C2-symmetric diphosphines

The catalyst-substrate complexes deserve some additional comments. The two possible diastereomers for C2-symmetrical diphosphines interconvert inter- and intramolecularly, the latter being the dominant mechanism [76] (Scheme 1.22). A second property - at least of some catalyst-substrate complexes - is that the reactivity of the minor diastereomer toward H2 is notably higher than that of the major diastereomer. 

A variety of C2 symmetrical diphosphine ligands with a ferrocenyl backbone (see Fig. 25.5) have recently been described and tested, with sometimes quite impressive results. Interesting examples are f-binaphane [11], ferrotane [12], L2... 

As many endeavors in transition-metal catalysis, the design, synthesis and screening of chiral ligands have played a pivotal role in the development of the asymmetric allylic alkylation reaction. A series of C2-symmetric diphosphines such as DiPAMP, chiraphos, DIOP. and BINAP,... 

J. Gao, T. Ikariya, and R. Noyori, Aruthe-niumjii) complex with a C2-symmetric diphosphine/diamine tetradentate ligand for asymmetric transfer hydrogenation of aromatic ketones, Organometallics 1996, 15, 1087-1089. 

The square-planar complex (70) incorporating a C2 symmetric diphosphine catalyzes the polymerization of ethene, which is somewhat surprising as diphosphine... 

Routes to the chiral ferrocenyldiphosphines 113 have been developed, via the use of the chiral oxazaphospholidine borane 114. Routes to other chiral ferrocenylphosphines have also been developed, including the boranato-functio-nalised systems 115, and the C2-symmetric diphosphine 116, having only the planar chirality of the ferrocene system. Full details have now appeared of the palladium-promoted asymmetric Diels-Alder reaction between l-phenyI-3,4-di-... 

A very simple C2-symmetric diphosphine 10 was prepared by Bosnich and Fryzuck in 1977 [33]. Hydrogenation of AT-acetyldehydrophenylalanine gaveN-acetylphenylalanine with an ee close to 99%. The authors proposed that the asymmetric induction originated from the twist conformation of the chelate ring X or 8 configuration), locked by the diequatorial orientation of the two methyl substituents. 

Extensive mechanistic studies have been performed on reactions catalyzed by rhodium and platinum complexes containing enantiopure C2-symmetric diphosphine ligands.As discussed above, (1) the formation of the Tr-olefin-Rh(H) complex 19, (2) stereospecific cis addition of the hydridorhodium to the coordinated olefin to form the alkyl-Rh complex 20 (and then 2, and (3) the migratory insertion of a carbonyl ligand giving the acyl-Rh complex 17 with retention of configuration, have been established in the hydroformylation of 1-alkenes or substituted ethenes. Thus, it is reasonable to assume that the enantioselectivity of the reaction giving a branched aldehyde is determined at the diastereomeric (1) TT-olefin-Rh complex 19 formation step, (2) alkyl-Rh complex 20 formation step, or (3) acyl-Rh complex 17 formation step. 

Azanorbornyl-3-methanol (93) shows its value as a chiral ligand in Ru-catalyzed transfer hydrogenation. On binding to Ru, C2-symmetric diphosphines (94, 95, etc.) and the ferrocene-based ligand 96 are able to exert their steric influences during hydrogenation of 1,3-diketones. 

Anderson BJ, Glueck DS, DiPasquale AG, Rheingold AL (2008) Substrate and catalyst screening in platinum-catalyzed asymmetric alkylation of bis(secondary) phosphines. Synthesis of an enantiomerically pure C2-symmetric diphosphine. Organometallics 27 4992-5001... 

Chen WP, McCormack PJ, Mohammed K, Mbafor W, Roberts SM, Whittall J (2007) Stereoselective synthesis of ferrocene-based c2-symmetric diphosphine ligands application to the highly enantioselective hydrogenation of a-substituted cinnamic acids. Angew Chem Int Ed 46 4141 144... 

The palladacycles discussed here are very effective for the resolution of bidentate mono- and diphosphines. For C2-symmetric diphosphines, a single pair of cationic complexes is formed but for Ci-bidentates two isomers can be formed. For the resolution of these complexes the chloride counterion is usually exchanged by hexafluorophosphate for solubility reasons. ... 

The generated phosphide boranes are extremely nucleophilic and reacted smoothly with activated alkyl halides (entries 3-7), epoxides (entries 1, 2 and 8) and molecules with activated multiple bonds such as ethyl acrylate (entry 9), benzyne (10) or even fullerene (entry 14). Some C2-symmetric diphosphine... 

Scheme 4.21 Preparation of C2-symmetric diphosphine boranes based on the ferro-cenyl backbone.

In 1992, Takaya et al. reported on the use of a chiral diphosphite derived from bisnaphthol in the asymmetric hydroformylation of vinyl acetate [11], but the enantioselectivity achieved was only 50%. They noted that diphosphites led to more stable hydroformylation catalysts than diphosphines. This observation prompted Takaya, Nozaki et al. to synthesize the chiral phosphine-phosphite ligands (R,S)- and (R,R)-BINAPHOS (24, figure 17), which were expected to combine the high enantioselectivity obtained with diphosphines such as BINAP in asymmetric hydrogenation, with the apparently efficient coordination of the phosphite moiety [38]. Indeed, the Rh(I) complex of Ci-symmetric (R,S)-BINAPHOS provided much higher enantioselectivities than either C2 symmetric diphosphine ligands or diphosphite ligands, viz. more than 90% ee for a wide variety of both functionalized and internal aUcenes [38,39,40,41]. 

Many chiral C2 diphosphines have been used as hgands in rhodium sterns for asymmetric hydroformylation. It can be observed that the chiral diphosphines providing the highest enantioselectivities in asymmetric hydrogenation lead to low chiral induction in hydroformylation. The C2-symmetric diphosphines, which were developed for square planar... 

Ligand TriFer is the first example of a C2-symmetric diphosphine that combines C-, P-central and planar chirality. Indeed, it is astonishing how straightforward and short is the synthesis of such a complex molecular assembly ... 

TriFer Ferrocene-based C2-symmetric diphosphine ligand... 

The palladium-catalyzed asymmetric allylic substitution of 1,3-diphenyl-2-propenyl pivalate was investigated with dimethyl mal-onate with various C2-symmetric diphosphines ligands having different spacer derived from valine. The cyclobutane moiety gave high selectivity for the (5)-product (eq 45). A reversal of the configuration of the substituted product has been observed in this reaction on changing the spacer. A stereochemical model was proposed to explain this phenomenon. ... 

The AAA of cyclohexyl pivalate using a C2-symmetric diphosphine ligand derived from valine has been performed with high enantioselectivity (eq 47). 

The current state of the art in the synthesis of 3,3-disubstituted (3-lactones was reported by the group of Kerrigan who showed the viability of a chiral C2-symmetric diphosphine 34 to catalyze the formal [2-1-2] cycloaddition between alkylarylketenes and various benzaldehyde derivatives (Scheme 3.16) [21]. anti (3-Lactones 67 were obtained in good yield (typically >60%) and in excellent diastereo-and enantiocontrol (up to 95 5 dr and up to 99% ee). 

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