Electron chain carriers

Organometallic compounds with a 17-electron configuration are often labile toward associative ligand exchange. Radical chain mechanisms are well established for phosphine substitution on metal carbonyl hydrides (Scheme 23), the 17-electron chain carrier being in most cases non hydridic. This mechanism, however, was also shown to operate for OsH2(CO)4 via the 17-electron hydride complex OsH(CO)4 [137]. Thus, phosphine addition to the radical prevails over the dimerization, which indeed occurs in the absence of phosphine [33] (section 6.5.7), and over other possible decomposition pathways. The second step of the chain propagation process in Scheme 23, for this osmium system, is another example of atom transfer to a hydride radical (section 6.5.6). 

Chain carriers are usually very reactive molecular fragments. Atomic species such as H and Cl, which are electrically neutral, are in fact the simplest examples of free radicals, which are characterized by having an impaired electron, in addition to being electrically neutral. More complex examples are the methyl and ethyl radicals, CH and QH, respectively. 

The half-order of the rate with respect to [02] and the two-term rate law were taken as evidence for a chain mechanism which involves one-electron transfer steps and proceeds via two different reaction paths. The formation of the dimer f(RS)2Cu(p-O2)Cu(RS)2] complex in the initiation phase is the core of the model, as asymmetric dissociation of this species produces two chain carriers. Earlier literature results were contested by rejecting the feasibility of a free-radical mechanism which would imply a redox shuttle between Cu(II) and Cu(I). It was assumed that the substrate remains bonded to the metal center throughout the whole process and the free thiyl radical, RS, does not form during the reaction. It was argued that if free RS radicals formed they would certainly be involved in an almost diffusion-controlled reaction with dioxygen, and the intermediate peroxo species would open alternative reaction paths to generate products other than cystine. This would clearly contradict the noted high selectivity of the autoxidation reaction. 

I consider there to be a sharp distinction between the most polar form of a molecule and its ionically dissociated form. The reason for this is empirical An ion is defined as a species carrying a charge equal to an integral multiple of the electronic charge, and this definition implies that it will have a characteristic predictable electronic spectrum and, under suitable conditions, mobility in an electric field. There is so far no evidence which would compel one to abandon this definition, and I think it is important to distinguish clearly in this context between reaction intermediates (chain carriers, active species) of finite life-time, and transition states. 

In a final confirmation, agents that inhibit the flow of electrons through the chain have been used in combination with measurements of the degree of oxidation of each carrier. In the presence of 02 and an electron donor, carriers that function before the inhibited step become fully reduced, and those that function after this step are completely oxidized (Fig. 19-6). By using several inhibitors that block different steps in the chain, investigators have determined the entire sequence it is the same as deduced in the first two approaches. 

In contrast to the flavin oxidases, flavin dehydrogenases pass electrons to carriers within electron transport chains and the flavin does not react with 02. Examples include a bacterial trimethylamine dehydrogenase (Fig. 15-9) which contains an iron-sulfur duster that serves as the immediate electron acceptor167 169 and yeast flavocytochrome b2, a lactate dehydrogenase that passes electrons to a built-in heme group which can then pass the electrons to an external acceptor, another heme in cytochrome c.170-173 Like glycolate oxidase, these enzymes bind their flavin coenzyme at the ends of 8-stranded a(i barrels similar... 

The initial step of this reaction consists of the one-electron oxidation of the substrate. The resulting cation radical of o-diethynylbenzene cyclizes to the fulvenyl form, which further reacts with the neutral substrate to yield the fulvenyl diradical and the substrate cation radical. The latter is the chain carrier. The fulvenyl diradical adds oxygen and transforms into the final product (Scheme 6-23). 

The results of Table 3 become clear when we consider these results. The complexes that react according to a valency change mechanism act as catalysts because of hydroperoxide decomposition, while in the case of zinc and copper another mechanism clearly operates, and we propose here that in this case the complex acts as a chain carrier, much like the example of HBr at the beginning of this section (for simplicity s sake the phthalocyanine ir-electron system is indicated as a square) ... 

EPR experiments have shown that the redox ability of WZ catalysts is sufficient to initiate a homolytic cleavage of C-H bonds in alkanes. Exposure of a WZ catalyst to n-pentane at 523 K led to the formation of W5+ species and organic radicals on the surface.27 The formation of organic radicals also occurred when WZ catalysts interacted with other hydrocarbons, including benzene.31 We therefore infer that one-electron transfer, although it is not regarded as a step in the catalytic cycle, can initiate catalysis by a process that leads to the formation of the carbenium ion chain carriers,27 as also occurs in acidic solutions.32 We emphasize that a strong redox reactivity is necessary but not sufficient for the catalytic activity of WZ. 

Although less widely employed than the related dialkylcuprates, zincates such as LiZnRs can also undergo electron transfer reactions, e.g. with free radicals as chain carriers [35]. 

Like a DO loop in FORTRAN, the chain must be started and eventually be terminated. It is started by an additional reaction that serves as source of chain carriers. This is called initiation. It is terminated by a reaction that, once in a while, consumes chain carriers without generating others. This is called termination. The steps of the DO loop are called propagation. Typically, chain carriers are species (atoms or free radicals) with unpaired electrons. They resemble catalysts in that they arise again after having been consumed, but differ in having extremely short life spans that prevent their isolation. Moreover, the initiation and termination steps and their kinetic implications set chain reactions apart from catalysis as well as from any other kinds of chemical reactions. 

As mentioned at the outset, certain chain reactions include steps in which more chain carriers are formed than consumed, and this may cause a detonation. In most cases, branching is caused by oxygen, whose atom has two unpaired electrons. 

Unlike ordinary chain reactions, chain-growth polymerization need not involve free radicals. The reactive center may instead be a carbanion or carbocation generated by intermolecular transfer of a proton or electron. Depending on the sign of the ionic charge on the chain carriers, the overall reaction is called anionic or cationic polymerization. As in free-radical polymerization, initiation is required. 

The primary process of initiation in this type of polymerisations consists in the abstraction of an electron from a monomer molecule with a consequent formation of the conesponding radical cation. A detailed knowledge of the nature of these species and of their subsequent reactions leading eventually to chain carriers implies the use of fast-... 

Funt and Blain reported the electropolymerisation of isobutyl vinyl ether in methylene chloride with various tetrabutylammonium salts and showed the unmistakably cationic character of the chain carriers in these processes. Similar studies by Mei oli and Vidotto " extended the range of monomers and solvents with sodium tetraphenyl borate as background electrolyte. The anodic initiation mechanism was postulated as a two-electron process ... 

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