Current Organic Chemistry - Volume 9, Issue 7, 2005

Advances in transition metal (TM)-catalyzed organic reactions are occurring at a very fast pace, and staying current with the primary literature is a formidable task. Besides, new developments are often published in the form of communications where the range of substrates and experimental conditions are necessarily narrow. Therefore, medium-sized reviews written by practitioners critically summarizing the recent advances in a given area are invaluable. It has been my aim to provide the readership of Curr. Org. Chem. with a selection (admittedly biassed by personal preferences) of topics within the realm of TMcatalyzed organic reactions that can be useful to many and convey where the borders are (or were, at the time manuscripts were finished!) in this fruitful and exciting area. Recent advances in TM-catalysis are highly influenced by general concerns such as atom and energy economy, cleaner (more efficient and selective, with greener solvents) syntheses, use of abundant yet less reactive feedstocks, etc. In this context, Kevin H. Saughnessy and Rebecca B. DeVasher (University of Alabama, USA) reviewed Pd-catalyzed cross-coupling reactions in water (2000-late 2004 period), and Janis Louie (University of Utah, USA) describes recent progresses in TMmediated reactions of carbon dioxide (excluding reductions or reactions that need an oxygen sink) and other heterocumulenes. I have mentioned the fast pace at which developments take place in TM-catalyzed organic reactions. The article by John P. Wolfe and Jennifer S. Thomas (University of Michigan, USA) is restricted to recent (January 2003- July 31, 2004) Pd-catalyzed heterocycle synthesis and functionalization. And it contains nearly 200 references!. Jason A. Halfen (University of Wisconsin-Eau Claire, USA) also needed to impose a tight time restriction to his review of C-N bond formation via aziridination and amidation. Current efforts in this area include the application of “new” metals, such as iron, the tailoring of new ligands to improve the performance of “old” metals such as copper, and an intense search for new nitrene sources. The status of C-H bond activation is changing from “Holy Grail” to that of a promising tool in organic síntesis, and the area has been the subject of a number of recent reviews. Laurel A. Goj and T. B. Gunnoe (North Carolina State University, USA) focused on TM-catalyzed activation of aromatic C-H bonds in reactions leading to hydroarylation of multiple bonds or oxidative coupling of aromatics and unsaturated substrates. Christopher M. Vogels and Stephen A. Westcott (Mount Allison University, Canada) describe recent advances in metal-catalyzed hydroboration. A few weeks after their paper was finished, the chemical community learned with sadness the passing away of H. C. Brown (Nobel laureate in 1979), whose seminal work in the area dates back to half a century ago. Hydroformylation is an even older reaction (and one for which rhodium is also the metal of choice). Whereas the linear version, a topic dealt with in many organometallic textbooks, remains the preferred one in bulk chemicals production, branched selective hydroformylation, the subject of the review by Matthew L. Clarke (University of St. Andrews, UK) is useful for asymmetric synthesis. In addition, the review will be useful for for any chemist interested in recent developments in phosphine syntesis. Joseph M. Fox and Ni Yan (University of Delaware, USA) discuss, with an emphasis on regio- and stereochemical aspects of the reaction, metal-promoted or catalyzed nucleophilic additions to cyclopropenes, one of the ways to construct cyclopropane rings. As the authors point out, the area is still particularly rich in challenges. I thank the authors for their excellent work, which I hope the readers will find both instructive and inspiring. Also, my gratitude to the anonimous reviewers for their helpful advice.

Water has attracted significant attention as an alternative solvent for homogeneous metal-catalyzed reactions because it is a non-toxic, nonflammable solvent that can allow for simplified recovery of homogeneous catalysts. The use of water as a solvent in palladium-catalyzed cross-coupling reactions is of great academic and industrial interest. Palladium-catalyzed coupling reactions are powerful methods to make carbon-carbon and carbon-heteroatom bonds under mild conditions. Separation of the catalyst from the organic product can be very difficult; however, constraining the catalyst to the aqueous-phase of an aqueous-biphasic solvent system can potentially simplify catalyst recovery. This review will focus on recent developments in the design and application of aqueous-phase, palladium-catalyzed coupling reactions over the period from 2000, through late 2004. A variety of new hydrophilic ligand architectures have been introduced recently that provide significantly more active catalysts towards industrially relevant aryl bromides and chlorides.

This review presents the recent progresses in the transition metal mediated synthetic transformations of heterocumulenes (e.g. carbon dioxide, isocyanates, carbodiimides, ketenes, etc.). Organic transformations featured include cycloadditions, nucleophilic addition, metathesis reactions, and polymerizations. Particular emphasis is placed on catalyzed reactions although in some cases, stoichiometric reactions are also described.

This review summarizes the recent developments in palladium-catalyzed heterocycle synthesis that have been described in the literature from January, 2003 to July 31, 2004. The strategies described herein include heterocycle synthesis via either carbon-carbon or carbon-heteroatom bond-forming ring closure, annulations and cycloadditions that form two bonds in a single process, and methods that permit the elaboration of existing heterocyclic frameworks.

This review describes recent progress in the development of homogeneous, metal-mediated, catalytic pathways to synthetically valuable aziridines and sulfonamides. Areas of investigation described within this review include: (a) catalytic copper, iron, and silver-mediated nitrene transfer from the iminoiodinanes to olefins, (b) development of alternative nitrene sources for olefin aziridination, including N-halo-p-toluene sulfonamides, iminoiodinanes featuring labile N-protecting groups, and sulfonylazides, (c) use of single-pot aziridination protocols for the in situ generation of iminoiodinanes, and (d) metal-mediated carbene transfer to imines.

Catalytic carbon-carbon bond formation using transition metal complexes typically incorporates the use of an aryl halide or triflate compound (electrophile) in combination with an organometallic reagent (nucleophile). Versatile synthetic methods that allow C-C bond formation of aromatic compounds that proceed through transformation of C-H bonds would be of substantial value. Potential routes for C-C bond formation with aromatic substrates include the hydroarylation of unsaturated compounds (e.g., olefins or alkynes) and oxidative coupling of aromatic compounds and olefins or alkynes. Such reactions can proceed through metal-mediated transformation of C-H bonds. The range of reported catalysts and catalytic cycles for these classes of reactions have been reviewed herein.

Among the most significant advances in organic synthesis over the past few decades are those associated with the development of transition metal complexes for their use as catalysts in organic reactions. The discovery that transition metals can also be used to catalyze the addition of B-H bonds to unsaturated substrates has provided the synthetic community with another valuable tool. This review examines recent advances in the field of catalyzed hydroborations and their applications in organic synthesis.

Hydroformylation reactions that show branched regioselectivity are reviewed. The mechanism of hydroformylation and the current state of the art in linear selective hydroformylation is briefly discussed prior to a more in depth discussion on synthetically useful branched selective hydroformylation reactions. The review divides these into several different classes; hydroformylation of internal cyclic alkenes, acyclic internal alkenes, aryl alkenes, terminal alkenes bearing electron withdrawing groups, terminal alkenes bearing groups that can potentially co-ordinate to the rhodium catalysts, and simple terminal alkenes. Recent advances in asymmetric hydroformylation are also discussed. The review aims to provide the non-specialist and expert reader with the information required to assess the type of substrate, catalyst and reaction conditions that are likely to be conducive to obtaining branched aldehydes in a hydroformylation reaction.

This review describes the literature of nucleophilic additions of organometallic and metal hydride reagents to cyclopropenes with a particular focus on the regiochemical and stereochemical aspects of the addition reactions.