Tungsten-Promoted Diels−Alder Cycloaddition of Pyridines: Dearomatization of 2,6-Dimethoxypyridine Generates a Potent 2-Azadiene Synthon

The complex TpW(NO)(PMe3)(DMP), where DMP is 2,6-dimethoxypyridine,
features an η2-coordinated pyridine ligand that chemically
resembles an electron-rich 2-azadiene. As such, it readily reacts
with a full range of dienophiles including examples of both alkenes
and alkynes to generate azabicyclo[2.2.2]octadiene complexes. Often,
one diastereomer spontaneously precipitates from the reaction solution.
The bicyclic ligand in these complexes can in some cases be directly
removed from the metal by the action of an oxidant (e.g., CAN) and
in other cases can be chemically modified prior to its removal. Crystal
structures of five tungsten cycloadduct complexes are included. A
related complex of 2,6-lutidine is found to react with the nitrile
dienophile ethyl cyanoformate to form a new dihapto-coordinated pyridine
complex that is derived from a purported diaza[2.2.2]octatriene intermediate.