Mesoionic Imines (MIIs): Strong Donors and Versatile Ligands for Transition Metals and Main Group Substrates

We report the synthesis and the reactivity of 1,2,3‐triazolin‐5‐imine type mesoionic imines (MIIs). The MIIs are accessible by a base‐mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O‐stretching frequencies in MII−CO₂ and −Rh(CO)₂Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N‐heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituent‐induced fluorine‐specific interactions and undergo C−H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO–LUMO gap compared to other related ligands. MIIs are potentially able to act as both π‐donor and π‐acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.

Reactions of 1-trifluoromethyl-prop-2-yne1-iminium salts with 2- and 3-aminopyridines

4-CF3-pyrido[1,2-a]pyrimidin-5-ium triflates could be prepared in fair to high yields from acetylenic iminium salts R-C≡C–C(CF3)=N+Me2·CF3SO3 − and 2-aminopyridinium triflate under thermal conditions. Their reactions with protic heteronucleophiles, leading to 4-CF3, 4-XR-substituted 1,4-dihydropyrido[1,2-a]pyrimidin-5-ium salts, have been studied and reversible addition as well as subsequent opening of the pyrimidine ring have been observed. The cyclocondensation of a 3-phenyl-propyniminium triflate and 3-aminoyridinium triflate required strong thermal activation and yielded a 4-CF3-and a 4-CF3,5-CH3-[1,7]naphthyridine. A 1-CF3-benzo[f][1,7]naphthyridine was obtained from the same propyniminium salt and 3-aminoquinolinium triflate.