Acetals hydrogen abstraction

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

By using various trapping reagents, it has been deduced that the transannular fragmentation is rapidly reversible. The cyclization of the fragmented radical C is less favorable, and it is trapped at rates which exceed that for recyclization under most circumstances. " Radicals derived from ethers and acetals by hydrogen abstraction are subject to fragmentation, with formation of a ketone or ester, respectively. 

In order to avoid competitive bimolecular photoreactions such as ketone reduction by hydrogen abstraction, poor hydrogen donating solvents are recommended (acetonitrile, acetic acid, tertiary alcohols). In those cases where ketene trapping is desired, solvents must also be miscible with water or other protic nucleophiles. 

For allyl acetate a significant deuterium isotope effect supports the hydrogen abstraction mechanism (Scheme 6,31).183 Allyl compounds with weaker CTT-X bonds (113 X=SR, S02R, Bi etc.) may also give chain transfer by an addition-fragmentation mechanism (Section 6.2.3). 

The N,0- and N,S-heterocyclic fused ring products 47 were also synthesized under radical chain conditions (Reaction 53). Ketene acetals 46 readily underwent stereocontrolled aryl radical cyclizations on treatment with (TMSlsSiH under standard conditions to afford the central six-membered rings.The tertiary N,0- and N,S-radicals formed on aryl radical reaction at the ketene-N,X(X = O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from (TMSlsSiH was investigated and found to provide higher selectivities than BusSnH. 

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

Three types of photochemical reaction of carbohydrate acetals have been investigated. Early studies centered on the photochemical fragmentation of phenyl glycosides, and the photolysis of o-nitrobenzyli-dene acetals. (The latter reactions will be discussed with the photolysis of other nitro compounds see Sect. VII,1.) Later experiments were concerned with hydrogen-abstraction reactions from acetal carbon atoms by excited carbonyl compounds. 

Study of model systems has revealed a tendency for photochemical abstraction of axial, rather than equatorial, acetal hydrogen atoms in... 

Scheme 15.—Rationalization for the Importance of Orbital Overlap to Radical Stabilization During Hydrogen Abstraction from an Acetal Carbon Atom.

In the case of t-butyl acetate, shown in reaction 41, three successive geminal hydrogen abstractions and insertions of 303 take place a product of reductive dimerization (306) is also formed in small yield626. 

Reactivity Is typical of an acrylamide. For example, compound 1 shows essentially 1 1 copolymerizablllty with butyl acrylate. Copolymerizablllty has also been demonstrated with styrene, other acrylates and methacrylates, vinyl acetate (VAc), VAc/ethylene and vinyl chlorlde/ethylene. High molecular weight polymers and copolymers remain soluble. Indicating any chain transfer to polymer, e.g. through abstraction of the acetal hydrogen. Is minor. 

The alkylation of quinoline by decanoyl peroxide in acetic acid has been studied kineti-cally, and a radical chain mechanism has been proposed (Scheme 207) (72T2415). Decomposition of decanoyl peroxide yields a nonyl radical (and carbon dioxide) that attacks the quinolinium ion. Quinolinium is activated (compared with quinoline) towards attack by the nonyl radical, which has nucleophilic character. Conversely, the protonated centre has an unfavorable effect upon the propagation step, but this might be reduced by the equilibrium shown in equation (167). A kinetic study revealed that the reaction is subject to crosstermination (equation 168). The increase in the rate of decomposition of benzoyl peroxide in the phenylation of the quinolinium ion compared with quinoline is much less than for alkylation. This observation is consistent with the phenyl having less nucleophilic character than the nonyl radical, and so it is less selective. Rearomatization of the cr-complex formed by radicals generated from sources other than peroxides may take place by oxidation by metals, disproportionation, induced decomposition or hydrogen abstraction by radical intermediates. When oxidation is difficult, dimerization can take place (equation 169). 

Additional evidence was obtained when the oxidation was carried out in the presence of copper(II) acetate, a common radical terminator. The oxidation proceeded unhindered and is not, therefore, dependent on hydrogen abstraction by free radicals. It can be represented by Eq. (15) or better by... 

In 1963, Elad and Youssefyeh80 described the photochemical conversion, in the presence of acetone, of a number of 2-alkyl- or 2-aryl-substituted 1,3-dioxolanes (see Scheme 3, n=0) or 1,3-dioxanes (Scheme 3, n = l) into carboxylic esters. They suggested that the reaction proceeds by initial, hydrogen abstraction from the acetal carbon... 

As already mentioned (see Section III, 3 and Refs. 41-45), an acetal can be added to a double bond under irradiation by initial hydrogen-abstraction from the acetal carbon atom. If both the acetal grouping... 

Altnough it does not involve hydrogen abstraction, it is worth noting that some ketones add alcohol on irradiation to give an acetal (4.41). The reaction is similar to the thermal, acid-catalysed formation of acetals, except that it occurs, especially for cyclic ketones, in solutions which do not have any strong acid added. There is an element of irreproducibility in the systems, and it seems that traces of acid in the alcohol initiate the reaction, which can be prevented by adding a small amount of solid potassium carbonate to the alcohol solution. 

Methyl ethers have been employed, but alkyl glycosides may be unstable following hydrogen abstraction from the anomeric center (see Section 11,9). Benzyl ethers and benzylidene (and other aldehyde-based) acetals, which themselves undergo ready radical bromination,27 can be expected to be entirely unsuitable protecting groups. Ketone-derived acetals, on the other hand, should be stable, at least in the absence of acid, and a few examples of successful brominations in their presence are reported in Section 11,4. 

In the presence of bromide, the oxidation of alkenes by Mn(OAc)3 in acetic acid readily occurs it 70-80 °C and produces allylic acetates in good yields. Thus cyclohexene is oxidized to cyclo-lexenyl acetate in 83% yield,508 and a-methylstyrene to j3-phenylallyl acetate in 70% yield,509 vith a mechanism involving allylic hydrogen abstraction by bromine atoms coming from the ixidation of bromide by MniU (equation 206). 

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