Steric Effects on the Synthesis, Structure, Reactivity and Selectivity of t-Phosphine Rhodium Complex Hydroformylation Catalysts

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Abstract
Bulky trivalent phosphorus ligands of transition metal complexes were often observed to affect catalyst activity and selectivity. In the area of the low pressure hydroformylation of α-olefins in the presence of rhodium complexes, Pruett and Smith observed early (1) that the use of bulky ortho-substituted phosphite ester ligands leads to a decrease of the ratio of straight chain versus branched aldehyde products (n/i ratio). More recently van Leeuwen and Roobek have shown (2) that rhodium complexes of such bulky ligands are much more active catalysts than the much studied triphenylphosphine-rhodium catalyst system, in the hydroformylation of linear internal and branched terminal olefins. They suggested that for steric reasons only two bulky ligands could be coordinated to the same rhodium in such complexes and that such ligands could increase the coordinative unsaturation of rhodium, thus leading to a higher reactivity. Indeed a number of coordinatively unsaturated rhodium carbonyl complexes of bulky trialkylphosphine ligands were described by Otsuka and coworkers (3) and by Freeman and Young (4).