Reactions of Zirconocene–1-Aza-1,3-diene Complexes with Acyl Cyanides: Substrate-Dependent Synthesis of Acyl- or Non-Acyl-Substituted Pyrroles

Insertion of acyl cyanides into azazirconacyclopentenes
derived
from 1,3-azadienes has been described, which affords acyl- or non-acyl-substituted
pyrroles upon acidic quenching. These reactions are initialized through
CO insertion into the azazirconacycle to afford seven-membered
oxaazazirconacycles. In the cases of 1,4- or 1,2,4-substituted azadienes,
addition of a second molecule of acyl cyanide followed by cyclization
upon acidic quenching leads to acyl-substituted pyrroles. In the cases
of 1,3,4-substituted azadienes, the addition of a second molecule
of acyl cyanide cannot proceed due to the steric hindrance caused
by the R3 group on the zirconium intermediate. Acidic quenching
of the resulting zirconium intermediate affords non-acyl-substituted
pyrroles.