6 E. Alacid and C. Na´jera, Adv. Synth. Catal., 2006, 348, 945–962.
7 A. Gordillo, E. de Jesu´s and C. Lo´pez-Mardomingo, Org. Lett., 2006, 8,
3517–3520.
8 M. T. Reetz and J. G. de Vries, Chem. Commun., 2004, 1559–1563;
J. G. de Vries, Dalton Trans., 2006, 421–429.
9 Selected recent reports on the Hiyama vinylation of aryl halides:
M. E. Mowery and P. DeShong, J. Org. Chem., 1999, 64, 1684–1688;
T. Jeffery, Tetrahedron Lett., 1999, 40, 1673–1676; H. M. Lee and
S. P. Nolan, Org. Lett., 2000, 2, 2053–2055; S. E. Denmark and
J. M. Kallemeyn, J. Am. Chem. Soc., 2006, 128, 15958–15959;
S. E. Denmark and C. R. Butler, Org. Lett., 2006, 8, 63–66.
10 T. Mizoroki, K. Mori and A. Ozaki, Bull. Chem. Soc. Jpn., 1971, 44,
581; R. F. Heck and J. P. Nolley, Jr., J. Org. Chem., 1972, 37,
2320–2322; I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000,
100, 3009–3066; S. Bra¨se and A. de Meijere, in Metal-Catalyzed Cross-
Coupling Reactions, ed. A. de Meijere and F. Diederich, Wiley-VCH,
Weinheim, 2nd edn, 2004, ch. 5.
enhancement in the environmental or economical efficiency of a
process because the impact of the overall process, including
workup, must be considered.20 The conversions and selectivities of
the reactions described here in water compete favourably with
those reported in organic solvents, and the use of water has specific
advantages over conventional organic solvents: (a) the activation
of aryl bromides under ligand-free conditions relies on the use of
high temperatures for which a nonflammable solvent such as water
is superior on safety grounds; (b) many of the alkenes reported,
especially 1,2-diarylethenes, are solids that precipitate from the
aqueous reaction medium and can therefore be separated by
simple filtration to afford crude products that, in many cases, are
1
pure by H NMR spectroscopy.21
In conclusion, we have provided an effective method for the
sequential performance of Hiyama and Heck reactions in aqueous
media that allows the preparation of a variety of symmetrically
and asymmetrically functionalized (E)-1,2-diarylethenes in one pot
from aryl bromides and triethoxy(vinyl)silane. The only additives
required are sodium hydroxide, palladium acetate (0.1 to 0.5 mol%)
and, in some cases, poly(ethylene glycol). The reactions are carried
out in air and the products can be isolated, in many cases, by
simple filtration of the aqueous solution.
11 Recent reports on Heck reactions between aryl halides and vinylarenes
in water: C. Na´jera, J. Gil-Molto´, S. Karlstro¨m and L. R. Falvello,
Org. Lett., 2003, 5, 1451–1454; L. Botella and C. Na´jera, Tetrahedron
Lett., 2004, 45, 1833–1836; R. B. DeVasher, L. R. Moore and
K. H. Shaughnessy, J. Org. Chem., 2004, 69, 7919–7927; H. Hagiwara,
Y. Sugawara, T. Hoshi and T. Suzuki, Chem. Commun., 2005,
2942–2944; S. Ogo, Y. Takebe, K. Uehara, T. Yamazaki, H. Nakai,
Y. Watanabe and S. Fukuzumi, Organometallics, 2006, 25, 331–338;
N. S. C. R. Kumar, I. V. P. Raj and A. Sudalai, J. Mol. Catal. A:
Chem., 2007, 269, 218–224.
12 Classical non-catalytic methods such as the Wittig or Horner–
Wadsworth–Emmons reaction usually afford poor control over the
(E) or (Z) configuration of the CLC double bonds. Catalytic methods
for the synthesis of stilbenes have been recently reviewed:
K. Ferre´-Filmon, L. Delaude, A. Demonceau and A. F. Noels,
Coord. Chem. Rev., 2004, 248, 2323–2336.
This work was supported by the Ministerio de Educacio´n y
Ciencia of Spain (project CTQ2005-00795/BQU) and the
Comunidad de Madrid (project S-0505/PPQ/0328-03). A. G. is
grateful to the MEC for a FPU Fellowship.
13 Asymmetric 1,2-diarylethenes are also prepared by titanium-catalyzed
McMurry coupling of aldehydes and ketones or by ruthenium-catalyzed
cross-metathesis of vinylarenes, but the first procedure suffers from poor
tolerance to functional groups and the second from low selectivities. See,
for example: K. Ferre´-Filmon, L. Delaude, A. Demonceau and
A. F. Noels, Eur. J. Org. Chem., 2005, 3319–3325.
14 Itami et al. used a Heck–Hiyama sequence of reactions in THF to
couple aryl iodides and 2-pyridyldimethyl(vinyl)silane using 10 mol% of
Pd(OAc)2 and tri-2-furylphosphine: K. Itami, T. Nokami, Y. Ishimura,
K. Mitsudo, T. Kamei and J.-I. Yoshida, J. Am. Chem. Soc., 2001, 123,
11577–11585.
15 Jeffery described the synthesis of (E)-3,49,5-trimethoxystilbene in 72%
yield by two sequential Heck reactions using aryl iodides, trimethyl-
(vinyl)silane and 4 mol% of Pd(dba)2: T. Jeffery and B. Ferber,
Tetrahedron Lett., 2003, 44, 193–197.
16 We started from the conditions established in our previous study on the
Hiyama arylation (ref. 7). Palladium acetate, palladium dichloride and
complexes of the latter with the soluble phosphines 1,3,5-triaza-7-
phosphaadamantane (PTA) and PPh2Ar* (Ar* = 3,4-[18-crown-6]-
phenyl) produced comparable results.
17 In contrast with our observations that short reaction times are key in the
efficient synthesis of vinyl derivatives, high yields of 4-vinylacetophenone
after 20 h of reaction at 120 uC have been reported during the course of
our work by Alacid and Na´jera using tetrabutylammonium bromide
(TBAB) as a nanoparticle stabilizer: E. Alacid and C. Na´jera, Adv.
Synth. Catal., 2006, 348, 2085–2091.
18 Poly(ethylene glycol) (PEG), pure or as a co-solvent, is an efficient
medium for palladium-catalyzed coupling reactions. S. Chandrasekhar,
Ch. Narsihmulu, S. S. Sultana and N. R. Reddy, Org. Lett., 2002, 4,
4399–4401; C. Luo, Y. Zhang and Y. Wang, J. Mol. Catal. A: Chem.,
2005, 229, 7–12.
19 PEG was a more efficient additive than TBAB in Suzuki reactions
catalyzed by palladium(II) acetate in water and ligand-free conditions.
L. Liu, Y. Zhang and Y. Wang, J. Org. Chem., 2005, 70, 6122–6125.
20 D. G. Blackmond, A. Armstrong, V. Coombe and A. Wells, Angew.
Chem., Int. Ed., 2007, 46, 3798–3800.
Notes and references
{ The synthesis of (E)-4-[2-(3-pyridyl)vinyl]acetophenone is given as
representative: in a 35 mL pressure tube, triethoxy(vinyl)silane (0.77 mL,
3.54 mmol) and a 0.5 M NaOH solution in water (10.70 mL, 5.35 mmol)
containing 20% (w/w) of PEG 2000 (2.14 g) were vigorously stirred for
5 min at room temp. 3-Bromopyridine (0.178 mL, 1.78 mmol) and
palladium acetate (0.4 mg, 1.8 mmol) were subsequently added and the
vigorous stirring was continued for 3 h at 140 uC. 4-Bromoacetophenone
(0.3615 g, 1.78 mmol) and palladium(II) acetate (0.4 mg, 1.8 mmol)
dissolved in 1 mL of a 20% (w/w) aqueous solution of PEG 2000 were
added at room temperature and the reaction warmed for 6 h at 140 uC. The
product was extracted with ethyl acetate (3 6 20 mL) and purified by flash
chromatography on silica gel with hexane–ethyl acetate 1 : 5 (Rf = 0.24).
Yield: 0.3467 g (87%).
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21 See, for example, the experimental procedures described in the ESI{ for
(E)-1,2-bis-[4-carboxyphenyl]ethene and (E)-1,2-bis-(3-quinolyl)ethene.
Carboxy derivatives, which are water-soluble in their basic form,
precipitate upon neutralization of the basic solution.
5 T. Huang and C. J. Li, Tetrahedron Lett., 2002, 43, 403–405; T. Koike
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4058 | Chem. Commun., 2007, 4056–4058
This journal is ß The Royal Society of Chemistry 2007