Phosphite-Phosphine Ligands

Figure 3.2. Chiral phosphine-phosphite ligands on a highly cross-linked polystyrene polymer...

Dieguez et al. reported that phosphine-phosphite ligand 88 (Figure 9) gave 62% ee in the reaction of triethyl-aluminum with 2-cyclohexenone.131... 

Scheme 44.10 Phosphine-phosphite ligand in enantioselective hydrogenation.

Abstract This chapter presents the latest achievements reported in the asymmetric hydroformylation of olefins. It focuses on rhodium systems containing diphosphites and phosphine-phosphite ligands, because of their significance in the subject. Particular attention is paid to the mechanistic aspects and the characterization of intermediates in the hydroformylation of vinyl arenes because these are the most important breakthroughs in the area. The chapter also presents the application of this catalytic reaction to vinyl acetate, dihydrofurans and unsaturated nitriles because of its industrial relevance. 

Inspired by the excellent results using the BINAPHOS ligands, new phosphine-phosphite ligands with different backbones have been developed in recent years. Unfortunately, their Rh-catalyzed hydroformylation provided low-to-moderate enantioselectivity (ee s from 20 to 62%) [35,36,65]. 

Based on the precedent of Van Leeuwen and Roobeek, livinghouse and co-workers screened a variety of electron-deficient phosphine/phosphite ligands for the rhodium-catalyzed [4-1-2] reaction, which provided an alternative catalyst system for the formation of 5,6- and 6,6-ring systems [13]. The most notable of these was the tris-(hexafluoro-2-propyl) phosphite-modified rhodium complex, which was applicable to both carbon- and oxygen-tethered substrates, and also provided the first example of a facial-directed diastereoselective intramolecular rhodium-catalyzed [4-i-2] reaction (Eq. 4). 

Figure 7 A rhodium complex of chiral phosphine-phosphite ligand (H,S)-BINAPHOS used as a catalyst for asymmetric hydroformylation.

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