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AgOTf as catalyst in the tandem acetalization and cycloisomerization reaction shows an inversion of the cycloisomerization regioselectivity from 6-endo-dig to 5-exo-dig with some amine additives.

In order to investigate the possible mechanism(s) by which CC bonds are cleaved or are formed on metallic surfaces, the reactions of linear and branched butenes and butanes over Ru/SiO2 catalysts are studied. At temperatures above 50–100°C and in the presence of hydrogen, 1-butene, cis-2-butene, trans-2-butene, or isobutane simultaneously undergo isomerization (cis-trans isomerization or double-bond migration), hydrogenation, hydrogenolysis, and homologation reactions. In the same conditions, n-butane and isobutane only undergo hydrogenolysis. The influence of the structure of the starting hydrocarbon, contact time, reaction temperature, and hydrogen/hydrocarbon ratio on conversions and selectivities has been examined. Mechanisms for these reactions of hydrocarbons over metallic surfaces in the presence of hydrogen are envisaged using the known concepts of molecular chemistry: the results suggest that metal-alkyl species are key intermediates. Concerning hydrogenolysis and homologation of butenes, it appears that these two reactions occur at comparable rates, which suggests that they are mechanistically related. These reactions involve: (i) the formation of C3 and C1 (probably methylene) fragments from a C4 olefin and (ii) the reaction of the C1 fragments with the starting olefin to give C5 hydrocarbons, in competition with the hydrogenation of these C1 fragments leading to methane. Two simple mechanisms can explain the simultaneous and reversible nature of these two reactions of CC bond cleavage and formation: (i) the insertion-deinsertion of a metallocarbene into (or from) a metal-alkyl species and (ii) the formation-rearrangement of a dimetallacyclopentane intermediate.

The metathesis of acyclic or cyclic olefinic sulfides is successfully achieved by using a chloroaryloxide neopentylidene complex of tungsten as catalyst. This reaction constitutes a new route to the synthesis of various olefins, dienes or unsaturated polymers containing one or more S atoms.

Although Schrock-type tungsten alkylidyne complexes react with inorganic oxides with formation of surface tungsten alkylidene species, the high catalytic activity of W(CCMe3)(CH2CMe3)3/Nb2O5 for the metathesis of Z-pent-2-ene does not seem to be correlated to the concentration of the alkylidene ligand, measured by specific reactions.

At the present time, our knowledge of the mechanism of action of most heterogeneous catalysts is limited. At the most primitive level, only the overall product distribution is known, while the nature of the catalyst-reactant interactions remains obscure. In a few cases, more detailed mechanistic proposals have been advanced, but they remain modest in scope compared to the detailed mechanisms that have been developed in molecular organic chemistry, and more recently, in organometallic chemistry. Indeed, even the apparently well-established catalytic mechanisms (for example, hydrogenation of olefins) are sometimes contested, indicating the fragility of the evidence on which these proposals are based. It must be recognized, however, that by comparison to strictly molecular systems, heterogeneous catalysts are intrinsically much more complicated. In addition, the application of physical methods to structural and mechanistic problems of heterogeneous systems, as well as interpretation of the results, can be difficult.

Various 2-benzylsulfanyl-1,3-oxazolines were synthe- sized and engaged in a copper-promoted palladium-catalyzed cross- coupling reaction with a range of organoboryl and organostannyl re- agents to produce 2-aryl- or 2-hetaryl-substituted oxazolines. Pro- tected and unprotected carbohydrate backbones were shown to be compatible with the reaction conditions. This approach opens a new versatile access to chiral oxazoline structures. The broad applicability of palladium-catalyzed carbon- carbon bond formation stems from the effectiveness of the process, in neutral medium, in either Suzuki, Stille or Hiyama reactions and there lie the main reasons why Pd cross-coupling has emerged over the past years as one of the major tools for the construction of complex mole- cules. 1 In most of these reactions, the electrophilic part- ners are aryl or alkenyl halides or sulfonates which have shown limitations depending on the nature of the sub- strates, mostly due to difficulties in their preparation or because of low stability. Tremendous efforts have been made to overcome this problem 2 and, recently, hetaryl alkylsulfides have been proposed as a new class of elec- trophile. 3 On the other hand, 1,3-oxazolidine-2-thiones (OZT) are non-aromatic heterocycles which have been used in a broad range of applications. 4 For example, these simple structures have been exploited in asymmetric Diels-Alder reactions or in stereoselective alkylations of their N-sub- stituted derivatives. 5 Regioselective S-alkylations of OZT were also the starting point for studying other reactions - namely cyclocondensations, Eschenmoser sulfur extru- sions or C-C bond formation with organolithium and Grignard reagents - in which the S-alkylated moiety was implicated as a leaving group. 6 The palladium cross-coupling methodology was devel- oped by Liebeskind and mostly applied to heteroaromatic templates. 3e-h,7 Applying the process to non-heteroaryl

Regioisomerically pure trifluoromethyl- and trifluoromethoxy-substituted aromatic and heteroaromatic aldehydes and carboxylic acids are valuable building blocks for the synthesis of biologically active molecules. They have been prepared by employing modern organometallic methods. On this basis, a novel access to 2,3-dihydro-5-(trifluoromethoxy)indole was developed which represents an intriguing example of how organometallic and radical chemistry can fecundate each other.