Radical displacement, pathway

The usual sulfone synthesis by displacement of halide by sulfmate is assumed to have a nucleophilic 8 2 mechanism However, in special cases of alkyl halides with additional, electron-withdrawing substituents a radical substitution pathway has been observed (equation 32). Correspondingly, substitutions under formation of sulfones take... 

The role of Coball-dioxygen complexes in autooxidations other than phenol oxidation is less certain, and ostensibly similar reactions appear to follow radically different pathways. Thus, in the oxidation of thiols to disulfide catalyzed by Co11 species catalysis by the phthalocyanine complex [Con(TSPc)]4 apparently proceeds via a Co1 intermediate and without participation of Co—02 species,680 whereas catalysis by [CoH(TPP)] appears to involve initial formation of an >/ cobalt-dioxygen complex from which Of is displaced by thiolate.681 Several reviews giving extensive coverage to oxidations catalyzed by cobalt(II) complexes are available.649,650,682 683... 

These stereochemical results, together with some further mechanistic studies on the same reactions (Labinger et ai, 1980) have led to the conclusion that the reactions of saturated alkyl halides (except methyl derivatives), vinyl and aryl halides and a-haloesters with rans-[IrCl(CO)-(PR3)2l proceed by a radical chain pathway, whereas methyl, benzyl and allyl halides and a-haloethers probably react by a nucleophilic displacement process. 

Evidence for a free radical pathway in the foregoing cycloalkyl bromide reactions was secured by San Filippo and coworkers, who found that cis and iraws-4-r-butylcydohexyl bromide afford nearly identical product mixtures with MesSnLi, MesSnNa and MesSnK under a given set of conditions (Table 7)45. Of the various combinations examined, that of di-bromide and Me3SnLi appeared to be most favorable to S/v2 displacement. 

As depicted in Scheme 11, ylides 39 derived from 4-methyl-[l,2,3]triazolo[l,5- ]pyridine react with Michael acceptors, which, upon nucleophilic attack at C3 and ring opening, lead to nucleophilic displacement of nitrogen. The intermediate diradical led to a mixture of compounds, including alkenes and a cyclobutane derivative when methyl acrylate was used, and the indolizine 40 with methyl propiolate as the electrophile <1998T9785>. Heating 4-methyl triazolopyridine with benzenesulfonyl chloride in acetone also confirmed decomposition via a radical pathway. 

The presence of the OH group in alcohols makes alcohol combustion chemistry an interesting variation of the analogous paraffin hydrocarbon. Two fundamental pathways can exist in the initial attack on alcohols. In one, the OH group can be displaced while an alkyl radical also remains as a product. In the other, the alcohol is attacked at a different site and forms an intermediate oxygenated species, typically an aldehyde. The dominant pathway depends on the bond strengths in the particular alcohol molecule and on the overall stoichiometry that determines the relative abundance of the reactive radicals. 

Unactivated aryl halides also undergo nucleophilic displacement via electron transfer in the initial step the so-called SRN1 mechanism. It is now clear that in the case of heteroaromatic compounds, nucleophilic substitution by the Srn process often competes with the addition-elimination pathway. The SRN reactions are radical chain processes, and are usually photochemically promoted. For example, ketone (895) is formed by the SRN1 pathway from 2-chloroquinoxaline (894) (82JOC1036). 

Each of these reactions and their reverse reactions may, in fact, be involved in major reaction pathways in coal conversion. Reaction 10, for instance, is expected to be a key step in H-transfer and aromatization, reaction 11 can lead to crosslinking and polymerization while reaction -11 breaks up complex molecules, and reaction 12 should provide a steady source of free radicals in many pyrolytic systems even in the absence of weak covalent bonds (vide infra). Furthermore, these free radicals contain weak C-H bonds that may rupture to yield H atoms, which can in turn lead to breaking of C-C or C-0 bonds through aromatic displacement reactions (reaction 5). 

The reactivity of 02 - with alkyl halides in aprotic solvents occurs via nucleophilic substitution. Kinetic studies confirm that the reaction order is primary > secondary > tertiary and I > Br > Cl > F for alkyl hahdes, and that the attack by 02 - results in inversion of configuration (Sn2). Superoxide ion also reacts with CCI4, Br(CH2)2Br, CeCle, and esters in aprotic media. The reactions are via nucleophilic attack by 02 on carbon, or on chlorine with a concerted reductive displacement of chloride ion or alkoxide ion. As with all oxyanions, water suppresses the nucleophilicity of 02 (hydration energy, lOOkcalmoL ) and promotes its rapid hydrolysis and disproportionation. The reaction pathways for these compounds produce peroxy radical and peroxide ion intermediates (ROO and ROO ). 

Three mechanisms are usually considered for oxidative addition of alkyl and aryl halides Sn2, radical and concerted. In the Sn2 pathway, the metal acts as a nucleophile, displacing the hahde from RX, followed by coordination of the halide to the metal. The oxidation state of the metal rises by two units. 

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