Current Organic Chemistry - Volume 7, Issue 8, 2003

New efficient synthetic routes for preparation of molecular and macromolecular organosilicon compounds based on silicon-containing alkene metathesis and metathesis polymerization catalyzed by transition metals (W, Mo, Re and Ru), which initate the formation of metal carbenes (ill-defined first generation catalysts), or by metallacarbene complexes (second generation catalysts, most of which are well defined) are reviewed.The group of reactions involve self-metathesis and cross- metathesis of alkenes (ring closing metathesis (RCM) of dienes, tandem ring opening of cycloalkene - cross-metathesis of alkenes (ROM/CM), as well as alkyne and enyne metathesis, which lead to molecular unsaturated organosilicon products. On the other hand, ring opening metathesis polymerization (ROMP) of silaalkenes and acyclic diene metathesis (ADMET) polymerization of siladienes yield linear macromolecular unsaturated organosilicon compounds. Polymer degradation via crossmetathesis with silicon-containing alkenes is also discussed in this review.All these reactions of alkenyl substituted (except vinylsubstituted) silicon compounds proceed according to the generally accepted metallacarbene mechanism. Vinylderivatives of organosilicon compounds while being inert to self-metathesis and ADMET polymerization, in the presence of complexes containing M-H and M-Si bonds (or initiating their formation) undergo effective transformations leading, in most cases, to metathesis-type products according to a non-metallacarbene mechanism.

This review introduces the recent progresses on the fixation of carbon dioxide (CO2) including 1) the reactions of carbon dioxide with alcohols in the presence of base; 2) the reactions of carbon dioxide with amines in the presence of base; and 3) the transition metal catalyzed carbon dioxide insertion reactions. In the section of transition metal catalyzed fixation of carbon dioxide, we mainly focus on the catalytic process of carbon dioxide insertion reactions promoted by metal catalysts and the emphasis is put on the preparation of the corresponding carbonates or carbamates from CO2 and their further transformations.

The radical nucleophilic substitution mechanism or SRN1 is a chain process, in which radicals and radical anions are intermediates. This process has been extensively used to effect substitution on a wide variety of substrates. The SRN1 reaction has been studied from both mechanistic and synthetic standpoints.The SRN1 mechanism requires an initiation step. Spontaneous electron transfer (ET) from the nucleophile to the substrate has been observed in a few systems; light stimulation, electrodes, alkali metals or inorganic salt-mediation is used otherwise.There are systems that are totally inert or undergo rather slow substitutions by classical polar mechanisms. Their lack of reactivity is usually due to strain (cycloalkyl and polycycloalkyl halides), steric (cycloalkyl, polycycloalkyl and neopentyl halides), or electronic factors (unactivated aromatic, vinyl halides and perfluoroalkyl halides). For these families of compounds, the nucleophilic substitution can be accomplished by the SRN1 mechanism. Conversely, there is a class of substrates, for which substitution can be achieved through both polar and ET mechanisms; however, the ET pathway is favored in some systems (i.e.: alkyl halides with EWG).We propose to undertake a compilation and critical review on SRN1 reactions dealing with substitutions on aromatic substrates, cycloalkyl, bridgehead, neopentyl, vinyl halides, perfluoroalkyl iodides, aliphatic substrates with EWG in the α position and N,N-dialkyl-p-toluenesulfonamides. With this aim in mind, we expect to cover recent SRN1 substitutions, with an emphasis on the scope of the process in terms of synthetic capability and target applications.

This article (the third of three parts) reviews the use of 1,3-dipolar cycloaddition reactions of azides, diazo compounds, nitrile imines, azomethine imines and azomethine ylides in the construction of carbohydrate mimics. Preparation of triazole, tetrazole and other modified nucleosides, azasugars, polyhydroxylated alkaloids and carbocycles, pyrazolidine and pyrazoline systems is described. It is organized depending on the dipole employed and subdivided into separate sections according to the inter- or intramolecular fashion applied and the nature of dipolarophiles incorporated. It is mainly dedicated to the synthetic preparations and stereoselectivities observed with limited reports to biological tests and results.