Rhodium-DlOP catalyst

An interesting case is the reduction of prochiral benzophenones with one parasubstituted phenyl ring. It is surprising that optical yields as great as 26% could be attained, allowing the preparation of chiral benz-hydrols (23,24). Any of the proposed schemes of asymmetric induction with the rhodium-DlOP catalyst (27, 28) can hardly explain such results. Obviously very subtle interactions between the ketone and the complex are involved. Maybe charge transfer interactions between aromatic... 

N-Acetyl-(R)-phanylalanlna (6). The rhodium catalyst was obtained by adding (-) dlop 5 (from diethyl tartrate) to a benzene solution of [RhCi(cyclooctene)2]2 under Ar, and stirring for tS mn A solution of the Rh catalyst (1 mM in EtOH PhH 4 1) was introduced under Hj to a solution of a-N acetylamino- phenytacrylic acid 4 (molar ratio Rh 4 1.540) The solvent was evaporated, the residue dissolved In 0 5 N NaOH, the catalyst was filtered and the solution acidified and concentrated to dryness to give 6 (81% ee) in 90 95% yield... 

The sol-gel entrapment of the metal complexes [Ru(p-cymene)(BINAP)Cl]Cl and the rhodium complexes formed in situ from the reaction of [Rh(COD)Cl]2 with DlOP and BPPM has been reported by Avnir and coworkers [198]. The metal complexes were entrapped by two different methods the first involved addition of tetramethoxysilane to a THF solution of the metal complex and triethylamine, while the second method was a two-step process in which aqueous NH4OH was added to a solution of HCl, tetramethoxysilane and methanol at pH 1.96 followed by a THF solution of the appropriate metal complex. The gel obtained by each method was then dried, crushed, washed with boiling CH2CI2, sonicated in the same solvent and dried in vacuo at room temperature until constant weight was achieved. Hydrogenation of itaconic acid by these entrapped catalysts afforded near-quantitative yields of methylsuccinic acid with up to 78% e.e. In addition, the catalysts were found to be leach-proof in ethanol and other polar solvents, and could be recycled. 

The DlOP-rhodium(I) complex attached to organic polymers , e.g., polystyrene resin and poly(methyl vinyl alcohol), exhibits good catalytic activity as a chiral catalyst comparable to the corresponding homogeneous catalyst. In contrast, the rhodium(I) complexes anchored on inorganic supports display only a low efficiency . Studies show that the steric requirements for a match of the chiral ligand, a hydrosilane and a ketone are of definite importance in bringing about effective asymmetric induction. 

Of interest is iso-selective hydroformylation of 1,3-butadienes, because a chiral product is produced. Thus, with a rhodium catalyst based on DlOP as a ligand, 41% yield of 2-methyl-butanal was formed, unfortunately with negligible optical... 

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