Oxonium special

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

In case of alkanols, the methylene oxonium ion, CH2=OH, m/z 31, deserves special attention. Resulting from a-cleavage, it undoubtedly marks spectra of primary alkanols, where it either represents the base peak or at least is the by far most abundant of the oxonium ion series (Fig. 6.8). [32] The second important fragmentation route of aliphatic alcohols, loss of H2O, is discussed in Chap. 6.10. 

The reaction starts of with a protonation - use the catalyst. Resist the urge to protonate the 4-hydroxyl, but go for the one at position 1 that has the added functionality of the hemiacetal linkage. It is going to be the more reactive one. Protonation is followed by loss of water as leaving group. The intermediate oxonium cation shown is actually a resonance form of the simpler carbocation now you can see the role of the adjacent oxygen. The reaction is completed by attack of the nucleophile, the 4-hydroxyl of another molecule. This is not special, but is merely another version of the hemiacetal synthesis done in part (a). 

A special case is the ring-forming reaction in the lactone acetal 568,69. When 5 is treated with trimethylsilyl trifluoromethanesulfonate (trimethylsilyl triflate TMSOTf) in the presence of triethylamine at 0°C the cisjtrans mixture 6a and 6b is formed in ca. 50% yield. In this intramolecular aldol reaction the probable intermediate is the oxonium ion 7. 

The epoxy-phosphonium salt adduct would then be vulnerable to attack by another epoxy molecule resulting in the formation of an oxonium ion (Eq. (26)) or it reacts with the anhydride to form a special monoester with hydroxy groups (Eq. (27)). 

The elements yield tetravalent derivatives m special circumstances, iu which they function as the basic elements. In the case of oxygen they are termed oxonium salts sulphur yields sulphonium salts whilst selenium and tellurium offer an even wider range of derivatives. Thus, the following types of compounds are now well known ... 

An illustration of Lowry s concept of acid catalysis is given in Figure 7, where S is the molecule to be converted (rearranged) to T. The proton donators cited as examples are the oxonium ion an undissociated acid molecule HA, and water as a special case of HA, while the proton acceptors cited as examples are water (transformed to H30 ), and the acetate ion (transformed to acetic acid). Contrary to the concept of Arrhenius, Lowry s concept can explain why water as a proton donator and acetate ions as proton acceptors represent a power-... 

Heterocyclics containing P-atoms are usually strong nucleophiles. For example the basicity of 2-methoxy-ODP (pKa = 3.1) is much higher than that of cyclic ethers or sulfides. Therefore, a wide range of initiators, i.e., carbenium or oxonium salts, Lewis and protonic acids, and relatively nonreactive alkyl halides and organoalu-minum compounds have been used. Usually, reactions were carried out in a N2 atmosphere, although no special precautions (e.g. vacuum) were used to avoid contamination with water. 

Whereas carotenoids in general are often considered by non-specialists as unstable compounds, they may easily be studied when particular precautions are made [1], This is also true for most subtypes with charged functional groups. The instability of carotenoid oxonium ions, radical cations, mono- and dications, however, represent special challenges. This review appears to be the first compilation on charged carotenoid species. 

Like simple dialkyl ethers, aliphatic and alicyclic crown ethers are chemically stable [179]. Aromatic crown ethers react like anisole, that is, they can be halo-genated or nitrated and they react with formaldehyde [182]. Hydrolysis takes place only in special cases [183]. Crown ethers are also thermally stable dibenzo-18-crown-6 can be distilled at 380°C without decomposition [184]. With hydrogen ions [185] and in the presence of Lewis acids (AICI3, TiCU), oxonium compounds are formed [186]. 

Tertiary oxonium ions that are active species in CROP of cyclic ethers are inherently stable (trialkyloxonium salts with stable counterions are commercially available and may be stored without special precautions for prolonged periods of time). Thus, if basic impurities are avoided in CROP of cyclic ethers, there is essentially no irreversible termination. Those polymerization are, however, not classified as living because reversibility of propagation and reversible chain transfer to polymer cause deviations from the ideal situation observed for, for example, anionic vinyl polymerization in which DP = [M]o/[I]o/ molecular weight distribution is close to Poisson distribution and the nature of end-groups may be strictly controlled. As will be discussed in subsequent sections, conditions of living polymerization may be more closely approached if polymerization proceeds by the AM mechanism. 

The H transfer, that is a typical route of transfer in cationic vinyl polymerization, is not important in the polymerizations proceeding with the participation of tertiary oxonium ions as active species. This type of transfer may, however, occur in special cases where the presence of carbenium ions cannot be excluded. Such situations may appear in the cationic polymerization of suitably substituted oxiranes when the presence of low concentration of carbenium ions in equilibrium with oxonium ions has been postulated (Scheme 13). 

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