Carbocations, benzylic bridged

When the bromination of the unsubstituted P-methy 1-styrenes (ref. 19) is carried out in methylene chloride, the two diastereoisomeric dibromides are obtained in ratios of 72 threo/28 erythro and 20 threo/80 erythro for the cis and trans isomers, respectively. This result agrees fairly well with a partially bridged intermediate, since the corresponding benzylic carbocation leads to a 65 erythro/35 threo ratio (ref. 20). When the same reactions are carried out in methanol, the... 

The results of ab initio calculations provide evidence that Me2NC(S)-[14+] is stabilized by resonance electron donation from the a-thioamide group (A, Scheme 12) and by covalent bridging of sulfur to the benzylic carbon (B, Scheme 12).96 Direct resonance stabilization of the carbocation will increase the barrier to the nucleophile addition reaction, because of the requirement for the relatively large fractional loss of the stabilizing resonance interaction (A, Scheme 12) at the transition state for nucleophile addition to a-substituted benzyl carbocations.8,13,28 91-93 If the solvent adds exclusively to an open carbocation that is the minor species in a mixture of open and closed ions, then... 

Ruasse et al, 1978) is totally regioselective and shows X-dependent chemoselectivity. This is partly in agreement with the kinetic data, which indicate no primary carbocation but rather a competition between the benzylic carbocation and the bromonium ion, depending on X. According to the data of Table 6, bridged intermediates would lead to more dibromide than open ions do. From these results and from those on gem-, cis- or frans-disubstituted alkenes, empirical rules have been inferred for chemoselectivity (i) more solvent-incorporated product is formed from open than from bridged ions (ii) methanol competes with bromide ions more efficiently than acetic acid. 

Phenol-formaldehyde prepolymers, referred to as novolacs, are obtained by using a ratio of formaldehyde to phenol of 0.75-0.85 1, sometimes lower. Since the reaction system is starved for formaldehyde, only low molecular weight polymers can be formed and there is a much narrower range of products compared to the resoles. The reaction is accomplished by heating for 2 1 h at or near reflux temperature in the presence of an acid catalyst. Oxalic and sulfuric acids are used in amounts of 1-2 and <1 part, respectively, per 100 parts phenol. The polymerization involves electrophilic aromatic substitution, first by hydroxymethyl carboca-tion and subsequently by benzyl carbocation—each formed by protonation of OH followed by loss of water. There is much less benzyl ether bridging between benzene rings compared to the resole prepolymers. 

Only symmetrical alkenes react by means of true bromonium ions (i.e., strongly bridged, symmetrical structures). If the alkene is unsymmetrically substituted with alkyl groups, the bromonium ion takes on the character of a /7-bromocarbocation with restricted rotation about the formerly olefinic carbon-carbon bond. If an aryl group is present on one or both of the olefinic carbon atoms, the evidence points to an intermediate that is even more nearly like a benzylic carbocation. 

The correlation with suggests that the equilibrium population of the benzylic carbocation (23) is enhanced by a substituent X that can stabilize the carbocation through a resonance interaction. The correlation with o-y suggests that substituent Y has a much weaker effect. This result is not compatible with an alternative mechanism involving rate-limiting formation of a bridged cationic intermediate such as 24, since in that case the X and Y substituents would be expected to have a similar effect on cation stability. ... 

---