Stereodivergent Syntheses of N-heterocycles by Catalyst-Controlled Reaction of Imidazolidines with Allenes

Reported herein is a catalyst-controlled reaction of imidazolidines with allenes, providing a general and efficient method to construct two series of N-heterocycles, 1,4-diazepanes via gold-catalyzed [5 + 2] cycloadditions and 1,4-diazabicyclo[4.3.1]decanes through iron-catalyzed [5 + 2] cycloaddition/Friedel–Crafts cyclization cascades, in moderate to high yields under mild reaction conditions. Mechanistic investigations indicate that water acts as a proton shuttle to assist the [1,3]-hydrogen shift in the Friedel–Crafts cyclization process. This strategy features the use of imidazolidines as stable 1,5-dipoles for [5 + 2] cycloadditions and the utilization of an iron catalyst to accomplish the [5 + 2] cycloaddition/Friedel–Crafts cyclization cascades in a highly diastereoselective manner for the synthesis of bridged-ring systems.