Ferrocenyl diphosphine ligands

Platinum complexes [PtCl2(diphosphine)] and [PtCl(SnCl3)(diphosphine)] of the ferrocenyl diphosphine ligands (35a), (35b), and (36) have been synthesized. Complexes [PtCl2(35)] and [PtCl2(36)] have been structurally characterized by XRD. Both the preformed and the in situ catalysts have been used in the hydroformylation of styrene.112... 

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). 

The Rh/Diop catalytic system is one of the fastest catalyzed gas-liquid asymmetric hydrogenations. A (R,S)-Cy-Cy-Josiphos ligand behaves almost as good as the Diop ligand and provides abetter enantioselectivity of 75% (Josiphos family of ferrocenyl diphosphine ligands cy cyclohexyl). The latter is the most active of the Josiphos family (88% conversion). The reproducibility of the data obtained has been checked with the Rh/Diop catalytic system. For more than five tests, the mean deviation was 2% for conversions and less than 1% for the enantiomeric excess that proved the reliability of this new microdevice. 

Figure 2.55 reports the preparation and structure of ferrocenyl diphosphine ligands and the dependence of the performances on the substituents in the ligand. 

A new class of chiral ferrocenyl diphosphine ligands (478) with an imidazole ring, have been prepared from acylferrocenes through a five-step synthesis and successfully applied in the Rh-catalyzed asymmetric... 

Scheme 17 Chiral ferrocenyl diphosphine ligands synthesised and applied in 1,2- and 1,4-additions by Harutyunyan and Minnaard...

In 1997, Inoue and co-workers achieved a methoxycarbonylation of styrene with a notable enantioselectivity (86% ee), when using the ferrocenyl diphosphine ligand 26 (Figure 14.7). However, this reaction only gave 46%... 

Godard C, Ruiz A, Claver C. Systematic study of the asymmetric methoxycarbonylation of styrene catalyzed by palladium systems containing chiral ferrocenyl diphosphine ligands. Helv. Chim. Acta 2006 89 1610-1622. 

A variety of C2 symmetrical diphosphine ligands with a ferrocenyl backbone (see Fig. 25.5) have recently been described and tested, with sometimes quite impressive results. Interesting examples are f-binaphane [11], ferrotane [12], L2... 

The most common chiral auxiliaries are diphosphines (biphep, binap and analogues, DuPhos, ferrocenyl-based ligands, etc.) and cinchona and tartaric acid-derived compounds. It is clear that the optimal chiral auxiliary is determined not only by the chiral backbone (type or family) but also by the substituents of the coordinating groups. Therefore, modular ligands with substituents that can easily be varied and tuned to the needs of a specific transformation have an inherent advantage (principle of modularity). 

The search for a commercially viable process took many years [126], Several approaches with Rh or Ir complexes using commercially available diphosphine ligands were not successful. A critical breakthrough was achieved when Ir complexes with a new class of ferrocenyl-based ligands (now called Solvias Josiphos) were used. Extremely active and productive catalysts were obtained, especially in the presence of acid and iodide ions. Different Josiphos ligands were tested and a selection of the best results obtained is shown in Table 37.5. 

A broad screening of ligands and ionic liquids was carried out by Feng et al. [104]. For rhodium-catalyzed hydrogenation of enamides the best catalysts were found to be the rhodium-ferrocenyl-diphosphine complexes with taniaphos, josiphos, walphos and mandyphos as ligands (Fig. 41.9). 

The research group of Van Leeuwen has focused on catalysis at the core of a carbosilane dendrimer in an effort to be able to control stereoselectivity [10]. To this end, a ferrocenyl diphosphine backbone was functionalized with different generations of carbosilane dendrons producing a series of dendrimer phosphine ligands with an increasing steric demand (see 7 for an example, Scheme 6). In situ... 

When the catalyst is located in the core of a dendrimer, its stability can also be increased by site-isolation effects. Core-functionalized dendritic catalysts supported on a carbosilane backbone were reported by Oosterom et al. 19). A novel route was developed to synthesize dendritic wedges with arylbromide as the focal point. These wedges were divergently coupled to a ferrocenyl diphosphine core to form dppf-like ligands (5). Other core-functionalized phosphine dendritic ligands have also been prepared by the same strategy 20). 

Ferrocenyl diphosphine core-functionalized carbosilane dendrimers have been prepared as ligands for homogeneous catalytic reactions applied in a CFMR by Van Leeuwen et al. [20,21,67,68]. The syntheses of these dppf-like ligands (Go-G2)-17 were performed using carbosilane dendritic wedges with an aryl bromide as focal point. These wedges were coupled to the core via quenching of the lithiated species with ferrocenyl phosphonites (Scheme 11). 

Dppf functions chiefly as a diphosphine ligand and usually no formal ferrocenyl-metal interaction occurs. Direct interaction between the ferrocenyl iron and the phosphine-bound metal center was first found in [Pd(dppf-P, P )(PPh3)][BFj2 3 [40]. This complex is prepared from ligand addition to [Pd(CH3CN)4][BF4]2... 

The carbon-carbon double bond of an enamine is also applicable for asymmetric hydrogenation leading to chiral amino acids. For example, hydrogenation of 13 by rhodium catalyst with ferrocenyl diphosphine 15 as a ligand was successful for the synthesis of methyl 3-amino-4-polyfluorophenylbutanoate 14 with excellent stereoselectivity (see Scheme 9.5) [15]. 

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