Microwave-enhanced cross-coupling of allyl chlorides with vinyltrifluoroborates

Article abstract of DOI:10.1016/j.tetlet.2006.08.043

The allylation of potassium alkenyltrifluoroborates with allyl chloride via a palladium catalyzed cross-coupling reaction occurs rapidly under microwave irradiation. The allylation reaction produces 1,4-pentadienes in high yields.

Full text of DOI:10.1016/j.tetlet.2006.08.043

Tetrahedron Letters 47 (2006) 7459–7461
Microwave-enhanced cross-coupling of allyl chlorides
with vinyltrifluoroborates
George W. Kabalka,a,* Eric Dadusha and Mohammad Al-Masumb
aDepartments of Chemistry and Radiology, The University of Tennessee, Knoxville, TN 37996-1600, USA
bDepartment of Chemistry, Tennessee State University, Nashville, TN 37209, USA
Received 6 July 2006; revised 31 July 2006; accepted 11 August 2006
Abstract—The allylation of potassium alkenyltrifluoroborates with allyl chloride via a palladium catalyzed cross-coupling reaction
occurs rapidly under microwave irradiation. The allylation reaction produces 1,4-pentadienes in high yields.
Ó 2006 Elsevier Ltd. All rights reserved.
Palladium catalyzed cross-coupling reactions between
organoboron compounds and organic halides are pow-
erful tools for carbon–carbon bond formation.1 A wide
variety of boron reagents, including trifluoroborates,
can be utilized in these Suzuki–Miyaura reactions.2 Tri-
fluoroborates can easily be prepared and are remarkably
stable, yet they are particularly useful since they are
quite reactive.3
as potential catalysts. Solvents such as tetrahydrofuran
and tert-amyl alcohol were also evaluated. Cesium car-
bonate, potassium carbonate and triethylamine were
evaluated as potential bases. The organotrifluoroborates
were readily prepared from the corresponding boronic
acids.3a
A series of allyl chlorides were then allowed to react with
various substituted potassium vinyltrifluoroborates
(Table 2). A catalyst loading of 2 mol % PdCl2(dppf)-
We have been investigating the use of microwave irradi-
ation as an alternative to thermal heating in reactions
including boron derivatives.4 Organic syntheses involv-
ing microwave systems, in contrast to classic thermal
conditions, provide the opportunity to complete com-
plex reactions in a course of minutes, as opposed to
hours when thermal conditions are used.5
CH Cl , along with 3.0 equiv of Hunig’s base (i-Pr NEt),
¨
2
2
2
in isopropanol/water (2:1), at 100 °C under microwave
irradiation was found to provide the coupled products
in good to excellent yields.
The reactions are rapid and straightforward. In a typical
experiment, the organotrifluoroborates (0.50 mmol) and
palladium catalyst (0.01 mmol) are placed in an argon
We recently reported the use of microwaves for cross-
coupling reactions involving potassium aryltrifluoro-
borates and vinyltrifluoroborates.4a,b We now wish to
describe the results of a study focused on the coupling
of allyl halides with potassium vinyltrifluoroborates.
As shown in Table 1, allyl chloride produces the highest
yields of the desired products. This study was initiated
by allowing potassium trans-2-[4-(trifluoromethyl)-
phenyl]vinyltrifluoroborate to react with allyl halides
in the presence of palladium catalyst. No carbon–carbon
bond formation occurred in the absence of palladium.
PdCl2(dppf)ÆCH2Cl2, Pd2dba3ÆCHCl3/dppf, Pd2dba3/
(o-tolyl)3, Pd(OAc)2/dppf and Pd(OAc)2 were examined
Table 1. Microwave-enhanced cross-coupling reactions of potassium
phenylvinyltrifluoroborate with allyl halides
2 mol % PdCl2(dppf)CH2Cl2
BF3K
X
Ph
+
Ph
i-PrOH-H2O (2:1), i-Pr2NEt
°
MW, 100 C, 20 min.
Entry
X
Product
Yield%a
1
I
55
65
81
Ph
Ph
Ph
2
3
Br
Cl
*
Corresponding author. Tel./fax: +1 865 974 3260; e-mail: kabalka@
utk.edu
a All yields are of pure products isolated by silica gel chromatography.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.08.043

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G. W. Kabalka et al. / Tetrahedron Letters 47 (2006) 7459–7461
Table 2. Microwave-enhanced cross-coupling reactionsa
Entry
1
Alkenyl trifluoroborate
BF K
Allylating agent
Product
Yield%b
71
3
Cl
Cl
Cl
BF K
3
Cl
Cl
2
3
4
5
79
89
46
54
CH3
CH
3
BF K
3
CF
CF
3
3
BF3K
BF3K
Cl
Cl
BF3K
BF3K
6
7
8
9
64
51
81
75
Cl
Cl
BF K
3
Cl
Cl
CH3
CF3
CH
3
BF K
3
CF
3
a All reactions were run in i-PrOH–H2O (2:1) at 100 °C for 20 min.
b Isolated yields.
VCH: Weinheim, Germany, 1998, Chapter 2; (d) Suzuki, A.
J. Organomet. Chem. 1999, 576, 147; (e) Littke, A. F.; Fu,
G. C. Angew. Chem., Int. Ed. 2002, 41, 4176; (f) Barder, T.
E.; Walker, S. D.; Martinelli, J. R.; Burchwald, S. L. J. Am.
Chem. Soc. 2005, 127, 4685.
flushed pyrex test tube. The aryl halide (0.50 mmol) is
then added along with diisopropyl ethyl amine
(1.5 mmol) and 5 mL of isopropanol/water (2:1). The
tube is capped with a plastic septum, placed in a CEM
microwave unit and allowed to react at 100 °C for
20 min. The product is isolated by adding water
(ꢀ25 mL) and ether (ꢀ35 mL), the ether layer separated,
and the organic phase dried over anhydrous sodium sul-
fate. The ether solution is filtered, and the product iso-
lated by column chromatography using hexane/ethyl
acetate (100/1). In some reactions, the conjugated diene
by-product was observed (5%) which is readily removed
by chromatography.
2. (a) Liu, W.-J.; Xie, Y.-X.; Liang, Y.; Li, J.-H. Synthesis
2006, 860; (b) Kudo, N.; Perseghini, M.; Fu, G. C. Angew.
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Acknowledgements
We wish to thank the US Department of Energy and the
Robert H. Cole Foundation for support of this research.
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