Benzyl cation substituent effects

A carbocation is strongly stabilized by a C substituent (Figure 7.1c) through n-type interactions which involve substantial delocalization into the substituent. The LUMO energy is relatively unchanged, but the reactivity of the electron-deficient center toward attack by nucleophiles is reduced because the orbital coefficients are smaller. Allyl and benzyl carbocations are prototypical of C -substituted carbo-cations. The effects of substitution are cumulative. Thus, the more C -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. A prime example is triphenyl carbocation. 

Effects of oxygen substitutents in an aromatic ring upon an exocyclic rather than endocyclic carbocation charge center have also been measured. The possibility of comparing HO, MeO, and O substituent effects for the benzylic cations is provided by recent studies of quinone methides, including the unsubstituted / -quinone methide 23, which may be considered as a resonance-stabilized benzylic cation with a /xoxyanion substituent. 

Combining the above values of pAR with the value for the p-methoxybenzyl cation measured by Toteva and Richard158 allows the effect of the three oxygen substituents on the stability of the benzyl cation to be compared in Scheme 22. The values of pAR may also be compared with effects of similar oxygen substitutions at the a-position of the benzyl cation from Table 3, which are also shown in Scheme 22. As expected, the relative magnitudes of the O-, HO, and MeO substituent effects exhibit similar patterns in the a- and... 

In general, polymerizations with these salts are much more efficient than those with the unsubstituted salt. Considering that the structure of the propagating species and the rate of the polymerization (kp) are expected to be the same in all three cases, the enhanced activity may be attributed to the stabilization of the benzyl cation by the substituents. Similar effects were observed with the benzylic sulphonium salts. However, more detailed studies [30] of polymerization and hydrolytic properties of various p-substituted... 

The difficulties encountered in using the analysis of substituent effects in solvolyses as a mechanistic probe mostly arise from the mechanistic involvement of the solvent (Shorter, 1978, 1982 Tsuno and Fujio, 1996). Consequently, the behaviour of benzylic carbocations in the gas phase should be the best model for the behaviour of the solvolysis intermediate in solution (Tsuno and Fujio, 1996). The intrinsic substituent effects on the benzylic cation stabilities in the gas phase have also been analysed by equation (2), and they will be compared here with the substituent effects on the benzylic solvolysis reaction. In our opinion, this provides convincing evidence for the concept of varying resonance demand in solvolysis. Finally, we shall analyse the mechanisms of a series of benzylic solvolysis reactions by using the concept of a continuous spectrum of varying resonance demand. 

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroanthnonate salt at —124 °C. It is nearly planar (8 ° twist), with a short bond between the C " and the ring (1.41 A), consistent with benzylic delocalization. The Me—C bonds are also shortened, indicative of hyperconjugative interaction. However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5). The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation. The heats of formation of some a-methylbenzyl cations indicate that the primary stabilization in these species comes from the a-substituents, and that the stabilization provided by the aromatic ring is secondary. ... 

The fact that the benzyl anion and benzyl cation should have opposite MCD signs according to the mirror-image theorem makes it easy to understand why benzene derivatives with mesomeric donor substituents, which are isoelectronic with the benzyl anion, and benzene derivatives with mesomeric acceptor substituents, which are isoelectronic with the benzyl cation, show opposite MCD signs. This fact can be used for a qualitative and even a quantitative characterization of mesomeric substituent effects. (See also Section 2.4 and Example 3.10.)... 

In p-silyl substituted benzyl cations with different a-aryl substituents the stabilizing effect of a P-silyl group depends on the electron donating ability of the a-aryl group. The better the a-aryl substituent can stabilize the positive charge, the lower is the need for P-o-hyperconjugative stabilization by the silyl group and the no-bond resonance structure becomes less important (Fig. 6). 

Polymer Tacticity. Our initial results on the polymerization of several different p-substituted-a-methylstyrene monomers indicated that there was some relationship between polymer stereoregularity and both the type of initiator and substituent in these monomers ( ). However, our recent investigations with a much wider variety of monomers, catalysts and cocatalysts revealed that the classical approach to analyzing substituent effects in organic reactions, the use of the Hammett pa relationship, gave no simple and self-consistent relationship between tacticity and the a (or a ) constant for the para-substituent. These results are summarized in the data in Table I for the cationic polymerization of a-methylstyrene and a series of five p-substituted-a-methylstyrene monomers initiated with two different Friedel-Crafts catalysts, TiCl and SnCl, either alone or with a cocatalyst benzyl chloride (BC) or t-butyl chloride (TBC), in methylene chloride at -78°C. Where a cocatalyst was used, the initiator was presumably a carbonium ion formed by the following reaction ... 

Fleming and co-workers previously reported that the photocleavage of benzyl-sulfur bonds proceeds via a radical mechanism [14,149,150]. As part of that study they irradiated benzylphenyl sulfide and several of its derivatives, 107, which allowed a comparison of the substituent effects on the photocleavage of benzyl sulfides. On the basis of this work and transient absorption studies [151], benzyl sulfides apparently undergo almost exclusive radical photocleavage. No products resulting from benzyl cations were detected. These irradiations were performed in r-butyl alcohol and acetonitrile. 

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