Carboxonium active species

Thus, the contribution of carboxonium active species to propagation is small, but not negligible. 

Both oxonium and carboxonium active species are relatively strong electrophiles (stronger than in the case of tetrahydrofuran). Thus, to avoid the termination by interaction with counterion, the stable counterions of low nucleophilicity are required. It has been shown that only the most stable complex anions SbF6 and AsF6 provide the living active species, whereas BF4, SbCl6, and even PF6 anions cause termination [98],... 

Taking advantage of the fast transacetalization (scrambling) accompanying the cationic polymerization of 1,3-dioxolane, the telechelic polymers containing two allyl ether groups were obtained in the polymerization carried out in the presence of bis(allyloxy)methane (the carboxonium active species showed in the scheme for simplicity) ... 

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

The other consequence of the presence of —O—CH2—O— group in the molecule is the possibility of unimolecular opening of tertiary oxonium ion active species to form corresponding carboxonium ion. 

The rate constants of the reaction modeling the propagation on carboxonium and oxonium active species were found equal (at the conditions given above) to ... 

The structure of active species in the homogenous polymerization of cyclic acetals (e.g., DXL) was discussed in detail in Adv. Pol. Sci. 37, Sect. 4.1.4. In short, the problem may be stated as follows. The polymerization of cyclic acetals involves an equilibrium between oxonium and carbenium (carboxonium) ions ... 

In eqn [5] and subsequent reaction schemes, ionic active species are shown for simplicity in the form of alkoxycarbenium (carboxonium) ions, although in reality a large majority of active species exist in the form of oxonium ions. There are several types of oxonium ions coexisting in the system (see Section 4.10.2.3.1) and, due to their multiplicity, schematic representation is difficult. 

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. 

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... 

As mentioned, 2-oxazolines may form a ring-opened distonic superelectrophile in reactions in superacid. These carboxonium-carbenium dications are capable of reacting with benzene and moderately deactivated substrates.2 For example, the optically active oxazoline (94) reacts in CF3SO3H to generate the chiral dication (95) and this superelectrophilic species is capable of reacting with o -dichlorobenzene in fair to modest yield and diastereoselectivity (eq 35). 

The observed electrophilic reactivity is indicative of superelectrophilic activation in the dication 173. Other ammonium-carboxonium dications have also been reported in the literature, some of which have been shown to react with benzene or other weak nucleophiles (Table 4).1 42b 57-60 Besides ammonium-carboxonium dications (175-179), a variety of N-heteroaromatic systems (180-185) have been reported. Several of the dicationic species have been directly observed by low-temperature NMR, including 176, 178-180, 183, and 185. Both acidic (175, 180-185) and non-acidic carboxonium (176-177) dicationic systems have been shown to possess superelectrophilic reactivity. The quinonemethide-type dication (178) arises from the important biomolecule adrenaline upon reaction in superacid (entry 4). The failure of dication 178 to react with aromatic compounds (like benzene) suggests only a modest amount of superelectrophilic activation. An interesting study was done with aminobutyric acid... 

The activating effects of ammonium groups on carboxonium electrophiles has also been exploited in the Friedel-Crafts acylations with amides.50 For example, in comparing the superacid-catalyzed reactions of acetanilide, the monoprotonated species (198) is found to be unreac-tive towards benzene (eq 67), while the diprotonated, superelectrophilic species (199) reacts with benzene to give the acyl transfer product in reasonably good yield (eq 68). 

As previously considered in this chapter, propagation proceeds via oxonium and/or carboxonium ions, and these species are reasonably assumed to remain stable and active. The formation of formaldehyde during the process, resulting from thermodynamic depolymerization of triox-ane and to a lesser extent from unzipping of open-chain active ends, indicates that carboxonium ions are in equilibrium with oxonium ions. This is also supported by the observation of methoxy and formate end groups resulting from hydride shift processes in transfer reactions to... 

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