Chlorine initiating substituent

Aryl and, more so, chlorine substituents on silicon enhance thermal stability of silacyclobutanes. The rate of the first-order thermal decomposition of silacyclobutanes varies inversely with the dielectric constant of the solvent used. Radical initiators have no effect on the thermal decomposition and a polar mechanism was suggested. Thermal polymerization of cyclo-[Ph2SiCH212 has been reported to occur at 180-200°C. The product was a crystalline white powder which was insoluble in benzene and other common organic solvents [19]. 

Halogenation, and particularly chlorination, unlike most radical reactions, is markedly influenced by the presence in the substrate of polar substituents this is because Cl, owing to the electronegativity of chlorine, is markedly electrophilic (c/. p. 314), and will therefore attack preferentially at sites of higher electron density. Chlorination will thus tend to be inhibited by the presence of electron-withdrawing groups, as is seen in the relative amounts of substitution at the four different carbon atoms in 1-chlorobutane (78) on photoehemically initiated chlorination at 35° ... 

A fact that can be put to preparative/synthetic use thus bromination of (CH3)3CH is found to yield only (CH3)3CBr (cf. chlorination, p.324). The effect is more pronounced when substituents are present that can stabilise the initial radical thus across the series, CH4, PhCH3, Ph2CH2 and Ph3CH the relative rates of bromination differ over a range of 109, but only over a range of 103 for chlorination. Selectivity decreases with rise of temperature, however. 

The steric course of this two-step process was examined with several chiral secondary propargylic phosphates (Eq. 9.35) [40], The derived propargylic stannanes were found to be formed with net inversion of configuration. Based on previous evidence that the initial formation of the allenyltitanium intermediate occurs with inversion, it can be deduced that stannylation proceeds by a syn pathway. This surprising result was attributed to coordination between the chlorine substituent of the Bu3SnCl and the electropositive titanium center (Scheme 9.11). 

Bromoarenes are converted into the corresponding chloroarenes on treatment with sodium hypochlorite in the presence of a catalytic amount of nickel(II) tetraphenyl-porphin (NiTPP) and benzyltributylammonium bromide [8]. Fluoro and iodo substituents are not replaced. The reaction involves chlorine radical attack via the initial formation of a Ni(II)-OCl complex. Although high conversions are recorded, the procedure has not been extended for synthetic purposes. 

Kinetic results on the chlorination of aniline by A-chloro-3-methyl-2,6-diphenylpiperi-din-4-one (3) suggest that the protonated reagent is reactive and that the initial site of attack is at the amino nitrogen. The effects of substituents in the aniline have been analysed but product studies were not reported. Zinc bromide supported on acid-activated montmorillonite K-10 or mesoporous silica (100 A) has been demonstrated to be a fast, selective catalyst for the regioselective para-bromination of activated and mildly deactivated aromatics in hydrocarbon solvents at 25 °C. For example, bromobenzene yields around 90% of dibromobenzenes with an ortholpara ratio of 0.12. 

Two mechanisms of reaction appear possible depending upon the reactants. Initial palladation of other the alkene or the arene may occur alkene palladation is proposed to occur initially in reactions of chlorinated alkenes.2-3 Alkenes without strongly electron-withdrawing chloride substituents and moderately reactive arenes generally react via an initial arene palladation, and the reaction takes the route shown in Scheme l.4... 

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