From mesomeric betaines to anionic N-heterocyclic carbenes

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Mesomeric betaines are versatile precursors of anionic heterocyclic carbenes. However, their synthetic potential is far from being exploited. By deprotonation, extremly pi-electron-rich anionic N-heterocyclic carbenes can be generated which also - depending on the type of conjugation – can possess high pi-electron densities on the carbene carbon atom. After preliminary work on sydnones and münchnones, this project deals with four additional mesomeric betaines as precursors of N-heteroccylic carbenes, which differ in their pi-architectures. They have been chosen by their CREF values, which have been introduced recently to quantify the ease of carbene formation from suitable precursors. The translation of the CREF values into chemistry and their relationship to TEP and HEP values, 1JCSe coupling constants and others will be discussed. Trapping reactions (Se, S), complexations (Rh, Pd, Pt, Au, Ir, …), and examinations of the catalytic activities will be performed. Surprisingly, the carbon dixide adducts of sydnone-carbenes catalyze Suzuki-Miyaura reactions in acids. The application of this cross-coupling to base-labile substrates such as amino acids and dipeptides (which racemate easily with bases) and isoxazoles as well as benzisoazoles (which suffer from rapid ring-cleavages with bases) will be studied.