A Computational Study of Model Parent Systems and Reported Aza‐(Iso)Nazarov/Aza‐(Iso)Piancatelli Electrocyclic Reactions

Structural variants of the (iso)‐Nazarov and aza‐(iso)Nazarov electrocyclic reactions, including monofunctional and difunctional pentadienyl cations with permutations of the heteroatom‐containing functional groups have been computationally studied at the ωB97XD/Def2TZVPP(SMD, THF)//ωB97XD/Def2TZVP level of theory. The relative location of two heteroatoms at either odd‐ or even‐numbered positions of the conjugated pentadienyl cation, including the Piancatelli and iso‐Piancatelli reactions and their variants, determines the feasibility of their rearrangement. In particular 4πe–‐electrocyclic reactions of formal enamines/protonated ketones are more favorable than those of the alternative enols/iminium ions. Both the classical aza‐Piancatelli and the aza‐iso‐Piancatelli electrocyclizations are highly favored. The key electrocyclic aza‐iso‐Piancatelli and aza‐Piancatelli steps of the few experimental systems successfully reported show activation energies that are in agreement with those of the simple model systems. On line with the computational predictions, the stabilization of the intermediates are key to ensure the success of these concerted electrocyclizations due to the positional location of the two heteroatoms and other factors (such as the release of strain of reacting allenes). Computations on (iso)‐Nazarov and aza‐(iso)Nazarov as well as Piancatelli and iso‐Piancatelli electrocyclic reactions, including the few systems successfully reported to undergo the process, have confirmed the key role played by the heteroatoms when located at either odd‐ or even‐numbered positions of the conjugated enols, enamines, protonated ketones and iminium ions reaction intermediates.