Carboxonium ion

The results of aetivatioii of aeyl cations led to our study of other carboxonium ions. Carboxonium ions are highly stabilized compared to alkyl cations. As their name indicates, they have both carbocationic and oxonium ion nature. 

Acetic acid and other carboxylic acids are protonated in superacids to form stable carboxonium ions at low temperatures. Cleavage to related acyl cations is observed (by NMR) upon raising the temperature of the solutions. In excess superacids a diprotonation equilibrium, indicated by theoretical calculations, can play a role in the ionization process. 

Protonation of formic acid similarly leads, after the formation at low temperature of the parent carboxonium ion, to the formyl cation. The persistent formyl cation was observed by high-pressure NMR only recently (Horvath and Gladysz). An equilibrium with diprotonated carbon monoxide causing rapid exchange can be involved, which also explains the observed high reactivity of carbon monoxide in supera-cidic media. Not only aromatic but also saturated hydrocarbons (such as isoalkanes and adamantanes) can be readily formylated. 

In the 1970s, Brouwer and Kifflin reported the reactions of saturated hydrocarbons with aliphatic aldehydes and ketones in superacidic media. Analysis of the products from these reactions suggested that the protonated aldehydes and ketones (carboxonium ions) were reacting at the carbon-hydrogen o-bonds of the alkanes. This was a surprising observation because carboxonium... 

Alkoxycarbenium ions are important reactive intermediates in modem organic synthesis.28 It should be noted that other names such as oxonium ions, oxocarbenium ions, and carboxonium ions have also been used for carbocations stabilized by an adjacent oxygen atom and that we often draw structures having a carbon-oxygen double bond for this type of cations.2 Alkoxycarbenium ions are often generated from the corresponding acetals by treatment with Lewis acids in the presence of carbon nucleophiles. This type of reaction serves as efficient methods for carbon-carbon bond formation. 

Another important field of work is the characterization of oxonium and carboxonium ions, especially the energetics of their formation and the ranking of their stability in relation to structure. 

In the polymerisation of compounds which polymerise through the carbonyl group, the active species is believed to be a carboxonium ion IX ... 

Since the carboxonium ion has been eliminated as a possible propagating species, one is left with two alternatives which we may call the Keele and the Mainz theories. Plesch and Westermann [6, 8] have suggested that the cyclic formals polymerise by a ring-expansion mechanism, in which no free end is ever formed. This is illustrated in Reaction (B), where Y = H if the initiator is a protonic acid, and Y = Et if the initiator is a... 

When one considers how cyclic polymers could be formed from propagating species (VII) one is faced with exactly the same problem as with the carboxonium ion theory either there must be a 100% efficient end-to-end ring-closure (which in any case would be confined to polymers with an initial hydroxyl group) or there must be back-biting and the formation of linear fragments, as illustrated by Jaacks [17a]. 

Subsequently, a series of regioisomeric a-phenanthrene-substituted carbocations were generated from their alcohols by ionization with FS03H/S02C1F. Model carboxonium ions were also generated by O-protonation of the isomeric acetyl- and... 

Fig. 13 NMR data for a-phenanthrene-substituted carbocations and carboxonium ions and A5 C values in parenthesis.

Fig. 14 Structures of cyclopenta[(2]phenanthrene derivatives and their carbocations and carboxonium ions.

Fig. 15 Structures of representative benzanthracene derivatives and their derived carbocations/carboxonium ions (and comparison with model anthracene cations).

The methoxy and methyl substituents directed the protonation to their respective ortho positions.Whereas parent Ch was protonated at C-6/C-12, the 5-methyl derivative was protonated at C-6 and at C-12, with the latter being the thermodynamic cation. The 2-methoxy-Ch was protonated at C-1 to give two conformationally distinct carboxonium ions. In the disubstituted Ch derivatives, the 2-methoxy substituent was able to override the 5-methyl group, and the predominant carbocations formed were via attack at the position ortho to methoxy. For the methano derivative 37 (Me at C-9), a 3 1 mixture of37aH+/37bH+ was formed. 

Very recently, a series of novel carbocations and carboxonium ions were generated from 77/-benzo[c]fluorene (80), 1 l//-benzo[Z)]fiuorene (81), ll//-benzo[a]fluorene (82), 2-methoxy- (83), 7-methoxy- (84), and 9-methoxy-l l//-benzo[u]fluorene (85), 7//-dibenzo-[c,g]fluorene (86), 137/-dibenzo[u,g]fluorene (87), 2-methoxy-13//-dibenzo [u,g]fluorene (88), and 5,6-dihydro-13//-dibenzo[u,g]fluorene (89) (Fig. 30). Charge delocalization modes in the resulting carbocations were derived based on experimental and/or computed (GIAO-DFT) A8 C NMR values and via the NPA-deiived changes in charges (A ). 

The four-, five-, and six-membered analogs (178,180, and 182) were also obtained from the diprotonation of squaric acid (3,4-dihydroxy-3-cyclobutene-l,2-dione, 177), tri-O-protonation of croconic acid (4,5-dihydroxy-4-cyclopentene-l,2,3-trione, 179), and tetra-O-protonated rhodizonic acid (5,6-dihydroxy-5-cyclohexene-l,2,3,4-tetraone, 181), respectively. These ions were prepared in either Magic Acid (1 1 FSOsH-SbFs) or fluorosulfuric acid at low temperature and characterized by NMR. Ab initio/IGLO calculations showed that di-O-protonated squaric acid (178) is planar and aromatic, whereas the polyprotonated croconic and rhodizonic acids (180 and 182) have more carboxonium ion character, and no indication was obtained for any significant contributing homoaromatic structures. 

By in situ MAS NMR spectroscopy, the Koch reaction was also observed upon co-adsorption of butyl alcohols (tert-butyl, isobutyl, and -butyl) and carbon monoxide or of olefins (Ao-butylene and 1-octene), carbon monoxide, and water on HZSM-5 (Ksi/ Ai — 49) under mild conditions (87,88). Under the same conditions, but in the absence of water (89), it was shown that ethylene, isobutylene, and 1-octene undergo the Friedel-Crafts acylation (90) to form unsaturated ketones and stable cyclic five-membered ring carboxonium ions instead of carboxylic acids. Carbonylation of benzene by the direct reaction of benzene and carbon monoxide on solid catalysts was reported by Clingenpeel et al. (91,92). By C MAS NMR spectroscopy, the formation of benzoic acid (178 ppm) and benzaldehyde (206 ppm) was observed on zeolite HY (91), AlC -doped HY (91), and sulfated zirconia (SZA) (92). 

At higher temperatures hydrolysis of the carboxonium ion and Baeyer-Villiger oxidation lead to the formation of acetone, methanol, and methyl acetate ... 

Species 17 exists in equilibrium with its isomeric open-chain carboxonium ion form, 18 which is responsible for further reaction. 

The linear acetals units of polymer segments can also stabilise the open chain carboxonium ion 18 (thereby accelerating its formation), and the species formed (i.e. 19) can be regarded as the effective active centre, able only to propagate and unable to participate in hydride ion transfer. For steric reasons 1,3-dioxolane cannot itself stabilise carboxonium ions in this way. 

G. A. Olah and A. M. White, "Carbon-13 Resonance Investigation of Protonated Carboxylic Acids (Carboxonium Ions) and Oxocarbonium Ions (Acyl Cations), J. Amer. Chem. Soc. 89, 7072 (1967). 

The extent of kinetically controlled formation of the carboxonium ions 31 depends on the nature of R1 and Yy. The possible existence of 31 allows formation of acylated enols 32 (Y = R3CO), which are analogous with w-acylaminostyrene derivatives. As is known, the latter compounds easily undergo an intramolecular acid-catalyzed cyclization to isoquinolines (the Pictet-Gams reaction) (80T1279). 

Upon acylation of some benzyl carbonyl compounds (25, R = H, Me 51, R = OH) dibenzo[a,tropylium salts 65 have been isolated in low yields (5-15 %) along with the major products, 2-benzopyrylium salts. Veratryl acetone 25 (R = Me) as well as homoveratric aldehyde 25 (R = H) (or carboxonium ions 31 which are formed from them) may undergo an oxidative a-cleavage, resulting in the benzyl cation 64. The formation of the same cation from homoveratric acid 51 is the result of decarbonylation of the acylium ion 63. Further interaction of the benzyl cation 64 with the substrate, followed by cyclization and oxidation, results in the polycyclic tropylium salts 65 (82ZOR589). 

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