Dealkylation, reversible

Lewis acid catalysts can also effect dealkylation, i.e. the reaction is reversible. Thus ethylbenzene (22) with BF3 and HF, is found to disproportionate ... 

In conclusion, it should be mentioned that though the great majority of aromatic electrophilic substitution reactions involve displacement of hydrogen, other atoms or groups can be involved. Thus we have already seen the displacement of S03H in the reversal of sulphonation (p. 140), of alkyl in dealkylation (p. 143), and a further, less common, displacement is that of SiR3 in protodesilylation (cf. also p. 161) ... 

In Fig. 19(6) the formation of a hydrogen bond is envisaged followed by the attachment of the positive P atom to the negative site and elimination of HF (fig. 19 (c)). Reversal, when possible,3 is represented in fig. 19(d). The inhibitor is of course hydrolysed at the end of the reaction. It may well be that (i 0)2P0(0H) is not actually recoverable, but that the regeneration process involves, first of all, a dealkylation which then facilitates the removal of the phosphate moiety from the enzyme.4... 

Di-rm-butylphenol dealkylation, 41 161, 171 Dodecacarbonylhexarhodium, reversible catalytic decarbonylation, 35 206-207 Dodecacarbonyltriiron, reaction on catalytic surfaces, 35 192-194 on alumina, 35 192 on silica, 35 190... 

Aromatic N,N-diaIkylamines react rapidly with HNOj and undergo ring nitration and nitro-sative dealkylation both reactions are linked through the formation of a nitrosoammonium ion RiR N+-N=0 (Ri=Ar, R =Alk). This nitrosoammonium ion then undergoes reversible homolysis to NO and a cation-radical (Loeppky et al. 1998). 

Reversal of Friedel—Crafts Alkylation Hydro-de-alkylation or Dealkylation... 

Quaternization of molecules containing alkoxy and thioalkoxy substituents can be complicated by dealkylation of the substituent. Such side reactions are especially facile in molecules that give rise to a carbonyl or thiocarbonyl group following dealkylation. The reaction between a 2-substituted pyridine and Mel is illustrative [Eq. (16)]. This sequence is reversible and can be used to advantage to compare the enthalpies of unquaternized starting material and product.137,138... 

In this section we shall deal only with the reverse reaction to dealkylation by cracking (Sect. 2.5), that is with additions of alkanes and aromatic compounds to the carbon—carbon double bond. The former reaction is described only in a single paper [339] the formation of 2,2,4-trimethyl-pentane from isobutane and isobutene... 

Note that these reactions are reversible, and indeed one of the best methods for the preparation of thioether complexes is by alkylation of a co-ordinated thiolate. In general, the dealkylation reactions require forcing conditions, but in some cases they proceed in an... 

There is some evidence that the alkylation of co-ordinated thiolate is a reversible process, and there are a number of examples known in which a co-ordinated thioether is de-alkylated to yield a co-ordinated thiolate. In general, these reactions are rather sluggish, but occur in hot dmf solution. Whether the mechanism involves simple thermal dealkylation, or the intermediacy of the dmf as an alkyl group acceptor is not clear. 

The first mechanism operates to remove ENA damage, whereupon the missing genetic information is resynthesized chemically according to instructions received from the complementary strand (e.g., excision repair or direct reversal of damage, as by photoreactivation and dealkylation). The second mechanism acts as a part of, or in close association with,... 

Photoreactivation and dealkylation by enzymes that directly reverse the altered DNA structure to its original state. This form of repair has been associated mainly with microorganisms and is thought to occur in mammalian cells.1 1 3 It is not involved currently in predictive tests for carcinogenicity or mutagenicity. 

The alkylation of thiurets (347) in basic media produces the imino derivatives (352) possibly via intermediates such as (350) and (351) (Nu = soft nucleophile), as indicated in Scheme 127 (74IJC134). The acid-catalyzed dealkylation of (352 R = (-butyl) results in a reverse isomerization to the thiuret (347). The reaction takes a different course when iV-alkyl thiurets of type (353) are alkylated in the presence of an amine. In this case intermediate (354) is postulated to exchange with the amine to form a guanidine which undergoes ring closure to yield (355 Scheme 128) (74JOC2235). Thermolysis of (356) also produces (355) presumably via a spiran intermediate. 

A mechanistic study by Deady and Korytsky, which showed that dealkylation is exactly the reverse of the alkylation reaction (79TL451), implies, according to the principle of microscopic reversibility, that the amount of strain in the TS for quaternization is two-thirds of that in the final quaternary salt, in close agreement with the early estimate by Brown. The ratio of substituent steric effects is 3/1 for disubstituted pyridines in methylation vs. demethylation. Because of smaller sensitivity to steric effects in thiazoles, steric acceleration in the reverse reaction is small (76AJC1745 79BSF(2)484). 

Complexes of alkyl-linked o-benzenedithiolates eliminate ethylene to give benzenedithiolato complexes (Eq. 27) (275-277). This reaction is the reverse of the known addition of alkenes to bis(dithiolene) complexes (278-281). Photolysis of the S-benzylated dithiolenes M(PhCH2S2C2Ph2)2 (M = Ni, Pd, Pt) results in S-dealkylation with elimination of benzyl radicals (282). In general, however, the properties of S-alkylated dithiolenes have not been... 

Under acidic conditions, the alkylation and dealkylation of aromatic compounds are reversible reactions involving several steps in which n- and CT-complexes are formed. However, dealkylation proceeds only under more drastic conditions compared with alkylation. Nevertheless, this is not always the case. For example, if the aromatic compound is of the DPM type, the dealkylation may proceed under mild conditions since the cations formed (Fig. 6.6.5) are resonance-stabilized. This statement is supported by the fact that DPM derivatives may be degraded even at room temperature by aluminum chloride to yield benzene, alkylbenzene, and alkyldiphenylmethane, together with some resinous substances (Tsuge and Tashiro 1962, 1965). 

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