Thermally Induced Denitrogenative Annulation for the Synthesis of Dihydroquinolinimines and Chroman‐4‐imines

Abstract

A rapid growth in synthetic methods for the preparation of diverse organic molecules using N-sulfonyl-1,2,3-triazoles is of great interest in organic synthesis. Transition metals are generally used to activate the α-imino diazo intermediates. Metal-free methods have not been studied in detail, but can be a good complement to transition metal catalysis in the mild reaction conditions. We herein report a novel method for the preparation of 2,3-dihydroquinolin-4-imine and chroman-4-imine analogs from their corresponding N-sulfonyl-1,2,3-triazoles in the absence of metal catalysts. To achieve intramolecular annulation, the introduction of an electron-donating group is required at the meta position of N-sulfonyl-1,2,3-triazole methyl anilines. The inclusion of tailored substituents on the aniline moieties and nitrogen atoms enhances the nucleophilicity of the phenyl π-electrons, thus allowing them to undergo a Friedel-Crafts-type reaction with the highly electrophilic ketenimines. This metal-free method was carefully optimized to generate a variety of dihydroquinolin-4-imines and chroman-4-imines in moderate-to-good yields.

Synthesis and Photophysical Characterization of 2,3‐Dihydroquinolin‐4‐imines: New Fluorophores with Color‐Tailored Emission

“…As compared to the reported thermal condition, Cu I ‐catalyzed annulation overcame the limitation of substrate scope, particularly for halo substituents ( 2 w and 2 x ) as well as the trifluoromethyl substituent ( 2 y ). Nevertheless, the introduction of a strong electron‐withdrawing nitro substituent ( 2 z ) only formed acrylamidine ( 3 z“ , 72 %).…”

“…To this end, we reported as imple methodology for preparing 2,3-dihydroquinolin-4-imine (DQI) from aniline-tethered Nsulfonyltriazole by thermal-assisted denitrogenativea nnulation (Scheme 1e). [10] The DQIs thus obtained could be easily transformed to various 2,3-dihydroquinolones. This metal-free approach involves the decompositiono fN-sulfonyltriazoles into a ketenimine intermediate by the heat obtained from the release of nitrogen, followed by the intramolecular trapping of the electrophilic ketenimine intermediate by the aryl ring.…”

“…As the thermalr eaction is significantly promoted by the presence of electron-donating groups (EDGs) at the meta-position of anilines, the substrate scope is restricted, considerably limiting its use. [10] For expanding the substrate scope under milder conditions, insteado fw ith thermala ctivation,t ransition-metal catalysis can be employed for constructing DQIs, which might overcome the need fort he electron-donating effect. In 2005, Chang and co-workersh ave reported ah ighly efficient coppercatalyzed multicomponent strategy,i nw hich sulfonyl azides react with aw ide range of alkynesa nd amines to form amidines.…”

“…To this end, we reported a simple methodology for preparing 2,3‐dihydroquinolin‐4‐imine (DQI) from aniline‐tethered N ‐sulfonyltriazole by thermal‐assisted denitrogenative annulation (Scheme e) . The DQIs thus obtained could be easily transformed to various 2,3‐dihydroquinolones.…”

“…This metal‐free approach involves the decomposition of N ‐sulfonyltriazoles into a ketenimine intermediate by the heat obtained from the release of nitrogen, followed by the intramolecular trapping of the electrophilic ketenimine intermediate by the aryl ring. As the thermal reaction is significantly promoted by the presence of electron‐donating groups (EDGs) at the meta ‐position of anilines, the substrate scope is restricted, considerably limiting its use …”