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By
John Tebby;
John Tebby
Division of Chemistry, School of Sciences, Staffordshire University, Stoke-on-Trent, Staffs
UK
ST4 2DE
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David Allen
David Allen
Biomedical Research Centre, Sheffield Hallam University
Sheffield
UK
S1 1WB
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This volume, covering the period July 2004–December 2005, represents the second of our attempts to present a more up to date and timely coverage of the literature of organophosphorus chemistry, which continues to generate a vast range of material. In future volumes, it is hoped that we will be able to present a more selective and critical coverage of the literature of a single calendar year, e.g., January–December 2006, the volume appearing in the later part of the following year. This volume follows the traditional approach and we welcome contributions from new authors. Professor György Keglevich (Budapest University of Technology and Economics) has provided coverage of phosphine chalcogenides, phosphonium salts and ylides, and Dr Alan Hewson (Sheffield Hallam University) has covered the chemistry of tervalent phosphorus acid derivatives. The phosphazene chapter is the final contribution from Dr Johan van der Grampel, who has contributed reviews of this area from volume 29 to the present, and we thank Johan for his input. In the next volume, the phosphazene area will be written by Professor Gabino Carriedo (Universidad de Oviedo). In addition, the hypervalent phosphorus chapter will also be contributed by a new author, Professor Gerd-Volker Roeschenthaler (Institute of Inorganic & Physical Chemistry, University of Bremen) and Dr R. Bodalski has retired and been replaced by K. Owsianic as co-author of the quinquevalent phosphorus acids.

The period under review has seen the publication of whole issues of major journals, focusing on organophosphorus chemistry. The papers from the 16th International Conference on Phosphorus Chemistry (July 2004, Birmingham, UK) appeared in a special issue of the Journal of Organometallic Chemistry (2005, 690, isssue 10) and a special issue of Tetrahedron: Asymmetry (2004, 15, issue 14) was devoted to the chemistry of chiral phosphorus ligands.

As in recent years, the drive for improved performance in transition metal ion-catalysed processes has continued to stimulate the synthesis of new types of organophosphine and tervalent phosphorus-ester and -amide ligands. Activity in the chemistry of heteroaromatic phosphorus ring systems and low-coordination number p π -bonded systems remains at a high level. New mechanistic insights into the Mitsunobu and Staudinger reactions have appeared, and interest in synthetic applications of Staudinger/Mitsunobu procedures has continued to develop.

The chemistry of phosphine chalcogenides and phosphonium salts has also continued to develop, and although no major advances have appeared, activity remains at a high level. In this volume, we have also provided coverage of the chemistry of phosphorus-based ylides and their applications and it is interesting to note a growing number of papers describing the reactions of ylides under aqueous conditions.

Activity in the area of nucleotide chemistry has remained at a high level, with continuing emphasis on the synthesis of modified nucleotide systems and related prodrugs. Work in the oligonucleotide area continues to expand rapidly, the emphasis being on the synthesis of modified nucleosides and their effects on oligonucleotides. Oligonucleotides are also playing a key part in developments in nanotechnology and single-molecule detection.

The level of interest in P(v) compounds remains quite high as a result of the diversity of their biological aspects and synthetic applications. Both of these areas involve many new important developments. A wide variety of natural and unnatural phosphates, phosphonates and phosphinates have been synthesized and their biological activities evaluated. Some of the compounds involved extensive structural modifications. Several compounds are novel prodrugs and promising antibacterial and anti-tumor reagents. Highlights include a synthetic phosphorylcholine-carbohydrate epitope, a model antigen for the rational design of a novel multipurpose vaccine against respiratory infections, and a novel series of cytostatic serine amide phosphates which are very effective in killing prostate cancer cells with low micromolecular cytotoxicity. The first fluorescent edelfosine analogue with apoptotic activity comparable with that of parent drug has been reported, as has a novel gemicitabinelipid conjugate (NE 06002) that exhibited promising in vivo human pancreatic tumor activity and lower toxicity when compared with Ganzar®. Numerous investigations of phosphate ester hydrolysis and related reactions have been described. They include an alternative mode of lithium-catalyzed cleavage of methyl phosphate that involves hydroxide attack at the carbon atom of methyl phosphate. New very effective metal catalysts for the cleavage of the P–O bond have been discovered. Two catalysts of special interest include a Zn(ii) complex bearing multiple auxiliary organic groups that greatly accelerated the cleavage of bis-p-nitrophenyl phosphate, as well as a Zn(ii) complex containing ligand-based hydrogen bond donors that was more reactive than dinuclear Zn(ii) complexes reported to date. An example of facile detoxification of PS insecticides using an ortho-palladated dimethylamine complex as catalyst was described. There have been interesting studies of biological phosphoryl transfer reactions. Special attention has been paid to the synthesis of all types of amino acids and related compounds. It has been shown that phosphinoylimines have substantial potential as useful synthetic intermediates. In addition a review has appeared covering phosphinoylimines as reactive intermediates in stereoselective organic synthesis. There have been many reports of asymmetric synthesis. Highlights include the use of a chiral catalyst based on a phosphoric acid with different functionalized binaphthylene systems in order to direct alkylation of α-diazoester via C–H bond cleavage, also a new asymmetric entry to the aza-Friedel-Crafts alkylation of furan, as well as cyclopropanation of alkenes. All these reactions proceed in highly enantioselective manner up to 99%. Enantioselective fluorination of β-keto phosphonates was catalyzed by chiral palladium complexes and construction of optically active CF3-containing quaternary carbon centered on a stereospecific SN2′ reaction. A number of other new and improved stereoselective synthetic procedure have also been elaborated.

Over the past year there has been increased interest in the role of hypervalent phosphorus compounds in phosphoryl transfer mechanism and the biological importance of hypervalent phosphorus intermediates is gaining wider support. In this respect, there have been reviews by R. R. Holmes, together with a number of theoretical studies, which include the role of solvation and protonation on the stability of phosphorane intermediates. The role of magnesium ions has also been studied. The energies and modes of pseudorotation of biologically important phosphoranes have also been determined. It has been concluded that the lifetimes of phosphoranes and thus occurrence of pseudorotation is pH dependant. There have been some interesting studies of Wittig reaction intermediates and a preliminary report on an imaginative solid phase peptide synthesis procedure based on bicyclic phosphoranes. A new and very useful method for deoxygenating alcohols has been reported. Further studies of the use of hexacoordinate phosphate anions, such as BINPHAT and TRISPHAT in stereochemical investigations include a review. The mode of their interactions with cations has been studied by various NMR techniques and other physical methods. Both enantiomers of TRISPHAT have been isolated, the process being readily scaled up. There have been a number of papers on phosphoranyl free radicals both with respect to their stability relative to carbon radicals and applications such as free radical polymerisation. It has been suggested that four membered cyclic phosphines may provide an alternative route to polymeric phosphines. The keen interest in phosphatranes has continued. There have been reviews of proazaphosphatranes as strong bases and of In-Out isomerism of bridge-head caged compounds.

In the phosphazene area there have been three reviews of the very important Staudinger reaction. Amongst several studies, a mechanistic investigation showed that the overall reaction rate is second order with the phosphoryl oxygen being derived from the water. A reaction intermediate was isolated using anhydrous reaction conditions. In the metal complex area, the synthesis of a new class of aryl ruthenium compounds involved migration of an aryl group from nitrogen to carbon. Incorporation of an anionic aluminium atom into the phosphazene ring produced the first example of a zwitterionic phosphazene. An interesting study showed that a zinc cyclophosphazene complex is an effective biocatalyst and can be considered as a synthetic phosphoesterase and nuclease. It has been found that the thermal and mesomorphic properties of hexa(organo)-substituted cyclotriphosphazenes vary markedly with the number of alkoxy chains. Azido cyclophosphazenes have been found to possess quite high positive heats of formation making them good candidates as high energy materials. Host guest relationships involving tris(o-phenylenedioxy)cyclotriphosphazenes are still of considerable interest and it has been found that carbon dioxide and methane show a preferential inclusion over hydrogen. The useful properties of sulphonated polyphosphazenes have promoted a number of investigations that include direct sulphonation. It has been shown that blends of sulphonated polyphosphazenes with polyfluoroalkylpolymers or polybenzimidazole have potential use as membranes in fuel cells. In the case of blends of protonated polyphosphazenes and poly(p-phenylenesulphide), their conductivity was the centre of interest. There has been much interest in the biological properties of polyphosphazenes. Thus in vivo trials of a polyphosphazene-bound Cisplatin exhibited a much higher anti-cancer activity that the non-bound drug. Also in the search for a control of cell adhesion for neuroblastoma, a series of ether derivatives of polyphosphazenes showed useful properties. In addition, coating vascular stents with trifluoroethoxy derivatives has been shown to reduce thrombogenicity.

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