Transition metal mediated reactions

Cu-mediated Ullman reaction has been used for the polymerization of dihaloaryls. For example, see ref. 3. This type of polymerization as well as other transition-metal-mediated reactions that involve radicals in the polymerization process is not included in this chapter. 

W-Heterocyclic Carbene Complexes in other Transition Metal Mediated Reactions... 

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... 

Ionic liquids are a versatile class of solvents in transition metal-mediated reactions because of many projected advantages compared with conventional media, including their non-volatility. The catalytic PKR was also tested in this media, but only a narrow range of substrates provided reasonable chemical yields. ... 

Transition metal-mediated reactions in organic synthesis... 

Transition Metal Mediated Reactions in Organic Synthesis... 

The same authors have demonstrated that 1,3-diynes behave in predictable yet distinctive manners compared to simple enynes under electrophilic transition metal-mediated reaction conditions. This characteristic behaviour of 1,3-diynes is presumably caused by the slightly electron-withdrawing nature of the alkynyl substituent, which not only renders preferentially the formation of 5-exotype alkylidenes but also allows for the subsequent [l,3]-metallotropic shift. Several salient features of reactions with this functionality include the following (a) an acetate is more reactive than the tethered alkene as an initiator, generating [l,2]-acetate migrated alkylidene intermediate, whereas an alkene is a better terminator than an acetate/bromide to generate the cyclopropane moiety (b) allene products are not formed at all under current reaction conditions (c) 5-exo/6-endo-type alkylidene formation depends on the heteroatom substituent in the tether (d) facile metallotropic [1,3]-shift of the intermediate alkylidenes occurred whenever possible. 

Although the process, which is called P-hydride elimination or simply p-elimination, is useful in organic synthesis, it competes with other reactions, thereby limiting its value. In general, since P-elimination is rapid, transition metal mediated reactions of species bearing P-hydrogens often fail. 

Although a radical intermediate can be written for this transition-metal-mediated reaction, this is not a radical chain reaction because the transition metal reagents are employed in stoichiometric amounts. In addition, if the reaction were to proceed by a chain reaction, one might reasonably expect to obtain a different product (7-59) as a result of hydrogen abstraction. 

Torrent, M., Sola, M., Frenking, G. Theoretical Studies of Some Transition-Metal-Mediated Reactions of Industrial and Synthetic Importance. Chem. Rev. 2000, 100, 439-493. 

The synthesis of -lactams has been achieved using a variety of transition metal mediated reactions, many of which are specific to this type of compound. The vinyl epoxide (15) reacts with [Fc2(CO)9] to... 

Some very widely used organic reactions are catalyzed or mediated by transition metals. For example, catalytic hydrogenation of alkenes, dihydroxylation of alkenes, and the Pauson-Khand reaction require Pd, Os, and Co complexes, respectively. The d orbitals of the transition metals allow the metals to undergo all sorts of reactions that have no equivalents among main-group elements. This doesn t mean that the mechanisms of transition-metal-mediated reactions are difficult to understand. In fact, in some ways they are easier to understand than standard organic reactions. A transition-metal-catalyzed or -mediated reaction is identified by the presence of a transition metal in the reaction mixture. 

Various bi- and oligocyclic ring systems, such as norcaradiene (1 bicyclo[4.1.0]hepta-2,4-diene), bicyclo[6.1.0]nona-2,4,6-triene (2), bullvalene (3 tricyclo[3.3.2.0 ]deca-3,6,9-triene), semibul-Ivalene (4 tricyclo[3.3.0" ]octa-2,7-diene), barbaralone (5 tricyclo[3.3.1.0 ]nona-3,6-dien-10-one) and many other systems containing divinylcyclopropane subunits, exhibit interesting transition metal mediated reaction pathways leading to new carbon skeletons. ... 

Several years ago transition metal mediated reactions in the area of C-glycoside synthesis were primarily limited to palladium and to a lesser extent nickel and manganese. Over the last few years several other metals, including chromium, molybdenum, tungsten, cobalt, and rhodium, have been utilized in C-glycoside synthesis. This section discusses the chemistry of palladium, which is divided into Stille-type couplings and 7T-allyl complexes. This is followed by considerations of the chemistry of chromium and the above-listed metals. A review by Frappa and Sinou entitled Transition Metal Catalysed Fimctionalization at the Anomeric Center of Carbohydrates appeared in early 1997 [55]. 

Type A PCET reactions describe amino acid radical generation steps in many enzymes, since the electron and proton transfer from the same site as a hydrogen atom [188]. Similarly, substrate activation at C-H bonds typically occurs via a Type A configuration at oxidized cofactors such as those in lipoxygenase [47, 48] galactose oxidase [189-191] and ribonucleotide reductase (Y oxidation at the di-iron cofactor, vide infra) [192]. Here, the HATs are more akin to the transition metal mediated reactions of Section 17.3.1 since the final site of the electron and proton are on site differentiated at Ae (redox cofactor) and Ap (a ligand). 

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