A Mechanistic Rationale for the Mode Selectivity in the Intramolecular Cyclization of Ethylene‐Tethered Iminoketenimines: [2+2] versus [4+2] Stepwise Cycloadditions

N‐Arylcarbodiimides react with PTAD to provide [1,2,4]triazolo[1,2‐a][1,2,4]benzotriazines by a [4+2] cycloaddition reaction. When asymmetrically substituted diarylcarbodiimides are used, the cycloaddition proceeds with total chemoselectivity because only the more electron‐rich aryl nucleus is involved. This observation has been rationalized by a computational study using DFT methods that shows that, in the reaction of HTAD with arylcarbodiimides, the magnitude of the energy barriers depend on the electronic features of the substituents at the aryl nucleus; the calculations also indicate that the reaction proceeds through asynchronous states with polar characteristics. The treatment of the final cycloadducts with potassium hydroxide affords 3‐aryl(alkyl)amino‐1,2,4‐benzotriazines.

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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

Alajarı́n

Bonillo

Ortin

et al. 2011

Eur J Org Chem

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Keywords: Density functional calculations / Reaction mechanisms / Ketenimines / Cyclization / Hydride shift / Hydricity 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 Npyrazolyl ketenimines. DFT calculations have established a two-step mechanism for those conversions, consisting of an

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A Mechanistic Rationale for the Mode Selectivity in the Intramolecular Cyclization of Ethylene‐Tethered Iminoketenimines: [2+2] versus [4+2] Stepwise Cycloadditions

Cited by 10 publications

References 35 publications

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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

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