Chemistry metal-mediated

Metal Ion-Promoted Reactions of Thiols. Metal ion-promoted reactions of thiols have been reviewed (53). The bulk of the coverage concerns metal ion promoted aspects of sulfur chemistry. The main topics of interest are the formation of sulfenamides, sulfides, and disulfides using metal-mediated reactions. 

The total synthesis of ( )-estrone [( )-1 ] by Vollhardt et al. is a novel extension of transition metal mediated alkyne cyclotrimeriza-tion technology. This remarkable total synthesis is achieved in only five steps from 2-methylcyclopentenone (19) in an overall yield of 22%. The most striking maneuver in this synthesis is, of course, the construction of tetracycle 13 from the comparatively simple diyne 16 by combining cobalt-mediated and ort/io-quinodimethane cycloaddition reactions. This achievement bodes well for future applications of this chemistry to the total synthesis of other natural products. 

A second route was devised using chiral /3-keto ester 14, which was identified as our precursor for 2 [7]. This idea was in analogy with the carbapenem chemistry [8], as depicted in Scheme 2.4, where Masamune reaction [9] for carbon elongation, diazo-transfer, and transition metal-mediated carbene insertion reaction [10] were employed as key steps sequentially. 

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... 

Although the generation of nitroalkyl radicals by oxidative transition-metal-mediated reactions is known for many years, their application in carbohydrate chemistry was not investigated until recently.41... 

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

Kinetic analyses and deuterium-labeling experiments have demonstrated that, remarkably, the reductive elimination of TEA and the formation of intermediate C is the rate-determining step in the (de)hydrogenation cycle. Accordingly, hydrogenation of the acceptor appears to be slower than dehydrogenation of the alkane substrate. This contrasts with the fact that catalytic olefin hydrogenation is well-established in transition-metal-mediated chemistry [10]. 

Metal-mediated processes represent an important advance in azomethine ylide chemistry, allowing reactions to proceed at ambient temperatures. They also allow for the possibility of chiral catalysis, which is a little explored area as yet, but represents an exciting development allowing for the induction of enantiocontrol without the need for stoichiometric stereocontrolling units. 

Although Grigg et al. (97) have been prolific authors in azomethine ylide chemistry, probably their greatest contribution has been in the development of metal-mediated technologies, leading to elegant solutions to the problems of regio-, stereo-, and enantiocontrol over reactions. Typically, EtaN or DBU deprotonation of amino esters, with subsequent metalation of the anion, led to the formation of stabilized, metallo-1,3-dipolar systems (340), often to the extent where isolation is possible (Scheme 3.115). 

The great diversity of systems in this chapter makes a comparison of synthetic approaches difficult and most of the best synthetic approaches have counterparts in the syntheses of simpler heterocycles. Since the area was last reviewed <1996CHEC-II(7)841>, established methods have continued to be of synthetic utilit r, in particular, the photocycliza-tion chemistry of l,2-bis(heteroaryl)ethene systems and the chemistry of bis-TTFs have been further explored. The focus, however, of these research efforts has tended to concentrate on the applications of these materials in electronic devices with less emphasis on novel synthetic approaches. A noticeable trend has been the increasing use of metal-mediated chemistry, in particular, chemistry involving transition metals. The diverse range of metals employed and the number of tricyclic systems synthesized admirably demonstrates the importance of this area. 

Obviously, we owe much to the pioneers of this area and it is a testimony of the importance of their work to find cross-coupling affecting so many areas of chemistry. Much progress has been made in the last decade, and at the look of things much will emerge in the very near future to solidify the crucial importance of metal-mediated cross-coupling in modern synthetic chemistry. 

Superheated and supercritical water are used in several applications. Supercritical water is most often used in the destruction of organic wastes, including some chemical warfare agents, as an alternative to incineration (Katritzky et al., 1996 Sherman et al., 1998). Recent reports describe the use of both forms as a solvent and as a reactant in synthetic chemistry (Katritzky et al., 1996 An et al., 1997). Some of the reactions investigated include metal-mediated alkyne cyclizations, Pd-catalyzed al-kene arylations, aldol reactions, the Fischer indole synthesis, and hydrolysis reactions. Waterborne coatings and the destruction of wastes in supercritical water are fully... 

R.A.J.O thanks the Australian Research Council for financial support for studies on metal-mediated chemistry (Grant DP0558430) and via the ARC Centres of Excellence program (ARC Centre of Excellence in Free Radical Chemistry and Biotechnology). 

Laboratory of Organic Chemistry, University of Utrecht, Dept. Metal Mediated Synthesis Padualaan 8, 3584 CH, Utrecht, The Netherlands. 

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