Reevaluation of the Palladium/Carbon-Catalyzed Decarbonylation of Aliphatic Aldehydes

Abstract

An improved method for the decarbonylation of aliphatic ­aldehydes by using a commercially available Pd/C catalyst is described. The reaction conditions are suitable for linear, cyclic, or sterically demanding substrates, as they afford the corresponding alkanes in yields of up to 99%. In addition, this Pd/C-catalyzed method exhibits good functional-group tolerance. A comparison of previously reported methods with the present one showed that the reaction conditions play a crucial role in the outcome of the reaction. The method can also be ­applied in a two-step reaction sequence for the synthesis of industrially important compounds.

Key words

decarbonylation - aldehydes - palladium catalysis - adamantine - alkylated aromatic compounds - natural products

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• 22 Pd/C-Catalyzed Decarbonylation of Aliphatic Aldehydes: Decarbonylation of 3-phenylpropanal to ethylbenzene (2a); Typical Procedure [CAS Reg. No. 100-41-4] A dry glass reaction tube, purged with argon and equipped with a magnetic stirrer bar, was charged with 3–4 Å MS (100 mg), aldehyde 1a (50 μL, 0.38 mmol), Pd/C (20 mg, 5 mol% Pd), and cyclohexane (1 mL). The tube was sealed and heated at 130 °C for 24 h. The mixture was then cooled to r.t., filtered through a pad of Celite, and washed with CH₂Cl₂ (15–20 mL), to give PhEt (2a) in 67% GC-MS yield (naphthalene as standard); GC-MS: m/z = 106.1 [M]⁺. NMR Spectroscopic Analysis: A dry glass reaction tube, purged with argon and equipped with a magnetic stirrer bar, was charged with 3–4 Å MS (100 mg), aldehyde 1a (40 μL, 0.30 mmol), Pd/C (16 mg, 5 mol% Pd), and C₆D₆ (1 mL). The tube was sealed and heated at 130 °C for 24 h. The mixture was then cooled to r.t. and filtered through a short pad of silica gel to remove the catalyst. The silica gel was then washed with C₆D₆ (2 × 0.5 mL) to give PhEt (2a) in 78% NMR yield (BzOMe as standard). ¹H NMR (500 MHz, C₆D₆): δ = 7.10–7.00 (m, 5 H), 2.43 (q, J = 7.5 Hz, 2 H), 1.06 (t, J = 7.5 Hz, 3 H). ¹³C NMR (125 MHz, C₆D₆): δ = 144.3, 128.6, 128.1, 125.9, 29.2, 15.8.