Radicals, formation intermediates initiators

As in the case of ceric and vanadium ions, the reaction of organic compounds with Co(III) proceeds via formation of an intermediate complex. Such a complex decomposes and produces free radicals capable of initiating vinyl polymerization. However, only a few reports on Co(IIl) ion-initiated grafting onto cellulose fibers are available [38]. 

Some evidence to suggest that peroxo complexes can be intermediates in the oxidation of Pt(II) by 02 has been presented. As shown in Scheme 41, a Pt(IV) peroxo complex was obtained by reacting cis-PtCl2(DMSO)2 and 1,4,7-triazacyclononane (tacn) in ethanol in the presence of air (200). An alkylperoxoplatinum(IV) complex is obtained in the reaction of (phen)PtMe2 (phen = 1,10-phenanthroline) with dioxygen and isopropyl-iodide. Under conditions that favor radical formation (light or radical initiators), an isopropylperoxoplatinum(IV) compound was obtained (201,202), depicted in Scheme 42. 

The formation of a radical cation intermediate normally initiates reactions that... 

In terms of nitration, the system (NaN02 + CF3SO3H) is of no interest. At the same time, dimerization in this system can be attractive. For the last direction, CF3SO3H (or FSO3H) is necessary to produce binaphthyl derivatives more preferentially than nitro compounds (Tanaka et al. 1996). This work was preceded by the observation that the reaction of NO+AICI4 with 1-methyl-, 1,2-dimethyl-, 1,3-dimethyl, or 1,8-dimethylnaphthalenes in liquid SO2 leads to a partial a,a-dimerization (see Borodkin et al. 1993). Ozeryanskii et al. (1998) published the dimerization of l,8-Af,Af-bis(d imethylamino)naphthalene by the action of NOj in CHCI3. This reaction is accompanied by the formation of 4-nitro-l,8-A,A-bis(dimethylamino)naphthalene. Both gronps of anthors consider cation-radicals of the initial substrates as intermediate species. 

Unlike thermal [2 + 2] cycloadditions which normally do not proceed readily unless certain structural features are present (see Section 1.3.1.1.), metal-catalyzed [2 + 2] cycloadditions should be allowed according to orbital symmetry conservation rules. There is now evidence that most metal-catalyzed [2 + 2] cycloadditions proceed stepwise via metallacycloalkanes as intermediates and both their formation and transformation are believed to occur by concerted processes. In many instances such reactions occur with high regioselectivity. Another mode for [2 + 2] cyclodimerization and cycloadditions involves radical cation intermediates (hole-catalyzed) obtained from oxidation of alkcnes by strong electron acceptors such as triarylammini-um radical cation salts.1 These reactions are similar to photochemical electron transfer (PET) initiated [2 + 2] cyclodimerization and cycloadditions in which an electron acceptor is used in the irradiation process.2 Because of the reversibility of these processes there is very little stereoselectivity observed in the cyclobutanes formed. 

Until now, the detailed mechanism involved in the MTG/MTO process has been a matter of debate. Two key aspects considered in mechanistic investigations are the following the first is the mechanism of the dehydration of methanol to DME. It has been a matter of discussion whether surface methoxy species formed from methanol at acidic bridging OH groups act as reactive intermediates in this conversion. The second is the initial C—C bond formation from the Ci reactants. More than 20 possible mechanistic proposals have been reported for the first C-C bond formation in the MTO process. Some of these are based on roles of surface-bound alkoxy species, oxonium ylides, carbenes, carbocations, or free radicals as intermediates (210). 

Detailed studies on the decomposition of organic peroxides are of fundamental interest and of high importance in polymerization reactions. The time-scales of intermediate radical formation and of their subsequent decomposition determine process parameters such as the initiator efficiency in radical polymerizations. An improved understanding of the mechanism and dynamics of photo-induced fragmentation is achieved by systematic investigations in which quantum-chemical calculations are carried out in conjunction with highly time-resolved experiments. 

Figure 18-22 Some possible intermediates in the action of extradiol (left) and intradiol (right) aromatic dioxygenases. Although the steps depict the flow of pairs of electrons during the formation and reaction of peroxide intermediates, the mechanisms probably involve free radicals whose formation is initiated by 02. The asterisks show how two atoms of labeled oxygen can be incorporated into final products.

Scheme 10.18 illustrates the key features of the intramolecular reaction in general and also the specific radical intermediates in the mechanistic study [18]. After initial radical formation from a suitable halide precursor, in the vast majority of cases using Bu3SnH and... 

To this day, redox systems are selected empirically. In spite of considerable efforts devoted to the generalization of past experience with redox initiation, neither the ionization potentials, electron affinities nor other molecular constants can be used at the moment as a reliable criterion for characterizing radical formation. The mechanism of interaction, the intermediates and the medium are all of great importance. The same is true of the monomer, which may even replace the reducing component in special cases [39]. General schemes are therefore of little importance and it will be more useful to discuss specific cases in greater detail. 

Alternatively, Lewis acids such as SbCl5 may initiate oligomerization directly by electron transfer from extremely reactive alkenes such as 1,1-diphenylethylene and 1,1 -di(p-methoxyphenyl)ethylene [28,143,144]. The dimeric tail-to-tail carbenium ion of 1,1-diphenylethylene shown in Eq. (32) was observed, and its formation explained by a radical cation intermediate. Because 1,1-diarylethylenes can not polymerize, only oligomerization was observed. 

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