Phosphorus-bound Ligands

Boschi, U. Belluco, T. Toniolo, R. Favez, and R. Rouiet, Inorg. Chim. Acta, 1979, 34, 37. 

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The carbethoxy tertiary phosphine Bu2P(CH2) C02Et (n — 1) forms an octahedral, cationic rhodium(III) complex (31), in which the carbonyl group coordinates to rhodium. Homologous ligands ( = 2, 3) from four-coordinate rhodium(II) complexes in which the ligands are only phosphorus bound.275... 

The high asymmetric inductions that can be attained in these reductions are above all to be attributed to steric interaetions of the substrate with the phosphorus-bound phenyl groups of the [Ru-f/fj-BINAP] complex R)-S (Scheme 5). It can be assumed that hydride, being the smallest ligand, coordinates cis to both phosphine ligands. The P-bound phenyl groups occupy equatorial (parallel to the substrate coordination plane) and axial positions (perpendicular to the substrate coordination plane) with respect to the metal-ligand cycle. The skewed BINAP skeleton requires the equatorial phenyl substituents to protrude directly into two of four coordination quadrants that are accessible to the substrate. 

The number of publications describing new ligands that allow the transfer hydrogenation of aromatic ketones with over 90 % ee has grown in leaps and bounds since 1996 [15]. In these reactions the use of ruthenium [15a-f] and iridium [15g] as the catalytically active metals has recently been augmented by the use of phosphorus-free ligands such as chiral diamines, amino alcohols, and bisthioureas such as 7 [15a,e-g]. A ruthenium-catalyzed transfer hydrogenation with 92 % ee has even been reported for the aliphatic ketone pinacolone (tert-butyl methyl ketone) [16]. 

Phosphorus ligands bound to cobalt are met more broadly in the parallel Comprehensive Organometallic Chemistry series, but some representative examples not also including Co—C bonded species are presented here. 

Diphosphine mono-chalcogenides are able to bind to metal centres from either one of or both the phosphorus(III) and chalcogen donor centres. This has led to their study as hemi-labile ligands in catalyst systems, whereby one of the donor centres binds strongly to the metal centre and the other one is weakly bound and hence can be readily dissociated or reattached to reveal or protect the active site at a metal centre.28... 

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