Researchers at the University of Münster have developed an unusual variant of the Staudinger reaction which takes advantage of frustrated Lewis pair (FLP) frameworks to provide internal stabilisation.
The Staudinger reaction represents an important tool for the generation of iminophosphoranes, which can be subsequently utilized in the synthesis of a wide range of functional motifs, including amines, imines, and amides. Additionally, with the variation of the phosphorus compound, the Staudinger reaction can be employed in bio-ligation processes under mild conditions.
Traditionally, the iminophosphorane product (3) is generated from the reaction of an organic azide (1) with a trivalent phosphorus compound. This is thought to proceed via transition state 2, from which elimination of dinitrogen yields the iminophosphorane (3). In the variation developed by the Erker group, such elimination of dinitrogen is avoided, generating products such as 4.
This unusual reactivity was cleverly obtained through the incorporation of the phosphorus moiety in an FLP; specifically, vicinal phosphane–borane FLPs (5). Upon reaction with mesityl azide, these FLPs led to the formation of 5-membered heterocycles (6). Following thermolysis or photolysis, phosphinimines (7) were obtained which benefit from internal stabilisation from borane. The by-product of this formation is indazole 8. While the mechanism of this unusual variation has yet to be confirmed, the authors propose a pathway proceeding via the ion pair 9.
This work contributes to the broadening utility of the Staudinger reaction, and illustrates the potential of FLP chemistry in the development of new reactivity.
Read the ‘HOT’ Chem Comm article today (Free to access until the 7th of December):
Anomalous Staudinger reaction at intramolecular frustrated P–B Lewis pair frameworks
Annika Stute , Lukas Heletta , Roland Fröhlich, Constantin G. Daniliuc, Gerald Kehr and Gerhard Erker
Chem. Commun., 2012, 48, 11739-11741
DOI: 10.1039/C2CC36782C
Published on behalf of Ruth Gilligan, Chemical Communications web science writer.
