Ferrocene-rhodium complexes

A first class of heteronuclear ferrocene-rhodium complexes, the electrochemical behavior of which has been studied, is shown in Scheme 7-26. 

Recently a novel chiral ferrocene-based amidinato ligand and its rhodium complexes have been described. The chiral N,N -bis(ferrocenyl)-substituted formamidine (N,N -bis[(S)-2- (lR)-l-(diphenylphosphino)ethyl ferrocen-l-yl]for-mamidine was prepared from commercially available (IR)-l-(dimethylamino) ethyl ferrocene by a multistep procedure in an overall yield of 29%. Deprotonation of the ligand with -butyllithium followed by addition of [RhCl2(COD)2] as illustrated in Scheme 167 yielded the corresponding (formamidinato)rhodium(l)... 

The last example of a dendritic effect discussed in this chapter is the use of core-functionalized dendritic mono- and diphosphine rhodium complexes by Van Leeuwen el al. [45] Carbosilane dendrimers were functionalized in the core with Xantphos, bis(diphenylphosphino)ferrocene (dppf) and triphenylphosphine (Figures 4.22, 4.32 and 4.33). 

The dithiophosphonic acid monoesters, RP(OR )(S)SH can be conveniently prepared by cleavage of dimeric, cyclic diphosphetane disulfides, [RP(S)S]2 with alcohols, silanols, or trialkylsilylalcohols180 and then can be converted into metal complexes M[SPR(OR )]2 without isolation.181 The substituted ferrocenyl anion, (N3C6H4CH20)(CpFeC5H4)PS2 has been prepared in two steps from P4Sio, ferrocene and hydroxymethylbenzotriazole (and its salt was used for the preparation of some nickel and rhodium complexes).182 Zwitter-ionic ferrocenylditiophosphonates,... 

While Josiphos 41 also possessed an element of atom-centered chirality in the side chain, Reetz reported a new class of ferrocene-derived diphosphines which had planar chirality only ligands 42 and 43, which have C2- and C -symmetry, respectively.87 Rhodium(i)-complexes of ligands (—)-42 and (—)-43 were used in situ as catalysts (0.75 mol%) for the hydroboration of styrene with catecholborane 1 for 12 h in toluene at — 50 °C. The rhodium/ i-symmetric (—)-43 catalyst system was the more enantioselective of the two - ( -l-phenylethanol was afforded with 52% and 77% ee with diphosphines (—)-42 and (—)-43, respectively. In both cases, the regioselectivity was excellent (>99 1). With the same reaction time but using DME as solvent at lower temperature (—60 °C), the rhodium complex of 43 afforded the alcohol product with an optimum 84% ee. 

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]. 

The use of chiral rhodium complexes fashioned from ferrocene derivatives has gained in popularity significantly in recent years 1651. 

A new family of chiral ligands for asymmetric homogeneous hydrogenation has been developed. The performance of mono- and bis-rhodium complexes of these chiral ferrocene tetraphosphine ligands in the hydrogenation of model substrates was surveyed in comparison to their ferrocene bis-phosphine analogs. 

For our initial studies we chose to evaluate the hydrogenation of two unsaturated carbonyl model prochiral substrates with rhodium complexes of chiral ferrocene diphosphine and tetraphosphine ligands using a standard set of conditions. The substrates screened were methyl a-acetamido cinnamate (MAC) and dimethyl iticonate (DIMI). The substrates, catalysts, conditions, and experimental results are shown in Table 1. 

Alternatively, the rhodium complexes of 4 exhibit enhanced performance relative to the ferrocene diphosphine analog 3 which cannot be explained simply as a switch from one binding mode to another. P NMR experiments on in situ formed rhodium complexes are useful for correlating rhodium binding of different ligands in solution when a single species dominates, but are inconclusive when multiple phosphorus resonances are observed. Preparation and isolation of preformed complexes may provide better systems for study by NMR spectroscopy. 

Ortho-directed metallations also allowed the synthesis of ferrocene-based hydroxamic acids such as 17 and 18 [23] as well as the preparation of planar-chiral carbenes 19 and 21 (which were trapped as chromium and rhodium complexes, 20 and 22, respectively) [24], In this context it is noteworthy that 19 was the first carbene with planar chirality ever reported. 

Covalent attachment has also been exploited for protein incorporation of non-native redox active cofactors. A photosensitive rhodium complex has been covalently attached to a cysteine near the heme of cytochrome c (67). The heme of these cytochrome c bioconjugates was photoreducible, which makes it possible for these artificial proteins to be potentially useful in electronic devices. The covalent anchoring, via a disulfide bond, of a redox active ferrocene cofactor has been demonstrated in the protein azurin (68). Not only did conjugation to the protein provide the cofactor with increased water stability and solubility, but it also provided, by means of mutagenesis, a means of tuning the reduction potential of the cofactor. The protein-aided transition of organometallic species into aqueous solution via increased solubility, stability and tuning are important benefits to the construction of artificial metalloproteins. 

The ferrocene-iridium complexes, analogs of the rhodium complexes illustrated in Scheme 7-26, are shown in Scheme 7-28. 

Ferrocene was the first organometallic guest incorporated and numerous spectroscopic and electrochemical studies have been performed on ferrocene, substituted ferrocene, and related metallocene (e.g. cobaltocene) inclusion complexes (444-469]. Half-sandwich cyclopentadienyl- and benzene-metal carbonyl complexes have also been studied quite extensively [470-479] as have // -allyl metal (palladium) complexes [480], diene metal (rhodium) complexes [481-484], acetylene cobalt carbonyl cluster complexes [485], and complexes with metal carbonyls, e.g. Fe(CO)5, Mn2(CO)io, and CoNO(CO)3 [485a]. 

This is in the chloroquine series that the advantages of the organometallic labeling have been by far the most fully explored [120,131]. Sanchez-Delgado et al., for example, have obtained the rhodium complex 44 by direct complexation of the chloroquine (Scheme 3.19) [131]. This complex has an efficacity in vitro comparable to that of chloroquine (IC50 = 72 and 73 nM respectively) but it is chiefly in the ferrocene series that the results are most spectacular. 

The cationic Rh and Ir complexes with chiral (iminophosphoranyl)ferrocene ligands (107) and (108) were found to be very powerful catalysts for asymmetric hydrogenation of a series of unfunctionalized di- and trisubstituted olefins with almost perfect enantiomeric excesses (up to 99% ee) under mild conditions. In some cases the rhodium complexes were even better catalysts than their iridium counterparts. ... 

Also, pincer ligands have been prepared from l,3-bis(diisopropyl or /-butyl)(phosphinomethyl)ruthenocene. Notably, the iridium and rhodium complexes of these have been discussed. The iridium hydride complex [Ir(H)(Cl)[ 2,5-(Bu PCH2)2C5H2 Ru(Cp)]] exists in the form of a mixture of endo- and o-isomers, the exo being the more thermodynamically favorable.Trimetallic ruthenocene/ferrocene ligands 23 have been prepared in three steps from l,l -diacetylruthenocene, and further derivatized to the pentametallic bis-palladium allyl complex 24. 

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