A palladium-based catalytic system is highly active in the synthesis of γ-keto acids of type ArCOCH2CH2COOH via carbonylation-decarboxylation of the corresponding α-chloride. Typical reaction conditions are: P(CO) = 20-30 atm; substrate/H2O/Pd = 100-400/800-1000/1 (mol); temperature: 100-110 °C; [Pd]=0.25 × 10-2-1 × 1O-2 M; solvent: acetone; reaction time: 1-2 h. A palladium(II) complex can be used as catalyst precursor. Under the reaction conditions above, reduction of the precursor to palladium metal occurs to a variable extent. High catalytic activity is observed when the precursor undergoes extensive decomposition to the metal. Pd/C is also highly active. Slightly higher yields are obtainable when the catalytic system is used in combination with a ligand such as PPh3. A mechanism for the catalytic cycle is proposed: (i) The starting keto chloride undergoes oxidative addition to reduced palladium with formation of a catalytic intermediate having a Pd-[CH(COOH)CH2COPh] moiety. The reduced palladium may be the metal coordinated by other atoms of palladium and/or by carbon monoxide and/or by a PPh3 ligand when catalysis is carried out in the presence of this ligand. It is also proposed that the keto group in the β-position with respect to the carbon atom bonded to chlorine weakens the CCl bond, easing the oxidative addition step and enhancing the activity of the catalyst. (ii) Carbon monoxide 'inserts' into the PdC bond of the above intermediate to give an acyl catalytic intermediate having a Pd-[COCH(COOH)CH2COPh] moiety. (iii) Nucleophilic attack of H2O to the carbon atom of the carbonyl group bonded to the metal of the acyl intermediate yields a malonic acid derivative as product intermediate. This, upon decarboxylation, gives the final product. Alternatively, the desired product may form without the malonic acid derivative intermediate, through the following reaction pathway: the acyl intermediate undergoes decarboxylation with formation of a different acyl intermediate, having a Pd-[CO-CH2CH2COPh] moiety, which, upon nucleophilic attack of H2O on the carbon atom of the carbonyl group bonded to the metal, yields the final product. © 1993.
Synthesis of γ-keto carboxylic acids from γ-keto-α-chloro carboxylic acids via carbonylation-decarboxylation reactions catalysed by a palladium system
TONIOLO, Luigi
1993-01-01
Abstract
A palladium-based catalytic system is highly active in the synthesis of γ-keto acids of type ArCOCH2CH2COOH via carbonylation-decarboxylation of the corresponding α-chloride. Typical reaction conditions are: P(CO) = 20-30 atm; substrate/H2O/Pd = 100-400/800-1000/1 (mol); temperature: 100-110 °C; [Pd]=0.25 × 10-2-1 × 1O-2 M; solvent: acetone; reaction time: 1-2 h. A palladium(II) complex can be used as catalyst precursor. Under the reaction conditions above, reduction of the precursor to palladium metal occurs to a variable extent. High catalytic activity is observed when the precursor undergoes extensive decomposition to the metal. Pd/C is also highly active. Slightly higher yields are obtainable when the catalytic system is used in combination with a ligand such as PPh3. A mechanism for the catalytic cycle is proposed: (i) The starting keto chloride undergoes oxidative addition to reduced palladium with formation of a catalytic intermediate having a Pd-[CH(COOH)CH2COPh] moiety. The reduced palladium may be the metal coordinated by other atoms of palladium and/or by carbon monoxide and/or by a PPh3 ligand when catalysis is carried out in the presence of this ligand. It is also proposed that the keto group in the β-position with respect to the carbon atom bonded to chlorine weakens the CCl bond, easing the oxidative addition step and enhancing the activity of the catalyst. (ii) Carbon monoxide 'inserts' into the PdC bond of the above intermediate to give an acyl catalytic intermediate having a Pd-[COCH(COOH)CH2COPh] moiety. (iii) Nucleophilic attack of H2O to the carbon atom of the carbonyl group bonded to the metal of the acyl intermediate yields a malonic acid derivative as product intermediate. This, upon decarboxylation, gives the final product. Alternatively, the desired product may form without the malonic acid derivative intermediate, through the following reaction pathway: the acyl intermediate undergoes decarboxylation with formation of a different acyl intermediate, having a Pd-[CO-CH2CH2COPh] moiety, which, upon nucleophilic attack of H2O on the carbon atom of the carbonyl group bonded to the metal, yields the final product. © 1993.
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