Tetraalkylammonium salt-based catalyst systems for directing Heck-type reactions. Arylation of allyltrimethylsilane

Article abstract of DOI:10.1016/S0040-4039(00)01498-2

An appropriate selection of the [Pd/base/QX] catalyst systems allows one to direct at will the palladium-catalysed arylation of allyltrimethylsilane towards the formation of either (E)-1-aryl-3-(trimethylsilyl)-1-propene or 3-aryl-1-propene, by preventing

Full text of DOI:10.1016/S0040-4039(00)01498-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8445–8449
Tetraalkylammonium salt-based catalyst systems for
directing Heck-type reactions. Arylation of
allyltrimethylsilane
Tuyet Jeffery*
ESCOM, D e´ partement de Chimie, 13, Boulevard de l’Hautil, 95092 Cergy Pontoise C e´ dex, France
Received 20 July 2000; accepted 6 September 2000
Abstract
An appropriate selection of the [Pd/base/QX] catalyst systems allows one to direct at will the
palladium-catalysed arylation of allyltrimethylsilane towards the formation of either (E)-1-aryl-3-
trimethylsilyl)-1-propene or 3-aryl-1-propene, by preventing or promoting efficiently the desilylation and
(
the double bond migration. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: ammonium salts; arylation; Heck reactions; palladium and compounds.
Palladium-catalysed arylation of allyltrimethylsilane has been the subject of several studies.
To achieve this type of reaction with a high selectivity, several problems have to be overcome.
The regioselectivity of both the insertion and b-hydride-elimination steps has to be controlled as
well as the double bond migration and the chemoselectivity of the reaction which depends upon
the relative rates of b-hydride and b-trimethylsilyl eliminations. Formation of a mixture of
silylated ( 16 + 26 +36 ) and desilylated products (46 +66 ) has been observed in the reaction of iodoben-
Scheme 1.
*
Corresponding author. Tel: (33) 01 30 75 61 96; fax: (33) 01 30 75 60 21.
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01498-2

8446
zene with allyltrimethylsilane (Scheme 1) under traditional Heck conditions, with 16 being the
1
major product. Addition of silver nitrate has been shown not only to suppress the desilylation
but also to affect the regiochemistry of both the insertion and subsequent b-hydride-elimination
1
steps as products 26 and 36 were then obtained predominantly (16 /26 /36 =3/70/27).
Highly regioselective internal arylation of allyltrimethylsilane has recently been realised using
aryl triflates as the arylating agent together with 1,1%-bis(diphenylphosphino)ferrocene (DPPF)
2
as the ligand for the palladium catalyst. Intramolecular arylation of allylsilanes with highly
3
selective construction of tertiary sp carbon centres has also been described in the presence of
3
,4
tetraalkylammonium or silver salts. Use of silver salts led to highly selective formation of
3
silylated cyclised products, although this effect is cancelled by addition of the ligand (S)-
3
BINAP resulting in highly regio and enantioselective formation of tertiary sp carbon centres
3
,4
which occurred with desilylation.
5
,6
We have recently shown that a suitable selection of quaternary ammonium salt-based
catalyst systems not only enhance the rate and selectivity of Heck-type reactions, but also allows
5
6
the choice of outcome of the reactions involving 2,3-dihydrofuran or vinyltrimethylsilane. In
this paper, we wish to report that tetraalkylammonium salt-based catalyst systems can also be
highly effective for Heck-type reactions involving allyltrimethylsilane, not only for achieving
them with a high selectivity, under mild conditions, but also for directing them at will towards
the formation of either silylated or non-silylated products.
Conditions were first determined for the reaction of iodobenzene with allyltrimethylsilane
(
Scheme 1 and Table 1). Highly selective formation of (E)-1-phenyl-3-(trimethylsilyl)-1-propene
1
6 was smoothly achieved upon treatment in acetonitrile, at room temperature, of iodobenzene
with allyltrimethylsilane, in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO), tetra-n-
butylammonium bromide and catalytic amounts of bis(dibenzylideneacetone) palladium (Table
1
, entry 1).
Arylation at the terminal position occurred exclusively as no formation of 2-phenyl-3-
(
trimethylsilyl)-1-propene 3
propene 2 was obtained as a minor product (1
desilylated product, but its isomer 5 was observed, although only as a secondary product.
6
was observed. The isomeric (E)-3-phenyl-1-(trimethylsilyl)-1-
6
6
/2=89/11). Allylbenzene 4 was not detected as the
6
6
6
Acetonitrile appears more effective than tetrahydrofuran, toluene or N,N-dimethylformamide
for obtaining both a high yield and a high selectivity (entry 1 compared with entries 2, 3 and 4).
Tetra-n-butylammonium chloride (entry 5) was slightly more efficient than tetra-n-butyl-
ammonium bromide (entry 1) giving slightly better results. Lower conversion and lower
selectivity were both observed in the absence of tetraalkylammonium salt (entry 6) leading,
moreover, to a mixture of three isomeric silylated products (16 /26 /36 =79/13/8). Use of DABCO as
base seemed essential for obtaining a high yield of silylated products as desilylation occurred
much more readily in the presence of other bases such as triethylamine or diisopropylethylamine
(
entries 7 and 8). Complex mixtures of silylated and desilylated products were then obtained, the
latter being constituted by allylbenzene 4, 1-phenyl-1-propene 5 and (E)-1,3-diphenyl-1-propene
. Formation of 7 most probably results from palladium-catalysed arylation of allylbenzene 4
formed in situ; this reaction being in competition with the isomerisation of 4 into 5, even when
reactions were performed with an excess (1.75 equiv.) of allyltrimethylsilane.
Conditions for achieving highly selective desilylation have also been found (entries 9–11).
Allylbenzene 4 was obtained without formation of silylated products when the reaction is
effected in the presence of tetra-n-butylammonium acetate. It is noteworthy that allylbenzene 4
was exclusively obtained; no isomeric products 5 and/or 6 were detected. The described reaction
6
6
76
6
6
6
6
6
6
6
6

Table 1
a,b
Palladium-catalysed phenylation of allyltrimethylsilane
d
c
Entry
Base
QX
Solvent
T
Conversion
(%)
Silylated products
yield (%)
Ratio
/2/3
Yield (%)
c
c
(
°C/h)
1
6
6
6
46
56
76
a
1
2
3
4
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
NEt3
n-Bu NBr
CH CN
THF
toluene
DMF
rt/21
rt/26
rt/21
rt/21
rt/19
rt/24
rt/24
rt/21
50/24
50/24
100
74
87
90
100
75
82
54
82
76
87
61
28
9
89/11/0
80/20/0
63/29/8
85/15/0
90/10/0
79/13/8
–
–
–
–
–
0
0
0
0
0
0
14
Traces
Traces
12
12
10
9
10
0
0
0
0
0
0
0
11
28
0
4
3
a
n-Bu NBr
4
a
n-Bu NBr
4
a
n-Bu NBr
4
a
e
5
n-Bu NCl
CH CN
4
3
a
6
7
None
CH CN
3
a
e
e
f
f
n-Bu NCl
CH CN
82
12
13
95
40
83
4
3
a
8
EtN(i-Pr)2
n-Bu NCl
CH CN
100
100
51
4
3
b
g
g
g
9
n-Bu NOAc
n-Bu NOAc
DMF
DMF
DMF
0
0
0
4
4
b
1
1
0
1
KOAc
n-Bu NOAc
0
0
0
0
4
b
h
n-Bu NOAc
n-Bu NOAc
rt/48
100
4
4
a
All reactions were carried out with PhI (1 equiv.), allyltrimethylsilane (1.75 equiv.), Pd(dba) (0.03 equiv.), base (2.5 equiv.), and tetraalkylammonium
2
salt (1 equiv.).
b
Unless otherwise stated, all reactions were carried out with PhI (1 equiv.), allyltrimethylsilane (5 equiv.), Pd(dba) (0.03 equiv.), PPh (0.09 equiv.), base
2
3
(
1.5 equiv.), and tetraalkylammonium salt (1 equiv.) in the presence of crushed 4 A
,
molecular sieves.
Determined by GLC analysis against an internal standard (dodecane or hexadecane).
c
d
e
f
Determined by GC/MS analysis.
Tetrabutylammonium chloride, tech., 95% from Acros.
Based on PhI.
Biphenyl as the only by-product.
g
h
In the presence of 0.04 equiv. of Pd(dba) and without PPh .
2
3

8448
has thus been realised with a very high regioselectivity in the insertion step and the desilylation
achieved without migration of the double bond. Use of tetra-n-butylammonium acetate both as
base and tetraalkylammonium salt (entry 9) is the most efficient, while a combination of
potassium acetate and tetra-n-butylammonium acetate is much less effective (entry 10). Forma-
tion of allylbenzene 4
6 can even be achieved under mild conditions, i.e. at room temperature
(
entry 11). Biphenyl was the only by-product observed in these reactions; its formation can be
minimised by effecting reactions in a strictly inert atmosphere.
Optimum conditions have thus been determined to induce at will the arylation of
allyltrimethylsilane towards the formation of either (E)-1-aryl-3-(trimethylsilyl)-1-propene or
3
-aryl-1-propene (Table 2 and Scheme 2).
Table 2
Palladium-catalysed arylation of allyltrimethylsilane
.

8449
Scheme 2.
The high regioselectivity observed in the described reactions (Scheme 2) is most probably due
to the mild reaction conditions made possible by the use of tetraalkylammonium salts. Although
an in-depth understanding of the exact role of tetra-n-butylammonium acetate and DABCO is
not yet available, the opposing nature of these bases might explain the high and complementary
chemoselectivities of the described reactions. An affinity of acetate ion for trimethylsilyl group
could account for the preferential desilylation while both the sterically hindered nucleophilicity
and the strong basicity of DABCO should favour the competitive b-hydride elimination which
leads to the formation of silylated products.
In conclusion, tetraalkylammonium salt-based catalyst systems can be highly useful for
Heck-type reactions involving allyltrimethylsilane as not only do they allow one to achieve these
reactions under mild conditions, with high regio- and chemoselectivity, but they also allow one
to direct them at will towards the formation of either silylated or non-silylated products.
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