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4,5‐Bis(dialkylamino)‐Substituted Imidazolium Systems: Facile Access to N‐Heterocyclic Carbenes with Self‐Umpolung Option
Author(s) -
Huber Stefan M.,
Heinemann Frank W.,
Audebert Pierre,
Weiss Robert
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201101999
Subject(s) - umpolung , carbene , nucleophile , chemistry , reactivity (psychology) , acceptor , transition metal carbene complex , medicinal chemistry , combinatorial chemistry , organic chemistry , catalysis , medicine , physics , alternative medicine , pathology , condensed matter physics
The first synthesis of 4,5‐bis‐ (dimethylamino)‐substituted imidazolium compounds was developed, which is based on the reaction of a 1,2‐diamino‐1,2‐bis(phosphonio)ethene with lithiated formamidines. This represents the first application of this class of ethene derivatives for the preparation of heterocycles. These N‐heterocyclic carbene (NHC) precursors show a remarkably reduced basicity and nucleophilicity of their NMe 2 groups, which is due to the strong anomeric interactions of the latter with the imidazolium core. According to DFT calculations, these NHCs are capable of self‐umpolung if sufficiently strong acceptor substituents are introduced at the carbene center. To test the self‐umpolung capabilities of the NHCs, various substituents were attached to the carbene center and the obtained compounds were characterized by single‐crystal X‐ray analysis as well as quantum chemical computations. Strong acceptor substituents are required to induce self‐umpolung, such as in the phosphonio‐substituted derivative, for which partial self‐umpolung was found. The N , N ′‐bis(4‐dimethylaminophenyl)‐substituted imidazolium compound represents a special case, as it incorporates as much as three two‐step redox systems within the NHC framework. This will probably result in a high electronic flexibility of the corresponding nucleophilic carbenes, especially when they serve as ligands in transition metal complexes.