Carbocations, benzylic opening

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. 

Energies for various possible arenium ions and regioisomeric benzylic cations were computed at the DFT B3LYP/6-31G(d) level or by AMI for comparison with the experimental results. These findings provided further evidence in support of the stability sequence 1-pyrenyl > 4-pyrenyl > 2-pyrenyl in a-pyrene-substituted carbocations as models for the intermediates arising from BaP-epoxide ring opening (Fig. 10). 

Conversions of 3-arylcyclopropenes into indenes using Ag(I) and Pd(0) have been reported " . The silver-catalysed opening of the three-membered ring in cyclopro-pabenzene proceeds via a benzylic carbocation. 

BES buffers (pKa<7.5) with 76 to form benzylic carbocation 77 is solely rate limiting, whereas increase in Tris, HEPES and CHES (p/fa >7.5) buffer concentrations bring about a change in mechanism from rate-determining epoxide ring opening at low buffer concentration to attack of solvent on the carbocation at high buffer concentration. 

Nucleophilic ring opening of epoxides usually occurs with anti stereochemistry, with nucleophilic attack at the less substituted carbon. On the other hand, the acid-catalyzed epoxidation-hydrolysis sequence is not always stereospecific. In the case of (5 )-l-phenyloxirane (styrene oxide), the acid-catalyzed ring opening is regiose-lective and proceeds through the more stable (benzylic) carbocation there is extensive racemization because of the involvement of a carbocation. ... 

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