Carbenium ions benzylic

With alkylaromatics, side chains are cleaved giving a benzylic carbenium ion (benzyl cleavage), which dominates as the tropylium ion m/z 91 in many spectra of aromatics. 

TFA, CH2CI2, rt, 5-30 min, 84-99% yield." An adamantyl glycoside was stable to these conditions. The reaction has also been performed in the presence of anisole to scavenge the liberated benzyl carbenium ion." ... 

BF3-Et20, NaCNBHs, THF, reflux 4-24 h, 65-98% yield. Functional groups such aryl ketones and nitro compounds are reduced and electron-rich phenols tend to be alkylated with the released benzyl carbenium ion. The use of BF3 Et20 and triethylsilane as a cation scavenger is also effective." ... 

Conclusive evidence for the stability of Si-H bond toward the benzyl carbenium ion was obtained by quantitative IR measurements. 

Having demonstrated that the Si-H bond is quite stable in the presence of Me3AI coinitiator and benzyl carbenium ion, further model experiments have been carried out to explore the stability of the Si-H bond in the presence of propagating carbenium ions. 

The various carbenium ions /erf-alkyl, bridgehead-, norbornyl-, allyl-, benzyl- or cyclopropylcarbinyl-cations, which are assumed to be intermediates in these decarboxylations are compiled in ref. [293]. 

This reaction type has been intensely studied °. The application of highly polar solvents, catalysis with tertiary amines" or with acids mesomeric stabilization of intermediate carbenium ions " (allylic and benzylic systems propargylic systems" ) as well as derivatives of sulfinic acids with increasing acidity - usually indicate an ionic pathway (intra- and/or inter-molecular) ... 

The data are summarized in Table II. They have been normalized to kx x s i for each zeolite catalyst. In general it is seen that the7transfer of an ethyl group (E,E E,X) occurs faster than that of a methyl group (X,E X,X). This is in agreement with the indicated mechanism for transalkylation (Figure 4) which involves a benzylic carbenium ion intermediate. In the case of methyl transfer, this is a primary cation,... 

Electrolysis of /J-hydroxy carboxylic acid (90) gave ketone (91) as the main product, which was converted to dl-muscone (Scheme 34) [113]. The migratory aptitude of alkyl substituents at the fi-position in the intermediate -hydroxy carbenium ion was scrutinized to show that vinyl, cyclopropyl, and benzyl groups rearrange predominantly. 

Fig. 6.28. El mass spectrum of (3-methylpentyl)-benzene. McLafferty rearrangement and benzylic cleavage are clearly dominating. In the low-mass range carbenium ions and the aromatic fragments are present. Spectmm used by permission of NIST. NIST 2002.

If this is the case, substituent effects in the distal ring of a 4-biphenylyl-nitrenium ion should resemble those of a benzylic carbenium ion. Ren and McClelland showed that substituent effects on log for a series of distally substituted 4-biphenylylnitrenium ions (75n, 75p-v) were carbenium-ion-like. A plot of log kg vs. cr was not highly scattered but was non-linear and similar to those previously described for diarylmethyl and triarylmethyl... 

Evidence for the SNl-type mechanism is provided by fluorodehydroxylation of diastereomeric benzylic alcohols both diastercomers give the same mixture of products which implies that bond breaking precedes bond making in precursors that readily form carbenium ions. Evidence for an SN2 displacement is based on the results of fluorodehydroxylation of epimeric a-amino-/ -hydroxypropanoic acids which proceeds with predominant inversion of configuration.40... 

There have been a number of reports of improved selectivity with sulfonic acid resin catalysts compared with conventional liquid acid catalysts[6—9]. Various explanations have also been proposed. If mechanisms usually postulated for condensation reactions with liquid Br0nsted acid [10] and solid acid catalysts [11] are adopted, the sequence of steps shown in Fig. 2 could be considered for the condensation of MFC. Both mechanisms incorporate the essential features of known carbenium ion chemistry, i.t., electrophilic attack on the aromatic ring by polar carbenium ion intermediates. Note that MDU is formed by this attack on the benzene ring of MPC, while the N—benzyl compound by the attack on nitrogen atom. 

The rearrangement of light and deuterium-labelled cis- and trans-2-methyl-3-phenyloxiranes (1, 2 and 1, 2 ) was studied on ZnO, Al-O-j and WO, and in the presence of BF,. Both in the gas phase (473-673 K) and in the liquid phase (298-413 K), l-phenyl-2-propanone (3) and 2-phenylpropanal (4) were formed with high selectivities (0-90% and 11-80%, respectively). Ring-opening was found to occur by selective fission of the benzyl C-0 bond. Mechanistic studies revealed the formation of an open carbenium ion or a double-bonded surface intermediate. The acidic (electrophilic) and basic characters of the oxides determine the product distributions by affecting the relative importance of the competing mechanisms. 

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