Carbocations aromatics

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

Three significant hydrocarbon ions, [S" ], [9 ] and [10 ], should be mentioned as additional examples of the formation of stable salts of the hydrocarbon ions. The triphenylmethylium ion [8" ], as a prototypical aromatic carbocation, reacts with the chloride ion to give the chloride, a... 

We consider the relatively high pKA values of 6-8 to be typical value for a cation-quinone methide equilibrium. The formation of a resonance-stabilized aromatic carbocation is one reason for these high pKA values. Another reason is the high energy of the quinone methide. The thermodynamic cycle shown in... 

In all these cases it is possible to determine Kk directly by combining kn2o with ku, the rate constant for carbocation formation. The latter constant is readily determined spectrophotometrically by monitoring acid-catalyzed dehydration of the aromatic hydrate to the corresponding aromatic product. In principle, as we have seen, when the dehydration product is aromatic, carbocation formation is the rate-determining step of the reaction. However, the finite values of kp/kn2o for the phenanthrenonium ion and other areno-nium ions leading to moderately stable aromatic products imply a small correction for reversibility of this reaction step. 

Hehre et 3 . who have used ion-cyclotron resonance techniques to obtain criteria of aliphatic and aromatic carbocation stabilities. 

Carbocations in which the empty C(p) orbital is part of an aromatic system (Chapter 1) are considerably stabilized over what one would expect just from resonance stabilization alone. The most important aromatic carbocation is the tropylium (cycloheptatrienylium) ion, which is so stable that one can buy its salts from commercial suppliers. Conversely, those carbocations in which the empty C(p) orbital is part of an antiaromatic system (e.g., cyclopentadienylium) are considerably destabilized. 

Nitrenium ions may occur during bioactivation of aromatic amines and amides, which are usuaUy A-oxidized into A-hydroxylated derivatives. By sulfation or esteriUca-tion foUowed by elimination of the newly formed leaving group, the latter may be transformed into highly reactive nitrenium ions. In the case of aromatic nitrenium ions they are in equiUbrium with their tautomeric aromatic carbocations, which react with ceUular nucleophiUc macromolecules (nucleic acids, etc.). 

Several linkers have been developed that rely on the formation of highly stabilized aromatic carbocations. The most frequently used are the eponymous Sieber amide linker 36 [3] and Barany s 3-XAL linker 6 [4]. Both are based on a 3-methoxyxanthine scaffold, which owing to the highly stabilized nature of the xan-thenium ion can provide primary amides on treatment with 1% TFA in DCM, making them excellent tools for the synthesis of protected peptide carboxamides. The Sieber amide resin has also been used to prepare secondary amides via reductive alkylation of the amino group, acylation of the resultant amine and cleavage with dilute TFA [88]. Brill et al. [67] have effected transamination of trifluoroacety-lated Sieber amide resin in good yield. This approach offers considerable potential for the immobilization of amines on this support. 

Table 7 Thermodynamic data for non-aromatic carbocations with formal rr bonds...

The aromatic carbocation is delocalized over the whole ring, making it a very stable intermediate and most easily formed in an SnI reaction. 

For the detection of stable carbocations, various physicochemical methods are used spectroscopy (aromatic carbocations possess a high extinction coefficient up to 10 l/(mol cm)), NMR spectroscopy, and methods of measuring the electroconductivity and the number of particles in a solution (cryoscopy). Many carbocations are very labile. To prove their formation, traps are used, reactants that react very promptly with carbocations Nj, tetrahydrothiophene. 

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. 

The carbocation is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is we are not comparing the stability of the two to each other Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic Cycloheptatrienyl cation although aromatic is still a carbocation and reasonably reac tive toward nucleophiles Its special stability does not imply a rock like passivity but rather a much greater ease of formation than expected on the basis of the Lewis struc ture drawn for it A number of observations indicate that cycloheptatrienyl cation is far more stable than most other carbocations To emphasize its aromatic nature chemists often write the structure of cycloheptatrienyl cation m the Robinson circle m a ring style... 

Alkyl halides by themselves are insufficiently electrophilic to react with benzene Aluminum chloride serves as a Lewis acid catalyst to enhance the electrophihcity of the alkylating agent With tertiary and secondary alkyl halides the addition of aluminum chlonde leads to the formation of carbocations which then attack the aromatic ring... 

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... 

Carbocations usually generated from an alkyl halide and aluminum chloride attack the aromatic ring to yield alkylbenzenes The arene must be at least as reactive as a halobenzene Carbocation rearrangements can occur especially with primary alkyl hal ides... 

Arenium ion (Section 12 2) The carbocation intermediate formed by attack of an electrophile on an aromatic substrate in electrophilic aromatic substitution See cyclohexadienyl cation... 

All lation of Phenols. The approach used to synthesize commercially available alkylphenols is Friedel-Crafts alkylation. The specific procedure typically uses an alkene as the alkylating agent and an acid catalyst, generally a sulfonic acid. Alkene and catalyst interact to form a carbocation and counter ion (5) which interacts with phenol to form a 7T complex (6). This complex is held together by the overlap of the filled TT-orbital of the aromatic... 

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

Measurement of (R /R ) can be accomplished by cyclic voltammetry for relatively Stable species and by other methods for less stable cations. The values obtained for AG -range from 83kcal/mol for the aromatic tropylium ion to 130kcal/mol for destabilized betizylic cations. For stable carbocations, the results obtained by this method correlate with cation stabiUty as measured by pKf.+. Some of these data are presented in Table 5.3. 

Very stable carbocation (stabilized by both alkoxy function and aromaticity)... 

The Friedel-Crafts reaction is a very important method for introducing alkyl substituents on an aromatic ring. It involves generation of a carbocation or related electrophilic species. The most common method of generating these electrophiles involves reaction between an alkyl halide and a Lewis acid. The usual Friedel-Crafts catalyst for preparative work is AICI3, but other Lewis acids such as SbFj, TiC, SnCl4, and BF3 can also promote reaction. Alternative routes to alkylating ecies include protonation of alcohols and alkenes. 

All these kinetic results can be accommodated by a general mechanism that incorporates the following fundamental components (1) complexation of the alkylating agent and the Lewis acid (2) electrophilic attack on the aromatic substrate to form the a-complex and (3) deprotonation. In many systems, there m be an ionization of the complex to yield a discrete carbocation. This step accounts for the fact that rearrangement of the alkyl group is frequently observed during Friedel-Crafts alkylation. 

Trialkyltin substituents are also powerful ipso-directing groups. The overall electronic effects are similar to those in silanes, but the tin substituent is a better electron donor. The electron density at carbon is increased, as is the stabilization of /S-carbocation character. Acidic cleavage of arylstannanes is formulated as an electrophilic aromatic substitution proceeding through an ipso-oriented c-complex. ... 

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