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Tandem 1,5‐Hydride Shift/6π Electrocyclization of Ketenimines and Carbodiimides Substituted with Cyclic Acetal and Dithioacetal Functions: Experiments and Computations
Author(s) -
Alajarin Mateo,
Bonillo Baltasar,
Ortin MariaMar,
SanchezAndrada Pilar,
Vidal Angel
Publication year - 2011
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201001372
Subject(s) - ketenimine , chemistry , electrocyclic reaction , acetal , ring (chemistry) , aryl , hydride , computational chemistry , medicinal chemistry , stereochemistry , organic chemistry , alkyl , hydrogen
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.