Aromatically Stabilized Carbocations

If a carbocation at the same time is also a Huckeloid (4n+2) n aromatic system, resonance can cause substantial stabilization. There were numerous aromatically stabilized Huckeloid systems128-13 s) generated in superacidic media in recent years and characterized by NMR spectroscopy. Some of the best known examples are the following. 

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A wide variety of carbocations and carbodications, including those that are aromatically stabilized as well those as stabilized by heteroatoms, were reported in the nearly 200 publications on the topic during my Cleveland years. 

Oxygen stabilized carbocations of this type are far more stable than tertiary carbocations They are best represented by structures m which the positive charge is on oxygen because all the atoms have octets of electrons m such a structure Their stability permits them to be formed rapidly resulting m rates of electrophilic aromatic substitution that are much faster than that of benzene... 

An electrophilic aromatic substitution reaction takes place in two steps—initial reaction of an electrophile, E+, with the aromatic ring, followed by loss of H+ from the resonance-stabilized carbocation intermediate to regenerate the aromatic ring. 

Because the initial electrophilic attack and carbocation formation results in loss of aromatic stabilization, the electrophiles necessary for electrophilic aromatic substitution must be more reactive than those that typically react with alkenes. Thus, chlorination or... 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

Substitution will occur in the most electron-rich aromatic ring by way of the most stabilized carbocation intermediates (see Problem 13.14). 

The neutral 1,4- and 1,2-quinone methides react as Michael acceptors. However, the reactivity of these quinone methides is substantially different from that of simple Michael acceptors. The 1,6-addition of protonated nucleophiles NuH to simple Michael acceptors results in a small decrease in the stabilization of product by the two conjugated 7T-orbitals, compared to the more extended three conjugated 7T-orbitals of reactant. However, the favorable ketonization of the initial enol product (Scheme 1) confers a substantial thermodynamic driving force to nucleophile addition. By comparison, the 1,6-addition of NuH to a 1,4-quinone methide results in a large increase in the -stabilization energy due to the formation of a fully aromatic ring (Scheme 2A). This aromatic stabilization is present to a smaller extent at the reactant quinone methide, where it is represented as the contributing zwitterionic valence bond structure for the 4-0 -substituted benzyl carbocation (Scheme 1). The ketonization of the product phenol (Scheme 2B) is unfavorable by ca. 19 kcal/mol.1,2... 

Like an alkene, benzene has clouds of pi electrons above and below its sigma bond framework. Although benzene s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabilized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. 

The basic concept underlying alkylation reactions of aromatics is the formation of a stabilized carbocation able to attack nucleophilic substrates. Hydrocarbon cracking and hydrocracking, alkane isomerization, and olefin alkylation are important processes based on related alkane carbocation chemistry in the production of various types of hydrocarbons such as the branched ones for high octane gasolines. Zeolites and metal oxides are the preferred catalysts. 

Significantly low r values have been observed in the protonation equilibria (p i BH values) of benzoyl compounds (Mishima et al., 1988, 1990c, 1996c). ITie trend in the r values mentioned above predicts that a stabilized carbocation will not require a large ir-delocalization of the positive charge. Substituent effects on Ae gas-phase basicities (AG(co>h+) of the aromatic carbonyl compounds [30(R)], ArCOR (30) have been studied. 

In bromination (Mechanism 18.2), the Lewis acid FeBr3 reacts with Br2 to form a Lewis acid-base complex that weakens and polarizes the Br- Br bond, making it more electrophilic. This reaction is Step [1] of the mechanism for the bromination of benzene. The remaining two steps follow directly from the general mechanism for electrophilic aromatic substitution addition of the electrophile (Br in this case) forms a resonance-stabilized carbocation, and loss of a proton regenerates the aromatic ring. 

This reaction is another example of electrophilic aromatic substitution, with the diazonium salt acting as the electrophile. Like all electrophilic substitutions (Section 18.2), the mechanism has two steps addition of the electrophile (the diazonium ion) to form a resonance-stabilized carbocation, followed by deprotonation, as shown in Mechanism 25.4. 

The best electron sinks, occurring almost exclusively in acidic media, are reactive cations such as carbocations (Section 4.2.4). Most are such good electron sinks that they can react with even very poor electron sources like aromatic rings. The most stable carbocations are the least reactive. Highly stabilized carbocations like +C(NH2)3 are so stable that they can exist in basic media and make very poor electrophiles. The following are some of the more common reactive carbocations. 

Friedel-Crafts—Aromatic Rings with Lone-Pair-Stabilized Carbocations as Sinks... 

Electrophilic aromatic substitution is a three-step process. First, a positive electrophile is generated. This is followed by two-step substitution. In the first step, the positive electrophile bonds to the benzene ring and produces a resonance stabilized carbocation. Then hydrogen ion is lost from the ring as the carbocation is neutralized and the benzene ring is regenerated. 

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