1404293-53-3Relevant articles and documents
Preparation of NH-pyrroles under superelectrophilic conditions by an Aza-Nazarov reaction cascade with indole as neutral leaving group: Experiment and theory
Narayan, Rishikesh,Daniliuc, Constantin-Gabriel,Wuerthwein, Ernst-Ulrich
, p. 6021 - 6032,12 (2020/09/02)
Aza-Nazarov reactions starting from 1-azapenta-1,4-dien-3-ones offer an attractive route to five-membered nitrogen heterocycles such as N-substituted pyrroles. It has now been discovered that N-indolinylhydrazone-derived 1-azapenta-1,4-dien-3-one 9a gives different products under conditions of varying acidity. Under strongly acidic conditions (2 equiv. of triflic acid, dilute solution, electrophilic conditions) it simply undergoes an aza-Nazarov reaction to give N-indolinylpyrrole 15a after workup with acetic anhydride. If, however, a large excess of triflic acid is used (7-10 equiv., concentrated solution, superelectrophilic conditions), 9a has been found to undergo an aza-Nazarov reaction cascade followed by N-N bond cleavage to give NH-pyrrole 10a and acetylated indole 8 as the final products (after workup with acetic anhydride). High-level quantum chemical calculations were used to explain this acid-concentration-dependent reaction cascade. The formation of the reaction products can be explained in terms of an electrocyclization reaction of the protonated starting material 9a and a subsequent N-N bond cleavage reaction involving either mono- or dicationic species under strongly acidic conditions. In an easy one-pot reaction sequence,1-azapenta-1,4-dien-3-ones 9a-i undergo aza-Nazarov reactions with subsequent acid-induced N-N bond cleavage in the presence of large excesses of triflic acid to give NH-pyrroles 10a-i and acetylated indole 8 (after acetic anhydride workup). Quantum chemical calculations were used to explain this acid-concentration-dependent reaction cascade.
