Alkyl group abstraction

Possible activation mechanism by alkyl group abstraction for an organozirconium complex on acid-treated montmorillonite passivated with Al Bug. 

In addition to MAO, boron compounds based on tris(pentafluorophenyl)boron and its derivatives, typically dimethylanilinium tetrakis(pentafluorophenyl) borate, have been used as cocatalysts for sPS polymerizations (40,41). Although MAO has been used in large molar excesses relative to the titanium complex, the boron compounds may be used in roughly equimolar amounts to the titanium catalyst. The boron cocatalyst reacts with a titanium alkyl species, either by protonation in the case of dimethylanilinium tetrakis(pentafluorophenyl)borate or by alkyl group abstraction in the case of tris(pentafluorophenyl)boron, to generate a titanium cationic species with a borate counterion (74-76). The esr spectral evidence has been reported for these systems, supporting a titanium(III) cationic active species (76). 

The removal of the two alkyl groups comprises the two primai y steps, the abstraction of the ethyl group being the dominant one. These products show up first, as can be seen on the left-hand side of Figure 10.3-14, It can also be seen that it is expected that after one year an apprceiable amount of s-triazine eom-pounds will still be found in the soil. 

Some details of the chain-initiation step have been elucidated. With an oxygen radical-initiator such as the /-butoxyl radical, both double bond addition and hydrogen abstraction are observed. Hydrogen abstraction is observed at the ester alkyl group of methyl acrylate. Double bond addition occurs in both a head-to-head and a head-to-tail manner (80). 

The effect substitution on the phenolic ring has on activity has been the subject of several studies (11—13). Hindering the phenolic hydroxyl group with at least one bulky alkyl group ia the ortho position appears necessary for high antioxidant activity. Neatly all commercial antioxidants are hindered ia this manner. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom from the substrate and thus produces an alkyl radical (14) capable of initiating oxidation (eq. 18). 

In ketones having propyl or longer alkyl groups as a carbonyl substituent, intramolecular hydrogen abstraction can be followed by either cleavage of the bond between the a and P carbon atoms or by formation of a cyclobutanol ... 

Alkyl radicals can be obtained by abstraction of a hydrogen atom from an alkyl group by another radical. This method was utilized for the generation of benzyl radicals from toluene with iert-butoxy radical obtained on heating di- er -butyl peroxide. BenzoyP and carboxymethyP radicals have also been obtained by this method. The reaction gives rise to a complex mixture of products and therefore is of rather limited use. 

The stereochemistry of the silyl ketene acetal can be controlled by the conditions of preparation. The base that is usually used for enolate formation is lithium diisopropyl-amide (LDA). If the enolate is prepared in pure THF, the F-enolate is generated and this stereochemistry is maintained in the silyl derivative. The preferential formation of the F-enolate can be explained in terms of a cyclic TS in which the proton is abstracted from the stereoelectronically preferred orientation perpendicular to the carbonyl plane. The carboxy substituent is oriented away from the alkyl groups on the amide base. 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

Shida and Hamill23 found that the positive and negative molecular ions of 1,3-butadiene and its homologs have similar absorption spectra. Band maxima of the anions are not sensitive to substituent alkyl groups, whereas those of the cations are red-shifted as the number of substituent methyl groups increases. In alcoholic matrices the butadiene anions abstract the alcoholic proton to form an allylic radical (equation 23), as was proven by ESR spectroscopy. 

One side reaction commonly encountered in the reaction of alkyl-substituted polyenes with oxygen-centered radicals is hydrogen abstraction from the alkyl group in positions adjacent to the polyene jr-system. For reactions of the ferf-butyloxy radical, this reaction becomes so dominant that it can be used to form polyenyl radicals by hydrogen abstraction... 

Dihydro-l, 2,3,5-thiatriazole 1-oxide radicals were generated from the thiatriazole oxides upon hydrogen abstraction in the reaction with thermally formed bis(4-methylphenyl)aminyl (4.20.5.1). Only radicals with aryl substituents in positions two and four are persistent, radicals with alkyl groups in these positions could not be detected. The radicals were investigated by ESR, ENDOR, and general triple resonance spectroscopy <92MRC84>. 

For instance, if the metal is lost by Sn2 attack on coordinated carbon, this constitutes R loss, and alkyl migration to an electrophilic centre such as coordinated CO may resemble R loss. R- loss may take place by simple homolysis, or by alkyl group transfer. Moreover, as Yamamoto has pointed out an electroneutral metal-carbon bond lengthening may be a prelude to more complex processes such as 0-elimination, or may lead to internal hydrogen abstraction rather than to actual free ligand release. 

The reaction of singlet oxygen with cis and trans alkenes shows an unexpected regioselectivity for hydrogen abstraction on the large alkyl group of the double bond . 

C-H. Since the alcohol O-H bond is stronger ( 104 kcal mol 1) than the alkyl C-H bond ( 96 kcal mol 1) (Lide, 1998-1999), one would expect that abstraction from the alkyl group would predominate, e.g., reaction (69a) would be faster than reaction (69b) ... 

The well-established chemistry of alkide abstraction by B(C6F5)3 allows the facile synthesis of betaine-type complexes where the perfluoroarylborate counteranion is covalently attached to the metal cation via either a reactive alkyl group or an ancillary Cp ligand (Scheme For example, B(G6Fs)3 is added exclusively at a terminal GH2... 

Reduction of ketones may occur if the alkyl group contains a /3-hydrogen that is available for abstraction. For example, addition of benzophenone to the in situ prepared complex BuMgN(SiMe3)2 results in /3-hydride transfer from the butyl group leading to the formation of the reduction product . This type of reaction has also been conducted in an... 

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