Acetals radical attack

Between 6 and 30% of the radical attack on butane may occur at the primary hydrogen atoms (213). Since ca 6% of the butane goes to or through butyric acid (22), the middle of this range does not seem unreasonable. Because it is much more resistant to oxidation than its precursors or coproducts, acetic acid (qv) is the main product of butane LPO. 

During this early period, a very ingenious free-radical route to polyesters was used to introduce weak linkages into the backbones of hydrocarbon polymers and render them susceptible to bio degradabihty (128—131). Copolymerization of ketene acetals with vinyl monomers incorporates an ester linkage into the polymer backbone by rearrangement of the ketene acetal radical as illustrated in equation 13. The ester is a potential site for biological attack. The chemistry has been demonstrated with ethylene (128—131), acryhc acid (132), and styrene (133). 

The formation of derivatives of this type by free-radical attack has been mentioned previously (see section E above). The most common route to vinylogous halo ketones is by halogenation of dienol acetates or ethers. Both free halogen and A -halo compounds may be employed, and this approach has frequently been used to obtain 6 (axial) halo compounds ... 

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

Although the reactivity of 1,2-disubstituted ethylenes in copolymerization is low, it is still much greater than their reactivity in homopolymerization. It was observed in Sec. 3-9b-3 that the steric hinderance between a P-substituent on the attacking radical and a substituent on the monomer is responsible for the inability of 1,2-disubstituted ethylenes to homopolymerize. The reactivity of 1,2-disubstituted ethylenes toward copolymerization is due to the lack of P-substituents on the attacking radicals (e.g., the styrene, acrylonitrile, and vinyl acetate radicals). 

Another effect of high oxygen concentrations was increased oxidative attack on the solvent. This is shown in Figure 2. Methane and methyl acetate were detected in most of the runs, and it is likely that these materials were formed by free-radical attack on acetic acid. Loss of nitric acid to the nonregenerable species nitrous oxide and nitrogen was reduced in the presence of high oxygen concentrations. 

As far as the mechanisms of branching and crosslinking are concerned, there appear to us to be certain weaknesses in those commonly accepted. With ethylenic monomers, there can be little doubt that if branching were to occur at all, it will arise from radical attack upon the polymer already formed. It would be immaterial whether this transfer takes place on backbone carbon atoms or via side chains, as is almost certainly true for, say, vinyl acetate. When dienes are present, it has been generally accepted that the residual double bonds are the main seat of reaction, thereby creating the immediate possibility of crosslinking. However, the internal residual double bonds—that is, those... 

A tin-free radical cyclization of the xanthate 272 using dilauroyl peroxide (DLP), as the radical initiator, in chlorobenzene was used to give the 5//-pyrido[2,3-A azepin-8-one 273 (Scheme 35) <20040L3671>. The xanthate 272 was also made by an intermolecular free radical addition to allyl acetate, using the xanthate 271, as the radical precursor. Somewhat surprisingly in this latter case, intramolecular free radical attack on the pyridine ring did not take place. 

In support of the intermediate formation of the acetate radical from N-nitrosoacetanilide is the evolution of carbon dioxide during the reaction. Moreover, metals such as copper, zinc, lead, antimony, and iron are attacked when N-nitrosoacetanilide is allowed to decompose in a non-polar solvent such as carbon disulfide in the presence of the metals, a behavior similar to that of metals in contact with active free radicals in the Paneth test. 

Propylene, a substrate with , /2 for oxidation at considerably higher potential than nitrate ion, gave the products indicated below eqn (68) on anodic oxidation in acetic acid containing a perchlorate or nitrate salt (Formaro et al., 1973). Both nitrates were postulated as originating from nitrate radical attack upon either an allylic hydrogen or the terminal carbon of the double bond. 

Concurrently, the hydroperoxide may be converted to methyl ethyl ketone (MEK). If the initial radical attack is at the primary rather than the secondary carbon, the process makes propionic and formic acids. Reaction conditions can be changed to produce more MEK at the expense of some acetic acid. The maximum acetic acid/MEK ratio is 6.5-7 on a weight basis. If ethyl acetate is also formed, the ratio can go down to acetic acid/(ethyl acetate + MEK) of 3.6-4, with MEK being about 55 percent of the byproduct. 

The complete mineralisation of pentachlorophenol has been achieved within 3 h [58]. As with chlorophenol both quinoidal and hydroquinone byproducts were detected which again suggested hydroxyl radical attack as the primary oxidation route. Subsequent ring cleavage was reported to be slow with cleavage products including acetate and formate being detected. 

Mechanistically, it was argued that the products are derived from enol acetate radical cations that are either attacked by nucleophiles (A) or by a base (B) (Scheme 9). 

This method could be successfully applied for a straightforward synthetic approach to 2,5,7,10-tetraoxabicyclo[4.4.0]decanes [256]. Some direct [257] and indirect [258] evidence has been found supporting attack of water as a nucleophile at the a-carbon, a reaction mode already identified for enol acetate radical cations [226,227], although this may not be general. 

Random copolymers will be formed, or course, if each radical attacks either monomer with equal facility (kn =k 2, kn = 21, 1 = 2 = I). Free-radical copolymerization of ethylene and vinyl acetate is an example of such a system, but this is not a common case. Random monomer distributions are obtained more generally if k /k 2 is approximately equal tok2i/k22- That is to say, r I jr2- This means that k /k22 and A 2i / 22 will be simultaneously either greater or less than unity or in other words, that both radicals prefer to react with the same monomer. 

Electron transfer from Me2C=C(OMe)OSiMe3 to Q is made possible by the strong interaction between Q" and Mg + (or 2Mg +) to produce the radical ion pair. Since the spin of the ketene silyl acetal radical cation is mainly localized on the terminal carbon atom [229], the carbon -oxygen bond is formed before the cleavage of the Si-0 bond to yield the adduct (Scheme 15). This contrasts with the 1,2-addition of nonsubstituted ketene silyl acetal [H2C=C(OEt)OSiEt3] via nucleophilic attack to the positively charged carbonyl carbon of the quinone rather than via an alternative electron transfer pathway [228]. 

Acetoxylation occurs in either of the following ways (1) nucleophilic attack of acetate ion on cationic intermediates formed by loss of electrons, or (2) radical attack by the acetoxy radical generated by loss of an electron from the acetate ion. 

For example the radical cation 7.131 is generated by oxidation of 2-methylnaphthalene. The odd electron is in the HOMO of naphthalene, the highest coefficient of which is at C-l. The methyl group, as an X-substituent, will further enhance the coefficient at this site relative to the other Q-positions thus, the total electron population at this site will be higher than at the other positions, and yet the nucleophile, an acetate ion, attacks at this site. That an anion should attack a site of relatively high electron population is easily accounted for by the SOMO/HOMO interaction. The intermediate radical 7.132 eventually gives l-acetoxy-2-methylnaphthalene when a radical abstracts the hydrogen atom. 

In another study, Burbano et al. [106] investigated the elimination of the MTBE by-products tBA, tBF, methyl acetate and acetone at a concentration of 0.0227 mM (1.3 lo 2.3 mg/L). The reaclion rales were slower than that of MTBE. The compounds containing a lerl-butyl group were observed to be more susceptible to OH radical attack. 

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