Oxycarbenium ions

Essentially all allylsilanes (M = SiR3, Section D.l.3.3.3.5.) with the exception of fluorosil-iconates11 and most of the trialkyl(allyl)stannancs (Section D.l. 3.3.3.6.), which have only very weak Lewis acidic properties, require a strong Lewis acid to trigger the reaction with a carbonyl compound by the preceding formation of an x-oxycarbenium ion, which attacks the allylic compound in an ionic open-chain pathway. These Lewis acid catalyzed carbonyl additions offer new possibilities for the control of the simple and induced diastereoselectivity12. 

There are two possible ways to interprete the decrease in the EQ-H% of the polymers with rise in temperature and/or the polarity of the solvent. The first is the concept that the growing chain end in the cationic polymerization of 2 consists of the cyclic trialkyloxonium ion 11 and the oxycarbenium ion 14, the latter of which is... 

The second explanation for the formation of the stereoirregular polymer is that the propagation of 2 is essentially an SN1 type reaction, and that attack d (broken arrow) in 14 is depressed by the interaction of the outer-ring oxygen atom with the positively charged carbon atom of the oxycarbenium ion from the upper side of the... 

A simplistic view of the mechanism is depicted in Scheme 3.5. The sulfoxide (I) is activated by triflic anhydride to generate a sulfonium salt (III). It was observed early on that the outcome of the glycosylation was not influenced by the configuration at the two variable stereogenic centers in the sulfoxide donor (i.e., anomeric carbon and sulfur).1 This was taken as evidence for the intermediacy of the oxycarbenium ion (V), which arises from rapid elimination of an alkylsulfenyl triflate(IV) from III. Reaction of V with a nucleophile then gives rise to a mixture of a- and P-glycosides (II). 

In attempts to improve the yields of problematic glycosylation reactions, it has become clear that the mechanism is much more complex than Scheme 3.5 suggests. In essence, the oxycarbenium ion(V) is susceptible to attack by nucleophiles other than the glycosyl acceptor (Scheme 3.6). Two intermediates for which there is experimental evidence are glycosyl triflates(VII)17 and glycosyl sulfenates(IX).18... 

Gildersleeve and coworkers have suggested that the formation of a sulfenate(IX) can be competitive with glycoside formation this has been confirmed by characterization of reaction products generated at low temperatures (-78°C).18 The sulfenate itself becomes a glycosylating agent at higher temperatures (-20-20°C). This is consistent with earlier reports that the activated sulfoxide appears to be extremely reactive at -78°C (oxycarbenium ion, V) and yet stable at room temperature (sulfenate, IX).1... 

If the reaction mixture contains er .-oxonium ions (V) (reactions 6a, b), or oxycarbenium ions (VI) (reaction 6c), or any other oxygen-containing cations which are stoichiometrically equivalent to these (see for example, (X) and (XI) below), the ethoxide ions combine with them and are thus incorporated in the polymer ... 

In the standard procedure the neutralised polymer is purified and hydrolysed and the hydrolysate is examined for ethanol. The absence of ethanol indicates that no tert.-oxonium ions containing a polymeric moiety and no polymeric oxycarbenium ions could have been present in the reaction mixture. 

Okada et al. [22], used the unpolymerisable 1,3-dioxan as a model compound and let it react with Et30+BF4 in CH2C12 at 21 °C for 24 h. On the basis of the H NMR spectrum of the resulting reaction mixture and its variation with time and temperature, and other evidence, they suggested that the oxycarbenium ion (XI), solvated by diethyl ether (from the Et30+), is the propagating species ... 

The oxycarbenium ion was the first concept in terms of which the polymerisation of DXL was interpreted by Yamashita and Okada and they quite logically adopted and adapted it from the theories of Kern and Jaacks about the polymerisation of 1,3,5-trioxan. It was subsequently favoured by many other authors in the context of DCA polymerisations. There appears to be no need at present to re-examine the theory of the trioxan polymerisation and it is necessary to mention that the following critique of the oxycarbenium theory as applied to the polymerisation of DCA does not necessarily apply to trioxan, because both the nature of the monomer and the polymerisation conditions are generally very different from those of the DCA. 

Or one can say that even if the equilibrium constant for the above reaction is negligibly small, the unsolvated oxycarbenium ion (XII) is a part of a relatively unimportant canonical form of the tert.-oxonium ion, which implies a certain probability that the oxycarbenium ion may react as such by detaching itself from the ether and finding some other basic site. The important point, however, is that under the normal conditions under which DCA are polymerised the attempts to distinguish experimentally between oxycarbenium ions and tert.-oxonium ions as the active species appear at present to be quite pointless. 

The situation would be completely different for oxycarbenium ions in a highly polar solvent such as sulphur dioxide which could compete effectively as solvating agent with the DCA and their polymers. In such systems one could envisage that both the solvent-solvated oxycarbenium ions and also the solvent-solvated teJt.-oxonium ions could coexist in a true equilibrium, and that each would react according to its own characteristics. This is an area which remains very largely unexplored, although Penczek has made a start in this direction and these considerations arose from discussions with him of his exploratory experiments with sulphur dioxide as solvent. 

Whereas TiCU interacts with the peroxide bridge yielding ethers, SnCl4 promotes a selective displacement of the alkoxide to form peroxides. Heterolysis of an 0-0 bond (Hock reaction) furnishes oxycarbenium ion intermediates via 1,2-shifts (path a), whereas acid-catalyzed C-O ionization affords carbenium ions (path b, Scheme 13). 

Treatment of the alduronic acid 180 with DAIB-I2 furnishes the formyloxylactone 182 (Scheme 52) (98JOC2099). This and related reactions proceed by a radical fragmentation-oxidation sequence, culminating in the formation and intramolecular capture of oxycarbenium ion intermediates (e.g., 181). 

Oxidations of this type, promoted by PhIO-I2, have been exploited for syntheses of densely functionalized pyrrolidines and piperidines, e.g., 184, from TV-protected amino sugars, e.g., 183 (Scheme 53) (97TA1971, 01JOC1861). The advantage of iodosylbenzene over DAIB in this context is presumably to circumvent capture of the oxycarbenium ion intermediates by the acetate ion (92AGE772). 

Some heterocyclic monomers may undergo random copolymerization with vinyl monomers. This is a case of cyclic acetals (e.g., 1,3-dioxolane) which forms the random copolymers with styrene [308,309] or isoprene [310], Apparently, the oxycarbenium ions, being in equilibrium with tertiary oxonium ions (cf., Section II.B.6.b), are reactive enough to add styrene ... 

A first hint comes from the fact that the carbonyl carbon of the acyl substituent in IV may be the a carbon of the enol ether moiety in I. In fact, the vinyl ether would be converted into a methyl ketone by breakage of the bond between the acetal carbon and the endocyclic oxygen. In that case, the resulting oxycarbenium ion would serve as a linking position for the carbocyclization that eventually affords the cyclohexyl ring of IV. 

On the other hand, 13C NMR spectroscopy has extensively been used to study the structure of oxonium, carboxonium and oxycarbenium ions and diprotonated carboxylic acids,144-146 since it allows the direct monitoring of the cationic centre and since the chemical shifts and coupling constants can be correlated with the geometry and hybridization of the cation. This technique has also been used by Olah et al. to provide... 

Aiming to prepare a library of derivatives of the medicinally relevant piperidine scaffold, Veerman [393] and co-workers exploited N-acyhminium ion chemistry, starting from a stable aminal precursor. Coupling of six different amino acetals onto sulfonylethoxycarbonyl-modified (SEC-modified) polystyrene resin afforded the potential precursors. However, attempts to transform these into the desired piperidine derivatives via one-pot generation of N-acyliminium ions and functionalization failed, essentially because direct attack of the nucleophile on the acid-mediated oxycarbenium ion took place at a similar rate to that of the intramolecular carbamate-nitrogen attack. 

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