Acyl cations superelectrophilic

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. 

As discussed in Chapter 1, Brouwer and Kiffen reported the observation that HF-BF3 promoted hydride transfer from isoalkanes to acyl cations. These results were later shown by Olah and co-workers to be due to superelectrophilic activation of the acyl cation (24, eq 13).37 Diproto-nated acetone and aldehydes were also shown to abstract hydride from isoalkanes in HF-BF3 solutions.38 Carboxonium ions (25) are generally... 

Related classes of gitonic superelectrophiles are the previously mentioned protoacetyl dications and activated acyl cationic electrophiles. The acyl cations themselves have been extensively studied by theoretical and experimental methods,22 as they are intermediates in many Friedel-Crafts reactions. Several types of acyl cations have been directly observed by spectroscopic methods and even were characterized by X-ray crystal structure analysis. Acyl cations are relative weak electrophiles as they are effectively stabilized by resonance. They are capable of reacting with aromatics such as benzene and activated arenes, but do not generally react with weaker nucleophiles such as deactivated arenes or saturated alkanes. 

There are several reports of activation of acyl cations by superacids, suggesting the involvement of gitonic superelectrophiles.61 As discussed in Chapter 2, hydride transfer from isobutane to the acetyl cation has been reported when the reaction is carried out in excess HF-BF3. At the same... 

Like other superelectrophiles, protonation of the acyl cations leads to decrease of neighboring group participation and a lowering of the LUMO energy level, which facilitates reactions with even weak nucleophiles. 

There have been several types of gitonic superelectrophiles having acyl cationic groups as part of a 1,3-dicationic system. Monocationic acyl cations (168) can be prepared as persistent species in superacidic media and salts have even been studied by X-ray crystallography.113 Much of the interest in superelectrophilic species from acyl cations has focused on the protioacyl dications (169, vide supra).61... 

Several types of onium dications have been studied in which a single acyl cationic center has been part of 1,3-dicationic superelectrophiles. For example, pyruvic acid has been studied in FSOsH-SbFs solution at low temperatures.34 Initially the diprotonated species is observed in equilibrium with some of the monocation (eq 55). 

This provides maximum charge-charge separation in the dication. Thus, it is expected that analogous distonic superelectrophiles will likewise tend to undergo protosolvation at the carbon(s) most distant from the acyl cation center. 

Monocationic acyl ions are readily prepared as persistent species in solutions of low nucleophile strength.68 These acyl ions have been thoroughly characterized by IR and NMR spectroscopy, and several acyl ion salts have been characterized by X-ray crystallography. The monocationic acyl ions are often prepared in situ from carboxylic acids, esters, or anhydrides, by the action of a strong Brpnsted acid, or the ions can be prepared from ionization of an appropriate acid halide with a strong Lewis acid. Both methods have been used to prepare acyl-centered dications, some of which can be considered distonic superelectrophiles. As described previously, dicarboxylic acids cleave to the bis-acyl ions in superacid (FSChH-SbFs) provided that the acyl cations are separated by at least three methylene units (eq 54).55 The first bis-acyl dications were reported by Olah and Comisarow, being prepared by the reactions of dicarboxylic acid fluorides with superacidic SbFs (eq 72).69... 

One of the proposed intermediates in this transformation is the super-electrophilic species (184), which undergoes deprotonation to give the 2-buten-oyl cation. Further evidence for the superelectrophile 184 is obtained from experiments in which the 2-butenoyl cation (185) is generated in DSOsF-SbFs. Significant deuterium incorporation is found at the a and y positions, suggesting equilibria involving 184-186. In a similar respect, formation of the 4-chloro-3-methylbutanoyl cation (187) in superacid leads to the two acyl dications (188-189, eq 57).69... 

In dication 188, the NBO charge at the carbenium ion center is +0.69 and at the acyl carbon is +1.09. The tert-butyl cation has been found to have NBO charge at its carbenium center of +0.67, suggesting a modest superelectrophilic activation of the carbenium ion center in 188, compared with the tert -butyl cation. When charges on the methyl groups are also considered, structure 188 is similar to the protosolvated tert -butyl cation 196. 

In contrast, the 1,1-diphenyl ethyl cation (92) is unreactive to benzene. This indicates that the carboxonium group (or the corresponding acyl ion) participates in the superelectrophilic activation of the adjacent carboca-tionic center. 

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