Covalent adducts of carbocations

Covalent Adducts of Carbocations with Representative Nucleophiles... 

When an appropriate alkene is added, color and conductance disappear due to the formation of covalent adducts (Scheme 34). For the success of the method it is essential that the chloride transfer from the complex anion to the new carbocation is fast and complete, because only then the selective formation of 1 1 products and the controlled decay of absorbance and conductance is warranted. Ideally, both quantities (absorbance and conductance) yield the same dependence of carbocation concentration on time, and the second-order rate law (18) is generally obeyed. 

The mechanism of type II IPP isomerase is currently not known. Two mechanisms have been proposed. Epoxy and fluorinated substrate analogues have been used to probe the mechanism of type II IPP isomerase. These compounds were irreversible inhibitors of the enzyme forming N5 covalent adducts with the reduced flavin. These studies were taken as evidence of a protonation/deprotonation mechanism in which protonation of the double bond of IPP is followed by deprotonation of the carbocation intermediate to form DMAPP, similar to the mechanism of type I IPP isomerase. Conversely, a radical mechanism has been... 

Diels-alder adducts at 0°C. This cation radical-vinylcyclobutane rearrangement is non-stereospecific, thus accounting for the formation of a cis-trans mixture of Diels-Alder adducts. Kinetic studies revealed (Scheme 8) that the ionization of these ethers involves an inner-sphere electron-transfer mechanism involving strong covalent (electrophilic) attachment to the substrate via oxygen (oxonium ion) or carbon (carbocation). 

This is clearly illustrated in the example given below for the reaction of the methoxymethyl cation with pivaldehyde. There are four possible reaction channels for the unimolecular dissociation of the initially formed adduct, which is presumed to be of the form of a covalent species resulting from the interaction of the carbocation center of the methoxymethyl cation with the carbonyl oxygen of the aldehyde, 2. For smaller carbonyl compounds this adduct has been demon-... 

Initially, Gandini and Plesch proposed that the perchloric acid-initiated low temperature polymerization of styrene is based on monomer insertion on the nonionic perchlorate chain ends, which was based on the observation that the polymerization mixture was not conductive [68, 69]. These nonionic polymerizations were referred to as pseudo-cationic polymerizations. However, more detailed investigations by stopped-flow UV-vis spectroscopy revealed the presence of short-lived carbocations indicating that these are the propagating species in the cationic polymerization of styrene [70, 71]. This was also confirmed for the polymerization of styrene with trifiic acid for which Matyjaszewski and Sigwalt showed that the covalent triflic ester adduct was unstable even at -78 °C leading to carbocationic propagating species [72]. 

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