1300044-09-0Relevant articles and documents
Tandem 1,5-hydride shift/6π electrocyclization of ketenimines and carbodiimides substituted with cyclic acetal and dithioacetal functions: Experiments and computations
Alajarin, Mateo,Bonillo, Baltasar,Ortin, Maria-Mar,Sanchez-Andrada, Pilar,Vidal, Angel
, p. 1896 - 1913 (2011/05/05)
N-Aryl ketenimines bearing five- and six-membered cyclic acetal functions - such as 1,3-dioxolane, 1,3-dithiolane, 1,3-dioxane, and 1,3-dithiane systems - at the ortho position of the N-aryl substituent transform on mild thermal treatment into quinolines, through a tandem sequence consisting of a [1,5]-H shift followed by a 6π electrocyclic ring closure. Structurally analogous N-aryl carbodiimides are converted into quinazolines in comparable tandem processes. Similar sequences can be successfully applied to N-thienyl and N-pyrazolyl ketenimines. DFT calculations have established a two-step mechanism for those conversions, consisting of an initial 1,5-hydride shift and subsequent 6π electrocyclization, and confirm the beneficial effect of the acetal function, which gives hydride character to the migrating hydrogen atom. The capability to promote the H shift depends on the type of acetal function (acetal better than dithioacetal), its ring size (five-membered better than six-membered) and the heterocumulenic fragment (ketenimine better than carbodiimide). Changing the benzene ring connecting the acetal and ketenimine functions for a heterocyclic ring has pronounced consequences for the magnitude of the energy barriers. Copyright
