By Transition Metal Catalysts

Coupling of butadiene with CO2 under electrochemically reducing conditions produces decadienedioic acid, and pentenoic acid, as weU as hexenedioic acid (192). A review article on diene telomerization reactions catalyzed by transition metal catalysts has been pubUshed (193). 

Synthesis of N-heterocycles with C—N bond formation catalyzed by transition metal catalysts 97SL749. 

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

The ruthenium-, rhodium-, and palladium-catalyzed C-C bond formations involving C-H activation have been reviewed from the reaction types and mechanistic point of view.135-138 The activation of aromatic carbonyl compounds by transition metal catalyst undergoes ortho-alkylation through the carbometallation of unsaturated partner. This method offers an elegant way to activate C-H bond as a nucleophilic partner. The rhodium catalyst 112 has been used for the alkylation of benzophenone by vinyltrimethylsilane, affording the monoalkylated product 110 in 88% yield (Scheme 34). The formation of the dialkylated product is also observed in some cases. The ruthenium catalyst 113 has shown efficiency for such alkylation reactions, and n-methylacetophenone is transformed to the ortho-disubstituted acetophenone 111 in 97% yield without over-alkylation at the methyl substituent. 

As mentioned above, MPVO catalysts are very selective towards carbonyl compounds. Alkenes, alkynes or other heteroatom-containing double bonds are not affected by these catalysts, while they can be reduced by transition-metal catalysts. Examples of the reduction of a,/ -unsaturated ketones and other multifunctional group compounds are compiled in Table 20.3. 

The reasons for the increasing acceptance of enzymes as reagents rest on the advantages gained from utilizing them in organic synthesis Isolated or wholecell enzymes are efficient catalysts under mild conditions. Since enzymes are chiral materials, optically active molecules may be produced from prochiral or racemic substrates by catalytic asymmetric induction or kinetic resolution. Moreover, these biocatalysts may perform transformations, which are difficult to emulate by transition-metal catalysts, and they are environmentally more acceptable than metal complexes. 

The discovery of the polymerization processes promoted by transition metal catalysts and the foundation of macromolecular stereochemistry represents a major breakthrough in chemistry in the second half of this century. Since the first discoveries by Ziegler in Miilheim and Natta in Milan there has been an enormous development of fundamental and applied research that still continues... 

Unactivated double bonds remote from a primary amino group can be used in cyclizations mediated by transition metal catalysts. Equation (2) shows a palladium(II)-catalyzed prepar-... 

Our literature search on alkene polymerization by transition metal catalysts yielded nearly 20,000 publications since 1993 even after the exclusion of the patents or patent applications, indicating a tremendous interest in this field. Three mutually interconnected trends were identified from the search. 

The initiation of thermo-oxidation of polyethylene films by transition metal catalysts during composting proceeded slowly compared to oxidation at the same temperature in an oven (60-70 °C) [56]. The starch-filled polyethylene bags exposed on the surface of the compost broke down into small pieces while the buried bags remained intact after 49 days of exposure [55,... 

Alkene metathesis, a remarkable reaction catalyzed by transition metal catalysts, can be traced back to Ziegler-Natta chemistry as its origin [11], In 1964, Natta et al. reported a new type polymerization of cyclopentene using Mo- or W-based catalyst, without knowing the mechanism. This was the first example of ring-opening metathesis polymerization (ROMP eq. 1.9) [12],... 

The dimerization of 1 -alkynes to enynes by transition metal catalysts occurs either via alkynyl-vinylidene M(C=CR)(=C=CHR) or alkynyl-alkyne M(C=CR)(if-HC=CR) coupling, by insertion into the a Ru-C bond. Selectivity control depends on the previous orientation of the alkyne/vinylidene moiety. 

Carbon-heteroatom coupling reactions including carbonylation and carboxy-lation polycondensations, promoted by transition metal catalysts, are becoming a promising route for various types of new condensation polymer. 

A comprehensive review on the whole field of polymerization of conjugated dienes by transition-metal catalysts was compiled by Porri and Giar-russo in 1989 [50]. 

Cleavage of carbon-carbon bonds by transition-metal catalysts is one of the major challenges in organic and organometallic chemistry [43]. For that purpose, strained small-ring ketones are useful substrates. Recently, some synthetic... 

The compehtion of one-electron pathways is sometimes detectable in the epoxidations catalyzed by transition metal catalysts [67]. However, in the epoxidahon of unhindered olefins on TS-1, the typical radical products are below the detection limits. Their presence could no longer be neglected when the rate of epoxidation is so slow as to become comparable to that of homolytic side reactions, for example with bulky olefins (see also Section 18.11). It is possible that, within these limits only, the epoxide is produced in part through the addition of a radical peroxy intermediate to the double bond [68, 69]. Even so, a homolytic pathway has again been proposed as a generally vahd epoxidation mechanism [7]. 

Catalytic functions of metal ions and their complexes The homogeneous catalytic isomerisation of olefins by transition metal catalysts Homogeneous catalysis... 

Trost, B. M. Cyclizations made easy by transition metal catalysts. Advances in Chemistry Series 1992, 230,463-478. 

Diazocarboxylate esters can be transformed by transition metal catalysts such as rhodium(II) acetate into alkoxycarbonylcarbenes that undergo a wide variety of synthetically useful C-H, C-C, C-X, X-H and X-X insertion reactions (where X = heteroatom) [99]. Chemoselectivity of rhodium carbenoids derived from Rh(II) carboxylates and carboxamides has been found to exhibit striking ligand dependency, for example in work by Padwa showing that perfluorocar-boxamide ligands exclusively promoted aromatic C-H insertions in Rh(II)-cat-alyzed decomposition of diazoamides to give oxindoles, whereas a carboxylate-based rhodium catalyst promoted other types of insertions and addition reactions [100]. 

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