Alacid and Na´jera
Only two examples have been described reporting the
vinylation of activated aryl bromides and chlorides using
vinyltrimethoxysilane and TBAF as activator.9 The use of
inorganic bases, such as sodium or potassium hydroxide and
potassium carbonate as activators for the cross-coupling of
arylhalosilanes,10 silicone,11 and aryltrimethoxysilanes,12 have
been carried out in water as solvent. We have recently described
the fluoride-free Hiyama reaction of aryltrialkoxysilanes13 and
vinyltrialkoxysilanes14 with aryl bromides and chlorides using
sodium hydroxide as activator under thermal and microwave
conditions. These processes took place using either Pd(OAc)2
or phenone oxime-derived palladacycles 1 and 2 as catalysts15
under low Pd loading and very simple reaction conditions.16
We report herein a full account about the vinylation of aryl
iodides, bromides and chlorides with different vinyltrialkox-
ysilanes promoted by aqueous sodium hydroxide, using either
palladium salts or palladacycle 1 directed to the synthesis of
styrenes. The regio- and diastereoselectivity in the arylation and
vinylation of alkenyltrialkoxysilanes for the synthesis of stil-
benes, alkenylbenzenes, and conjugate dienes has also been
studied. Attempts for simple product separation and Pd recycling
experiments under these reaction conditions were also consid-
ered.
Results and Discussion
Initial studies were performed with 4-bromoacetophenone as
an activated aryl bromide with different vinyltrialkoxysilanes
and alkaline hydroxides on water as solvent under microwave
irradiation at 120 °C during 10 min (Table 1). In the presence
of 0.5 mol % of palladium acetate, the cross-coupling with
vinyltrimethoxysilane (3a) took place in higher yield using
sodium rather than potassium hydroxide (Table 1, entries 1 and
2). When 0.5 mol % of other palladium sources, such as
palladium dichloride, palladium on carbon, and palladacycles
1 and 2, were used, only complex 1 gave a quantitative yield
(Table 1, entries 3-6). Using complex 1 as precatalyst and the
several commercially available vinylalkoxysilanes 3b-i (Table
1, entries 7-14), only vinyltriethoxysilane (3b) and 3i gave
yields similar to those of 3a (Table 1, compare entry 1 with 7
and 14, respectively). The cross-coupling with vinyltrialkox-
ysilane 3i was also performed using palladium acetate as
catalyst, giving 4-vinylacetophenone in slightly lower yield than
with complex 1 (Table 1, compare entries 14 and 15).
Next, the Pd loading of complex 1, for the cross-coupling of
the best silane 3a, was decreased to 0.1 mol %, resulting in a
lowered yield from 99 to 71%, which could be restored to 99%
by addition of 0.5 equiv of tetra-n-butylammonium bromide
(TBAB) (Table 1, entries 16 and 17). In the case of using 0.1
mol % of palladium diacetate, a 90% yield was obtained in the
presence of TBAB (Table 1, entry 18). However, a poor yield
was obtained when 0.01 mol % of palladium was employed
(Table 1, entry 19).
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(14) For a preliminary communication, see: Alacid, E.; Na´jera, C. AdV.
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(15) For applications of oxime-derived palladacycles, see: Mizoroki-
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2000, 2, 1823-1826. (b) Iyer, S.; Ramesh, C. Tetrahedron Lett. 2000, 41,
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45, 1833-1836. (g) Botella, L.; Na´jera, C. Tetrahedron 2004, 60, 5563-
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(i) Botella, L.; Na´jera, C. J. Org. Chem. 2005, 70, 4360-4369. (j) Botella,
L.; Na´jera, C. Tetrahedron 2005, 61, 9688-9695. (k) Cao, X.-P.; Barth,
D.; Kuck, D. Eur. J. Org. Chem. 2005, 3482-3488. (l) Alacid, E.; Na´jera,
C. Synlett 2006, 2959-2964. (m) Climent, M. J.; Corma, A.; Iborra, S.;
Mifsud, M. AdV. Synth. Catal. 2007, 349, 1949-1954. (n) Alacid, E.; Na´jera,
C. AdV. Synth. Catal. 2007, 349, 2572-2584. Suzuki-Miyaura reaction:
(o) Botella, L.; Na´jera, C. Angew. Chem., Int. Ed. 2002, 41, 179-181. (p)
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(r) Costa, A.; Na´jera, C.; Sansano, J. M. J. Org. Chem. 2002, 67, 5216-
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2004, 69, 439-446. (t) Ortiz, R.; Yus, M. Tetrahedron 2005, 61, 1699-
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A.; Na´jera, C.; Pacheco, M. C. AdV. Synth. Catal. 2003, 345, 1146-1158.
Alkyne acylation reaction: (w) Alonso, D. A.; Na´jera, C.; Pacheco, M. C.
J. Org. Chem. 2004, 69, 1615-1619.
The scope of the vinylation reaction was studied with aryl
iodides under thermal and microwave conditions using both
(16) For recent works using similar reaction conditions, see: (a) Gordillo,
A.; de Jesu´s, E.; Lo´pez-Mardomingo, C. Org. Lett. 2006, 8, 3517-3520.
(b) Gordillo, A.; de Jesu´s, E.; Lo´pez-Mardomingo, C. Chem. Commun. 2007,
4056-4058.
2316 J. Org. Chem., Vol. 73, No. 6, 2008