Ethers reactivity toward oxygen

There is a scant data on the radical behavior of thianthrene radical cation [58]. As pointed out above, thianthrene radical cation couples with radicals. It shows little reactivity toward oxygen but this may be ascribed to its positive charge which should render it an electrophilic radical. It initiates the polymerization of styrene [59,60], a-methyl styrene [59], and ethyl vinyl ether [59,60],but not that of vinyl pyridine [61], vinyltrimethylsilane [59], methyl acrylate [59, 61], or acrylonitrile [59,61 ]. These results can be explained by cationic rather than radical polymerization. 

Trichloromethyl groups linked to an ether oxygen have high reactivity towards hydrogen fluoride. Thus, complete Cl-F replacement is achieved by heating the reactants in an autoclave, e.g. reaction of 5226 (see also Houben-Weyl, Vol. E4, pp 627-628). 

The mechanism of the formation of the tetrahydropyranyl ether (see Figure 23.1) is an acid-catalyzed addition of the alcohol to the double bond of the dihydropyran and is quite similar to the acid-catalyzed hydration of an alkene described in Section 11.3. Dihydropyran is especially reactive toward such an addition because the oxygen helps stabilize the carbocation that is initially produced in the reaction. The tetrahydropyranyl ether is inert toward bases and nucleophiles and serves to protect the alcohol from reagents with these properties. Although normal ethers are difficult to cleave, a tetrahydropyranyl ether is actually an acetal, and as such, it is readily cleaved under acidic conditions. (The mechanism for this cleavage is the reverse of that for acetal formation, shown in Figure 18.5 on page 776.)... 

Oxygen or, particularly, nitrogen adjacent to a carbon-fluorine bond greatly increases reactivity towards nucleophiles. Hydrolysis of a,a-difluoro ethers occurs under acid conditions [39] (Figure 5.19). Orthoesters are produced by reaction with aUcoxides such reactions may, however, occur via initial elimination of hydrogen fluoride, rather than by direct nucleophilic displacement of fluoride [40] (Figure 5.20). 

Upon treatment with HF-SbFj, para-substituted phenols (or their methyl ethers) can be diprotonated [66], first on the oxygen atom and then on the meta carbon [67]. The resulting dipositively charged species are exceedingly reactive towards a variety of arenes [67], 4-Arylcyclohexanones, the primary products of the reactions, can be further transformed to 3-arylcyclohexanones. The ratio of the two isomers depends on conditions such as the reaction time, amount of acid, and the nature of the substrates. For example, when p-cresol is reacted with benzene in the presence of HF-SbFs, 4-methyl-4-phenylcyclohexenone and 3-phenyl-4-methylcyclohexenone are obtained in yields of 29 and 33 %, respectively, after 90 s. By increasing the reaction time to 15 min, the yield of 3-phenyl-4-methylcyclohexenone is increased to 90 % whereas that of 4-methyl-4-phenylcyclohexenone decreased to 2-3 % (Eq. 27). 

The methylene unit attached to the ethereal oxygen is particularly reactive towards radical attack and thus chain-transfer processes would be expected to substantially reduce the molecular weight of the final polymer. Some reduction is noticed but both this solvent and toluene can be used for free-radical polymerization (and note the successful polymerization of benzyl acrylate in Protocol 1). 

HO shows little selectivity in H abstractions from pyranose rings, but shows enhanced reactivity towards the 4 -positions of aldofuranosides and the 5 -positions of ketofuranosides such as sucrose. This appears concordant with studies on H abstractions by (CH3)3C-0, where an enhanced reactivity of C-H bonds antiperiplanar to a p-type lone pair on oxygen was noted. As discussed in Section 7.5.1, this could be a least-motion effect as much as a manifestation of ALPH. The labilisation of hydrogens a to an ether linkage -the cause of the well-known peroxidation of ethers - was apparent even in hydrogen abstraction by the normally avid and unselective tert-butoxy radical, with reactivities, relative to the C-H bonds in cyclohexane, of 2.7,0.14 and 0.21 for the ot, p and y hydrogens of tetrahydropyran. ... 

Gorman, A. A., Gould, I. R., Hamblett, I., Time resolved Study of the Solvent and Temperature Dependence of Singlet Oxygen ( Ag) Reactivity toward Enol Ethers Reactivity Parameters Typical of Rapid Reversible Exciplex Formation, J. Am. Chem. Soc. 1982, 104, 7098 7104. 

Fluorinated alkenes are particularly reactive towards ethers in free-radical processes why should this be so Stabilisation of a radical centre by adjacent oxygen is, of course, well known but if we represent this in... 

The high reactivity of acetylenic ethers toward both electrophilic and nucleophilic reagents is determined by the highly polarized character of the triple bond due to the contribution of the resonance structure RO =C=C R. The carbon atom p to oxygen is the usual site of electrophilic attack, while the a-carbon is reactive toward nucleophiles (equation 49). 

As a result of resonance, the oxygen of methyl vinyl ether carries a partial positive charge, which makes it less reactive toward an electrophile. The oxygen of methyl vinyl ether is also less basic than the oxygen of ethyl methyl ether. 

It is interesting to note that compared to the oxygen analogues (vinyl ethers) the vinyl sulfides and divinyl sulfides are much more reactive toward a growing polystyryl radical during the copolymerization process [16]. 

Unsaturated polyesters (see Section 16.7) can be used in UV curing as well, but only in combination with a special reactive diluent system, which is based on vinyl ethers. Kinetic analysis has revealed that in that specific case the fumarate forms a charge transfer complex with the vinyl ether and a homopolymerization of this charge-transfer complex takes place as depicted in Scheme 16.34. An advantage of a charge-transfer complex radical polymerization is that it is less susceptible toward oxygen inhibition than methacrylate systems. This is illustrated by the fact that the first commercial UV powder system, in which the powder is sprayed (much oxygen), melted, and then UV-cured, was based on a fumarate-vinyl ether system. 

Mocella, Okamoto and Barefield advise that the use of nitrosonium salts in synthesis requires careful selection and handling of solvents. Some solvents (for example, acetonitrile, nitromethane, dichloromethane, ethyl acetate, benzene, hexane, and carbon tetrachloride), when carefully purified, do not react with NOX salts. However, common solvents like acetone, pyridine, ethers, alcohols, and water are reactive towards nitrosonium salts, even when the solvents have been carefully purified. Reactions of NOX salts are best carried out in an oxygen-free atmosphere, so that the production of nitrate ion is avoided. Nitrate ion is generated in a stepwise manner, Eqs. 8.2 -8.4 ... 

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