Mizoroki–Heck Reactions Catalysed by (N-Heterocyclic carbene)PdCl₂(Et₃N) Complexes

Abstract

Complexes of general formula (NHC)PdCl₂(Et₃N) were used as pre-catalysts in Mirozoki–Heck couplings of activated aryl chlorides and deactivated aryl bromides as electrophiles, without the need of an inert atmosphere and in relatively short reaction times. The protocol was extended to the use of microwave heating, reducing further the reaction times with no detriment in the yields of the desired products.

Key words

carbene - palladium - homogeneous catalysis - alkenes - halides

• References and Notes

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• 12 General Procedure A (Conventional Heating): A 4-mL screw-capped vial equipped with a magnetic stirring bar was charged with potassium carbonate (1 mmol), TBAB (0.5 mmol), aryl halide (0.5 mmol), catalyst, anhyd DMF (1 mL) and the alkene (0.55 mmol). The vial was sealed with a screw-cap fitted with a septum and the reaction mixture was allowed to stir at 140 °C using conventional heating and monitored by GC. When the reaction was complete or there was no further increase in conversion, it was cooled to r.t., poured into H₂O (10 mL) and extracted with Et₂O or EtOAc (3 × 10 mL). The combined organic layers were washed with H₂O (3 × 10 mL) and brine (10 mL), dried over MgSO₄ and filtered. The resultant solution was concentrated onto Celite under reduced pressure for purification by column chromatography. All reported yields are an average of two runs. General Procedure B (Microwave Heating): A 35-mL microwave vial equipped with a magnetic stirring bar was charged with potassium carbonate (1 mmol), TBAB (0.5 mmol), aryl halide (0.5 mmol), catalyst, anhyd DMF (2 mL) and the alkene (0.55 mmol). The vial was sealed and the reaction mixture was heated at the required temperature using a microwave reactor (200 W, PowerMax off). When the reaction was complete the reaction was cooled to r.t., poured into H₂O (10 mL) and extracted with Et₂O or EtOAc (3 × 10 mL). The combined organic layers were washed with H₂O (3 × 10 mL) and brine (10 mL), dried over MgSO₄ and filtered. The resultant solution was concentrated onto Celite under reduced pressure for purification by column chromatography. All reported yields are an average of two runs.
• 13 1-(4-Styrylphenyl)ethanone (5): ¹H NMR (500 MHz, CDCl₃): δ = 7.96 (d, J = 8.3 Hz, 2 H), 7.60 (d, J = 8.2 Hz, 2 H), 7.55 (d, J = 7.6 Hz, 2 H), 7.40 (t, J = 7.6 Hz, 2 H), 7.32 (t, J = 7.3 Hz, 1 H), 7.24 (d, J = 16.3 Hz, 1 H), 7.14 (d, J = 16.3 Hz, 1 H), 2.62 (s, 3 H). ¹³C{¹H} NMR (126 MHz, CDCl₃): δ = 197.4, 142.0, 136.7, 136.0, 131.5, 128.9, 128.8, 128.3, 127.5, 127.4, 126.8, 126.5, 26.5.