Josiphos ligand compounds

Recently Togni et al. [19] focussed on the preparation of asymmetric dendrimer catalysts derived from ferrocenyl diphosphine ligands anchored to dendritic backbones constructed from benzene-1,3,5-tricarboxylic acid trichloride and adamantane-l,3,5,7-tetracarboxylic acid tetrachloride (e.g. 11, Scheme 11). In situ catalyst preparation by treatment of the dendritic ligands with [Rh(COD)2]BF4 afforded the cationic Rh-dendrimer, which was then used as a homogeneous catalyst in the hydrogenation reaction of, for example, dimethyl itaconate in MeOH. In all cases the measured enantioselectivity (98.0-98.7%) was nearly the same as observed for the ferrocenyl diphosphine (Josiphos) model compound (see Scheme 11). 

Because a comprehensive review on the catalytic performance of Josiphos ligands has been published,20 we restrict ourselves to a short overview on the most important fields of applications. Up to now, only the (7 )-(S)-family (and its enantiomers) but not the (R)-(R) diastereoisomers have led to high enantioselectivities (the first descriptor stands for the stereogenic center, and the second stands for the planar chirality). The most important application is undoubtedly the hydrogenation of C = N functions, where the effects of varying R and R1 have been extensively studied (for the most pertinent results see Table 15.5, Entries I—4). Outstanding performances are also observed for tetrasubstituted C = C bonds (Entry 5) and itaconic and dehydroamino acid derivatives (Entries 6 and 7). A rare example of an asymmetric hydrogenation of a heteroaromatic compound 36 with a respectable ee is depicted in Scheme 15.6.10b... 

Rh complexes of ferrocene-based ligands are very effective for the hydrogenation of several types of dehydroamino (2,3,29,41,42,44) and itaconic acid derivatives (4,5,28) as well as for enamide 45, enol acetate 26, and a tetrasubstituted C = C-COOH 21. Of particular interest are substrates that have unusual substituents (41,42,44) at the C = C moiety or are more sterically hindered than the usual model compounds (21,42). Table 15.10 lists typical examples with very high ee s and often respectable TONs and TOFs. Several industrial applications have already been reported using Rh-Josiphos and Ru-Josiphos (see Figure 15.7) as well as Rh-Walphos (Scheme 15.8). 

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