Methyl group nucleophilic attack

When considering the rate constants for deprotonation reported in Table 2 one should, however, take into account that in methylbenzene radical cations with < 3 methyl groups, nucleophilic attack of water on the aromatic ring can compete efficiently with side-chain deprotonation. An elegant explanation which accounts for this competition has been provided for the toluene radical cation on the basis of the three-electron three-orbital three-configuration approach [135, 136]. Three electrons are involved in the deprotonation reaction—the unpaired electron delocalized over... 

Example (c) uses HBr as the reagent. This is a strong acid with an electrophilic proton. The best nucleophile in the organic compound is the oxygen atom so the first step is a proton transfer and the second step uses the bromide ion, as a nucleophile for the methyl group. Direct attack at would give impossible four-valent oxygen. 

As we have seen the nucleophile attacks the substrate m the rate determining step of the Sn2 mechanism it therefore follows that the rate of substitution may vary from nucleophile to nucleophile Just as some alkyl halides are more reactive than others some nucleophiles are more reactive than others Nucleophilic strength or nucleophilicity, is a measure of how fast a Lewis base displaces a leaving group from a suitable substrate By measuring the rate at which various Lewis bases react with methyl iodide m methanol a list of then nucleophihcities relative to methanol as the standard nucleophile has been compiled It is presented m Table 8 4... 

A related but distinct rhodium-catalyzed methyl acetate carbonylation to acetic anhydride (134) was commercialized by Eastman in 1983. Anhydrous conditions necessary to the Eastman acetic anhydride process require important modifications (24) to the process, including introduction of hydrogen to maintain the active [Rhl2(CO)2] catalyst and addition of lithium cation to activate the alkyl methyl group of methyl acetate toward nucleophilic attack by iodide. 

In the alcohol oxidations, the sulfonium intermediate (2, nucleophile = R2C(OH)) loses a proton and dimethyl sulfide to give the carbonyl compound (42). The most common mechanism for the decomposition of (2) is attack by a mild base to remove a proton from one of the methyl groups. Subsequent cycHc coUapse leads to the carbonyl compound and dimethyl sulfide (eq. 9) ... 

In many cases, substituents linked to a pyrrole, furan or thiophene ring show similar reactivity to those linked to a benzenoid nucleus. This generalization is not true for amino or hydroxyl groups. Hydroxy compounds exist largely, or entirely, in an alternative nonaromatic tautomeric form. Derivatives of this type show little resemblance in their reactions to anilines or phenols. Thienyl- and especially pyrryl- and furyl-methyl halides show enhanced reactivity compared with benzyl halides because the halogen is made more labile by electron release of the type shown below. Hydroxymethyl and aminomethyl groups on heteroaromatic nuclei are activated to nucleophilic attack by a similar effect. 

Halide ions may attack 5-substituted thiiranium ions at three sites the sulfur atom (Section 5.06.3.4.5), a ring carbon atom or an 5-alkyl carbon atom. In the highly sterically hindered salt (46) attack occurs only on sulfur (Scheme 62) or the S-methyl group (Scheme 89). The demethylation of (46) by bromide and chloride ion is the only example of attack on the carbon atom of the sulfur substituent in any thiiranium salt (78CC630). Iodide and fluoride ion (the latter in the presence of a crown ether) prefer to attack the sulfur atom of (46). cis-l-Methyl-2,3-di-t-butylthiiranium fluorosulfonate, despite being somewhat hindered, nevertheless is attacked at a ring carbon atom by chloride and bromide ions. The trans isomer could not be prepared its behavior to nucleophiles is therefore unknown (74JA3146). 

Insertion of a methyl group at the site where nucleophilic attack (by OH or H2O) occurs during hydration considerably hinders the addition of water, thus lowering the percentage of the hydrated... 

Pyridinium chloride ([PyHjCl) has also been used in a number ofcyclization reactions of aryl ethers (Scheme 5.1-4) [4, 18]. Presumably the reaction initially proceeds by deallcylation of the methyl ether groups to produce the corresponding phenol. The mechanism of the cyclization is not well understood, but Pagni and Smith have suggested that it proceeds by nucleophilic attack of an Ar-OH or Ar-0 group on the second aromatic ring (in a protonated form) [4]. 

The stereochemical course of the reaction may be rationalized by assuming a six-mem-bered chelate which presumably exists in a chair-like conformation. Molecular models support the assumption that the nucleophile preferentially attacks from the bottom face of the complex due to the steric hindrance of the axial A-methyl group. 

The cationic species [IrX2(CO)(PMe2Ph)3]+ (XVI, Q = CO) undergoes nucleophilic attack by methoxide forming the carboxylate complex (XXI). In the presence of very strong base, a cyclometallation of a methyl group occurs forming (XXIV) [165],... 

We examined the possibility of a direct formation of two C-C bonds by reaction of a carbanion with [Fe(arene)2]2+ in which the arene bears methyl groups. We could indeed repeat Hellings s experiments but found that mesitylene was the only aromatic allowing this possibility in reasonable yields. With p-xylene, a low yield of an unstable complex was obtained corresponding to double nucleophilic attack of phenyllithium on the same ring in spite of the bulk of the methyl groups [23]. Eq. (4) ... 

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