Ferrocene-based ligands

The rhodium complexes of the ferrocene derivatives 39 have shown useful characteristics for the reduction of itaconates as well as dehydroamino acid derivatives [15, 167-170]. These compounds are hybrids between ferrocene-based ligands and the various other types. The P-chiral compounds, which in some ways are DIPAMP hybrids, showed tolerance for the reduction of N-methyl en-amides to produce N-methyl-a-amino acid derivatives [169-171]. 

Enantioselective Hydrogenation of Alkenes with Ferrocene-Based Ligands... 

As a general mle, most phospholanes are relatively air-sensitive and usually sold as rhodium-diene complexes. The ferrocene-based ligands, as well as phos-phoramidites and phosphites, are relatively easy to handle, and the corresponding Rh, Ir and Ru complexes are usually prepared in situ. Even though most biaryl type ligands are quite insensitive, most of them are applied as preformed Ru complexes. 

Figure 4.20. Commercialised asymmetric hydrogenations using ferrocene based ligands...

Ikeda and co-workers examined ferrocene-based ligands 11a and 11b in the same allyhc substitution reaction of racemic acetate 136 (Fig. 9.44, Table 9.24). This catalytic system resulted in quantitative yield of 137 with ee of 96 and 99%, respectively. 

The palladium based catalyst systems were also effective in the construction of carbon-nitrogen bonds on benzannulated five membered heterocycles. The 4-chloroindole derivative shown in 6.78. was coupled with piperazine in excellent yield, using a ferrocene based ligand.109 The analogous 5-bromo-benzimidazole derivative gave similar results (6.79.),110... 

Kumada and co-workers have introduced the ferrocene-based ligand BPPFOH (62) for the rhodium-catalyzed asymmetric hydrogenation of carbonyl compounds. 

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). 

Other ligands giving >90% ee for the alkylation or amination of 19 have been reported but will not be described in detail. They include derivatives of 3 [36], ferrocene-based ligands [54-57], l.l -binaphthyl-based ligands [58-60], natural product-based ligands such as fenchone [61], cholesterol [62], or carbohydrates [63,64], chiral aryl chromium complexes [65,66], chiral sulfimides [67], newP,N-ligands [19,22,52,53,68-78], and others [79-82]. 

Recently, Trost reported an efficient Pd-catalyzed asymmetric allylic alkylation of 1-tetralones [175] and a-arylketone 83 creating a quaternary center as exemplified by the synthesis of 84 (Scheme 7) [176]. Two a-heterocyclic ketones were also alkylated with similar results. Ferrocene-based ligands are effective in pro-... 

Figure 6.9 Chiral ferrocene-based ligands tested in allylic alkylation...

The use of ferrocene-based ligands such as 3 and PPFA (8) result in the formation of catalysts that extended the scope of the arylation reaction to more difficult transformations [37,38,56]. For example, di-n-butylamine could now be effectively coupled with electronically neutral as well as electron-deficient aryl bromides. Reaction of 4-ferf-butylbromobenzene with di-n-butylamine with the ( )-BlNAP/Pd- or DPPF/Pd-based catalysts resulted in significant amounts of terf-butylbenzene formation, however the use of ligands 3 and 8 resulted in formation of the desired product in excellent yield, Eq. (25). 

Hartwig reported the first amination of an aryl tosylate with an aliphatic amine. Utilizing electron-rich ferrocene-based ligand 20, the coupling with hexylamine shown below proceeded in 83 % isolated yield, Eq. (119) [72]. 

The use of ferrocene-based ligands 26 and 27 also effected the Pd-catalyzed formation of diaryl ethers,Eq. (193) [148,149]. The 27/Pd-catalyst is more reactive and mediates the transformation in better yield and at lower temperatures. 

Note Transition metals containing ferrocene-based ligands. . . often show superior catalytic activity when compared to complexes containing other types of bidentate. . . ligands [165] . 

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. 

---