Palladium-Catalyzed Synthesis of 1,3-Dienes from Allenes and Organic Halides

A wide range of aryl and vinylic halides react with 1,1-dimethylallene (2a) and potassium carbonate in the presence of Pd(dba)₂ (dba = dibenzylideneacetone) in N,N-dimethylacetamide (DMA) at temperature 100−120 °C to give the corresponding dienes CH₂C(CH₃)CRCH₂ (3a−o), where R is aryl or vinylic, in good to excellent yields. Higher yields of diene products were obtained for aryl bromides than for the corresponding aryl iodides and chlorides. Under similar reaction conditions, tetramethylallene (2b), 1-methyl-1-phenylallene (2c), 1-methyl-3-phenylallene (2d), and 1-cyclohexylallene (2e) also react with aryl and vinylic halides to give diene products (3p−w). For 2d, both E and Z isomers 3t and 3u of the diene product were observed. For 2e, two regioisomers 3v and 3w were isolated with 3w likely from alkene isomerization of 3v. Various palladium systems were tested for the catalytic activity of diene formation. In addition to Pd(dba)₂/PPh₃, Pd(OAc)₂/PPh₃, PdCl₂(PPh₃)₂, and PdCl₂(dppe) are also very effective as catalysts for the reaction of 2a with p-bromoacetophenone (1a) to give 3a. Studies on the effect of solvents and bases show that DMA and K₂CO₃ are the solvent and base that give the highest yield of diene 3a. Possible mechanisms for this catalytic diene formation are proposed.