Cation-type reactivity

The products isolated upon photolysis of phenyl azide 10 in addic medium agree with this description. At low pH (aq. HCl), o- andp-chloroaniline 113 and 114 and 4-aminophenol 115 were among the main products. All of these products can be rationalized in terms of cyclohexadienyl-cation-type reactivity of the phenylnitrenium ion. Aminophenol 115 is formed via the iminocyclohexadienol 116, which was... 

The oils and waxes described as lubricants in section 10.10.1, as well as talc, can be used as softeners but have now been superseded by more effective products. These may be non-reactive or reactive and may be cationic, anionic, nonionic or amphoteric. Although many compounds have been patented, by far the most important are cationic quaternary ammonium compounds and various silicones. Until quite recently the field was led by the cationic types but there is now evidence that aminofunctional polysiloxanes have become the most important product group [482]. 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

Interaction between the radical-cations of primary alkyl halides and acetonitrile is on the verge of Sivj2 versus S l type reactivity. Some product is formed by direct substitutiorr, but rearrangement also occurs to give the most stable carbonium ion [20, 21], Thus 1-bromopentane yields a mixture of three N-pentylacetamides on... 

All this leaves little doubt about the SNl-type reactivity of compounds 147. However, the actual nature of the intermediate does not correspond to that of a digonal vinyl cation. The following facts are indicative. The solvolysis in acetone-methanol mixtures (equation 27) and in acetic acid is entirely stereospecific and retention of the trans configuration is involved. Migration of the arylthio residue was found to occur in the... 

A similar study has been performed on EPI blends in which the vinyl ether was replaced by an acrylate monomer (HDDA) to produce, by different mechanisms, two interpenetrating polymer networks. With the onium salt as sole photoinitiator, the cationic polymerization of the EPI epoxy groups occurred as fast in the formulation containing 20% of HDDA by weight as in the EPI/DVE-3 blend, to reach nearly 100% conversion within 0.6 s (Fig. 11). The polymerization quantum yield was found to be similar to that measured in the EPI/vinyl ether blend Op 650 mol E. By contrast, the acrylate double bonds were found to polymerize at a much slower pace, most probably because of the low reactivity of the free radicals generated by the cationic-type photoinitiator. 

Another quantitative approach to the reactivity of thiazole (381) in reactions involving a cationic transition state, though not exactly of the electrophilic substitution type, deserves to be mentioned here because of... 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

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