Carbonium substitution with

For unsubstituted PAH, such as benzo[a]pyrene (BP), pyridinium or acetoxy derivatives are formed by direct attack of pyridine or acetate ion, respectively, on the radical cation at C-6, the position of maximum charge density (Scheme 1). This is followed by a second one-electron oxidation of the resulting radical and loss of a proton to yield the 6-substituted derivative. For methyl-substituted PAH in which the maximum charge density of the radical cation adjacent to the methyl group is appreciable, as in 6-methylbenzo[a]-pyrene (6-methylBP) (Scheme 2), loss of a methyl proton yields a benzylic radical. This reactive species is rapidly oxidized by iodine or MnJ to a benzylic carbonium ion with subsequent trapping by pyridine or acetate ion, respectively. 

If C(6) is substituted with very good carbonium ion stabilizers the transition-state structure [173] becomes more stable than the bicyclo[3.1.0]-hexenyl cation structure. Thus treatment of a series of 5-acyl-1,2,3,4,5-pentamethylcyclopenta-1,3-dienes with AlgCle produced cleanly the corres-... 

Similar to ferrocene, the ruthenocene derivatives (89) substituted with a CR R group are greatly stabilized in comparison to noncomplexed carbenes or carbonium ions. In fact, from the enhanced barrier to internal rotation in the a-ruthenocenyl carbonium ion Ru-CHMe+ (>130kJmol ) compared with the related ferrocene derivative (70kJmol ), even greater stabilization in ruthenocene is expected. 

The behaviour of 3j8-amino-A5 compounds (15) reveals the typical participation of the Tc-electrons of the C<5)-C(6) bond in the intermediate carbonium ion see p. 236), giving either substitution with retention of configuration at C(3> [6] or the 6jS-hydroxy 3,5-cyclosteroid [21]. The 3,5 cyclo product appears to result from the normal kinetically controlled reaction... 

In general, treatment of carbonium ions with nitriles under a variety of conditions yields A -substituted amines. Anodic oxidation in acetonitrile converts selected aliphatic compounds to acetamides, e.g. adamantane to adamantylacetamide (equation 22). ... 

When E+ is a proton, equilibrium C lies to the left and routes A and B are followed. Examples are given in Table 13(a). When R is phenyl, the combination of / -silyl hyperconjugative stabilization and carbonium ion stabilization by the benzene ring are sufficient to drive equilibrium C to the left, and substitution with retention of configuration dominates, as shown in Table 13(b). 

Therefore, in order to determine the MT s of substituted phenyl groups it seems expedient to measure directly the rate of the 1,2-shift in P-aryt-substituted carbonitm ions (8). True, this approach is not free of complications either. Of these the principal ones are the very high rate of 1,2-shift which fails to be measured by ordinary methods and the isomerization of the parent carbonium ion with an aryl group at the p-carbon atom into an a-aryl-substituted carbonium ion (9) or into a pheno-... 

The other common mechanism for substitution at saturated carbon, SnI (3.81a) also has its analogue in phosphorus chemistry. Moreover it is generally believed that, in the case of both elements, substitution reactions intermediate in mechanism between SnI and Sn2 may sometimes take place. In carbon chemistry, the SnI mechanism involves an intermediate planar carbonium ion. Since the nucleophilic entering group may attack either face of the planar carbonium ion with equal probability, a racemic mixture is expected to be obtained. In practice this is not always achieved completely, because the nucleophile may have attacked before the carbonium ion was produced. 

Stabilization of a carbonium ion intermediate by conjugation with an aromatic ring, as in the 1-phenylethyl system shown in entry 8, 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... 

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

Hydroxypyrroles. Pyrroles with nitrogen-substituted side chains containing hydroxyl groups are best prepared by the Paal-Knorr cyclization. Pyrroles with hydroxyl groups on carbon side chains can be made by reduction of the appropriate carbonyl compound with hydrides, by Grignard synthesis, or by iasertion of ethylene oxide or formaldehyde. For example, pyrrole plus formaldehyde gives 2-hydroxymethylpyrrole [27472-36-2] (24). The hydroxymethylpyrroles do not act as normal primary alcohols because of resonance stabilization of carbonium ions formed by loss of water. 

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

Vinylsilanes react readily with a range of electrophiles to give products of substitution (1). The overall stereochemistry of such substitution will depend on a number of factors, including the stereochemistry of addition and subsequent elimination when 1,2-adducts are discrete species. However, the regiochemistry of substitution is normally unambiguous, the -effect ensuring that carbonium-ion development on attack by the electrophile will occur at the carbon terminus remote, i.e. /3, to silicon ... 

Two reasons may be offered for the enhanced /3-deuterium isotope effect in vinyl cations as compared with carbonium ions (193). As pointed out by Noyce and Schiavelli (21), in the transition state of a vinyl cation, the isotopically substituted C—H bond is ideally suited for overlap with the developing vacant p orbital, as the dihedral angle between the empty p orbital and C—H bonds is zero in the intermediate, as shown in structure 239. Shiner and co-workers (195)... 

A specific case of the carbonium ion mechanism [Eq. (5)] with reasonable plausibility is decarboxylation of metal arenoates by classic electrophilic aromatic substitution [Eq. (12)]. This mechanism would be favored by electron-donating substituents and has been invoked to explain the relative ease of decarboxylation of p-methoxybenzoic acid in molten mercuric trifluoroacetate (77) as well as the very facile decarboxylation on reaction of polymethoxybenzoic acids with mercuric acetate (18) (see below). 

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