- STABLE HETEROARENIUM IONS - VIII SOME TRANSFORMATIONS OF ALKYLTHIOPHENIUM IONS AND NEW SYNTHESIS OF 2-t-BUTYLTHIOPHENE
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The ratio of isomeric ?-complexes formed from thiophene, t-butyl chloride and AlCl3 was found to be changed while keeping at room temperature whitin 1-2 days.This allows to obtain after deprotonation 2-t-butylthiophene containing only 3percent of 3-isomer.Transformations of ?-complexes formed by protonation of individual 2- and 3-t-butylthiophenes in the presence of AlCl3 have been studied and there was found that isomerization and deprotonation took place during their storage resulting in mixtures of 2- and 3-t-butylthiophenes (the ratio 97:3) which contained considerable quantities of 2,4-di-t-butylthiophene.Deprotonation of t-butylthiophenium ions in the presence of acetone leads to the formation of respective t-butylsubstituted dimethyldithienylmethanes.
- Belen'kii, L. I.,Yakubov, A. P.
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p. 2471 - 2477
(2007/10/02)
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- Gas-phase Heteroaromatic Substitution. Part 4. Electrophilic Attack of t-Butyl Cation on Pyrrole, N-Methylpyrrole, Furan, and Thiophene
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t-Butyl cation, obtained in the gas state from the γ-radiolysis of neopentane has been allowed to react with pyrrole (1), N-methylpyrrole (2), furan (3), and thiophene (4).Experiments have been carried out in the pressure range 50-760 Torr and in the presence of variable concentrations of a gaseous base (NMe3).The reactivity of simple heteroaromatic compounds relative to toluene in competition experiments is found to depend markedly upon the composition of the gaseous reaction environment.The apparent kS/kT ratios decrease slightly with increasing pressure.At atmospheric pressure, these ratios increase with the NMe3 concentration levelling off to a value of 1.0 (1), 2.2 (2), 2.6 (3), and 0.6 (4), at the highest base concentration (PNMe3=10 Torr).Under the same conditions, predominant β substitution occurs in the pyrroles , whereas α-attack is favoured in the case of furan (β:α=9:91) and thiophene (β:α=20:80).The gaseous elecrophile appears rather unselective between the n- and the ?-type centres of furan (O:ring=50:50) and thiophene (S:ring=40:60), while it displays no or very limited substitution on the heteroatom of pyrroles.A mechanism based upon reversible addition of the t-butyl cation to the heteroaromatic nucleus, involving premilinary formation of an electrostatic adduct, is used to account for the formation of the t-butylated products and their isomeric composition.It is concluded that the gas-phase t-butyl alkylation of simple five-membered heteroaromatic compounds is regulated by electrostatic interactions established within the encounter pair.A close correspondence exists between this rationalization of the present gas-phase results and recent theoretical and experimental evidence concerning related alkylation processes.
- Margonelli, Andrea,Speranza, Maurizio
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p. 1491 - 1498
(2007/10/02)
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