Tandem 1,5‐Hydride Shift/6π Electrocyclization of Ketenimines and Carbodiimides Substituted with Cyclic Acetal and Dithioacetal Functions: Experiments and Computations

摘要: N-Aryl ketenimines bearing five- and six-membered cyclic acetal functions - such as 1,3-dioxolane, 1,3-dithiolane, 1,3-dioxane, 1,3-dithiane systems at the ortho position of N-aryl substituent transform on mild thermal treatment into quinolines, through a tandem sequence consisting [1,5]-H shift followed by 6π electrocyclic ring closure. Structurally analogous carbodiimides are converted quinazolines in comparable processes. Similar sequences can be successfully applied to N-thienyl N-pyrazolyl ketenimines. DFT calculations have established two-step mechanism for those conversions, an initial 1,5-hydride subsequent electrocyclization, confirm beneficial effect function, which gives hydride character migrating hydrogen atom. The capability promote H depends type function (acetal better than dithioacetal), its size (five-membered six-membered) heterocumulenic fragment (ketenimine carbodiimide). Changing benzene connecting ketenimine heterocyclic has pronounced consequences magnitude energy barriers.