Phosphorus ligands tertiary phosphines

The two-coordinate phosphorus atom of a phosphonio-phospholide moiety is generally a weaker Lewis-base than a tertiary phosphine as a consequence, the complexes are thermally less stable (this holds in particular for metal fragments with low back donation capability, e.g. Mn(CO)4Cl) [35, 43], benzophospholide ligands are easily displaced by tertiary phosphines such as PhsP [27, 44], and bidentate ligands comprising a phosphonio-ben-zophospholide and a phosphine site react with transition metal fragments preferentially at the phosphine site coordination of both phosphorus atoms is only observed if the substrate offers two vacant coordination sites [27, 47]. 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphorus, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

For the sake of completeness the ultimate, encapsulated phosphorus ligand in [Rh9(CO)21P]2 is described.327 Like [Co2(CO)5(PPh3)(P2)], [Rh CO P]2- is formed by stripping the substituents from a tertiary phosphine ligand in tetraethyleneglycol dimethyl ether solution (equation 97). 

Because of its potential application to the synthesis of estersfor lubricating oils, the dimerization-carbonylation of butadiene has received special attention. Basic phosphines such as PBun3 and weakly basic tertiary amine solvents (quinoline, N, N-diethylaniline) were found to improve both the stability and activity of the catalyst system.S3° In a further report in which PPr 3 was used as phosphorus ligand it was found that the addition of maleic anhydride caused a marked increase in the catalytic activity. It was believed that through coordination it stabilized the palladium(O) complexes formed against precipitation as metal.s 1... 

Since 1948-50, by using as reactants isonitriles, phosphorus trihalides, and tertiary phosphines, we have gained important insight into the dependence of reactions of metal carbonyls with bases upon the nature of the ligand. Organophosphines were introduced into carbonyl chemistry even prior to 1948 by Reppe and Schweckendiek (7). In general, these ligands react only by substitution of CO, and do not cause disproportionation. Thus nickel carbonyl frequently reacts with complete displacement of carbon monoxide, as we were first able to demonstrate in the reaction with phenyl isonitrile (8). 

The relatively high optical purities obtained with the Rh-NMDPP system are particularly interesting from a practical viewpoint since the NMDPP ligand is prepared from an inexpensive, commercially available, chiral precursor, (-)-menthol (17). Tertiary phosphines chiral at phosphorus, on the other hand, are much less accessible and require a classic resolution step (see later discussion for details Section II. B). 

Two kinds of chiral tertiary phosphine ligands have been used in asymmetric hydrogenation experiments involving rhodium complexes the Horner and Monsanto groups have concentrated on ligands whose chirality is centered at an asymmetric phosphorus atom, and the New Hampshire and Paris groups have focused their attention mainly on phosphides that carry chiral carbon moieties. 

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