Biphosphines complex

MandyPhos Initial attempts to synthesize defined chelating biphosphine complexes by reacting (1) with established Ruthenium(II) precursors such as [Ru(COD)Cl2]x, [Ru(benzene)Cl2]2, [Ru(p-cymene)Cl2]2, (COD)Ru(Methylallyl)2,... 

Then, the olefin insertion and the reductive elimination take place from the resultant cis biphosphine complex. 

The above feature of the Brown mechanism that reductive elimination is more difficult than olefin insertion may be related to the nature of catalyst having a chelating bidentate ligand such as DIPHOS. Halpern have also investigated the hydrogenation (Equation 3) (2), where isomerization from the trans- to cis-biphosphine complex is not necessary. 

Scheme 6. Conformational isomerism in a 6-ring chelate biphosphine complex...

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.

TABLE IV. Hydrogenations catalysed by larger-ring chelate biphosphine complexes... 

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

The enantioselective hydrogenation of 3,4-dihydroxy-N-acetylamino dnnamic acid is catalyzed by the cationic Rh-biphosphine complex DIPAMP, in which the enantioselectivity is introduced by the chiral phosphine1104, 105l The hydrogenation proceeds quantitatively with 94% ee. The optically pure L-dopa is separated from the catalyst by crystallization. 

Related solvate complexes have been formed from many other asymmetric chelate biphosphine complexes and with other coordinating solvents. 

Similarly, a-cyanophosphonates were submitted to enantioselective fluorination by NFSi in the presence of organic base with palladium-biphosphine complexes (eq 46). The Pd(II) complexes activate the substrates through coordination of the nitrile group, and the base abstract an acidic proton, thereby affording the desired fluorinated products in high yields with good to high enantioselectivities. 

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