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Synlett 2006(18): 3130-3134  
DOI: 10.1055/s-2006-951493
LETTER
© Georg Thieme Verlag Stuttgart · New York

Efficient Heterogeneous Palladium-Montmorillonite Catalysts for Heck Coupling of Aryl Bromides and Chlorides

Árpád Molnár*, Attila Papp
Department of Organic Chemistry, University of Szeged, Dóm tér 8, Szeged, 6720, Hungary
Fax: +36(62)544200; e-Mail: amolnar@chem.u-szeged.hu;
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Publication History

Received 17 April 2006
Publication Date:
25 October 2006 (online)

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Efficient Heterogeneous Palladium-Montmorillonite Catalysts for Heck Coupling of Aryl Bromides and ChloridesSynlett 2006; 2006(19): 3368-3368
DOI: 10.1055/s-2005-956470

Abstract

New palladium catalysts were prepared using ion exchange or intercalation of Pd species into montmorillonite. The catalysts promote Heck reaction of various aromatic halides including aryl chlorides to give coupling products in high yields at low catalyst ratios down to 0.001 mol%.

Key words

C-C coupling - Heck reaction - palladium - montmorillonite - heterogeneous catalysts

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Preparation of Catalysts. Before catalyst synthesis Na-montmorillonite (Bentolite H, Laporte) was suspended in distilled water overnight with stirring at r.t.
Pd-montm1 (2.16% Pd loading): Pd(NO3)2·3H2O (0.4 g) dissolved in water and acidified with concd HNO3 (0.4 mL) was added to Na-montmorillonite (8 g). After stirring for 4 h the solid was separated by centrifugation and washed thoroughly with water.
Pd-montm2 (1.29% loading): Pd(NO3)2·3H2O (0.33 g) dissolved in water was added to Na-montmorillonite (5 g). After stirring for 4 h the solid was separated by centrifugation and washed thoroughly with water. Finally, the wet products were oven-dried (95 °C, 1 Torr, 6 h). The palladium content was determined by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES). Catalysts used in recycling experiments were reduced in flowing hydrogen (200 °C, 2 h).

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General Procedure for Heck Coupling: Reactions were carried out in 5 mL or 25 mL glass vials equipped with a Teflon screw cap with magnetic stirring using aryl halide (0.5-10 mmol) in NMP solvent. No special precaution was taken to exclude air or moisture. Reaction conditions (reactant and catalyst ratios, reaction time and temperature) are given in tables. Toluene or biphenyl was used as internal standard to calculate conversions and yields. All products are known compounds and were identified by GC-MS (HP 5890 GC +HP 5970 mass selective detector) and 1H NMR (Bruker Avance DRX 500).

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Catalyst Recycling: Bromobenzene (1 mmol), styrene (1.5 mmol), Na2CO3 [1.2 mmol or Et3N (1 mmol)-Na2CO3 (0.5 mmol)], and Pd catalyst (0.3 mol%) were reacted in NMP (2 mL). Catalyst samples after reaction were separated by centrifugation, washed successively with acetone, water, acetone and, finally, dried under an infra lamp and reused.