Dialkylation equilibration

Silica gel is an effective catalyst for the t-butylation of thiophene and benzo[/ ]thiophene using t-butyl bromide. 2,5-Di-r-butylthiophene and 3-f-butylbenzo[b]thiophene can be prepared easily by this procedure (84JOC4161). Alkylation of thiophene with f-butyl chloride, isopropyl chloride or ethyl chloride at - 70°C in the presence of A1C13 produced a-complexes under kinetic control. On thermal equilibration, migration of alkyl from position 3 to position 2, as well as disproportionation to dialkyl-and trialkyl thiophenes can occur (86T759). 

More recently some further interesting features of 4,4-dialkylation have been uncovered. As mentioned in the earlier discussion of enolisation, the reaction proceeds through the kinetically-preferred A. -fiienolate anion which is permitted to equilibrate in favour of the stable A qsomer by the low reactivity of the alkyl halide. The intervention of the A -diene was demonstrated by the total loss of deuterium when a 2jS-deutero-A 3 ketone was 4,4-dimethylated [no]. Studies on the stereochemistry of alkylation have revealed a strong preference for introduction of the second alkyl group at the 4 -position (7) [163], as would be predicted in view of the... 

Finally like methyllithium (ref. 121) ammonium fluoride (ref. 122), tris-(dialkylamino)sulfonium salts (ref. 123) or alkali alkoxides (ref. 124), alkali amides in liquid ammonia are able to cleave the silicium-oxygen bond of silyl enol ethers (refs. 125, 126) leading to enolates. The sodium enolate obtained (Fig. 27) by treatment of a silyl enol ether with NaNH2 can be equilibrated in the medium, leading to two alkylated products, nevertheless no polyalkylated species is detected. With the use of LiNH2 only the expected reaction product is prepared but the use of KNH2 leads to a mixture of C-mono and dialkylated and O-alkylted products (ref. 125). 

Equilibration of the Z-E isomers also occurs in acetonitrile.25 With 2-acetoxyphenylmethylene oxazolones, isomerization to the E isomer was observed.63 Isomerizations to benzamidocoumarin derivatives under photochemical conditions have also been reported 64,65 While the 2-oxazolin-5-ones are thermally stable, the introduction of a 4-acyl or aroyl group, a potential site for further reaction, markedly decreases the stability. Thus, when 2,4-dialkyl-4-aroyl-2-oxazolin-5-ones are heated to 180°, decarboxylation occurs readily with the formation of trisubstituted oxazoles in yields of 71-95%66 [Eq. (13)). 

Salts of mono- and dithiophosphoric acid 0,0-dialkyl esters react with A -benzylidene-2,2,2-trifluoro-l-chloroethylamine at the sp carbon to give compounds (155 a-c) when X = S. these can i.somerize to (156 a,b) in the presence of triethylamine and are then rapidly transformed into (157 a,b). When R = EtO, an equilibration occurs between (157 a) and (15S a). ... 

Determination of the acidity of water-immiscible HA encounters difficulties similar to those in the determination of water-immiscible amine basicity and can be approached similarly. By analogy to pHhn measurements, we have equilibrated HA-containing organic phases with aqueous NaOH solutions and determined aqueous phase pH. The acidity sequence followed that of the water-soluble analogs carboxylic acids < ot-halo carboxylic acids < dialkyl phosphoric acid esters < sulfonic acids [44]. 

Another interesting manifestation of the m-dialkyl effect appears in the series of epoxide migrations studied by Payne [see Eqs. (9)-(13)] in which the most highly substituted epoxides were found to be most stable at equilibrium. The epoxides were equilibrated in a deficit 0.5 M sodium... 

Chemical equilibrations and Hn.m.r. spectra have been used in an investigation of 2-alkyl- and 2,2-dialkyl-5-methyl-l,3-oxathiolans (63). ... 

Y.-X. Chen, T. J. Marks, Constrained geometry dialkyl catalysts. Efficient syntheses, C-H bond activation chemistry, monomer-dimer equilibration, and a-olefin polymerization catalysis. OrganometalUcs 16, 1997 3649. 

Dialkyl and tetraalkyldiboranes have been shown to equilibrate with boric, alkylboronic, and dialkylborinic esters as illustrated in Eqs. (29-31) 81, 82,130). 

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