Hydroformylation of styrene and 1-octene catalyzed by binuclear and oligomer rhodium(I) complexes containing the bis-p-phosphinito ligands [(p-Ph2POC6H4)(2)X] (X = O, CMe2, S)

The reactions of rhodium(I) substrates with the ligands [(p-Ph2POC6H4)(2)X] (X = O, 1; X = CMe2, 2; X = S, 3) have been studied with the aim to obtain rhodium(I) complexes to utilize as precatalysts in hydroformylation of ole fins. The reactions of [Rh(CO)(2)Cl](2) with ligands 1-3, in a Rh to ligand 1:1 molar ratio, in toluene, at room temperature afford the products [Rh(C O)(Cl)(mu-L)](2) (L = 1-3). The reactions of [Rh(acac)(CO)(2)] with 2 are s trongly dependent on the ligand nature and experimental conditions. The pro duct formed in a 1:0.5 Rh to ligand molar ratio is dimer with the metal cen ters held together by the ligand, while the product formed in a 1:1 molar r atio is oligomer. The in situ catalytic systems formed either by [Rh(CO)(2) Cl](2) and ligands 1-3 or by [Rh(acac)(CO)(2)] and 2, at variable ligand-to -metal molar ratio, have been employed in the hydroformylation of styrene a nd 1-octene. Almost quantitative conversion of styrene was achieved with th e catalytic system formed by [Rh(acac)(CO)(2)] and 2, in a 1:0.5 molar rati o, operating at 60 degrees C and 40 atm. The chemoselectivity of the reacti on was very high being the linear (L) and the branched (B) aldehydes about 99.9% of the reaction products. The terminal aliphatic olefin 1-octene was hydroformylated with lower conversion in the aldehydes with respect to styr ene and isomerization and hydrogenation of the double bond occurred in grea t extent. Lowering the temperature, the conversion of 1-octene increases bu t the chemoselectivity drastically decreases owing to the formation of more isomerizarion products, The results have been explained considering both t he flexibility and the number of phosphorus atoms (considering each of them as a monodentate ligand) coordinated to each metal center in the trigonal bipyramidal hydridorhodium intermediates formed, namely {[Rh(H)(CO)(3)](2)( mu-2)} (1:0.5 [Rh(acac)(CO)(2)] to 2 molar ratio) and the oligomer [Rh(H)(C O)(2)(mu-2)](n) (1:1 [Rh(acac)(CO)(2)] to 2 molar ratio). In the presence o f an excess of ligand 2 the conversion in the aldehydes drastically decreas es owing to the rigidity of the intermediate hydridorhodium oligomer specie s formed. (C) 1999 Published by Elsevier Science B.V. All rights reserved.