Alkyl groups removal

The second type of oxidative biotransformation comprises dealkylations. In the case of primary or secondary amines, dealkylation of an alkyl group starts at the carbon adjacent to the nitrogen in the case of tertiary amines, with hydroxylation of the nitrogen (e.g lidocaine). The intermediary products are labile and break up into the dealkylated amine and aldehyde of the alkyl group removed. 0-dealkylation... 

Extensive studies have revolved around the ability of alkyl substituents on benzotriazole nitrogen to be deprotonated for example, 1-methylbenzotriazole is lithiated readily with -BuLi or LDA at the methyl group and the subsequent reactions with various electrophiles provide 1-substituted benzotriazoles, e.g., 799 <1997JOC4142>. 1-Alkyl- and 1-benzylbenzotriazoles react with borane to give air- and water-stable complexes 800 these can also be deprotonated at the alkyl group removal of the borane unit is achieved by refluxing in ethanol <19980PP325>. 

Alkyl groups removed with thiophenol and triethylamine... 

The exceptional selectivity exhibited by disiamylborane in the hydroboration of terminal olefins enables the efficient conversion of such olefins into aldehydes, since disiamylboranes have been shown to be oxidized readily by pyridinium chlorochromate. Thexylalkylchloroboranes, reduced in the presence of an alkene, produce mixed thexyldialkylboranes, which can be transformed into ketones by carbonylation or cyanidation. This advance in the preparation of trialkylboranes bearing three different alkyl groups removes limitations previously imposed on ketones available by carbonylation of boranes [equation (11)]. 

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. 

The methylthio group is removed by treatment with zinc powder in HCl (276) to give the 2-unsubstituted thiazole. The action of aluminum-mercury amalgam in methanol on various thioethers is reported to yield the expected thiazole (108) when Rj is an alkyl group and the corresponding A-4-thiazoline-2-thione (109) when Rj PhCH - (Scheme 55) (169). 

Deall lation, Transall lation, and Disproportionation. The action of aluminum chloride also removes alkyl groups from alkylbenzenes (dealkylation, disproportionation) (12). Alkylbenzenes, when heated with AlCl, form mixtures of benzene and polyalkylated benzenes ... 

Protonolysis. Simple trialkylboranes are resistant to protonolysis by alcohols, water, aqueous bases, and mineral acids. In contrast, carboxyUc acids react readily with trialkylboranes, removing the first alkyl group at room temperature and the third one at elevated temperatures. Acetic and propionic acids are most often used. The reaction proceeds with retention of configuration of the alkyl group via a cycHc, six-membered transition state (206). 

Another indole/oxindole synthesis achieves the critical ortho-substitution by Sommelet-Hauser rearrangement of an anilinosiilfonium ion intermediate. Use of P-thioketones (G = R, an alkyl group) generates 2-substituted indoles, whereas P-thioesters (G = OR) lead to oxindoles. In each case, a 3-thio substituent must be removed by desulfuri2ation. 

Removal of tert-AW Groups, Alkyl groups on a phenol nucleus can be removed selectively to produce a desired synthetic result. The oxidative coupling of phenol offers a good example. 2,6-Di-/ f2 -butylphenol can be coupled under oxidative conditions to... 

A small fraction of the hydrocarbons decompose and deposit on the catalyst as carbon. Although the effect is minute ia terms of yield losses, this carbon can stiU significantly reduce the activity of the catalyst. The carbon is formed from cracking of alkyl groups on the aromatic ring and of nonaromatics present ia certain ethylbenzene feedstocks. It can be removed by the water gas reaction, which is catalyzed by potassium compounds ia the catalyst. Steam, which is... 

The reaction is irreversible and can be used to synthesize aUphatic and aromatic esters. In addition, there are no complications involving water removal or azeotrope formation. Boron tribromide can be used ia place of boron trichloride, but the bromide has a stronger tendency to halogenate the alkyl group of the alcohol (26). Boron tritiuoride does not give the ester, but gives either a complex or dehydrated product. 

The nitrosation of pyrroles and indoles is not a simple process. The 3-nitroso derivatives (84) obtained from indoles exist largely in oximino forms (85) (80IJC(B)767). Nitrosation of pyrrole or alkylpyrroles may result in ring opening or oxidation of the ring and removal of the alkyl groups. This is illustrated by the formation of the maleimide (86) from 2,3,4 -trime thylpyrrole. 

Nitroalkanes show a related relationship between kinetic acidity and thermodynamic acidity. Additional alkyl substituents on nitromethane retard the rate of proton removal although the equilibrium is more favorable for the more highly substituted derivatives. The alkyl groups have a strong stabilizing effect on the nitronate ion, but unfavorable steric effects are dominant at the transition state for proton removal. As a result, kinetic and thermodynamic acidity show opposite responses to alkyl substitution. 

The regioselectivity of the reaction with unsymmetrical alkynes is poor. Mixtures of isomers are obtained with alkyl substituted acetylenes, if the alkyl groups do not differ much in size. A solution to this problem has been reported by Semmelhack et al. The reactants are connected by a -OCHaCHaO-tether, which can later be removed the coupling step thus becomes intramolecular ... 

Alkyl group (Section 3.3) The partial structure that remains when a hydrogen atom is removed from an alkane. 

The carbon-centered radical R, resulting from the initial atom (or group) removal by a silyl radical or by addition of a silyl radical to an unsaturated bond, can be designed to undergo a number of consecutive reactions prior to H-atom transfer. The key step in these consecutive reactions generally involves the intra-or inter-molecular addition of R to a multiple-bonded carbon acceptor. As an example, the propagation steps for the reductive alkylation of alkenes by (TMSfsSiH are shown in Scheme 6. 

Alkyl groups can be cleaved from aromatic rings by treatment with proton and/or Lewis acids. Tertiary R groups are the most easily cleaved because this is true, the tert-butyl group is occasionally introduced into a ring, used to direct another group, and then removed. For example, ... 

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