Intermolecular Vicinal Diaminative Assembly of Tetrahydroquinoxalines via Metal-free Oxidative [4 + 2] Cycloaddition Strategy

Abstract

Reported herein is the first metal-free oxidative [4 + 2] coupling of o-phenylenediamines with various alkenes. Differing from the known strategy that hinged on reactive π-allyl Pd intermediates from restrained allylic alcohol/acetate and diene substrates, this metal-free method features easy accessibility of starting materials, step economy, benign reaction conditions, and more importantly broad C−C double bonds (styrenes, vinyl (thio)ethers, benzofurans, indoles) with diastereospecificities. Mechanistic studies suggest the intermediacy of the benzoquinone diimides, a class of useful but yet underexploited synthons. Of note, they efficiently furnished functionalized tetrahydroquinoxalines and complement the well-studied alkene vicinal diamination typically toward acyclic diamine derivatives.

Energy Transfer-Enabled Unsymmetrical Diamination Using Unprecedented Bifunctional Nitrogen-Radical Precursors

“…[9][10][11][12][13][14][15][16][17][18][19][23][24][25]28,[33][34][35][36][37][38][39][40][41] In addition, the utilization of stoichiometric amounts of metal reagents (e.g. osmium or cobalt), [7][8] chemical oxidants, 11,15,22,23,27,28,33,34,37,38 or azide reagents, [14][15][16][18][19][20]26,28,40,41 leads to the issues on cost, environment, and safety. Second, the established protocols are often limited to symmetrical diamination, [7][8][9][10][11]…”

“…osmium or cobalt), [7][8] chemical oxidants, 11,15,22,23,27,28,33,34,37,38 or azide reagents, [14][15][16][18][19][20]26,28,40,41 leads to the issues on cost, environment, and safety. Second, the established protocols are often limited to symmetrical diamination, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][26][27][28] namely installing two undifferentiated amino functionalities across alkenes.…”

Synthesis of [60]- and [70]Fullerene-Fused Tetrahydroquinoxaline Derivatives by Oxidative [4 + 2] Cycloaddition with Unusual Reactivity and Regioselectivity

“…Recently one reactive intermediate diimine, oxidized from protected o -phenylenediamines, was reported to diaminate the double bonds of various alkenes . Here we report a hetero-Diels–Alder cycloaddition to synthesize [60]- and [70]­fullerene-fused tetrahydroquinoxaline derivatives, with the reactive diimines from an oxidative process in situ, as shown in Scheme b.…”

Regioselective Three-Component Synthesis of Vicinal Diamines via 1,2-Diamination of Styrenes

“…For example, cyclic vicinal diamines can be accessed through intramolecular vicinal diamination of an inherent CC bond under metal-free conditions or in the presence of a metal catalyst . Two-component vicinal diamination protocols have been successfully developed for the synthesis of acyclic diamines, cyclic diamines, imidazolines, imidazolidin-2-ones, indolines, pyrrolopyridines, pyrrolidines, piperidines, piperidin-2-ones, isoxazolidines, dihydroisoquinolin-1­(2 H )-ones, tetrahydroquinoxalines, etc . Three-component vicinal diamination of alkenes however is a more powerful method with which to assemble multifunctional structures from abundant chemical feedstocks, and this methodology can greatly broaden the availability and utility of diamines (Scheme a).…”