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

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.

Key words

tetramethylguanidine - Heck reaction - palladium - aryl halides - ligand

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12

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) ų, R1 = 0.0309, wR = 0.0894.

13

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.