| Abstract: | The concept of 1,3-dipolar cycloaddition was developed, starting in the late 1950s, largely by Rolf Huisgen and his students in Munich, and it has led to one of the most versatile methods for the construction of five-membered ring heterocycles. Although first known only as transient intermediates, nitrile imines have been at the heart of mechanistic studies of this type of cycloaddition reactions. Hundreds of mechanistic papers appeared in 1960s and 1970s; reliable spectroscopic observations were achieved in the early 1980s both at low temperatures and in the gas phase; finally, the first crystalline nitrile imine was reported in 1988. The unusual structures found by X-ray analyses as well as the facile rearrangements observed experimentally have fostered a new interplay between experiment and theory. The story of nitrile imines, from matrix characterization to stable compounds, nicely illustrates the role that main group elements can play in organic chemistry. |
