Current Organic Chemistry - Volume 12, Issue 14, 2008

The present issue of Curr. Org. Chem. comprises four reviews. Their subjects are the recent application of “old” compounds, high oxidation molybdenum and rhenium complexes, for reductions, the metal-based methods for the synthesis of conducting polymers, the chemistry of s-block metals, and the complexes formed between halide anions and supramolecular receptors. Guodong Du and Mahdi M. Abu-Omar* (Purdue University, USA) contributed the paper Oxo and Imido Complexes of Rhenium and Molybdenum in Catalytic Reductions. Until recently, high oxidation state metal complexes like the ones dealt with in this review have been used as catalyst for oxidation reactions and atom transfer; however, the impressive results obtained by Toste prompted several groups, including the author's group, to apply these compounds in catalytic hydrosilylation and hydrogenation reactions of unsaturated organic substrates. A variety of mechanistic pathways (investigated both by means of kinetic experiments and theoretical calculations) have been found for these reactions, and the catalysts proved to be both versatile and stable (air and moisture tolerant). This review focuses mainly on Mo(VI) and Re(V) complexes with oxo, imido and nitrido ligands, most of which were already well known compounds. Jose Luis Garcia-Alvarez (Hospital Clinico San Carlos, Madrid, Spain) wrote the review Synthesis of Conducting Organic Polymeric Materials Mediated by Metals: How the Organic Chemists Make Some of the Most Advanced Polymeric Materials. The paper, with 151 references, summarizes, from a synthetic perspective, work on the polymerization of acetylenic compounds (including simple acetylenes and diynes), polyaromatic polymers, and polymers in which there is an alternation between an aromatic ring and a vinylidene or ethylidene group. The reactions leading to the formation of polymers can proceed via insertion or metathesis mechanisms. Also, polymers can be synthesized using Ni- or Pd-catalyzed C-C coupling such as Heck, Stille, Sonogashira, Yamamoto, Kumada, and Suzuki reactions. The polymeric products dealt with here have been selected because of their potential usefulness as materials, therefore, not only yields, but also molecular weights are given in many instances. Martyn P. Coles (University of Sussex, UK) wrote Recent Developments in the Synthetic and Applied Chemistry of the SBlock Metals, a concise review with more than one hundred references. The fact that most of them are from the literature of the period 2006-2007 reflects the high current interest in this area. The author selected a number of examples to illustrate some of the main recent advances in the molecular chemistry of the metals of groups 1 and 2, focusing on synthesis of their coordination compounds, their structures, and applications in organic synthesis, both stoichiometric and catalytic. A distinctive feature of this review is that, with a somewhat inorganic perspective, it focuses on well-characterized products, and considers the solid state and (when information is available) solution structure, the nature of the metal-solvent and metal-ligand interaction, the differences between metals within each group, and the effects of these factors on the reactivity. Md. Alamgir Hossain (Jackson State University, USA) has contributed the paper titled Inclusion Complexes of Halide Anions with Macrocyclic Receptors. It focuses in synthetic (i.e., abiotic) receptors of anions (halides) such as protonated macrocyclic amines, quaternary ammonium complexes, polyamides, mixed quaternized amineamides, polythioamides, polypyrroles and receptors that use metal sites to bind anions. For some of these types of receptors, sub-sections are presented for monotopic, ditopic, sandwich, or cascade receptors. These receptors employ positive charge (quaternary ammonium complexes), hydrogen bonds (e.g., polyamides), a combination of both (protonated amines), or metalanion bonds to bind the anionic guests. Abundant structural information of the anion-receptor complexes is provided. It is my hope that, thanks to the great job of the authors and the help of the anonymous reviewers, these papers will be useful to many readers of Curr. Org. Chem.

High oxidation state transition metal oxo and imido complexes are ubiquitous in catalytic oxidations and oxygen atom transfer reactions. Recently these complexes, particularly those of rhenium and molybdenum, have been shown to catalyze reductions of organic compounds bearing C=O, C=N, C=C, and C≡C functional groups. High oxidation state catalysts are convenient because they are often air and moisture stable; they also provide a new paradigm for transition element chemistry that is attractive for in-depth mechanistic investigation. In this review, we summarize the latest development in the field with emphasis on the current understanding of the operative hydrosilylation mechanisms for oxo and imido catalysts.

Since the discovery of the electronic conducting properties of poly(acetylene), many other organic conjugated systems have been studied, having acquired increased importance in the current technology. In this context, organometallic chemistry has played an important role in the catalytic version as well as in the stoichiometric version of their synthesis. In this review, from the first Ziegler catalyst used for Shirakawa to the new palladium based catalysts which allow smooth cross-coupling reactions of complex systems will be discussed. General mechanisms showing how the metal organometallic complexes work in these reactions are presented for a better understanding of why these systems, which have been developed for a general organic synthetic purpose, are used in polymerization.

From an historical perspective, much of the chemistry associated with the s-block elements has centred on the synthesis and structural characterization of an eclectic selection of compounds, with less effort directed towards developing their chemistry with regards to synthetically useful applications. Whilst obvious exceptions to this general statement exist, and the fundamental academic studies are invaluable in their own right, it is only in recent years that a renewed interest in the application of metal-ligand compounds of these elements has been evident. This short review is focussed on progress made in the molecular1 chemistry of the s-block metals, as presented in the literature in the period 2006-2007. Each section is divided into two parts: (i) advances in the synthesis of new complexes and ligand design, leading to a better understanding of the chemistry of these elements is initially presented; (ii) the latter part of each section concentrates on the application of these compounds in a number of synthetically important areas. Clearly with such a broad topic as this, not all of the literature can be covered, and it is important to state that this review does not include the vast literature concerning the organic transformation chemistry of organolithium and related reagents. Where possible, however, the author has selected examples that he feels will be of interest to the readers of Current Organic Chemistry.

This review highlights the binding and structural aspects of halide complexes with synthetic macrocyclic receptors with different functional groups including charged amine (protonated or quaternary form), amide, thioamide and pyrrole. Different binding modes in solution and solid states in terms of monotopic, ditopic, cascade and sandwich complexes of halides are discussed. The sizes and functional groups of the macrocycles, as well as solvents have been shown to play a critical role in binding and selectivity for anionic guests. In general, macrobicyclic or tricyclic hosts tend to form monotopic complexes with anions of comparable sizes, while an expanded cavity is needed for the higher degree of complexation (e.g., ditopic or cascade). On the other hand, the formation of a sandwich complex is favored by monocyclic ligands with hydrogen donor groups.