Carbocations acyl cations

Unlike alkyl carbocations, acyl cations do not rearrange. An acyl cation is so strongly stabilized by electron delocalization that it is more stable than any other ion that would arise from it by a hydride or alkyl group shift. 

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

Alkenes can be acylated with an acyl halide and a Lewis acid catalyst in what is essentially a Friedel-Crafts reaction at an aliphatic carbon. ° The product can arise by two paths. The initial attack is by the acyl cation RCO (or by the acyl halide free or complexed see 11-14) at the double bond to give a carbocation ... 

When acid catalysts are employed, in the absence of nickel carbonyl, the mechanism involves initial attack by a proton, followed by attack of the resulting carbocation on carbon monoxide to give an acyl cation, which, with water, gives the product ... 

The reaction is also called hydrocarboxylation. According to a later modification, the alkene first reacts with carbon monoxide in the presence of the acid to form an acyl cation, which then is hydrolyzed with water to give the carboxylic acid.97 The advantage of this two-step synthesis is that it requires only medium pressure (100 atm). Aqueous HF (85-95%) gave good results in the carboxylation of alkenes and cycloalkenes.98 Phosphoric acid is also effective in the carboxylation of terminal alkenes and isobutylene, but it causes substantial oligomerization as well.99 100 Neocarboxylic acids are manufactured industrially with this process (see Section 7.2.4). The addition may also be performed with formic acid as the source of CO (Koch-Haaf reaction).101 102 The mechanism involves carbocation formation via protonation of the alkene97 103 [Eq. (7.10)]. It then reacts with carbon monoxide... 

Trivalent ( classical ) carbenium ions contain an, vp2-hybridized electron-deficient carbon center that tends to be planar in the absence of constraining skeletal rigidity or steric interference. (It should be noted that ip-hybridized, linear acyl cations and vinyl cations also show substantial electron deficiency on carbon.) The carbenium carbon contains six valence electrons, and thus it is highly electron-deficient. The stmcture of trivalent carbocations can always be adequately described by using two-electron, two-center bonds (Lewis valence bond structures). 

The Koch-Haaf reaction397 for the preparation of carboxylic acids from alkenes uses formic acid or carbon monoxide in strongly acidic solutions. The reaction between carbocations and carbon monoxide affording oxo-carbenium ions (acyl cations) is a... 

The electrophile, an acyl cation, is generated in a manner similar to that outlined in Figure 17.4 for the generation of the carbocation electrophile from an alkyl halide. First the Lewis acid, aluminum trichloride, complexes with the chlorine of the acyl chloride. Then A1C14 leaves, generating an acyl cation. The acyl cation is actually more stable than most other carbocations that we have encountered because it has a resonance structure that has the octet rule satisfied for all of the atoms ... 

The synthesis starts with the addition of senecioyl chloride (6.96) to isoprene (6.97). The acyl cation derived from the acid chloride adds to isoprene with a high degree of regioselectivity, for reasons which will be explained in the next paragraph. The carbocation produced by addition of the senecioyl ion to isoprene is trapped by the chloride anion to give... 

Although the mechanism for the formation of acetylcyclohexane is known, an alternative mechanism is proposed here for the acylation of cyclohexene with acetyl chloride. In this reaction, after the acyl cation adds to double bond, the elimination of the a-proton gives an a,)0-unsaturated ketone, whereas the attack of chloride produces a 0-chloroketone. Since acetyl is an electron-withdrawing group, it is plausible that the positive charge migrates to form a more stable carbocation, which electrophilically alkylates with benzene. 

Typical cations of such salts are the trityl carbocation, (C6H5)3C , the tropylonium carbocation, C7H , and the aryl diazonium cation, ArNf On the other hand, the acyl cations, RCO , probably belong to the class of covalent compounds. 

Superacids such as Magic Acid and fluoroantimonic acid have made it possible to prepare stable, long-lived carbocations, which are too reactive to exist as stable species in more basic solvents. Stable superacidic solutions of a large variety of carbocations, including trivalent cations (also called carbenium ions) such as t-butyl cation 1 (trimethyl-carbenium ion) and isopropyl cation 2 (dimethylcarbe-nium ion), have been obtained. Some of the carbocations, as well as related acyl cations and acidic carboxonium ions and other heteroatom stabilized carbocations, that have been prepared in superacidic solutions or even isolated from them as stable salts are shown in Fig. 1. 

The mechanism involves carbocation formation via protonation of the alkene (87,89) (eq. 62) followed by its reaction with carbon monoxide to form acyl cation (acylium ion) 23 (eq. 63). In the final step (eq. 64), 23 is quenched with water to 3ueld the product a-methyl carboxylic acid. 

Cocatalysts of two types occur (/) proton-donor substances, such as hydroxy compounds and proton acids, and (2) cation-forming substances (other than proton), including alkyl and acyl haUdes which form carbocations and other donor substances leading to oxonium, sulfonium, halonium, etc, complexes. 

Other, removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a sdyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. The key step, treatment of (257) with methyl aluminum chloride in methylene chloride at —78° C, followed by acylation and chromatographic separation, affords (258) in 55% yield (two steps). When this cyclization was attempted on similar compounds that did not contain the C7P-silicon substituent, no tetracycHc products were observed. Steroid (258) is converted to lanosterol (77) in three additional chemical steps (225). 

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