Carbocations carboxonium ions

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

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. 

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.

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 transient nature of carbocations arises from their extreme reactivity with nucleophiles. The use of low-nucleophilicity counterions, particularly tetrafluorobo-rates (B I, ), enabled Meerwein in the 1940s to prepare a series of oxonium and carboxonium ion salts, such as R30+BF4 and HC(OR)2+BF4, respectively.13 These Meerwein salts are effective alkylating agents, and they transfer alkyl cations in SN2-type reactions. However, simple alkyl cation salts (R 1 BF4 ) were not obtained in Meerwein s studies. The first acetyl tetrafluoroborate—that is, acetylium tetrafluor-oborate—was obtained by Seel14 in 1943 by reacting acetyl fluoride with boron trifluoride at low temperature [Eq. (3.1)]. 

The reaction proceeds via a pentacoordinate hydroxycarbonium ion transition state, which cleaves to either fert-butyl alcohol or the tert-butyl cation. Since 1 mol of isobutane requires 2 mol of hydrogen peroxide to complete the reaction, one can conclude that the intermediate alcohol or carbocation reacts with excess hydrogen peroxide, giving fcrt-butyl hydroperoxide. The superacid-induced rearrangement and cleavage of the hydroperoxide results in very rapid formation of the dimethylmethyl-carboxonium ion, which, upon hydrolysis, gives acetone and methyl alcohol. 

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). 

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... 

Reactions of carbocations with acetal bonds, aldehydes, ketones, etc. yield carboxonium ions. Various structures have been assigned to these ions. They most probably actually exist in various variants, mutually connected by equilibria. The proportion and structure of the predominant form depend on the structure of the original particles and on their neighbourhood. 

Even with reactions of a non-radical active centre, the generated polymer is not always inert. Carbanions react with —C=N and —COOR sub-stitutents, carboxonium ions produce less acid centres by reaction with an ether-type chain (see Chap. 4, Sect. 2.3), carbocations alkylate aromatic groups, etc. All these reactions affect propagation. Sometimes the physical effect of the generated insoluble polymer is combined with its ability to react chemically in a certain way. 

CARBOCATIONS, CARBANIONS, FREE RADICALS, CARBENES, AND NITRENES nitrogen. A silylated carboxonium ion, such as 19, has been reported. ... 

The available NMR data for persistent carbocations derived from various classes of PAHs are compiled and their key features are highlighted and compared. The review covers PAH arenium ions, PAH carboxonium ions and a-PAH-substituted carbocations. Charge delocalization mode and substituent effects are evaluated on the basis of the A8,iC values. 

Figure 9 is a summary of the NMR data for a series of regioisomeric a-phenanthrene-substituted tertiary carbocations that were generated from their alcohols.19 20 Based on the AS values (Fig. 9), it is concluded that positive charge delocalization (p-7r overlap) is most effective from the meso positions (C-9/C-10) and least effective from C-2. The presence of a a-CF3 group destabilizes the carbocation and increases electron demand. Regioisomeric acetylphenanthrenes are CO protonated to give carboxonium ions (Fig. 10).19 The COH+ chemical shift is consistent with the carboxonium character, resulting in limited delocalization into the PAH. Nevertheless, the pattern of delocalization is similar to that of a-carbocations.

The available NMR data for persistent carbocations derived from the benz[a]anthracene (BA) skeleton and for a series of secondary a-substituted carbocations (and related model carbocations) and a-carboxonium ions are gathered in Figs 37 and 38.32 The me so positions (C-7/C-12) in BA are most reactive. The conformation(s) of the carboxonium group was deduced on the basis of NOED experiments. The A513C values (Fig. 38) indicate the strong anthrecenium ion character in benz[a]anthrenium and their a-carbocations. 

Fig. 37. H NMR data for persistent carbocations from the benz[a]anthracene derivatives and for related a-substituted carbocations and a-carboxonium ions (A5 H in parentheses).

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. 

In the case of Brpnsted acid catalysts, cationic electrophiles may be generated by the direct protonation of a functional group (Fig. 1.1). This type of chemistry is especially important in the SgAr reactions of carbonyl compounds and olefins. The carboxonium ions (8 and 9) and nitrihum ion (10) are formed by protonation at a nonbonding electron pair, while protonation at the olefinic x-bond gives the carbocation (11). Both sohd (i.e., zeolites) and liquid Brpnsted acids may generate electrophiles by this chemistry. 

Phenylethyl-substituted pyridinecarboxaldehydes (69) were shown to generate dicationic electrophiles such as the dicationic carboxonium ion (70)." ° These undergo cyclization to the dicationic carbocation (71), which may be trapped by arenes or water. The chemistry provides a useful route to triarylmethanes or 10,ll-dihydro-5//-benzo[4,5]cyclohepta[l,2-( ]pyridin-5-ones. 

Other monomers that are suitable for cationic polymerization include cyclic ethers (like tetrahydrofuran), cyclic acetals (like Irioxane), vinyl ethers, and N-vinyl carbazole. In these cases the hetero atom is bonded directly to the electron deficient carbon atom, and the respective carboxonium ion (9-13) and immonium ion (9-14) are more stable than the corresponding carbocations. 

Cationic polymerizations are presently overshadowed by other methods of polymer synthesis. The main reason is the extraordinary complexity of the chemical processes in cationic polymerizations, and the consequent difficulty in controlling technological problems. Some monomers cannot, however, be polymerized in any other way. This is a sufficient reason for studying the generation and reactions of carbocations (and also carboxonium and oxonium ions),... 

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