Subject atom transfer radical

It is possibly to carry out chain transfer catalytically. The process is related to atom transfer radical polymerization (ATRP) [67, 68] and related living polymerizations which keep the concentration of chain-carrying radicals low. ATRP employs a halide complex (often Ru X) that is subject to facile one-electron reduction that complex reversibly donates X to the chain-carrying radical (1.23) and thereby decreases the concentration of the latter [69, 70]. 

Atom or radical transfer reactions generally proceed by a SH2 mechanism (substitution, homolytie, bimolecular) that can be depicted as shown in Figure 1.6. This area has been the subject of a number of reviews.1 3 27 97 99 The present discussion is limited, in the main, to hydrogen atom abstraction from aliphatic substrates and the factors which influence rate and specificity of this reaction. 

Direct metathesis reactions are characterized by tight transition states and involve the transfer of atoms or radicals as a consequence of the close proximity of the two reactants, and they are the only type of reactions that are not subject to becoming energy-transfer-limited at high temperatures and low pressures. A typical energy diagram for metathesis reactions is shown in Fig. 8c. Examples of such reactions include... 

Free radical promoted hydrosilation of olefins typically gives poorer conversions than are produed by hydrosilation mediated by metal catalysts. However, they are not subject to isomerization of the olefin, a side reaction that can be a nuisance in metal-catalyzed hydrosilations. This is due to the extremely efficient hydrogen atom transfer reaction from Si-H to carbon radicals and the resulting very short lifetimes of the latter. The generally accepted mechanism for these processes is illustrated in Scheme 1. 

Atom transfer polymerizations are often subject to problems arising from solubility, the initial presence of metal ions in the higher oxidation state, multiple complex equilibria, and a variety of side effects.137-139 Quite often, diverse broken reaction orders are observed with respect to catalyst and initiator, and they are difficult to analyze.140 Nevertheless, with the methods outlined above for nitroxides, some quantitative data for the reversible radical formation steps were obtained. 

It is possible to generate an a-heteroalkyl radical by a 1,5-hydrogen atom transfer from the radical obtained from an o-iodobenzyl protected amine (eq 50). It can then be subjected to several reactions such as condensation with a ketone. ... 

There is a second type of hydrogen atom in 146 H),. If a chlorine radical reacts with Hj, by an atom transfer reaction, the products are HCI and radical 148. The reaction of 148 with additional diatomic chlorine leads to a different product, 93. Therefore, if all the different hydrogen atoms in 146 react with chlorine, two products are formed 149 and 93. Note that the reaction of 141 gives four products and 146 gives two products, and both are radical substitution processes. In both cases, the substitution reaction first requires formation of a radical to initiate or start the process. The relative percentages of products were given for the reaction of 141 but not for the reaction of 146. Is it possible to determine that all these products can form and which may be the major product This is the subject of the next section. 

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