Rhodium biphosphine complexes

Coordination of Unsaturated Carboxylic Acids in Rhodium Biphosphine Complexes... 

Typical square-planar rhodium-olefin complexes such as acetylacetonates (48) have a stoichiometry of two coordinated olefins per metal-atom. Since chelating olefins are bidentate in their cationic rhodium biphosphine complexes, it would be surprising if bis-olefin complexes were never found under hydrogenation conditions. It seems clear, in fact, that they can be the major coordinated species under certain conditions. Thus examples of 2 1 rhodium enamide complexes with biz-diphenyl-phosphinopropane have been observed (49), although the majority of cases involve a8-unsaturated acids co-complexed with DIOP. 

Figure 4. Rhodium biphosphine complexes of unsaturated carboxylic acids and carboxylates phosphorus-31 NMR spectra, MeOH, conditions as shown.

It is now possible to state the minimum requirements for effective asymmetric hydrogenation, using existing rhodium biphosphine complexes. These are as follows ... 

At this point mechanistic studies have reached an impasse. All of the observable intermediates have been characterized in solution, and enamide complexes derived from diphos and chiraphos have been defined by X-ray structure analysis. Based on limited NMR and X-ray evidence it appears that the preferred configuration of an enamide complex has the olefin face bonded to rhodium that is opposite to the one to which hydrogen is transferred. There are now four crystal structures of chiral biphosphine rhodium diolefin complexes, and consideration of these leads to a prediction of the direction of hydrogenation. The crux of the argument is that nonbonded interactions between pairs of prochiral phenyl rings and the substrate determine the optical yield and that X-ray structures reveal a systematic relationship between P-phenyl orientation and product configuration. 

A fortunate feature of asymmetric catalysis is that chiral biphosphine rhodium diene complexes, especially with norbornadiene, crystallize well... 

The apparent formation of arene complexes on hydrogenation in benzene (e.g., in Reference 61) seems to be an anomaly, since their solution nmr spectra are very different from authentic rhodium biphosphine tetraphenyl-... 

Morimoto T, Chiba M, Achiwa K. Efficient synthesis of natural (-f)-collinusin using catalytic asymmetric hydrogenation with a chiral biphosphine-rhodium(I) complex. Chem. Pharm. Bull. 1989 37(12) 3161-3163. 

In the absence of reactant, (cationic) biphosphine rhodium complexes exist in methanol, the generally preferred reaction medium, as a Hs-solvate. The affinity for dihydrogen is low. 

Catalysis and Binding by 5-Ring Chelate Biphosphine Rhodium Complexes... 

The hydroxyproline-derived biphosphine prepared by Achiwa (44,45,46) and co-workers presents an interesting case. Its rhodium complexes are only effective for asymmetric hydrogenation in the presence of triethylamine, and esters are reduced in poor optical yield. The N-H function coordinates to rhodium, as has been shown by X-ray analysis, and two diastereomers may occur in complexes of N-acyl derivatives which are related by rotation about the amide bond. 

Rhodium Complexes with Larger-ring Chelates of Chiral Biphosphines... 

Figure 4.7. Approximate crystal structures of chelating biphosphine rhodium complexes (other ligands omitted for clarity).

Allylamines are isomerized by cationic rhodium complexes to the corresponding -enamines.Using the catalyst [Rh( -binap)(COD) containing the optically pure biphosphine ligand -binap, high asymmetric induction in this isomerization can be achieved (Scheme 9). 

Achiwa, K. (1976) Asymmetric hydrogenation with new chiral functionalized biphosphine- rhodium complexes, J. Am. Chem. Soc. 98,8265-8266. 

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