Abstract:
Criegee intermediates or carbonyl O-oxides are reactive transients in ozonolysis reactions. Their aminosubstituted analogs, formally derived from N-heterocyclic carbenes might be stabilized through effective delocalization of electron density, but have remained elusive to this day. The utility of ozonolysis of adequate precursors, namely N-heterocyclic olefins and N-methoxyguanidines, towards the preparation of N-heterocyclic CIs was explored in this work. It was discovered in a DFT study that due to a high degree of polarization, such double bonds may accept a single oxygen atom from ozone, producing singlet dioxygen and zwitterionic intermediates, which in the case of N-heterocyclic olefins are precursors to Breslow intermediates. A switch in ozone reactivity towards polarized double bonds was explained by the concept of a post-TS bifurcation, where one TS can lead to (3+2) cycloaddition or O-atom transfer products, depending on the shape of the PES and the dynamic effects in operation. A formation of a monoxidized derivative of an N-heterocyclic olefin was identified in both, the oxidation with mCPBA and an ozonolysis reaction.
Sulfines or thiocarbonyl S-oxides are related species, that are usually isolable as opposed to Criegee intermediates. Their N-heterocyclic analogs are known but poorly studied compounds, mostly due to a prior lack of preparative synthetic procedures. In this work, they have been successfully prepared through two alternative procedures, namely thiourea oxidation and a formal SO transfer. The oxidation of thioureas usually leads to desulfurization caused by overoxidation, but it could be effectively halted with the help of steric protection and usage of protic solvents. The oxidant-free SO transfer reaction between a thiirane S-oxide and NHCs takes place at room temperature, which implies a concerted SO transfer mechanism. Structures of sulfines derived from various carbenes have been confirmed through X-ray crystallography. Unlike classical sulfines, they are primarily nucleophilic species, which can form Lewis acid-base complexes, and undergo formal (2+2) cycloaddition with electron-poor alkynes under fission of the S−O bond.
Criegee Intermediate