DFT characterization of the mechanism for Staudinger/aza-Wittig tandem organocatalysis

A computational simulation with DFT calculations and microkinetic modeling is carried out on a complete catalytic cycle, involving Staudinger ligation, aza-Wittig condensation and phosphine oxide recycle, for non-truncated substrates and catalyst. The Staudinger reaction produces phosphazenes (R₃P = NR), also known as iminophosphoranes, from phosphines and organic azides. Electrophilic carbonyl groups react with phospazenes to produce imines and phosphine oxides. Recently the Staudinger reaction and the aza-Wittig condensation have been combined to spawn intramolecular tandems producing cyclic molecules of great pharmaceutical interest (e.g. benzoxazoles). The release of diatomic nitrogen combined with the formation of phosphine oxide represents the driving force of the reaction. The implementation of in situ recycling of the exhausted phosphine oxide into the Staudinger/aza-Wittig tandem improves the scopes and the applicability of the reaction, transforming it into a powerful and versatile synthetic tool.