De- and Rearomatisation of Pyridine in Silylene Chemistry.

ABSTRACT

Traditional methods relying on metal-ligand cooperation for activating pyridine bonds in de- and rearomatisation are being challenged by the abundant metal-free element species as alternatives. Here, we investigated the de/re-aromatisation of pyridine facilitated by pyridylamino-functionalised silylene reactions with ketones and ketene. The reactivity outcome is highly dependent on the substituents on the ketones. By carefully tuning the steric demand of the ketone, each intermediate of the reaction sequence could be isolated. At room temperature, benzophenone and acetophenone substrates led to dearomatisation of the pyridine moiety, with the case of acetophenone showing an intermediate silaoxirane preceding dearomatisation. However, when subjected to acetone or diphenylketene, only silaoxiranes were formed without dearomatisation of the pyridine moiety. Notably, only benzophenone-derived dearomatised species demonstrated rearomatisation upon heating. Furthermore, the reduced steric bulk of the ketene facilitated further ring expansion with another equivalent of the substrate, forming sila-1,3-dioxolanes. Both steric hindrance and aromatic groups collectively influence the dearomatisation of pyridine in pyridylaminosilylene reactions.