Carbocations, benzylic aryl groups

Electrophilic addition (Section 11 16) An aryl group stabilizes a benzylic carbocation and con trols the regioselectivity of addition to a double bond involving the benzylic carbon Markovni kov s rule is obeyed... 

The diarylmethyl cations listed in Table 5.1 are 6-7pATr+ units less stable than the corresponding triarylmethyl cations. This indicates that the additional aryl group has a cumulative, although not necessarily additive, effect on stability of the carbocation. Primary benzylic cations (monparylmethyl cations) are generally not sufficiently stable for determination of pATr+ values. A particularly stable benzylic ion, the 2,4,6-trimethyl-phenylmefliyl cation, has a pATr+ of — 17.4. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

The SN1 reaction is almost always followed by tertiary halides, and also by secondary halides and, for example, tosylates, especially when at least one of the substituents is an aryl group. From this latter case the carbocation is now benzylic. The nature of the intermediate carbocation in an SN1 reaction is illustrated in Figure 7.3, which shows the three substituents in a coplanar arrangement and with an empty p-orbital. This structure provides a clue as to the stereochemistry of the subsequently formed tetrahedral product. 

The second acceleration effect is based on the radical promotion and mainly depends on nature of the substituents on the benzene ring of the benzyl alcohol. In this case, benzyl ether groups containing hydrogen atoms, which are labile to abstraction, are attached to ends of the polymer chains (see Equation 11.106 in Scheme 11.38). In the subsequent stage, benzyl ether hydrogens are abstracted by aryl radicals produced as a result of the photolysis of the onium salt, and benzyl ether radicals are formed (Equation 11.107). These more stable radicals, in turn, are oxidized by the onium salt to corresponding carbocations, which are also stable and able to initiate a new polymerization chain more rapidly (Equation 11.108), as described by Ledwith [82], This process also results in increase in the quantum yield of the photolysis of the onium salt. 

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 extent of participation of the carbon-carbon double bond in the ionization of anti-7-norbomenyl systems is a function of the substitution at C-7. The placement of an aryl substituent at C-7 diminishes the relative rate acceleration due to participation by the double bond. Evidently, the extent of participation is a function of the stability of the potential carbocation. When an aryl group is present at C-7, the resulting benzyl-type stabilization decreases the importance of participation by the double bond. The degree of stabilization is sensitive to substituents on the phenyl ring. For p-methoxyphenyl, phenyl, and p-trifluoromethylphenyl, the rate factor for the unsaturated relative to the saturated system is 3,40, and 3.5 x 10, respectively. The double bond clearly has a much larger effect on the poorly stabilized p-trifluoromethyl-substituted system. This dependence of the extent of participation on other stabilizing features is a general trend and has been observed with other types of carbocations. ... 

In the same way that an aromatic ring activates a neighboring (benzylic) C H toward oxidation, it also activates a benzylic carbonyl group toward reduction. Thus, an aryl alkyl ketone prepared by Friedel-Crafts acylation of an aromatic ring can be converted into an alkylbenzene by catalytic hydrogenation over a palladium catalyst. Propiophenone, for instance, is reduced to propylbenzene by catalytic hydrogenation. Since the net effect of Friedel-Crafts acylation followed by reduction is the preparation of a primary alkylbenzene, this two-step sequence of reactions makes it possible to circumvent the carbocation... 

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