Initiators, cationic, from electrophilic

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

The key of alkane transformation was assigned to the formation of CX3+-type cations that are electrophilic enough (probably due to a second complexation of A1X3), to abstract a hydride anion from linear and cycloalkanes. When these cations are generated in superacidic media, a protosolvation induces a superelectrophilic character, which was supported by Olah on the basis of high-level ab initio calculations 65 The generation of these cations was also used by various teams66,67 to initiate selective low temperature alkane activation. 

The competing pathways in the reactions of intermediate l-alkylidene-2-oxyallyl cations (47) with furan have been investigated.54 Cycloaddition pathways compete with the electrophilic substitution pathway which initially forms the cation (48) from which a number of products may form. 

By theoretical calculations (B3LYP/6-31G ) four reaction pathways were investigated formation of endo or egzo product with initial bond formation to C2 or C3 in indole. For each mechanism theoretical 13C KIE were analysed and the best agreement of theoretical and experimental KIEs was found for the reaction involving the intermediacy of the radical cation 11, resulting from electrophilic aromatic substitution of indole at C3 by cyclohexadiene in the rate-limiting step ... 

Exactly how the stabilized aromatic cation radical is converted into the nuclear chlorinated product, is not at present fully understood. As represented in eqn (135), nucleophilic substitution could arise from initial capture of the aromatic cation radical by chloride ion involving appropriate substituted cyclohexadienyl-type radicals ( ArHCl), in which case the substitution pattern (at least the ortho/para ratio of products) might be expected to resemble more those from typical homolytic aromatic substitution processes rather than those from electrophilic substitutions, as observed experimentally. At present, there is a scarcity of significant mechanistic information relating to nucleophilic capture of aromatic cation radicals, although in every reported case [vide infra) the position of substitution corresponds with that arising from comparable electrophilic processes. 

In 2009, Wiemer and coworkers [12] reported the termination of an epoxide-initiated cationic cyclization by electrophilic aromatic substitution (Scheme 14.5). In this novel domino transformation, a MOM (methoxymethyl) protected phenol was cychzed with concomitant loss of the MOM group, which was then harnessed intramolecularly to forge a new C-C bond. The methodology was applied to the total synthesis of (-l-)-angelichalcone (27), a natural chalcone isolated from the... 

Since the early reports of the Fujiwara-Moritani reaction [2], catalytic alkenylation procedures for a broad range of aromatic substrates with various alkenes have been developed. A proposed mechanism for these Fujiwara-Moritani-type reactions is illustrated in Scheme 18.4 [4]. The reaction is initiated by electrophilic attack on an arene by a cationic palladium species [PdOAc]+, generated in situ from Pd(OAc)2, to form an arylpalladium intermediate. Subsequent alkene insertion and fi-hydrogen elimination may occur to produce an alkenylarene derivative and HPdOAc. The latter may be reoxidized by an oxidant to regenerate Pd(OAc)2. 

Using both computational and experimental techniques, initial studies demonstrated that the position of reaction with a given electrophile was dependent on its size [39]. Related studies, using the prochiral cation derived from decarboxylation of Mosher s acid as a probe, computationally estimated the difference in energy between the adducts formed from Re and Si face attack on this cation as an indicator of stereoinductive potential [42]. 

Tertiary nitrogen and iodine initially form a /i-o-complex, from which a strongly reactive iodine cation is produced this cation can bring about electrophilic substitutions on aromatic systems or cause oxidations [2]. 

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

This is a further example of a carbonyl-electrophile complex, and equivalent to the conjugate acid, so that the subsequent nucleophilic addition reaction parallels that in hemiacetal formation. Loss of the leaving group occurs first in an SNl-like process with the cation stabilized by the neighbouring oxygen an SN2-like process would be inhibited sterically. It is also possible to rationalize why base catalysis does not work. Base would simply remove a proton from the hydroxyl to initiate hemiacetal decomposition back to the aldehyde - what is needed is to transform the hydroxyl into a leaving group (see Section 6.1.4), hence the requirement for protonation. 

To be really satisfactory, a Friedel-Crafts alkylation requires one relatively stable secondary or tertiary carbocation to be formed from the alkyl halide by interaction with the Lewis acid, i.e. cases where there is not going to be any chance of rearrangement. Note also that we are unable to generate carboca-tions from an aryl halide - aryl cations (also vinyl cations, see Section 8.1.3) are unfavourable - so that we cannot nse the Friedel-Crafts reaction to join aromatic gronps. There is also one further difficulty, as we shall see below. This is the fact that introduction of an alkyl substitnent on to an aromatic ring activates the ring towards fnrther electrophilic substitution. The result is that the initial product from Friedel-Crafts alkylations is more reactive than the... 

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