Current Organic Chemistry - Volume 11, Issue 1, 2007

After the success of the 1st Organophosphorus (OP) special issue of Current Organic Chemistry consisting of two parts published back to back it is my pleasure to announce the 2nd. OP special issue approved by the Editor-in-Chief, Professor Attaur- Rahman and the management of Bentham. The first three papers cover relatively large molecules, such as phosphorus-containing cyclodextrins, calixarenes and cyclophanes, as well as dendrimers. The phosphorus-containing macrocycles form an important group due to supramolecular host-guest interactions allowing enantioselective syntheses based on chiral discrimination and making possible selective extractions and application as sensors. Phosphorus containing dendrimers represent a marvellous field involving the features of art and a real architecture. The next article is on the fragmentation and rearrangements of -hydroxyiminoalkylphosphonates. The generation of metaphosphates is important from the synthetic point of view as the low-coordinate P-species are efficient phosphorylating agents; nevertheless the mechanism for the fragmentation is also of interest. The rich chemistry of N-vinylic phosphazenes is reflected in the next contribution, as they can be excellent building blocks in the synthesis of a variety of acyclic and heterocyclic compounds. The issue of stereogenic phosphorus remains an evergreen topic from the point of view of chirality, especially if it is discussed in a broader context including tricoordinated-, tetracoordinated-, pentacoordinated- and hexacoordinated phosphorus. Two papers cover the chemistry of sterically congested phosphorus compounds, such as tert-butyl substituted phosphinic- and phosphinous acid derivatives, as well as crowded triarylphosphines. The presence of bulky P-substituents is important from stability and mechanistic points of view, but may bring about advantages connected with the physical properties of the substrate. The next three articles fall in the scope of P-heterocyclic chemistry, but from entirely different approaches. Recent developments of the family of azaphospholes, that can be regarded as a modification of the most common P-heterocycles, phospholes, are discussed. Then an up-to-date method for the synthesis of P-ligands applying zirconium reagents is shown and this is followed by the presentation of chiral heterocyclic phosphines together with asymmetric catalytic applications. Finally, the phosphorus aspects of green chemistry have been surveyed. The use of quaternary phosphonium salts and 1,3- dialkylimidazolium hexafluorophosphates are summarized including the major synthetic applications. The 1. Organophosphorus Special Issue has been divided into two parts published back to back. The six reviews included in this issue (Part II) have been preceded by six other articles in Volume 10, Number 18 (Part I). The previous papers are the following: Organophosphorus Supramolecular Chemistry; Part 1. Receptors for Organophosphorus Compounds by ukasz Berlicki, Ewa Rudzi ska, Piotr M ynarz and Pawe Kafarski Phosphorus-Containing Chiral Macrocycles by Sergey Cherenok, Jean-Pierre Dutasta and Vitaly Kalchenko Synthetic Pathways towards Phosphorus Dendrimers and Dendritic Architectures by Anne-Marie Caminade, Cedric-Olivier Turrin, Regis Laurent, Alexandrine Maraval and Jean-Pierre Majoral Fragmentations and Rearrangements of α-Hydroxyiminoalkylphosphonates by Eli Breuer N-Vinylic Phosphazenes. A Useful Tool for the Synthesis of Acyclic and Heterocyclic Compounds by FranciscoPalacios, Domitila Aparicio, Gloria Rubiales, Concepción Alonso and Jesus M. de los Santos Stereogenic Phosphorus by Robert Engel and JaimeLee I'olani Rizzo

Basic synthetic procedures commonly applied for the preparation of racemic and optically active phosphinic and phosphinous acid derivatives containing a t-butyl group as a single bulky substituent are presented. Mechanistic and stereochemical aspects of the presented protocols are discussed. Structural studies, which include the X-ray and vibrational circular dichroism-based determinations of the absolute configuration at the stereogenic phosphorus atom and calculations of conformational equilibria for a few model compounds are also mentioned. The use of the optically active tbutylphenylthiophosphinic acid as a chiral solvating agent is illustrated.

Crowded triarylphosphines have attracted broad interest for a long time since they have unique structure, properties, and reactivity. This review article focuses on synthesis, structure, properties, and application of crowded triarylphosphines represented by trimesitylphosphine and tris(2,4,6-triisopropylphenyl)phosphine, which are of interest from contemporary viewpoint of material science rather than ligands for transition metal catalysis. Synthesis of the crowded triarylphosphines by reaction of various aryl organometallic reagents with phosphorus halides, structures of the crowded triarylphosphines in solid and solution, redox properties and structure-property relationship, cation radicals of crowded triarylphosphines, and construction of the functional molecules carrying crowded triarylphosphines are described.

Azaphospholes are 6π aromatic heterocycles incorporating at least one σ2, λ3-phosphorus atom and one σ3, λ3- nitrogen atom. This review covers the synthesis, structural characterization, reactions and theoretical calculations of azaphospholes anellated to a six - or five-membered heterocyclic ring, reported during the last 10-12 years. Three synthetic methods, namely [4+1] cyclocondensation, [3+2] cyclocondensation and 1,5-electrocyclization make a variety of these phosphorus heterocycles available. The aromatic character of these compounds has been confirmed by carrying out density functional theory (DFT) calculations. Mainly 1H, 13C and 31P NMR studies have been employed for the structural characterization. Almost all the functionalities present in the azaphosphole ring have been investigated and electrophilic substitution reactions of the 1-unsubstituted compounds and Diels-Alder reactions as well as 1,2-addition reactions to the >C=P- moiety have been accomplished successfully. The stereo- and regioselectivities observed in the Diels-Alder reactions have been explained on the basis of DFT calculations. Cheletropic cycloadducts formed from [4+1] cycloaddition of o-quinones on the σ2, λ3-P atom have been obtained. A variety of coordination compounds formed by σ- coordination of the σ2, λ3-P atom have been prepared.

The use of several zirconium reagents as the Schwartz reagent [Cp2ZrHCl]n, the dimethylzirconocene Cp2ZrMe2, the zirconocene [Cp2Zr], and the diphenylzirconocene Cp2ZrPh2 allows the synthesis of a variety of new linear and cyclic P-C and P-N ligands. 1,1 diphosphines can be prepared from unsaturated monophosphines. A useful methodology involving ring opening reactions permits the preparation of phosphinite-phosphine and bis-phosphinite ligands. N-functionalized imines and β-diimines substituted with phosphino groups are obtained from cyanophosphines or dinitriles. Carbozirconation of chlorophospha-imine leads to the unprecedented P-metalated iminophosphorane which presents a versatile reactivity towards electrophiles and small unsaturated molecules. Constraint three and four phosphorus cycles can be easily isolated using [Cp2M] (M = Zr or Ti). Interactions between benzynezirconocene and unsaturated phosphines afford numerous methods to prepare linear and cyclic phosphorus reagents. Enantiomerically pure iminophosphines can be synthetized from phosphinozirconocene complexes.

The synthesis of chiral heterocyclic phosphines, such as phosphiranes, phosphetanes, phospholes and phosphinanes will be reviewed. These compounds play a pivotal role as ligands and organocatalysts, respectively, in asymmetric reactions. Synthetic approaches will be analyzed in the framework of several sub-categories e.g. the preparation of mono-, bis- and poly(phosphines). General synthetic strategies for the preparation of large families of related compounds will be considered such as 1.) stereoselective -alkylation of phosphine oxides or phosphine boranes, 2.) ring closure by substitution reactions and 3.) hydrogenations of unsaturated P-heterocylic compounds, followed by epimerization reactions. Besides these general methodologies also protocols leading to a single phosphine will be discussed. In some cases also the preparation of such phosphines will be presented, which have not yet been used in asymmetric catalysis, however, which have - owing to their structure - some relevance to synthetic methodologies or compounds discussed herein. For most phosphines presented applications in asymmetric catalytic reactions will be briefly mentioned together with characteristic enantioselectivities achieved.

The search for suitable ionic liquids that are non-volatile, non-toxic, dissolve a variety of reactants and are recyclable is a real challenge for chemists to be able to replace the traditional solvents that are volatile, flammable and toxic. This segment of chemistry has, these days, become a firm part of green chemistry attracting much attention. The present review summarizes and critically discusses the data accumulated in recent years on the synthesis and utilization of phosphorus containing ionic liquids, such as imidazolium hexafluorophosphates and phosphonium salts. As regards the preparation of ionic liquids, the key steps are quaternization and the introduction of the corresponding anion. Physical properties of the ionic liquids can be fine-tuned by selecting suitable cation-anion pairs. The reactions studied in ionic liquids embraces catalytic hydrogenations, transfer hydrogenations, oxidations, hydroformylations, carbonylations, alkylations, acylations, nucleophilic substitutions, halogenations, condensations, Diels-Alder reactions, Michael additions, coupling reactions and transformations in organophosphorus chemistry. Ionic liquids are especially suitable to accommodate catalytic reactions where one of the components is a gas. Stress was laid to highlight the chemo-, regio-, diastereo- and, especially, the enantioselectivity of the reactions performed in ionic liquids. The data on the ecotoxicity of imidazolium hexafluorophosphates and phosphonium salts are briefly discussed.