Browsing by Author "Krebs, Andreas"

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    Carbodesilylierung von (Trimethylsilyl)imidazolen und -pyrazolen
    (1991) Effenberger, Franz; Roos, Michael; Ahmad, Roshan; Krebs, Andreas
    The preparation of the 1-methyl(trimethylsilyl) (TMS)-substituted imidazoles 3a, 4a, 8, 9, and 11a by silylation of the corresponding metallated imidazoles is described. Carbodesilylation of 3 with aldehydes or carboxylic halogenides occurs selectively in 2-position. In the presence of a strong base (CsF) the reactivity against carbon electrophiles correlates well with the stability of the imidazolyl anions; regioselective carbodesilylation in 2-, 5-, or 4-position of the twofold TMS-substituted imidazoles 3a and 9 therefore is possible, which allows the synthesis of a great variety of hydroxyalkyl-substituted imidazoles and of acylimidazoles. By using the dimethylsulfamoyl substituent as an N-protecting group, the N- unsubstituted 5-benzoylimidazole (26) as well as the comparable 5-benzoyl-pyrazole (30b) and 5-(hydroxyphenylmethyl)-pyrazole (30a) are accessible.
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    Regioselective halo- and carbodesilylation of (trimethylsilyl)-1-methylpyrazoles
    (1984) Effenberger, Franz; Krebs, Andreas
    The isomeric 3-, 4-, and 5-(trimethylsily1)- as well as the 3,4-, 3,5-, and 4,5-bis(trimethylsilyl)-l-methylpyrazoles (2,7,3,5,9,a nd 10, respectively) are obtained by methylation of the corresponding (trimethylsily1)-1H-pyrazoles or by silylation of Grignard or lithio derivatives of appropriate 1-methylpyrazoles with chlorotrimethylsilane. 5 and 10 are halodesilylated regioselectively by Br2 or ICl in the 4-position, yielding 13 and 15. With additional bromine, these monobromo compounds suffer exclusively bromodesilylation to give 3,4- and 4,5-dibromo-lmethylpyrazole (14 and 16, respectively). These findings are in accord with the electrophilic substitution reactivity indices for 1-methylpyrazole (8) and with ipso-directing influence of the MeBSi group. The reaction of 5 with Iz, unexpectedly, attacks preferentially at the 3-position. Regioselective carbodesilylation in the 5-position is observed in the fluoride-catalyzed reactions of 3, 9, and 10 with carbon electrophiles. The high regiospecificity of this reaction is rationalized in terms of carbanion stabilization at the individual pyrazole positions.
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    Zum Mechanismus der Carbodesilylierung 4- bzw. 5-substituierter 2-(Trimethylsilyl)pyridine mit Benzaldehyd
    (1992) Effenberger, Franz; Krebs, Andreas; Willrett, Peter
    An "ylide mechanism" is proposed for the carbodesilylation of 2-(trimethylsily1)pyridines with benzaldehyde. In contrast, 3- and 4-(trimethylsilyl)pyridines, react only in the presence of a base catalyst via pyridyl anions with electrophiles. The rates of the uncatalyzed carbodesilylation reactions of 4-substituted 2-(trimethylsily1)pyridines 2 with benzaldehyde correlate very well with the resonance parameters of the substituents σ1, whereas the rates of 5-substituted 2-(trimethylsily1)pyridines 7 correlate with the inductive substituent parameters σ1 in the Taft equation.