl bromides
to be carried out under mild condi[*] Prof. Dr. A. M. Echavarren
Institute of Chemical Research of Catalonia
43007 Tarragona (Spain)
Departamento de Qumica Orgnica
Universidad Autnoma de Madrid (Spain)
Fax: (+ 34) 977-920-225
aryl chlorides relies on the
use of fluoride anion to enhance the reactivity of the
stannane. Fu and co-workers
found that activation by fluoride anion is also crucial in
Ar Br
Ar Cl
the coupling of stannanes
with primary alkyl bromides
[Pd(PPh3)4] (2 %),
toluene, 110 8C
that bear b-hydrogen atoms
1).[10] This result
[Pd2(dba)3] (0.5–1.5 %),
as for Ar Br,
represented a significant dePtBu3 (1.1–3.3 %),[b]
but in dioxane
velopment in this area.[11, 12]
toluene or NMP, RT,
at 100 8C with
Fluoride ion probably gives
(CsF, 2.2 equiv for R = Ar) CsF (2.2 equiv)
rise to pentacoordinated speVerkade[7] [Pd2(dba)3] (1.5 %),
as for Ar Br,
cies with enhanced reactivity
but in dioxane
L[c] (3 %),
in the transmetalation step,
CsF (2.2 equiv),
at 110 8C
as demonstrated by Garca
Martnez et al. using hypervalent difluorostannate re[8]
as for Ar Br,
PdCl2 (2 %),
agents 1, which are easily
but at 100 8C
PtBu3 (4 %),
prepared by reaction of triCuI (4 %),
CsF (2 equiv),
DMF, 45 8C
(Scheme 2).[13]
Fouquet et al. reported
[a] Bn = benzyl, dba = dibenzylideneacetone, NMP = Nmethylpyrrolidone, DMF = N,N-dimethylformamide.
that monoalkylstannanes 2,
[b] [Pd(PtBu3)2] could also be used as catalyst.
prepared in situ from RX
and Lapperts stannylene Sn[(Me3Si)2N]2, are activated by
[c] L =
tetrabutylammonium fluoride (TBAF) to form reagents 3, which react with
tions.[8] For the coupling of aryl chlor- alkenyl or aryl iodides (Scheme 2).[14]
ides, the least reactive of the common The accelerating effect of fluoride ion
aryl electrophiles,[9] relatively harsh con- in the Stille coupling reaction had been
discovered earlier using TBAF by Fuditions are still required.
Part of the success of these new gami et al.[15] Significantly, selective aryl
methods for the coupling of ordinary transfer was attained in the palladium-
Table 1: Comparison of some of the best methods for Stille
couplings with aryl bromides and chlorides.[a]
Scheme 1. Cy = cyclohexyl, pyrr = pyrrolidinyl, MS = molecular sieves.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200500918
Angew. Chem. Int. Ed. 2005, 44, 3962 –3965
monoorganostannanes RSnCl3 by redistribution of commercially available tetraorganostannanes such as 4 and 5 with
SnCl4 (Scheme 5).[23]
As shown in the coupling of cholesteryl iodide 6, retention of configuration
was observed to give 7 (Scheme 5). It
remains to be seen if the C-3 epimer of
6, with an axial leaving group, would
also react with retention of configuration. Importantly, note that the Suzuki
and Hiyama couplings of exo- (8) and
endo-2-bromonorbornene (9) give selectively the exo-coupled product 10
(Scheme 6).[21b,c] These and other results[23] point to a radical mechanism
for the oxidative addition step involving
NiI species, which is supported by the
recent finding that NiI complex 11 is the
Scheme 2. a) See ref. [13]; Tf = trifluoromethanesulfonyl. b) See ref. [14]; TMS = trimethylsilyl.
catalyzed reaction between di-n-butylp-tolylstannyl chloride and p-iodoanisole to give 4-methoxy-4’-methylbiphenyl. Recently, Thiele and Mitchell reported palladium-catalyzed alkenyl transfers
from an alkenyldi-n-butylstannyl chloride or bromide in the presence of
excess TBAF, although the yields of
product were only low to moderate.[16]
Bumagin and co-workers[17] and Collum and co-workers[18] succeeded in
performing Stille reactions with monoorganotin
(Scheme 3). These couplings were performed under basic conditions in aqueous solutions at 90–100 8C with PdCl2 or,
preferentially, with palladium complexes of water-soluble phosphanes.[19]
As part of a study on the crosscoupling of alkyl halides, the groups of
Knochel[20] and Fu[21] found that nickel
catalysts were particularly effective in
this context.[22] This led Fu et al. recently
to develop a coupling of secondary alkyl
bromides with monoorganostannanes
RSnCl3 that drastically departs from
(Scheme 4).[23] In this case, activation
of the stannane is carried out with
KOtBu. The byproducts of the reaction
are nontoxic inorganic tin species that
can be easily removed. This and related
methods are significant steps forward in
synthetic methodology.
Nickel-catalyzed couplings with different Ar M reagents are compared in
Table 2.[22] Although the Suzuki-type
coupling (entry 1) proceeds with only
4 mol % catalyst,[21b] reaction with the
Angew. Chem. Int. Ed. 2005, 44, 3962 –3965
stannane (entry 3) uses a catalyst
formed from bipyridine and NiCl2,[23] a
less sensitive and much cheaper source
of nickel than [Ni(cod)2].
Another distinct advantage of this
coupling is the ready availability of
Scheme 3. a) See ref. [17]; L = Ph2P(m-C6H4SO3Na). b) See ref. [18]; L’ = PhP(m-C6H4SO3Na)2.
Scheme 4.
Table 2: Nickel-catalyzed cross-couplings of secondary alkyl bromides with Ar M reagents.[a]
Ni catalyst
KOtBu (1.6 equiv)
CsF (3.8 equiv)
[Ni(cod)2] (4 %),
bphen (8 %)
NiBr2·diglyme (6.5 %),
bphen (7.5 %)
NiCl2 (10 %),
bpy (15 %)
KOtBu (7 equiv)
tBuOH/iBuOH (1:1)
[a] cod = 1,5-cyclooctadiene, bphen = 4,7-diphenylphenanthroline (bathophenanthroline),
glyme = diethylene glycol dimethyl ether, DMSO = dimethyl sulfoxide, bpy = 2,2’-bipyridine.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
foreseen by using nickel catalysts and
monoorganotins RSnX3 or related reagents.
Published online: June 1, 2005
Scheme 5.
Scheme 6. [Ni] = Ni catalyst.
active catalyst in the coupling of npentylzinc bromide with alkyl bromides
and iodides.[24]
Nickel catalysts in Stille-type couplings had already been used by Shirakawa et al. for the synthesis of styrenes
from aryl chlorides (Scheme 7 a).[25, 26]
The reactive Ni0 catalyst, presumably
[Ni(PPh3)4], was prepared from [Ni(acac)2] (acac = acetylacetonate) and
PPh3 in the presence of iBu2AlH as
reductant. Aryl bromides also react with
alkenyl-, allyl-, and alkynylstannanes in
the presence of this catalyst. Stannanes
are also able to reduce NiII in situ as
shown in the coupling of hypervalent
iodonium salts with aryltri-n-butyltin
(Scheme 7 b).[27] Aryl–aryl couplings
have also been reported for arylboronic
acids in the presence of nickel catalysts.[28]
The original Stille protocols can
hardly be recognized in the Ni-catalyzed
reaction of monoorganotin compounds.[23] This coupling raises intriguing mechanistic questions and paves the
way to the development of asymmetric
couplings of secondary alkyl halides by
using chiral ligands. Indeed, recently
Fisher and Fu have successfully reported
asymmetric Ni-catalyzed Negishi couplings of a-bromoamides (Scheme 8).[29]
New procedures for the formation of
Csp2–Csp2 and Csp2–Csp3 bonds can also be
[1] J. K. Stille, Angew. Chem. 1986, 98, 504 –
519; Angew. Chem. Int. Ed. Engl. 1986,
25, 508 – 524.
[2] V. Farina, V. Krishnamurthy, W. K.
Scott, Organic Reactions, Vol. 50, Wiley,
New York, 1997.
[3] T. N. Mitchell in Metal-Catalyzed CrossCoupling Reactions (Eds.: A. de Meijere, F. Diederich), Wiley-VCH, Weinheim, 2004, chapt. 3.
[4] P. Espinet, A. M. Echavarren, Angew.
Chem. 2004, 116, 4808 – 4839; Angew.
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[5] D. R. McKean, G. Parrinello, A. F. Renaldo, J. K. Stille, J. Org. Chem. 1987, 52,
422 – 424.
[6] a) A. F. Littke, G. C. Fu, Angew. Chem.
1999, 111, 2568 – 2570; Angew . Chem.
Int. Ed. 1999, 38, 2411 – 2413; b) A. F.
Littke, L. Schwarz, G. C. Fu, J. Am.
Chem. Soc. 2002, 124, 6343 – 6348.
[7] a) W. Su, S. Urgaonkar, J. G. Verkade,
Org. Lett. 2004, 6, 1421 – 1424; b) W. Su,
S. Urgaonkar, P. A. McLaughlin, J. G.
Verkade, J. Am. Chem. Soc. 2004, 126,
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[8] S. P. H. Mee, V. Lee, J. E. Baldwin,
Angew. Chem. 2004, 116, 1152 – 1156;
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Scheme 7. a) See ref. [25]; DME = 1,2-dimethoxyethane. b) See ref. [27].
Scheme 8. See ref. [29]. DMI = dimethylimidazolidin-2-one.
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[9] A. F. Littke, G. C. Fu, Angew. Chem.
2002, 114, 4350 – 4386; Angew. Chem.
Int. Ed. 2002, 41, 4176 – 4211.
[10] a) K. Menzel, G. C. Fu, J. Am. Chem.
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K. Menzel, G. C. Fu, Angew. Chem.
2003, 115, 5233 – 5236; Angew. Chem.
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[11] A. C. Frisch, M. Beller, Angew. Chem.
2005, 117, 680 – 695; Angew. Chem. Int.
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[12] For pioneering work on the coupling of
stannanes with secondary alkyl bromides that comprise b hydrogens, see:
R. Sustmann, J. Lau, M. Zipp, Tetrahedron Lett. 1986, 27, 5207 – 5210.
[13] a) A. Garca Martnez, J. Oso Barcina,
A. de Fresno Cerezo, L. R. Subramanian, Synlett 1994, 1047 – 1048; b) A. Garca Martnez, J. Oso Barcina, M. R.
Colorado Heras, . de Fresno Cerezo,
Org. Lett. 2000, 2, 1377 – 1378; c) A.
Garca Martnez, J. Oso Barcina, M. R.
Colorado Heras, . de Fresno Cerezo,
Organometallics 2001, 20, 1020 – 1023.
[14] a) E. Fouquet, M. Pereyre, A. L. Rodriguez, J. Org. Chem. 1997, 62, 5242 –
5243; b) E. Fouquet, A. L. Rodriguez,
Synlett 1998, 1323 – 1324; c) A. Herve,
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A. L. Rodriguez, E. Fouquet, J. Org.
Chem. 2005, 70, 1953 – 1956.
K. Fugami, S. Ohnuma, M. Kameyama,
T. Saotome, M. Kosugi, Synlett 1999, 63 –
C. M. Thiele, T. N. Mitchell, Appl. Organomet. Chem. 2004, 18, 83 – 85.
A. I. Roshchin, N. A. Bumagin, I. P.
Beletskaya, Tetrahedron Lett. 1995, 36,
125 – 128.
R. Rai, K. B. Aubrecht, D. B. Collum,
Tetrahedron Lett. 1995, 36, 3111 – 3114.
For palladium-catalyzed couplings of
ArGeCl3 with ArX under aqueous basic
conditions, see: T. Enokido, K. Fugami,
M. Endo, M. Kameyama, M. Kosugi,
Adv. Synth. Catal. 2004, 346, 1685 – 1688.
a) A. Devasagayaraj, T. Stdemann, P.
Knochel, Angew. Chem. 1995, 107,
2952 – 2954; Angew. Chem. Int. Ed.
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1998, 120, 11 186 – 11 187; c) M. Piber,
A. E. Jensen, M. Rottlnder, P. Knochel,
Org. Lett. 1999, 1, 1323 – 1326; d) R.
Giovannini, T. Stdemann, A. Devasagayaraj, G. Dussin, P. Knochel, J. Org.
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[21] a) J. Zhou, G. C. Fu, J. Am. Chem. Soc.
2003, 125, 14 726 – 14 727; b) J. Zhou,
G. C. Fu, J. Am. Chem. Soc. 2004, 126,
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[22] M. R. Netherton, G. C. Fu, Adv. Synth.
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[23] D. A. Powell, T. Maki, G. C. Fu, J. Am.
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[24] T. J. Anderson, G. D. Jones, D. A. Vicic,
J. Am. Chem. Soc. 2004, 126, 8100 –
[25] a) E. Shirakawa, K. Yamasaki, T. Hiyama, J. Chem. Soc. Perkin Trans. 1 1997,
2449 – 2450; b) E. Shirakawa, K. Yamasaki, T. Hiyama, Synthesis 1998, 1544 –
[26] For coupling of (h3-allyl)nickel intermediates with alkynyltins, see: D.-M.
Cui, N. Hashimoto, S. Ikeda, Y. Sato, J.
Org. Chem. 1995, 60, 5752 – 5756.
[27] S.-K. Kang, H.-C. Ryu, S.-W. Lee, J.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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