Subject phosphorus ligands

Several recent reviews on catalytic asymmetric hydrogenation are available. The mechanistic studies have been summarized by Halpern.81,259 The synthesis and use of phosphorus(III) ligands have been reviewed by Horner.270 Ferrocene-based and nitrogen-containing phosphorus ligands have also been covered.271 General reviews of the subject have appeared.272"275... 

The interest in all these themes really is not decreasing, indeed some fascinating areas of research are emerging or are the subject of many investigations the medicinal chemistry of bisphosphonates, the role of phosphorus in biology, phosphorus ligands in radiopharmaceutical chemistry, phosphorus in material science, new polymers and dendrimers incorporating phosphorus, and asymmetric catalysis to name but a few. 

Phosphorus is a key element in catalysis, and the last two Nobel prizes in molecular chemistry were awarded to Noyori, Sharpless and Knowles (2001) for their work on enantioselective catalysis and to Grubbs, Schrock and Chauvin (2005) for their work on the chemistry of transition metal carbene complexes and their applications in metathesis. In both cases the development of highly efficient, specifically tailored phosphorus based ligands are of paramount importance The book opens with an account of the recent studies on a new family of air-stable chiral primary phosphines based on the binaphthyl backbone and their applications in asymmetric hydrosilylations (Chap. 1). The concept of applying phosphorus ligands to enantioselective catalysis is also the main subject of Chaps. 5 and 10, dealing with P-based planar chiral ferrocenes and chiral phosphorus ligands for enantioselective enyne cycloisomerizations, respectively. 

Although the subject of stability of complexes will be discussed in greater detail in Chapter 19 it is appropriate to note here some of the general characteristics of the metal-ligand bond. One of the most relevant principles in this consideration is the hard-soft interaction principle. Metal-ligand bonds are acid-base interactions in the Lewis sense, so the principles discussed in Sections 9.6 and 9.8 apply to these interactions. Soft electron donors in which the donor atom is sulfur or phosphorus form more stable complexes with soft metal ions such as Pt2+ or Ag+, or with metal atoms. Hard electron donors such as H20, NH3( or F generally form stable complexes with hard metal ions like Cr3+ or Co3+. 

Tetracarbonyl cations [Cr(CO)4(L2)]+, where L2 represents arylphosphines, alkyl, or aryl phosphites, or a bidentate ligand with phosphorus or arsenic donor atoms, have been produced by both chemical and electrochemical means. However, oifly one of these complexes, namely, tra i -[Cr(CO)4(PPh3)2]+, is stable enough to be isolated as the perchlorate salt. Exposure to light and moisture produces tra i -Cr(CO)4(PPh3)2 via disproportionation. Just as easily isolated, but somewhat less sensitive, are the [Cr(CO)3(PR3)3]+ cations. " Oxidation with a silver ion or NO+ converts the /uc-(R3 = Me2Ph, (OMe)3) and mer-(R3 = (OMe)3, (OMe)2Ph, (OPh)3) complexes into Cr products with mer structures. Light and heat promote the formation of reduced wer-Cr(CO)3 (PR3)3. These tetra(carbonyl) and tri(carbonyl) cations have been the subject of ESR spectroscopy and theoretical study. 

Other interests in the area of phosphorus acids and their derivatives has centered on the following subjects synthesis of new ligands and catalysts for known reactions, synthesis of supramolecular logic gates, use of phosphates as electrophiles in palladium-catalyzed cross-coupling reactions, synthesis of phosphate based dendrimers and conjugates with oligosaccharides. 

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