Chiral biphosphines

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 new type of iodonium salts constitute the conformationally rigid, tetranu-clear macrocyclic ring systems dubbed molecular boxes. The relatively simpler tetraaryltetraiodonium salts were obtained from 4,4 -bis(diacetoxyiodo)bi-phenyl and 4,4 -bis(trimethylsilyl)biphenyl [119]. The iodonium salt derived from 4-(4 -lithiophenyl)pyridine was made using the method of /J-(dichloroio-do)chloroethylene and it was used for the construction of hybrid iodonium-platinum (or palladium) cationic tetranuclear macrocyclic squares including some in which the ligand of the metal was a chiral biphosphine [120,121]. 

Matteoli, U., Beghetto, V., Schiavon, C., Scrivanti, A., and Menchi, G. Synthesis of the new chiral biphosphine ligand 2,3-bis(diphenylphosphino)butane (CHIRAPHOS), Tetrahedron Asymmetry 1997, 8, 1403-1409. 

Modified chiral biphosphinic dioxolane ligands and their use in the asymmetric synthesis of natural lignans 92H(33)435. 

Successively a rhodium chiral biphosphine catalyst ([Rh(l,5-COD)bisphosphine] + BF allowed better selectivity in reducing a-acylaminoacrylic acids (95-96% e.e.)193. 

There have been considerable developments in the last two years. Most of these concern the synthesis and application of new types of chiral chelating biphosphine and will be discussed in context. It is now possible to make some generalizations concerning current knowledge about asymmetric hydrogenation ... 

When the biphosphine is chiral, but posesses a symmetry axis, there are only two possible diastereomeric modes of binding in an enamide complex in which olefin and amide groups are coordinated to the metal. R - Phenylbis (diphenylphosphino) ethane complexes are disymmetric and consequently there are four possible enamide complexes - either face of the olefin may be cis or trans to PI. Complexation of methyl a-acetamidoacrylate gives only one of these but complexation of acetamidocinnamic acid or its esters leads to two diastereomeric complexes (21) (Figure 2). A combination of labelling experiments demonstrates that these are regioisomers (same olefin face coordinated to the metal) rather than stereoisomers. 

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

Numerous biphosphines appear to be good chiral ligands for the near-quantitatively enantioselective hydrogenation of amino acid precursors The two phosphorus atoms are connected by a chiral backbone having one asymmetric center or two. The general structure is easily prepared by alkylation of a disubstituted phosphide ... 

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. 

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

Sun and Fu [107] developed the first example of asymmetric catalytic y-sulfenylation of carbonyl compounds catalyzed by chiral biphosphine 38 (Scheme 14.39). This type of carbonyl compound functionalization has been poorly... 

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

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