Nucleophile-vinyl cation reaction relative nucleophilicities

The nucleophilicities are found to be dependent on electronic, steric, and symbiotic effects, and limited series obeyed a constant selectivity , a reactivity-selectivity or a dual-parameter linear free-energy relationship. The conclusion made was that because of different blends of the effects, the construction of a substrate-independent nucleophilicity scale was impossible at present, but an approximate scale was presented. In nucleophilic reactions on relatively long lived vinyl cations, the steric effects predominate, but at constant steric effects, reactivity-selectivity relationships were found for very short series of substrates. Additional data are required for constructing more reliable nucleophilicity scales toward neutral and positively charged vinylic carbons. 

Four different probes gave short reactivity orders toward vinyl cations (1) common ion rate depression in solvolysis (2) competitive capture of solvolytically generated ions (3) direct reaction of a vinyl cation with nucleophiles and (4) competition between intra- and intermolecular nucleophilic capture. A short reactivity order is obtained in each case, but because of the different solvents and conditions the orders cannot be combined to a single series. However, a selectivity rule that governs the relative reactivities toward different vinyl cations in terms of a constant selectivity or a reactivity-selectivity relationship can be determined. 

Interestingly, because of the lower stability of vinyl cations relative to alkyl carbenium ions, these structures are sometimes not on the reaction s energy surface, and instead concerted reactions occur. For example, the electrophilic addition of HCl to 3-hexyne in acetic acid gives predominately the anti addition product, indicating that protonation occurs simultaneously with nucleophilic attack (Eq. 10.14). 

Probably both reactant stabilization and the already evaluated relative instability of the cationic transition state contribute to the slowness of the solvolysis of vinyl components, but other factors are certainly involved. The most obvious experimental problem is whether the compounds compared react by a unimolecular mechanism or nucleophilic attack by the solvent is involved to a certain extent. In the case of vinylic systems, for instance, nucleophilic solvation from the rear is in general much more hindered than in the case of saturated compounds and the transition state is likely to be stabilized only by electrophilic solvation of the leaving group (Rappoport and Atidia, 1970). The low m values observed in the case of vinyl halides or sulphon-ates may be taken as a strong indication of poor solvation of the transition state in solvolytic reactions of vinyl derivatives. These and other complications, such as differences in hyperconjugation, differences in electronegativity of the -—C= and —- bonds (Jones and Maness,... 

An important advantage of the use of such added nucleophiles is that it allows controlled/living cationic polymerization of alkyl vinyl ethers to proceed at +50 to +70°C [101,103], relatively high temperatures at which conventional cationic polymerizations fail to produce polymers but result in ill-defined oligomers only, due to frequent chain transfer and other side reactions. Recently, initiators with functionalized pendant groups [137] and multifunctional initiators [ 138—140] have been developed for the living cationic polymerizations with added nucleophiles. 

In (A), the reaction will predominate on the left-hand side. Exceptions appear to be olefins with strong electron-releasing substituents that confer thermodynamic stability to the newly formed cation, -CH2-CHR . This either results from a suitable charge delocalization over the 7r-electron system or from the presence of a heteroatom. Accordingly, only those olefins that possess relatively strong nucleophilic characteristics can be polymerized by stable carbon cations. Such olefins are alkyl vinyl ethers, A -vinyl carbazole, p-methoxystyrene, indene, and vinylnaphthalenes. Styrene and cf-methylstyrene, however, will not polymerize, because they are less reactive. 

With ammoifia and primary alkylamines having functional groups such as hydroxy, vinyl, ester, cyano, and acetal groups, photoamination occurred without other side reactions, since the amino group has the stronger nucleophilicity and photoaminadon can be performed under mild conditions. However, the secondary amines whose oxidation potentials are relatively low could not be used because of the occurrence of the electron exchange of the cation radicals of the substrates with the amines. 

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