title: Formation of N-Heterocyclic Carbenes by Tautomerization of Mesomeric Betaines: Cyclic Boron Adducts and Palladium Complexes From 2-(Imidazolium-1-yl)phenolates.
abstract: 2-(Imidazolium-1-yl)phenolates are conjugated heterocyclic mesomeric betaines in tautomeric equilibrium with the corresponding N-heterocyclic carbenes (NHCs), 3-(2-hydroxyphenyl)-imidazol-2-ylidenes. The carbene tautomers can be trapped as thiones (X-ray analysis). Moreover, bis(triphenylphosphine)palladium(II) dichloride in THF trapped the carbene tautomer as a palladium complex without participation of the phenolate group (X-ray analysis). The corresponding anionic NHCs, 2-phenolate-substituted imidazol-2-ylidenes, can be trapped by triethylborane or triphenylborane to form 4,4-diethyl- or 4,4-diphenyl-4H-benzo[e]imidazo[2,1-c][1,4,2]oxaza-borininium-4-ides, respectively (two X-ray analyses). These tricyclic systems are the first representatives of a new heterocyclic ring system. The results of DFT calculations concerning the HOMO/LUMO profiles and partial charges are also presented.
authors.affiliation: Clausthal University of Technology, Institute of Organic Chemistry, Leibnizstrasse 6, 38678, Clausthal-Zellerfeld, Germany), Fax: (+49)-5323-722834.
authors.forename: Ming
authors.initials: M
authors.lastname: Liu
authors.affiliation: University of Helsinki, Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55 (A.I. Virtasen aukio 1), 00014, Helsinki, Finland.
authors.forename: Martin
authors.initials: M
authors.lastname: Nieger
authors.affiliation: Clausthal University of Technology, Institute of Organic Chemistry, Leibnizstrasse 6, 38678, Clausthal-Zellerfeld, Germany), Fax: (+49)-5323-722834.
authors.forename: Eike G
authors.initials: EG
authors.lastname: Hübner
authors.affiliation: Clausthal University of Technology, Institute of Organic Chemistry, Leibnizstrasse 6, 38678, Clausthal-Zellerfeld, Germany), Fax: (+49)-5323-722834. schmidt@ioc.tu-clausthal.de.
authors.forename: Andreas
authors.initials: A
authors.lastname: Schmidt
keywords: borates; carbenes; heterocycles; imidazol-2-ylidenes; mesoionic compounds