Highly regio- and stereoselective silylstannation of allenes catalyzed by phosphine-free palladium complexes

Article abstract of DOI:10.1039/b206488j

The addition reaction of silylstannanes with allenes in the presence of Pd₂(dba)₃·dba in toluene affords (E)-alkenylsilanes having an allylstannane moiety exclusively in good to excellent yields.

Full text of DOI:10.1039/b206488j

Highly regio- and stereoselective silylstannation of allenes catalyzed by
phosphine-free palladium complexes†
Masilamani Jeganmohan, Muthian Shanmugasundaram, Kuo-Jui Chang and Chien-Hong Cheng*
Department of Chemistry, Tsing Hua University, Hsinchu, Taiwan 300. E-mail: chcheng@mx.nthu.edu.tw;
Fax: +886-3-5724698; Tel: +886-3-5721454
Received (in Cambridge, UK) 4th July 2002, Accepted 16th September 2002
First published as an Advance Article on the web 1st October 2002
The addition reaction of silylstannanes with allenes in the
presence of Pd2(dba)3·dba in toluene affords (E)-alkenylsi-
lanes having an allylstannane moiety exclusively in good to
excellent yields.
reaction mixture. Control experiments revealed that in the
absence of palladium catalysts, no reaction occurred. The
catalytic reaction is highly regioselective with the silyl group
adding to the middle carbon and the stannyl group adding to the
unsubstituted terminal carbon of 2a. The structure of 4a was
1
The addition of main group metal–metal bonds such as Si–Si,
Sn–Sn, Si–Sn, Ge–Sn, B–Si, B–B bonds to allenes is a powerful
strategy for the construction of organometallic compounds
having both vinylic and allylic metal moieties that are versatile
synthetic reagents in organic synthesis.1,2 Though the silyl-
stannation of allenes catalyzed by Pd(PPh3)4 is known, the
observed regio- and stereoselectivity was poor (Scheme 1).3,4
The initial kinetic product 3a undergoes 1,3-shift of the stannyl
moiety upon heating to give a mixture of E/Z isomers 3b.
Recent effort in our laboratories revealed that phosphine-free
palladium complexes effectively catalyzed three-component
coupling reactions of allenes.5 These observations and our
interest in metal-mediated allene chemistry5,6 promoted us to
explore the reaction of silylstannane with allenes in the presence
of phosphine-free palladium complexes. Herein, we wish to
report a highly regio- and stereoselective addition of silyl-
stannanes to allenes by using phosphine-free palladium com-
plexes as catalysts to give (E)-alkenylsilanes having an
allylstannane moiety in high yields.
ascertained by H and 13C NMR and mass spectral data. The
stereochemistry of 4a was unequivocally established by typical
1H NMR NOE techniques.
To understand the effect of catalyst, several phosphine-free
palladium catalysts were tested. All these complexes PdCl2,
Pd(acac)2, Pd(OAc)2, PdCl2(PhCN)2 and PdCl2(CH3CN)2 in
toluene are lower in catalytic activity relative to Pd2(dba)3·dba
for the addition of 1a to allene 2a affording 4a in 70, 65, 61, 23
and 20% yields, respectively. Examination of the influence of
solvent on the catalytic activity revealed that toluene was the
solvent of choice. Other solvents such as EA, CH2Cl2 and THF
were also effective, giving 4a in 88, 85 and 73% yields,
respectively. Coordinating solvents CH3CN or DMF were less
suitable for the present catalytic reaction furnishing 4a in 27 and
19% yields, respectively. The same reaction when carried out at
80 °C in the presence of Pd2(dba)3·dba in toluene proceeded
quantitatively to afford single product 4a.
Table 1 delineates the results obtained for the reaction of
silylstannanes 1a,b with allenes 2a–l. Aryl allenes with an
electron-withdrawing or an electron-donating group on the
aromatic ring react efficiently with 1a to give the corresponding
allylstannanes 4b–f in high yields indicating that this catalytic
reaction is insensitive to electronic effects (entries 2–6).
Similarly, the addition is also insensitive to steric substituents.
1-Naphthylallene 2g and tert-butylallene 2h having a bulky
group react smoothly with 1a to afford the corresponding
products 4g and 4h in 82 and 80% yields (entries 7 and 8).
Table 1 Results of palladium-catalyzed addition of silylstannane (1) with
allene (2)a
Yieldb
(%)
Scheme 1
Entry
1
R1
2
R
Product E/Z
The addition reaction of trimethyl(tributylstannyl)silane 1a
with phenylallene 2a in the presence of Pd2(dba)3·dba (5 mol%)
(dba = dibenzylideneacetone) in toluene at ambient tem-
perature proceeded quantitatively to give an allylstannane
derivative 4a (Scheme 2). No other regioisomer or stereoisomer
other than 4a was detected in the 1H NMR spectrum of the crude
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1a -Bu
1a -Bu
1a -Bu
1a -Bu
1a -Bu
1a -Bu
1a -Bu
1a -Bu
1a -Bu
1a -Bu
1a -Bu
1a -Bu
2a
2b
2c
2d
2e
2f
2g
2h
2i
-C6H5
4a
4b
4c
4d
4e
> 99
91 (99)
82
91
90
89
88
82
80
90 (96)
85
87
88 (95)
90
83
87
2-Me-phenyl
3-Me-phenyl
4-OMe-phenyl
4-Br-phenyl
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
4-COCH3-phenyl 4f
1-naphthyl
tert-butyl
n-butyl
4g
4h
4i
4j
4k
4l
4m
4n
4o
2j
2k
2l
n-octyl
cyclopentyl
cyclohexyl
-C6H5
tert-butyl
n-butyl
1b -Me 2a
1b -Me 2h
1b -Me 2i
a The reaction of silylstannane (1.00 mmol) with allene (1.30 mmol) was
carried out at rt for 8 h in toluene (2.0 ml) and Pd2(dba)3·dba (5 mol%).
b Isolated yields; yields in the parentheses were measured from the crude
products by the 1H NMR integration method using mesitylene as an internal
standard.
Scheme 2
† Electronic supplementary information (ESI) available: synthesis and
characterization of compounds 4. See http://www.rsc.org/suppdata/cc/b2/
b206488j/
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This journal is © The Royal Society of Chemistry 2002

Under similar reaction conditions, the addition reaction of
aliphatic allenes n-butylallene 2i, n-octylallene 2j, cyclopenty-
lallene 2k and cyclohexylallene 2l with 1a also proceeds
smoothly to give the corresponding allylstannanes 4i–l in high
yields (entries 9–12).
The catalytic addition can be extended to trimethyl(trimethyl-
stannyl)silane 1b. Thus, the reaction of 1b with 2a, 2h and 2i
gave the corresponding silylstannation products 4m–o in 90, 83
and 87% yields. The reaction is highly regio- and ster-
eoselective affording (E)-alkenylsilanes having an allylstan-
nane moiety exclusively in all of these reactions (Table 1).
Comparison of the present results with those reported
previously3,4 reveals marked difference between these two
catalytic reactions. First, for the present Pd2(dba)3·dba cata-
lyzed silylstannation, the reaction is highly regioselective, with
the stannyl group always connecting to the unsubstituted
terminal carbon of the allene moiety, irrespective of the
substituent on the silylstannane and allene moieties. This is in
sharp contrast to the reported reaction using Pd(PPh3)4 as the
catalyst giving initially kinetic product 3a with the stannyl
group attaching to the substituted terminal carbon of the allene
group. The latter then undergoing 1,3-shift of the stannyl group
to regioisomer 3b with the stannyl group attached to the
unsubstituted terminal carbon of the allene moiety. The ratio of
the regioisomers was highly influenced by the substituents on
both the allene and silylstannane moieties. Secondly, only E
stereoselectivity of the reaction products 4 was observed for the
present catalytic reaction, but a mixture of E and Z isomeric
products 3b was isolated for the reported reaction.
Based on the known palladium chemistry, a mechanism
involving face-selective coordination of allene to the palladium
center is proposed to account for the observed regio- and
stereochemistry of products (Scheme 3). The catalytic reaction
is likely initiated by the oxidative addition of silylstannane to
Pd(0) to give Pd(II) intermediate 5.7 The terminal double bond
of allene is then coordinated favorably to the palladium center
of 5 at the face opposite to the substituents R to avoid steric
congestion. Insertion of the coordinated double bond of allene
to the Pd–Si bond affords p-allyl palladium complex 6 with the
R group anti to the SiMe3 moiety on the allyl group. Subsequent
reductive elimination of 6 gives the desired product 4 and
regenerates the Pd(0) catalyst. The anti form of 6 is solely
responsible for the exclusive formation of (E)-vinylsilane
derivatives 4.
Scheme 3
stannane moieties present in these products allow a large variety
of chemical modifications. Further work is in progress to study
the application of these products.
We thank the National Science Council of the Republic of
China (NSC 90-2811-M-007-034) for the support of this
research.
Notes and references
1 (a) For the applications of alkenyl- and allyl-metals in organic synthesis,
see Comprehensive Organometallic Chemistry II, ed. E. W. Abel, F. G.
A. Stone, G. Wilkinson and A. McKillop, Pergamon, Oxford, 1995, vol.
11; (b) R. Zimmer, C. U. Dinesh, E. Nandanam and F. A. Khan, Chem.
Rev., 2000, 100, 3067.
2 (a) H. Watanabe, M. Saito, N. Sutou, K. Kishimoto, J. Inose and Y.
Nagai, J. Organomet. Chem., 1982, 225, 343; (b) T. N. Mitchell, U.
Schneider and B. Frohling, J. Organomet. Chem., 1990, 384, C53; (c) S-y
Onozawa, Y. Hatanaka and M. Tanaka, Chem. Commun., 1999, 1863; (d)
M. Suginome, Y. Ohmori and Y. Ito, Synlett, 1999, 1567; (e) F.-Y. Yang
and C.-H. Cheng, J. Am. Chem. Soc., 2001, 123, 761.
3 T. N. Mitchell, H. Killing, R. Dicke and R. Wickenkamp, J. Chem. Soc.
Chem. Commun., 1985, 354.
4 T. N. Mitchell and U. Schneider, J. Organomet. Chem., 1991, 407,
319.
5 (a) M.-Y. Wu, F.-Y. Yang and C.-H. Cheng, J. Org. Chem., 1999, 64,
2471; (b) F.-Y. Yang, M.-Y. Wu and C.-H. Cheng, Tetrahedron Lett.,
1999, 40, 6055; (c) F.-Y. Yang, M.-Y. Wu and C.-H. Cheng, J. Am.
Chem. Soc., 2000, 122, 7122; (d) T.-H. Huang, H.-M. Chang, M.-Y. Wu
and C.-H. Cheng, J. Org. Chem., 2002, 67, 99.
In summary, a highly regio- and stereoselective silylstanna-
tion of allenes has been demonstrated using phosphine-free
palladium complexes as catalysts. The nature of the ligands on
palladium complexes influences tremendously the regio- and
stereochemistry of the reaction. The vinylsilane and allyl-
6 (a) H.-M. Chang and C.-H. Cheng, J. Org. Chem., 2000, 65, 1767; (b) H.-
M. Chang and C.-H. Cheng, Org. Lett., 2000, 2, 3349.
7 S.-K. Kang, Y.-H. Ha, B.-S. Ko, Y. Lim and J. Jung, Angew. Chem., Int.
Ed., 2002, 41, 343.
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