Tetraphosphine ligands

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. 

Figure 1. Structures of chiral ferrocene bisphosphine and tetraphosphine ligands. Results and Discussion...

Figure 2. Binding modes for mono-Rh and bis-Rh complexes of ferrocene tetraphosphine ligands.

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. 

The hydrogenation reactions were all performed at the same substrate-to-rhodium ratio and, since the tetraphosphine ligands 2, 4, and 6 can form bidentate... 

From these initial survey results, we postulate that the dominant species formed by the rhodium complexes of tetraphosphine ligand 2 are the same as the rhodium complex formed by Josiphos ligand 1 and the two catalytic sites in ligand 2 act essentially independently. 

However, because the rhodium complexes of tetraphosphine ligand 4 behave differently than the rhodium complex of Josiphos ligand 3 for both substrates, we postulate that either a) different rhodium complexes are being formed (different binding modes as discussed above, see Fig. 2) by 3 and 4, or b) the substituents on the second cyclopentadiene (cp) ring influence the catalytically active site in the rhodium complexes of 4. 

The major resonances suggest that complexation of two Rhodium ions by the tetraphosphine ligand 4 occurs in the same manner as binding of one rhodium by the diphosphine Josiphos ligand 3 (see Fig. 2). While the minor resonances observed... 

Actually a square pyramidal structure has been found in the complex [NiBrLJBr (168) which has been prepared with the tetraphosphine ligand Me4p41377 and in the complexes [NiXL]BPh4 (169) with the mixed-donor ligand 2,3,2-p2S2.1382,1383... 

The rhodium-catalyzed hydroformylation of 1-hexene using a polar-phase solvent system (30% water by volume in acetone) has resulted in improvements in rate and chemoselectivity for catalysts supported by the tetraphosphine ligand et,ph-P4. A cationic rhodium dimer comprising... 

Laurenti and Santelli have recently introduced the tetraphosphine ligand (Tedicyp) 57... 

A recent example of another class of interesting catalyst was reported by Stanley, namely a bimetallic Rh complex provided by a tetraphosphine ligand. Up to 85% ee has been achieved in the hydroformylation of vinyl carboxylates, although the applicable substrates are still limited [39]. As a bimetallic system, the chiral dithiolate-bridged dinuclear Rh complex has been described with results of up to 10% ee for styrene [40]. Addition of chiral bisphosphine to the dithiolate-Rh system improved the ee up to 55% [41],but in this case, the dissociation of the two Rh centers into monometallic species has been claimed recently [42]. In this article, emphasis is placed on the third generation Rh(I) catalyst and the earlier generations are mentioned only briefly. Excellent review articles are available on the latter topics [10,36,43,44,45,46,47,48]. 

Tetraphosphine ligands, 995 Tetrapyridyls metal complexes, 91 geometry, 91 Tetrapyrroles metal complexes, 79 T etraselenoether metal complexes, 666 Tetrasulfur tetranitride metal complexes, 799 Tetrathiolane, bis(imino)-metal complexes, 591 Tetrathiooxalic acid metal complexes, 585, 606 Tetrathiosquaric acid metal complexes, 606 Tetrazole, 2-methyl-metal complexes, 77 Tetrazole, pentamethyl-metal complexes, 77 Tetrazoles metal complexes, 76 pK,ll... 

Our work into bimetallic cooperativity in homogeneous catalysis has concentrated on the binucleating tetraphosphine ligands meso- and racemic-et,ph-P4, shown in Scheme 3 [25, 26]. These ligands are designed to chelate two metal centers via a single, conformationally flexible, methylene bridge. 

To these also belong systems based on Doucet-Santelli tetraphosphine ligands 41-43, with Tedicyp (41) being studied in depth (Figure 2.5) [90]. Ligand 41 was tested extensively in reactions of aryl bromides with unprecedented variety in the choice of the alkenes ... 

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