Aims: The aims of this study were the synthesis of palladium(II) complexes incorporating 1-substituted-3-methyl-3-phospholene ligands, and the evaluation of these Pd-complexes as catalysts in hydroalkoxycarbonylation of styrene.
Background: In the literature, a few palladium-complexes incorporating 4-6-membered heterocyclic ligands and their catalytic activity in hydroalkoxycarbonylation are described. The Pd-complexes of five-membered P-ligands (phospholenes or phospholanes) are the least studied group. Previously, the 3-phospholene-platinum complexes were good catalysts in the hydroformylation of styrene, which prompted us to prepare palladium(II) complexes incorporating 3-phospholenes as P-ligands, and to evaluate their catalytic activity.
Objective: The objectives of this study were to prepare palladium(II) complexes incorporating 3-phospholene ligands, and to investigate the catalytic activity of these species in hydroalkoxycarbonylation of styrene.
Method: Racemic 3-methyl-3-phospholene oxides were deoxygenated by Cl3SiH, and the phospholenes were reacted with bis(benzonitrile)palladium dichloride to give the corresponding Pd-complexes. Two phospholene-palladium complexes were chosen for testing as catalysts in hydroalkoxycarbonylation. The carbonylation reaction of styrene was performed in the presence of O-nucleophiles, such as methanol, ethanol or phenol in order to synthesize the esters of 3-phenyl- and 2-phenylpropionic acid.
Result: Eight derivatives of palladium(II) complexes incorporating 3-phospholene ligands were prepared in racemic form in a yield of 69-90%. The optically active propyl-phospholene-Pd-complex was also made available. The PdCl2(phospholene)2-type catalysts showed interesting catalytic results in terms of regioselectivity towards the esters of linear or branced phenylpropionic acid, but the activity remained low.
Conclusion: A series of palladium(II) complexes incorporating racemic 1-substituted-3-methyl-3-phospholene ligands were prepared in good or excellent yields. An optically active Pd-complex containing was also prepared. Catalytic activity of the 3-phospholene-palladium(II)-complexes was evaluated in hydroalkoxycarbonylation of styrene.