Ferrocene Ligands

An aromatic system can also provide a rigid backbone, as seen with Phane-Phos (28a) [122-124], 

Kumada s use of a ferrocene moved away from the C2-symmetrical motive, as planar chirality can result from the two ferrocene rings having different substituents. The development of this class of ligand is well documented [5, 125-127]. The best-known uses of these ligands are for reductions of carbon-heteroatom multiple bonds, as in the synthesis of the herbicide, Metolachlor [128, 129]. 

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], 

Developments after these Ugi derivatives have taken a number of pathways. The MandyPhos family of ligands (32) have been used to reduce enamides to 01-amino acids as well as an enol acetate to produce an a-hydroxy ester [134—140]. The substituents R and R1 can be used for the fine-tuning of a specific substrate. Many of the family have R1 as a secondary amine, relating the family back to PPFA. For confusion, MandyPhos has also been called FerriPhos, while the derivative 32 (R = Rx = Et) is known as FerroPhos. 

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Gomez Arrayas R, Adrio J, Carretero JC (2006) Recent applications of chiral ferrocene ligands in asymmetric catalysis. Angew Chem Int Ed 45 7674—7715 Dai LX, Hou XL (2010) Chiral ferrocenes in asymmetric catalysis. Wiley-VCH, Weinheim Rigaut S, Delville MH, Losada J, Astrac D (2002) Water-soluble mono- and star-shaped hexanuclear functional organoiron catalysts for nitrate and nitrite reduction in water syntheses and electroanalytical study. Inorg Chim Acta 334 225-242... 

Togni and co-workers have used the convergent methodology to link phosphine-containing chiral ferrocene ligands on the cyclophosphazene core to obtain dendrimeric structures of the type 37 (Fig. 21) (201). The reaction with the cyclophosphazene end occurs by the replacement of the P-Cl bond and by the formation of the P-0 bond. The dendrimers contain twelve and sixteen ferrocene moieties respectively. The phosphine units present can coordinate to Rh(I) to afford metallic dendrimers, which have been shown to be excellent catalysts for the enantioselective hydrogenation of dimethyl itaconate. The product... 

We like to conclude the present section with an example which points out the role played by the electronic effects of ferrocene ligands in stabilizing uncommon oxidation states in metal complexes rather than their electrochemical properties. Figure 10 shows the molecular structure of the Ir(I) monocation [Ir(dppf)2]+ (dppf= l,l -bis(diphenylphosphi-no)ferrocene)7 and its electrochemical behaviour in thf solution.8... 

Scheme 8.21. Allylic substitution in the presence of ferrocene ligand 35.

Bidentate ferrocene ligands containing a chiral oxazoline substituent possess both planar chiral and center chiral elements and have attracted much interest as asymmetric catalysts.However, until recently, preparation of such compounds had been limited to resolution. In 1995, four groups simultaneously communicated their results on the asymmetric synthesis of these structures using an oxazoline-directed diastereoselective lithiation (Scheme 8.141). " When a chiral oxazolinylferrocene 439 was metalated with butyllithium and the resulting aryllithium species trapped with an electrophile, diastereomer 442 was favored over 443. The structure of the major diastereomer 442 was confirmed, either by conversion to a compound of known stereochemistry or by X-ray crystallography of the product itself or of the corresponding palladium complex. ... 

Figure 1.5 Commercially available chiral ferrocene ligands.

Bincoletto C, Terariol ILS, Oliveira CR, Dreher S, Fausto DM, Soufen MA, Nascimento FD, Caires ACF (2005) Chiral cyclopalladated complexes derived from N, N-dimethyl-1-phenethylamine with bridging bis(diphenylphosphine)ferrocene ligand as inhibitors of the cathepsin B activity and as antitumoral agents. Bioorg Med Chem 13 3047-3055... 

Metzler-Nolte N, Salmain M (2008) The bioorganometallic chemistry of ferrocene. In Stepnicka P (ed) Ferrocenes ligands, materials and biomolecules. Wiley, Chicester, pp 499-639... 

Stepnicka P (2008) Ferrocenes ligands, materials and biomolecules. Wiley, Chicester,... 

The first example of a C C bond-forming reaction catalysed by gold was the asymmetric aldol condensation developed in 1986.30 The addition of an isocyano acetate to an aldehyde produces the A -oxazole as the major and Z-oxazole as the minor product in excellent enantiomeric excess (ee) in the presence of a cationic gold catalyst, [Au(CyNC)2]BF4, and a chiral diphosphanyl ferrocene ligand (see Scheme 12.5). 

Many of the ferrocene ligand families described above are derived from a resolved chiral precursor (i.e. 289). Efforts to (76) prepare planar chiral ferrocenes also employ other strategies that rely on a directed metalation (see Orthometalation). Sulfoxide (338), acetal (339), and oxazolines of type (321)... 

Electrochemical and X-ray Structural Aspects of Transition Metal Complexes Containing Redox-Active Ferrocene Ligands... 

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