Current Organic Chemistry - Volume 8, Issue 9, 2004

An overview of recent research on isopolyhalomethanes is presented that examines their formation, structure, properties and cyclopropanation reactions with olefins. Timeresolved experimental results show isopolyhalomethanes are formed vibrationally hot within a few picoseconds following solvent induced geminate recombination of the initially produced haloalkyl radical and halogen atom fragments of the parent polyhalomethane molecule photodissociation. The carbenoid behavior of the CH2I-I isomer toward C=C bonds can be largely attributed to the CH2I+I- radical ion pair character that activates the CH2I moiety and the sp2 bonding character of the C atom in the isopolyhalomethane species. Time-resolved resonance Raman and density functional theory studies on selected isopolyhalomethanes and their cyclopropanation reactions with olefins indicates the isopolyhalomethane lifetime and chemical reactivity relative to the intitially formed haloalkyl radical fragments helps to determine the efficiency of the isopolyhalomethane cyclopropanation reactions. The chemical reactivity of the CH2I-I and the classic Simmons-Smith IZnCH2I carbenoids towards olefins can be largely explained by their differing energetics for releasing the leaving group, steric demands and electrophilic character of the methylene group to be transferred.

This article summarizes the recent (1999 to mid-2003) advance of intramolecular aryl radical reactions in the synthesis of nitrogen- and oxygen-containing heterocyclic molecules including natural products and other biologically interesting compounds.

This review summarises the development of the intramolecular Heck chemistry from its first report in 1977 to the present status of the reaction as a versatile methodology for the construction of carbocycles and heterocycles. Examples of the application of intramolecular Heck cyclisations as the key steps in the preparation of many complex natural product syntheses are given. The development of the intramolecular asymmetric Heck (IAH) reaction will also be detailed. The development of the IAH is summarised by focusing on the optimisation of palladium catalysts derived from a range of diphosphine and phoshinamine ligands for the preparation of a range of cyclic products, e.g. cis-decalins, hydrindans, indolizidines, diquinanes and the synthesis of quaternary carbon centres. The exploitation of this methodology to natural product synthesis will also be reviewed. Mechanistic insights (neutral versus cationic pathway) will be discussed. New ligands for the IAH will be described and will include recent results from our own work in the IAH.

The catalysis of organic reactions by gold-catalysts has received in the past only little attention because of the preconceived opinion that gold is expensive and inert. The successful application of supported gold catalysts to pollution abatement changed this assessment and spurred on the explosion of interest in gold catalysis towards organic synthesis. Supported Au catalyst and homogeneous gold(III)- catalysts are effective in oxidation processes based on the most attractive oxidant, O2 / air, that proceed at a satisfactory rate at room temperature, selectively, and without rapid catalyst decomposition. Gold (III) salts are very effective catalysts for amination reaction of carbonyl compounds. The regio- / chemoselective interaction to the acetylenic bond is another of the more interesting features of gold catalysis. Activation of alkynes towards nucleophilic attack proved that gold salts are soft, exceptionally carbophilic Lewis acids. The dual role of Au(III) as a Lewis acid and as a transition-metal catalyst accomplished the synthesis of a variety of heterocycles through domino reactions.

Selenabenzene is one of the heterobenzenes in which a selenium atom has replaced a carbon atom of benzene. Selenabenzenes and their benzo-analogs stabilized by an electron-withdrawing group were synthesized in order to study their structures, properties, and reactivities. Cyano-substituted 1- and 2-selenanaphthalenes were isolated as orange crystals, while selenabenzenes with an electron-withdrawing group were too unstable to be isolated at room temperature. Selenabenzenes bearing two stabilizing substituents were recently prepared. The structure of 2,6-dibenzoyl-1-methylselenabenzene is discussed based on IR and NMR spectral data and X-ray crystallographic data. The thermal reaction of selenabenzenes and selenanaphthalenes caused the rearrangement of the Se-alkyl group and dealkylation. They reacted with oxygen to form oxygenated ring-opened products. The reaction of selenanaphthalenes with electron-deficient alkenes and alkynes gave novel ring-transformation products. Selenabenzene derivatives were ylide compounds and changed into selenonium salts on treatment with an acid.

Several methodologies based on the aza Wittig-type reaction of bis(iminophosphoranes) have been used for the preparation of a wide variety of nitrogen-containing heterocyclic ring systems. The resulting aza Wittig products, functionalized heterocumulenes, undergo cyclization either by electrocyclic ring-closure, intramolecular amination, [2+2] cycloaddition or intramolecular Diels-Alder cycloaddition. The synthetic utility of bis(iminophosphoranes) when the two iminophosphoranes groups show different reactivity (N-iminophosphorane > C-vinyliminophosphorane > C-aryliminophosphorane) toward the same functionality is also reported.