Current Organic Chemistry - Volume 12, Issue 16, 2008

The issue number 16 of Curr. Org. Chem. 2008 (volume 12) features four reviews. Their subjects are iron catalysis in organic synthesis, chiral diaminocarbene metal complexes for asymmetric catalysis, the reactivity of early transition metal M-C bonds with N-heterocycles, and the Ti- or Zr-mediated or catalyzed umpolung reactions. Eike B. Bauer (University of Missouri at Saint Louis, USA) contributed the paper Recent Advances in Iron Catalysis in Organic Synthesis, which deals with developments in the field of homogeneous catalysis with iron compounds since 2004, avoiding overlap with previous reviews. C-C forming reactions (including cyclizations, cyanohidrin formation, and allylation reactions), formation of C-O and C-N bonds, reactions involving sulfur, polymerizations, redox reactions, and biomimetic oxidations are included. The catalysts range from simple mixtures of FeCl3 and ligands such as amines to well-characterized complexes, including porphirine complexes and organometallic well-characterized complexes. Ligands used in combination with iron sources include phosphines and N-heterocyclic carbenes as well as polymer-inmobilized ligands. For all these transformations, advantages of iron-based catalysts over more widely used systems are low price and low toxicity. David R. Snead, Hwimin Seo and Sukwon Hong* (University of Florida, USA) wrote Recent Developments of Chiral Diaminocarbene-Metal Complexes for Asymmetric Catalysis. N-heterocyclic carbenes are the type of ligands deserving more attention in recent times, due to their desirable properties in catalysis. This paper reviews, with special attention to the most recent literature not covered in previous reviews, asymmetric reactions catalyzed by transitions metal complexes of chiral monodentate or bidentate NHC (and some isolated example of non-cyclic analog) ligands. These are classified by metal (Ru, Rh, Ir, Pd, Cu and Ni) and, within each metal, reactions type. Some of these reactions show excellent enantioselectivities. Paula L. Diaconescu (University of California at Los Angeles, USA) is the author of Reactions of Early Transition Metal- Carbon Bonds with N-Heterocycles, a review divided into two sections: (a) reactions of early transition metal (groups 3-5 and fblock metals) complexes featuring M-C σ bonds leading to ortho-metalation or insertion to yield functionalized heterocycles, and (b) heterocycle ring-opening via reaction with metal complexes containing metal-element multiple bonds. The interest in these processes stems from the search of methods that allow heterocycle functionalization, and from the difficulty of cleaving the C-N bonds of nitrogen heterocycles, a process that takes place during industrially-relevant, heterogeneously-catalyzed hydrodenitrification (HDN), required to avoid catalyst poisoning in subsequent industrial steps and generation of toxic nitrogen oxides. Hairong Guan (University of Cincinnati, USA) is the author of Titanium and Zirconium Mediated or Catalyzed Umpolung Reactions, centered on polarity reversal of carbonyl and imine derivatives. Acyl anions, a-amino carbanion or a-hydroxy carbanion equivalents, homoenolates and homoenolate equivalents are the most useful organometallic reagents in C-C bond forming reactions. Due to the high electropositive character of Ti and Zr, the carbanionic character of the metal-bonded carbon overcomes the electronegativity of N or O atoms, achieving the polarity reversal. The selectivity (chemo-, regio- and stereo-) is highlighted as an advantage of some reactions. I am sure that, thanks to the great job of the authors and the help of the anonymous reviewers, these reviews will be of interest to the broad readership of Curr. Org. Chem.

Transition metal complexes play a vital role as catalysts in organic syntheses, enhancing selectivity and rates of many transformations. Typically iron is not the first choice when it comes to development of novel catalyst systems. However, iron has a number of advantages to other transition metals typically used in catalysis: iron is cheap, non-toxic, environmentally friendly and abundant. Accordingly, an increasing number of publications describe novel applications of iron catalysts in organic synthesis. The review article covers recent developments in the field since 2004. Iron catalysis in carbon-carbon bond formations, cyclizations, carbon-heteroatom bond formation, oxidations, biomimetic oxidations, reductions and polymerizations is summarized. The article demonstrates the versatility of iron in organic synthesis and the vigorous research activities in the area.

N-heterocyclic carbenes (NHCs) serve as highly proficient ligands in many transition metal catalyzed reactions. As such, significant efforts have been devoted to the extension of NHCs into the field of asymmetric catalysis. Development was slow at first but is now rapidly accelerating, evidenced by the increase in number of transformations displaying excellent enantioselectivities. Chronicled here are the attempts at asymmetric catalysis with NHC ligands, and the results are categorized by the metal effecting catalysis as well as the type of reaction. Ruthenium, rhodium, iridium, palladium, and copper complexed with NHCs have all demonstrated excellent enantioselectivities in catalysis, and fundamental reactions such as olefin metathesis, alkene hydrogenation, kinetic resolution of secondary alcohols, and conjugate addition have been carried out with high selectivity.

Heterocycles are ubiquitous in biological compounds and methods to functionalize them have been of long standing interest. Reactions of early transition metal-carbon bonds with heterocycles are important in two major areas: (1) activation and functionalization of heterocycles and (2) ring-opening of aromatic heterocycles. In the first area, metalcarbon σ bonds determine ortho-metalation and insertion of unsaturated substrates to yield functionalized heterocycles. In the latter area, metal-element multiple bonds become important and provide the driving force for ring-opening reactions. This review will focus on reactions of nitrogen-containing heterocycles. Early transition metals are defined as elements belonging to groups III-V, the lanthanides, and the actinides; the reactivity of complexes featuring the metal in its highest oxidation state (d0fn electron count) and a carbon-based ligand will be described.

This review presents current strategies of using titanium and zirconium reagents to achieve umpolung of carbonyl and imine functionalities. The reactions of organometallic compounds, including those that are equivalent to acyl anions, α-amino carbanions, α-hydroxy carbanions, and homoenolates, are among the most powerful methods to form carbon-carbon bonds. The remarkable chemoselectivity, regioselectivity, and stereoselectivity observed in many cases once more demonstrate great synthetic application of organotitanium and organozirconium compounds.