Synlett 2005(12): 1885-1888  
DOI: 10.1055/s-2005-871581
LETTER
© Georg Thieme Verlag Stuttgart · New York

Tetramethylguanidine as an Inexpensive and Efficient Ligand for the Palladium-Catalyzed Heck Reaction

Shenghai Lib, Haibo Xiea, Suobo Zhang*a, Yingjie Linb, Jianing Xub, Jungang Caob
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Changchun 130022, P. R. China
b College of Chemistry, Jilin University, Changchun 130023, P. R. China
e-Mail: sbzhang@ciac.jl.cn;
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Publikationsverlauf

Received 6 May 2005
Publikationsdatum:
07. Juli 2005 (online)

Abstract

An inexpensive and efficient Pd(OAc)2/tetra­methylguanidine (TMG) or PdCl2/TMG catalytic system has been developed for the Heck reaction of an olefin with an aryl halide. The TONs were up to 1000000 when iodobenzene was used as the ­substrate with butyl acrylate as the reactant. In addition, [Pd(TMG)4]Cl2(H2O)8, an air-stable compound, effectively pro­moted the Heck reaction, which demonstrated that TMG acted as a ligand in this reaction system.

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Single crystals of complex [Pd(TMG)4]Cl2(H2O)8 suitable for X-ray diffraction were obtained by slow diffusion of hexane into a dichloromethane solution of PdCl2/TMG. The compound crystallizes in the triclinic space group P-1 with a = 8.9997 (11) Å, b = 10.3817 (13) Å, c = 11.6080 (14) Å, α = 78.040 (2)°, β = 68.603 (2)°, γ = 82.922 (2)°, V = 986.5 (2) Å3, R1 = 0.0309, wR = 0.0894.

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Typical Procedure for the Heck Reaction: An oven-dried Schlenk flask was charged under N2 with styrene (6.0 mmol), bromobenzene (5.0 mmol), and NaOAc (6.0 mmol). A solution of catalyst Pd(OAc)2/TMG (1:4) or PdCl2/TMG (1:4) in DMA (5.0 mL, 1.0 × 10-6 M) was then added via syringe. The flask was sealed and placed in a 140 °C oil bath and stirred for 20 h. The course of the reaction was monitored by periodically taking samples and analyzing them by gas chromatography [di(ethylene glycol)-Bu2O as internal standard]. After cooling to r.t., the reaction mixture was poured into H2O (50 mL) and extracted with EtOAc (3 × 40 mL). The combined organic extracts were washed with brine, dried (Na2SO4), and concentrated. Purification by flash chromatography on silica gel (hexanes-CH2Cl2,
1:1) gave trans-stilbene (859 mg, 95.5%), estimated to be >99% pure by 1H NMR and GC.