Electrophilic addition solvent participation

Multiple-bond participation in solvolysis may be looked upon as a competition in a nucleophilic displacement between solvent and the tr electrons of the multiple bond or as an electrophilic addition of a carbonium ion, or carbonium ion like species, to the multiple bond. 

ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS A. Chlorinating agents Sodium hypochlorite solution 7V-Chloro succi n i m i de Antimony pentachloride Formation of chlorohydrins from alkenes Chlorination with solvent participation and cyclization Controlled chlorination of acetylenes... 

In addition to polarity and nucleophilicity toward the bromonium ion, other solvent properties may come into play. The rate-limiting step in electrophilic bromine addition is thought to be the conversion of a CT complex to a cation-bromide ion pair, which is consistent with the observation that the rate constant for bromine addition increases as solvent polarity increases. Solvent may also electrophilically assist the removal of the bromide ion from the alkene-Br2 CT complex by hydrogen bonding to a developing bromide ion (Figure 9.6). It has been estimated that this electrophilic solvent participation can lower the energy of bromonium ion formation by 60 kcal/mol. ° Additional support for the role of electrophilic... 

Given their extraordinary reactivity, one might assume that o-QMs offer plentiful applications as electrophiles in synthetic chemistry. However, unlike their more stable /tora-quinone methide (p-QM) cousin, the potential of o-QMs remains largely untapped. The reason resides with the propensity of these species to participate in undesired addition of the closest available nucleophile, which can be solvent or the o-QM itself. Methods for o-QM generation have therefore required a combination of low concentrations and high temperatures to mitigate and reverse undesired pathways and enable the redistribution into thermodynamically preferred and desired products. Hence, the principal uses for o-QMs have been as electrophilic heterodienes either in intramolecular cycloaddition reactions with nucleophilic alkenes under thermodynamic control or in intermolecular reactions under thermodynamic control where a large excess of a reactive nucleophile thwarts unwanted side reactions by its sheer vast presence. 

Winstein et al. [45] first presented evidence for the concept that different types of electrophilic species, each with distinct reactivities, may participate in reactions involving cationic intermediates. As shown in Eq. (36), Winstein et al. proposed that four species are in equilibrium, including covalent electrophiles, contact ion pairs, solvent-separated ion pairs, and free ions. In addition, ion pairs may aggregate in more concentrated solutions- According to this concept, electrophilic species do not react with a continuous spectrum of charge separation, but rather in well-quantified minima in the potential energy diagram. 

Two of the factors that determine the reactivity of tethered ir-nucleophiles in Mannich-type cycliza-tions have been emphasized stereoelectronic effects and reaction medium effects. The stereoelectronics of orbital overlaps between the ir-nucleophile and the iminium electrophile are best evaluated by considerations such as antiperiplanar addition trajectories and Baldwin s rules for ring formation. The critical importance of the reaction medium has received serious attention only recently. However, it already appears clear that Tr-nucleophiles that would lead, upon cyclization, to relatively unstable carbocations can have their reactivity markedly increased by carrying out the cyclization in the presence of a nucleophilic solvent or additive which, by nucleophilic participation, can obviate the formation of high energy cyclic carbenium ion intermediates. 

A Pd-catalyzed cyanation procedure using aryl halides and potassium cyanide was reported in 1973 (Scheme 3.79) [258, 259]. Many studies investigated effective Pd catalysts, metal cyanides, solvents, or additives, intending to provide the efficient catalytic reactions of a variety of carbon electrophiles including aryl halides under rrald conditions. In addition to Zn(CN)2, KCN, and NaCN, several other cyanide sources including NaCN, K4[Fe(CN),5], cyanohydrines, and trimethylsilyl cyanide (TMSCN) have been shown to participate in Pd-catalyzed cyanations [258b]. 

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