Atropisomeric diphosphines

MeOBIPHEP is the atropisomeric diphosphine 2,2,-bis(diphenylphosphino)-6,6 -dimethoxy-l,-l -biphenyl (100), has been synthesized. In the presence of SnCl2, this species is an efficient catalyst for the asymmetric hydroformylation of styrene. Asymmetric inductions are higher than those attainable using the system [PtCl2 (i )-(+)-BINAP ]/SnCl2, where BINAP is 2,2 -bis(di-phenylphosphino)-l,l,-binaphthyl. The influence of CO and H2 partial pressures on the catalytic activity of the (99)/SnCl2 system has also been studied.328 Complexes [PtMeCl(P-P)][(101), P-P = (5)-6,6,-(dimethoxybiphenyl)-2,2,-diylbis(diphenylphosphine) ((5)-MOBIPH) (102),... 

In the hydrogenation of diketones by Ru-binap-type catalysts, the degree of anti-selectivity is different between a-diketones and / -diketones [Eqs (13) and (14)]. A variety of /1-diketones are reduced by Ru-atropisomeric diphosphine catalysts to indicate admirable anti-selectivity, and the enantiopurity of the obtained anti-diol is almost 100% (Table 21.17) [105, 106, 110-112]. In this two-step consecutive hydrogenation of diketones, the overall stereochemical outcome is determined by both the efficiency of the chirality transfer by the catalyst (catalyst-control) and the structure of the initially formed hydroxyketones having a stereogenic center (substrate-control). The hydrogenation of monohydrogenated product ((R)-hydroxy ketone) with the antipode catalyst ((S)-binap catalyst) (mis-... 

The sense of diastereoselectivity in the dynamic kinetic resolution of 2-substi-tuted / -keto esters depends on the structure of the keto ester. The ruthenium catalyst with atropisomeric diphosphine ligands (binap, MeO-biphep, synphos, etc.) induced syn-products in high diastereomeric and enantiomeric selectivity in the dynamic kinetic resolution of / -keto esters with an a-amido or carbamate moiety (Table 21.21) [119-121, 123, 125-127]. In contrast to the above examples of a-amido-/ -keto esters, the TsOH or HC1 salt of /l-keto esters with an a-amino unit were hydrogenated with excellent cwti-selectivity using ruthenium-atropiso-... 

Ruthenium(II) complexes bearing atropisomeric diphosphine ligands have proved to be efficient systems for the hydrogenation of a wide range of prochiral substrates. A new catalytic system has been developed based on ruthenium complexes having SYNPHOS and DIFLUORPHOS as chiral diphosphanes (Figure 3.6). 

Synthesis of chiral atropisomeric diphosphine type ligands is a current challenge in chemical research because their late transition metal complexes usually provide high enantioselectivity in homogenous catalytic reactions. [28] In practical point of view, preparation and optical resolution of racemic diphosphine oxides followed by the reduction of the separated enantiomers are usually more advantageous than an expensive enantioselective synthesis of one diphosphine enantiomer. [29]... 

Girard, C., Genet, J.-P., Bulliard, M. Non-linear effects in ruthenium-catalyzed asymmetric hydrogenation with atropisomeric diphosphines. Eur. J. Org. Chem. 1999, 2937-2942. 

Other olefins lacking a carboxylic or amido fimctionality give less satisfactory optical results. The Ru complexes of atropisomeric diphosphines circumvent this problem, at least with prochiral ally lie alcohol. Either enantiomer of citronellol is obtained in 96-99% e.e. starting from geraniol or nerol ° . 

The catalyst is prepared conveniently from equimolar amounts of Ru(OAc)2 [BINAP] and Another method uses the easily accessible complex (COD)2 Ru2 (OCOCp3)4 as a precursor for in situ preparation of the dichloro complex with BINAP. The COD ligands are displaced at 40°C by atropisomeric diphosphines. Both optical forms of BINAP are available. Ketones are reduced in predictable manner with maximal optical yields. 

Several new, water-soluble atropisomeric diphosphines in the biphenyl series, such as (S)-(+)- and (R)-(—)-MeO-BIPHEP tetrasulfonate (32), were prepared and used as components of Ru(II)- and Rh(I)-based hydrogenation catalysts [98]. Several C=C unsaturated substrates (enamides, unsaturated acids) could be hydrogenated biphasically with high rates and enantioselectivities (Eqs. 33 and 34). In some cases high substrate/catalyst ratios could be used (up to 10000 1), a strong requirement for practical applications [99]. 

Bronco, S. Consigho, G Regio- and stereoregular copolymerisation of propene with carbon monoxide catalysed by palladium complexes containing atropisomeric diphosphine ligands. Macromol. Chem. Phys. 1996,197,355-365. 

In this case also, bimetallic complexes of atropisomeric diphosphines proved to be optimal pre-catalysts. (/ )-Xylyl-BINAP and (/ )-DTBM-4Segphos (L4-b) gold(I) complexes have been applied to the synthesis of 8 and 7-membered rings, respectively, with enantiomeric excesses in the range 85-92%. Selected results are given in Fig. 10.51. 

From this short overview it appears that the majority of the recent studies on enantioselective cycloisomerizations have been focused so far on asymmetric Alder-ene type cyclizations with Pd and Rh catalysts, since these reactions represent an economical access into synthetically usefiil cyclopentene and cyclohexene frameworks (Sects. 10.2.1 and 10.3.1). For these processes, efficient chiral catalysts have been afforded mainly by atropisomeric diphosphines, but also DuPHOS, Skewphos and phosphine-oxazolines can occasionally represent suitable auxiliaries. 

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