BPPFA

One of the branches in ligand design was provided by Kumada and his introduction of the ferrocene backbone for BPPFA [99-101] (20a) and BPPOH [102] (20b). This development leads us to the next class of ligands - ferrocene-based. Other variations for development include changes in the backbone and incorporation of the phosphorus into a phospholane (see Section 23.6). 

The key access compound to the early members of the class, and indeed some later ones, is the Ugi amine (29) and its relationship to PPFA (30) and BPPFA (20a) can be clearly seen [100, 130, 131]. 

Analogues of BPPFA and BPPFOH have been prepared, but for many applications these two ligands still prove to be the best for enantioselective hydrogenations [125]. The introduction of another functional group into the side chain, as in 31, provided the first catalysts capable of hydrogenating the tetra-substi-tuted a,/ -unsaturated acids with high enantioselectivity, even though the activity was very low (turnover frequency, TOF, 2 h-1) [132, 133],... 

Bppfoh and bppfa derivatives have been applied most successfully for the Rh-catalyzed hydrogenation of dehydro amino acid derivatives such as MAC (ee 97%) and of functionalized ketones [7]. The nature of the amino group has a significant effect on enantioselectivity and often also on activity, and is used to tailor the ligand for a particular substrate. Rh-bppfa complexes were among the first catalysts able to hydrogenate tetrasubstituted C=C bonds, albeit with relatively low activity (Table 25.2, entries 2.1-2.3). Ferrophos, one of the very few li-... 

Fig. 25.6 Structures of specific bppfa, ferrophos, and mandyphos ligands and of test substrates.

Ferrocene-based complexes have some potential for the enantioselective reduction of ketones, but compared to other ligand classes this is relatively limited [3]. Rh complexes of bppfa, bophoz and josiphos are among the most selective catalysts for the hydrogenation of a-functionalized ketones (Table 25.9 Fig. 25.18, 30-32). Ru complexes of walphos and ferrotane are quite effective for... 

Rh complexes with ChiraPhos, PyrPhos, or ferrocenyl phosphines lacking amino alkyl side chains (such as BPPFA) are much less active toward tetrasubstituted olefins. Table 6-1 shows that in asymmetric hydrogenations catalyzed by 5a-d, the coordinated Rh complex exerts high selectivity on various substrates. It is postulated that the terminal amino group in the ligand forms an ammonium carboxylate with the olefinic substrates and attracts the substrate to the coordination site of the catalyst to facilitate the hydrogenation. 

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