Einfache Synthese von 1,3‐Dialkyl‐4,5‐dihalogen‐2(3H)‐imidazolonen

Abstract

Professor Rolf Appel zum 60. Geburtstag gewidmet Nachdem Maleinsaureanhydride und -imide[2a1 sowie Vinylencarbonate[2b1 und deren Dihal~genderivate~',~], 4-Oxazolin-2-0ne[~"~, 1,3-Dioxole und 1,4-Di0xen[~~' als cyclophile Molekule untersucht worden sind, fehlte noch eine ergiebige Synthese der potentiell cyclophilen 4,5-Dihalogen-2(3 H)-imidaz~lone'~~.Wir fanden einen einfachen Zugang zu den Titelverbindungen (4). Trichlor-(la)61 oder Tribromimidazol ( 1~)~'~ werden rnit Dialkylsulfat zu den Alkylimidazolen (2a-d) alkyliert und sodann mit Trialkyloxonium-tetrafluoroborat in hohen Ausbeuten zu den kristallinen, stabilen, hochschmelzenden Imidazoliumsalzen (3a-d) quaternisiert. Die in Dimethylformamid gut loslichen Sake (3a-d) tauschen in verdiinntem Natriumhydroxid den 2-Halogensubstituenten selektiv und nahezu quantitativ gegen die Hydroxygruppe aus. Die intermediaren 2-Hydroxyimidazoliumsalze spalten mit NaOH sogleich NaBF, ab und ergeben die 4,5-Dihalogen-2(3H)-imidazolone ( 4 -d ) (siehe Tabelle l)[*]. Wahrend die Chlorderivate (4a, b) stabil sind, zerset-

Synthesis of 2-Imidazolones and 2-Iminoimidazoles

“…At this stage, we do not know whether this reaction proceeds mechanistically via carbene 14 as depicted in Scheme or whether the alternative mechanism outlined in Scheme is more likely. Related observations of nucleophilic substitution of 2-haloimidazolium salts have been reported in the literature. , While these observations do not prove that the 2-chloroimidazolium salt is formed in the course of the oxidation, they demonstrate that it is a competent intermediate in this chemistry. In the latter case, hydoxide adds to the imidazolium salt 15 to form 19…”

“…It is of note that related observations of nucleophilic substitution of 2-haloimidazolium salts have been reported in the literature. 20, 21 While these observations do not prove that the 2-chloroimidazolium salt is formed in the course of the oxidation, they demonstrate that it is a competent intermediate in this chemistry. In the latter case, hydoxide adds to the imidazolium salt 15 to form 19 , 22 O -chlorination and then base-induced elimination of HCl from 20 results in the formation of the carbonyl moiety.…”