1- phenylethyl systems, substitution reactions

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

Saunders and co-workers (Amin et al., 1990) used E2 elimination reactions in the p-substituted 2-phenylethyl system to test the new criteria for tunnelling suggested by the above calculations. The actual substrates and base/solvent systems they used were (2-phenylethyl-2-f)-trimethylammonium bromide, [19], with sodium ethoxide in ethanol, 2-phenylethyl-2-f bromide, [20], with potassium t-butoxide in t-butyl alcohol and 2-(p-chlorophenyl)ethyl-2-f tosylate, [21], with potassium t-butoxide in t-butyl alcohol. When equation (57) was applied to the experimental secondary (kB/ S) KIEs in Table 39, the calculated /th h KIEs were 1.106 0.033 and 1.092 0.026 for [19] and [21],... 

Stabilization of cationic intermediates by conjugation with an aromatic ring, as in the 1-phenylethyl system, leads to nucleophilic substitution with diminished stereospecificity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. For the ion-pair equilibria... 

Like kact, kjeact depends upon the nature of the transferable atom. For instance, the deactivation of 1-phenylethyl radicals (structurally similar to the propagating polystyrene radicals) with the bromide complex Cu°(dNbpy)2Br (kjeact = 2.5 x lO s ) is almost 6 times faster than for the analogous chloride complex Cu°(dNbpy)2Cl (k eact = 4.3x 10 M s ). It is consequently expected and indeed observed that polymerization control is better if alkyl bromides and copper bromide-containing catalysts are used in ATRP provided that no side reactions take place in the system. However, if the alkyl halide initiator or polymeric dormant state can easily participate in nucleophilic substitution reactions, the use of chloride-based initiator and/or catalyst is advantageous due to lower reactivity of alkyl chlorides in nucleophilic substitution. For instance, the ATRP of 4-vinylpyridine using... 

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