Carboxonium ions superelectrophilic

Diprotonated, superelectrophilic intermediates were suggested to be involved in both conversions. Considering protonated aldehydes, benzal-dehyde gives a carboxonium ion that is significantly resonance stabilized and thus unreactive towards aromatic substrates such as o-dichlorobenzene or nitrobenzene. Pyridinecarboxaldehydes, however, show much higher electrophilic reactivities due to their ability to form via TV-protonation the superelectrophile (5, eq 8).10 A similar situation is seen in the hydroxyalkylation reactions of acetyl-substituted arenes. Acetophenone is fully protonated in excess triflic acid, but the resulting carboxonium ion (6) is... 

Carboxonium ions are indicated to be involved in a number of super-electrophilic reactions. In several cases, the direct observation of the superelectrophiles and reactive dications has been possible using low... 

The proposed dicationic, carboxonium ion intermediates (58-62) have been directly observed by 13C NMR. A more detailed description of this superelectrophilic chemistry is found in chapters 5-7. 

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... 

Vicinal 1,2-carbodicationic systems are some of the most important and thoroughly studied superelectrophiles. They include inter alia 1,2-ethylene dications, related carbon-nitrogen superelectrophiles (diprotonated imines and nitriles), protosolvated carboxonium ions, and superelectrophilic tri-halomethyl cations. It is understood that many of these systems involve extensive charge delocalization, and in a sense may not be considered formal 1,2-dicationic systems. For example, diprotonated 2,3-butanedione (34) may be represented formally as a 1,2-ethylene dication (35a, a gitonic superelectrophile), but even in monocationic carboxonium ions, there is a significant amount of double-bond character retained in the carbon-oxygen bond (eq 5).22... 

A wide variety of these superelectrophilic carboxonium ions have been studied. It has long been recognized that carboxonium ions are highly stabilized by strong oxygen participation and therefore are much less reactive than alkyl cations. For example, trivalent carbocations are efficient hydride abstractors from tertiary isoalkanes (eq 41). 

Besides promoting reactions with weak nucleophiles, several molecular rearrangements have been reported with superelectrophilic carboxonium ions suggested as intermediates. This includes the superacid promoted rearrangment of 2,2,2-triphenylacetophenone (157) with subsequent formation of the 9,10-diphenylphenanthrene (160, eq 43).30... 

There have been two reports involving gitonic superelectrophiles composed of carboxonium ions and vinylic carbocations in a 1,3-relationship. In the reaction of 3-phenylpropynoic acid (65) with benzene in superacid the novel carboxonium-vinyl dication 66 is generated, followed by reaction with benzene and then cyclization (eq 22).26a Likewise, the unsaturated amide (67) gives the cyclization product in high yields (70-97%) in very strong acids (polyphosphoric acid, CF3SO3H, Nation SAC-13, or HUSY eq 23).30... 

Carboxonium-Ammonium and Related Dications A wide variety of species have been generated in which the 1,3-dicationic structure arises from carboxonium ion centers being adjacent (separated by one carbon) to an ammonium or related charge center. These intermediates may be described as reactive dications, yet they have been shown to exhibit electrophilic reactivities comparable to superelectrophiles. 

Several gitonic superelectrophiles have been reported having closely oriented oxonium and carboxonium ion centers, some of which may be considered 1,3-dications. A series of hydroxy-substituted carboxylic acids were studied in FSOsH-SbFs in solution and the oxonium-carbonium dications could be directly observed at low temperature.57 In the case of lactic acid, dication 147 is a persistent ion at — 80°C, but at temperatures above — 60°C, formation of the diprotonated lactide (148) is observed (eq 48). 

It was previously noted that superelectrophilic carboxonium ions may be generated from suitable precursors, including amino-ketones, /V-heteroaromatic ketones and aldehydes, amino-acetals, and other substrates.45 In their superacid-promoted condensation reactions, these compounds often produce ammonium-carbenium superelectrophiles as intermediates in the reactions. As an example, the amino-acetal (229) reacts with arenes in the presence of superacid to give the arylated product (231, eq 72).43... 

The conversion is thought to involve formation of the carboxonium ion (77) by protonation of the carbonyl oxygen, and subsequent protonation then occurs at the C-H bond. The resulting carboxonium-carbonium dication (78) possesses the maximum possible charge-charge separation for this bicyclic framework. Subsequently, an intermediate carboxonium-carbenium dication (79) is produced, which isomerizes to the tertiary -carbenium ion, and deprotonation provides the product enone (80). Similar distonic superelectrophiles are proposed in other rearrangements of terpenes in superacid.28... 

Among the reported distonic superelectrophiles, a significant number of ammonium-carboxonium dications and related species have been studied. It has been shown that these electrophiles show enhanced reactivities compared with monocationic carboxonium ions. For example, 4-piperidone (172) is diprotonated in superacidic CF3SO3H to give the distonic superelectrophile (173), which condenses with benzene in high yield (eq 59).56 In contrast, cyclohexanone forms the monocationic carboxonium (174) ion, but ion 174 is not sufficiently electrophilic to react with benzene (eq 60). 

The electrophilicity index also accounts for the electrophilic activation/deactivation effects promoted by EW and electron-releasing substituents even beyond the case of cycloaddition processes. These effects are assessed as responses at the active site of the molecules. The empirical Hammett-like relationships found between the global and local electrophilicity indexes and the reaction rate coefficients correctly account for the substrate selectivity in Friedel-Crafts reactions, the reactivity of carbenium ions, the hydrolysis of esters, the reactivity at the carbon-carbon double bonds in conjugated Michael additions, the philicity pattern of carbenes and the superelectrophilicity of nitronium, oxonium and carboxonium ions. This last application is a very promising area of application. The enhanced electrophilicity pattern in these series results from... 

Two types of interactions have been shown to be involved in superelectrophilic species. Superelectrophiles can be formed by the further interaction of a conventional cationic electrophile with Brpnsted or Lewis acids (eq 16).23 Such is the case with the further protonation (protosolvation) or Lewis acid coordination of suitable substitutents at the electron deficient site, as for example in carboxonium cations. The other involves further protonation or complexation formation of a second proximal onium ion site, which results in superelectrophilic activation (eq 17).24... 

Among other distonic superelectrophiles described in the literature, there are carbo-onium dications. These include carbo-carboxonium dications, carbo-ammonium dications, and related ions. Despite the separation of charge in these superelectrophiles, some have been shown to have very high electrophilic reactivities. I. ike the carbodications described previously, the discussion here is limited to those systems that have been shown to have electrophilic reactivities greater than the related monocationic onium ions, as well as structural criteria supporting their designation as a distonic superelectrophilic species. 

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. 

There have been several distonic superelectrophiles described in the literature that are oxonium-centered dications. For example, a series of diols were solvated in FSOsH-SbFs-SC and the dicationic species were persistent at —80°C.77 Upon warming to 25°C, the bis-oxonium ions undergo rearrangement to the more stable carboxonium monocations (eq 77). Barring a concerted mechanism, the transformations are thought to involve the carbo-oxonium dications (i.e., 225) and concomitant hydride shifts. Interestingly, 2,5-hexanediol ionizes in superacid, and with warming the cyclic oxonium ion (229) is formed (eq 78). 

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