Resonance benzylic cation

Draw all reasonable resonance contributors for both planar and perpendicular conformers of benzyl cation. Identify the site(s) of the positive charge in each. Which cation would you expect to be more stable Which is the more stable Compare energies of planar and perpendicular conformers of benzyl cation. 

We saw in Section 6.9 that the stability order of alkyl carbocations is 3° > 2° > 1° > —CH3. To this list we must also add the resonance-stabilized allvl and benzyl cations. Just as allylic radicals are unusually stable because the... 

Experimental observations,23 supported by high-level ab initio calculations, 24 indicate that two extreme resonance forms contribute to the general energy of the benzyl cation the aromatic form A, in which the positive charge is concentrated at the methylene group, and the nonaromatic, methylene arenium form B with a sp2 ipso-carbon atom and ring-localized charge (Scheme 3.13). Unlike benzyl cations of the form A, which were isolated and studied, especially by Olah and coworkers,23 compounds represented by the form B remained elusive. Thus, metal complexation... 

Estimating stability it is possible to apply criteria commonly used in organic chemistry. Tertiary alkyl carbocation is more stable than the secondary one which is in its turn more stable than the primary one. For the carbon ions of this type the row of the stability is reversed. Allyl and benzyl cations are stable due to the resonance stabilization. The latter having four resonance structures may rearrange to be energetically favorable in the gas phase tropilium cation possessing seven resonance forms (Scheme 5.3). 

Due to the benzylic p-Tc resonance stabilization the C+-C,pso bond has partial double bond character and the ortho, ortho and meta, meta methine groups syn and anti to the silyl group are non-equivalent. The effect pf the a-silyl group on the positive charge in benzyl cations can be estimated by comparison of the NMR spectroscopic data of the 1 -phenyl-1 -(trimethylsilyl)ethyl cation 1 with those for the 1-phenylethy 1 cation 5 (P) and the cumyl pation 3 (15, 16, 17) (Table 1). 

It is into the LUMO, the energetically most accessible unfilled molecular orbital, that any further electrons will go. Hence, it may be thought of as demarking the location of positive charge in a molecule. The LUMO in planar benzyl cation is delocalized away from the formal cation center and onto the ortho and para ring carbons, in accord with classical resonance structures. On the other hand, the LUMO in perpendicular benzyl cation remains primarily localized on the benzy lie carbon. Resonance theory suggests that delocalization of the positive charge leads to stabilization. Thus, planar benzyl cation is more stable than perpendicular benzyl cation. 

Molecular orbital descriptions offer a number of significant advantages over conventional resonance structures. For one, they often provide more compact descriptions, e.g., the LUMO in planar benzyl cation conveys the same information as four resonance structures. Second, orbital descriptions are quantitative, compared to resonance structures which are strictly qualitative. Finally, molecular orbital descriptions may be applied much more widely than resonance descriptions. Of course, molecular orbital descriptions cannot be generated using a pencil as can resonance structures, but rather require a computer. It can be argued that this does not constitute a disadvantage, but rather merely reflects a natural evolution of the tools available to chemists. 

The latter reveals heavy concentration of positive charge (blue color) on the benzylic carbon and perpendicular to the plane of the ring. This is consistent both with the notion that only a single resonance structure may be drawn, as well as with the fact that the LUMO is localized almost entirely on the benzylic carbon (see discussion earlier in this chapter). On the other hand, planar benzyl cation shows no such buildup of positive charge on the benzylic carbon, but rather delocalization onto ortho and para ring carbons, exactly as suggested... 

An organic chemist would know that benzyl cation is planar and not perpendicular because four resonance structures are better than one . A physical chemist would reach the same conclusion based on Coulomb s law separation of charge requires that energy be expended . 

Both bromides give benzylic cations which are resonance stabilized by the benzene rings. The nitrogens of A are more electronegative than the carbons of C which should destabilize the ion somewhat. The greatest stabilization comes from the fact that the three-membered cationic ring of A is a lit aromatic system. This aromatic stabilization of the cation makes its formation much more rapid than the cation from C. 

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. 

In a cation such as the (2,4-di-ferf-butyl-6-methyl)benzyl cation 147, a high rotational barrier around the v/r-hybridized atom is observed. The methylene protons are found magnetically nonequivalent in the1H NMR spectrum.356 Recent combined experimental and theoretical studies for the related cation 143 suggest357 that structure 143b is an important resonance contributor. 

Reactions at Saturated Carbons trans ting junction as the new formed bond would be pseudoequatorial. The five-membered ring D is produced because the resonance-stabilized benzyl cation 240 is formed in preference to the six-membered ring homobenzylic cation 241. 

Benzyl-type linkers are the most common anchoring groups for various kinds of functionality. Esters, amides, amines, alcohols, and thiols, in particular, can be immobilized by this linker family. This was demonstrated by Merrifield [2] and Wang [19] and is the starting point of modern linker development. Benzylic linkers are typically cleaved by strong acids (for example trifluoroacetic acid, TFA), which cause protonation and subsequent elimination. A nucleophilic scavenger usually quenches the resonance-stabilized cation thus formed. 

The four resonance forms of the benzylic cations (bottom of Table 2.2) allow for the prediction of structural details. They are confirmed, for example, by crystal structure analysis of the... 

Why then are trityl cations still more stable than benzylic cations An aryl residue that is rotated out of the nodal plane of the 2py AO of the benzylic cation center by an angle % provides resonance stabilization that is decreased by cos2 -fold. Three aryl residues in a trityl cation can thus provide up to 3 x cos2(30°) = 2.25 times more resonance stabilization than one... 

In Section 3.5.1, it was mentioned that Br2 and Cl2 form resonance-stabilized benzyl cation intermediates with styrene derivatives and that gem-dialkylated alkenes react with Br2 hut not... 

In Section 3.5.1 it was mentioned that Br2 and Cl2 form resonance-stabilized benzyl cation intermediates with styrene derivatives and that gem-dialkylated olefins react with Br2 but not with Cl2 via halonium ions. Because C—Cl bonds are shorter than C—Br bonds, chloronium ions presumably have a higher ring strain than bromonium ions. Accordingly, a /3-chlorinated tertiary carbenium ion is more stable than the isomeric chloronium ion, but a /3-brominated tertiary carbenium ion is less stable than the isomeric bromonium ion. 

Aromatic aldehydes and aromatic ketones also can be reduced to hydrocarbons in a completely different manner, namely via the so-called ionic hydrogenation followed by an ionic hydrogenolysis. This kind of reduction is possible only if it can proceed via resonance-stabilized cationic intermediates. This resonance stabilization is readily achieved in a benzylic position, and it is therefore advantageous to employ aromatic carbonyl compounds in this kind of reduction. The carboxonium ion A, formed... 

The following represent the resonance forms of the benzyl cation ... 

Compounds containing aromatic rings tend to fragment at the carbon (called a benzylic carbon) next to the aromatic ring. Such a cleavage forms a resonance-stabilized benzylic cation. 

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. 

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