Current Organic Chemistry - Volume 5, Issue 7, 2001

Development of new methodology for the preparation of enantiomerically pure compounds (EPCs) is at the forefront in the realm of synthetic chemistry in the 21st century. This is partly in response to the requirement for pharmaceuticals to be single entities. The preparation of both enantiomers of a target molecule is a challenging endeavor. Of a plethora of methodologies available for the preparation of EPCs, catalytic methods using metal salts as Lewis acids in conjunction with chiral ligands have received the most attention. A variety of simple, complex and polymeric ligands have been Developed for use in asymmetric transformations in which Lewis acids or transition metals play a key role. Traditional methods for the synthesis of enantiomeric series of products require chiral starting materials with opposite configurations. However, some chiral sources, for instance sparteine, sugars, amino acids, etc., only one.enantiomer is readily accessible. Therefore, the development of general strategies to produce enantiomeric products from a single chiral starting material is important. In principle, access to enantiomeric series of products during chiral Lewis acid catalysis can be achieved by careful manipulation of the important components of the reaction: Lewis acid, ligand, and the substrate. Additional factors such as additives and reaction conditions also play a role. Strategies, which have been examined in detail to access enantiomeric products, are:Control of architecture of the ligand-Lewis acid-substrate complex (geometry control)Structural modifications of the ligand Modification of the substrate using different achiral templates The use of additivesThis review will focus on recent progress of such examples in asymmetric catalysis. It is a compilation of results from various labs organized according to bond formations

eta6-Arene ruthenium (II) complexes are versatile and potentially valuable synthetic Intermediates that have seen applications in diverse areas of organic chemistry. Though not as widely utilized as other arene metal complexes, arene ruthenium derivatives exhibit unique and useful reactivity patterns, and are typically easily handled air- and moisture-stable materials. This review summarizes the salient features of three common types of arene ruthenium (II) complexes: piano stool complexes, bis (arene) complexes, and (arene)Ru(II) cyclopentadienyl complexes. Typical preparative methods and common reactions of these ruthenium derivatives are presented. In addition, selected applications of arene ruthenium (II) complexes in organic chemistry are discussed. These applications include the use of arene ruthenium (II) complexes in “bioorganometallic” chemistry, as intermediates in the synthesis of macrocyclic peptides, as models suitable for mimicking the action of hydrotreating catalysts, as intermediates and constituents of organometallic polymers, and as building blocks in supramolecular chemistry.

Cobalt complexes are now used to mediate a host of organic reactions. This review Attempts to cover advances in this field, which have occurred in the last five years except where other reviews have been published more recently. In those cases where reviews have been published recently, the literature since the last review has been covered. Reaction coverage includes, but is not limited to Diels-Alder reactions, cobaloxime pi-cation mediated cyclizations, the Pauson-Khand reaction, cobalt mediated (2 + 2 + 2) and related higher order couplings, the Nicholas reaction, and cobalt mediated oxidations and reductions.

The chemistry of propargyldicobalt cations, also known as the Nicholas reaction, is Reviewed, with a focus on the developments since 1995. Advances in the understanding of the fundamental properties, such as structure, stability, and reactivity, of both the hexacarbonyl complexes and those bearing other ligands are discussed. All reactions involving propargyl cation dicobalt complexes are covered, including those stemming from ionization of propargylic leaving groups and those created by electrophilic addition to enyne complexes. Migration reactions involving either initiation or termination by propargyl cation complexes are included, as are the genera-tion and reactions of propargyldicobalt radicals. Cyclization reactions employing these cations have received much attention, in cases with the alkynedicobalt unit located in both exocyclic and endocyclic positions, and these reports are described. Particular attention is paid to preparation of medium ring cycloalkyne complexes and their heterocyclic analogues. In addition, there is discussion of the progress in the in selectivity of these reactions, especially in terms of introduction of asymmetry at the propargylic site. Finally, recent applications of Nicholas reaction chemistry in the synthesis of natural products and Related compounds are reported.

The (4+3)cyclocoupling reaction of allyl cations to 1,3-dienes is an efficient and easy route for the stereoselective synthesis of seven-membered ring compounds. Annulations using these species have opened easy access to a wide range of organic frameworks that have been utilized in the synthesis of natural products, pharmacologically active compounds and key intermediates for various useful organic and alkaloid compounds.