Pd/C as a Highly Active Catalyst for Heck, Suzuki and Sonogashira Reactions

Abstract

A specially optimized air-stable Pd on activated carbon catalyst is demonstrated to be a highly active (TON up to 36,000), selective and convenient heterogeneous catalyst for CC couplings of aryl halides in Heck, Suzuki, and Sonogashira reactions. The Pd/C catalyst developed allows extremely low Pd concentrations (down to 0.0025 mol% for Heck coupling, 0.005 mol% for Suzuki coupling) and high conversions of aryl bromides within a few hours. Easy and complete Pd separation and recovery is possible.

Key words

aryl halides - catalysis - coupling - heterogeneous - palladium

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10

Due to dissolution/re-precipitation processes (Pd leaching) during the reaction no exact number of catalytically active species or sites can be given. For this reason ‘turnover numbers’ (TON) and ‘turnover frequencies’ (TOF) per Pd atom are given as a measure of the catalytic activity that refer to the total amount of Pd applied (and not to active species or sites): TON = conversions of bromo-/chloroarene per total amount of palladium in the catalyst (TOF: per hour).

11

General Procedure for Heck Reaction : Reactions were performed in sealed pressure tubes after 5 min purging with argon using non-dried solvents (Only a small decrease in activity but a large increase in Pd leaching was detected without argon atmosphere). Yields and product identification were determined by GLC and GC-MS, separation of catalyst by filtration. Typical reaction conditions: 10 mmol bromobenzene, 15 mmol styrene, 12 mmol NaOAc, 0.0025-0.05 mol% Pd (E 105 CA/W 5% Pd, product of Degussa AG), 10 mL NMP (N-methyl-2-pyrrolidone) or DMAc (N,N-dimethylacetamide) for thermally treated(reduced) catalysts; T = 140 °C. Conversion of the bromoarenes and yields of the products 3-5 using diethylene glycol-n-butyl ether as internal standard.

12

General Procedure for Suzuki Coupling : Reactions were performed in sealed pressure tubes after 5 min purging with argon using non-dried solvents. Yields and product identification were determined by GLC and GC-MS, separation of catalyst by filtration. Typical reaction conditions: 5 mmol bromobenzene, 6 mmol phenylboronic acid, 6 mmol base, 0.005-0.25 mol% Pd (E 105 CA/W 5% Pd, product of Degussa AG), T = 120 °C. Conversion of arylhalides and yield of the product 7 using diethylene glycol-n-butyl ether as internal standard.

14

General Procedure for Sonogashira Coupling : Reactions were performed in sealed pressure tubes after 5 min purging with argon using non-dried solvents. Yields and product identification were determined by GLC and GC-MS, separation of catalyst by filtration. Typical reaction conditions: 5 mmol iodobenzene, 6 mmol phenylacetylene, 6 mmol base, 0.125-0.50 mol% Pd (E 105 CA/W 5% Pd, product of Degussa AG), 10 mL solvent; T = 100 °C. Conversion of iodobenzene and yield of the product 10 using diethylene glycol-n-butyl ether as internal standard.