Divergent Carbocatalytic Routes in Oxidative Coupling of Benzofused Heteroaryl Dimers: A Mechanistic Update

Abstract

Mildly thermal air or HNO3 oxidized activated carbons catalyse oxidative dehydrogenative couplings of benzo[b]fused heteroaryl 2,2’‐dimers, e.g., 2‐(benzofuran‐2‐yl)‐1H‐indole, to chiral 3,3’‐coupled cyclooctatetraenes or carbazole‐type migrative products under O2 atmosphere. DFT calculations show that the radical cation and the Scholl‐type arenium cation mechanisms lead to different products with 2‐(benzofuran‐2‐yl)‐1H‐indole, being in accord with experimental product distributions.

Carbocatalytic Cascade Synthesis of Polysubstituted Quinolines from Aldehydes and 2‐Vinyl Anilines

“…We focused our attention on the three-step cascade reaction between o-vinyl anilines and aldehydes (Figure 1) where the first step is dehydrative formation of an imine intermediate, which thermally electrocyclizes to a dihydrointermediate, that is finally oxidized to a quinoline. Plausibly, the carbocatalyst could participate in each of the three steps: i) either acids or the graphitic surface could assist in the imine formation, [22] ii) the latter could stabilize the reaction intermediates as well, [23,24] iii) and quinoidic groups could assist in the final oxidation of the dihydrointermediate to the target molecule. [25] As the annulative electrocyclization, which is the most probable rate-determining step (RDS) in this cascade, is known to typically require harsh conditions such as high temperatures (155-200 °C) [26] or strong acid catalysis (p-TsOH, CF 3 CO 2 H, TfOH) [27] there is a strong urge to develop milder methods so that more sensitive derivatives (e. g., alkynes and furans) can be accessed with this synthetic strategy.…”

Carbon Materials as Catalytic Tools for Oxidative Dehydrogenations and Couplings in Liquid Phase

“…2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ), phenanthrenequinone (PQ), anthraquinone (AQ)) and "heterogeneous quinones" (Figure 2a). 5,6,16,17,18 However, obtaining a clear mechanistic picture remains challenging as it involves the interplay of several different characterization methods, kinetic measurements, and…”

“…42 Moreover, the mechanism was explored through both experimental kinetic studies and DFT calculations with o-benzoquinone and PQ as computational models for the carbocatalysts. 5,6,16,40 Scheme 13 Cascade synthesis of substituted quinolines via AC catalyzed aerobic oxidative electrocyclization of aldehydes and 2-vinyl anilines…”

“…42 Moreover, the mechanism was explored through both experimental kinetic studies and DFT calculations with o -benzoquinone and PQ as computational models for the carbocatalysts. 5 6 16 40…”

“…Interestingly, during the optimization of the reaction conditions for COTs, it was observed that carbazole-type migrative products 44 could also be obtained if the reaction was conducted in the presence of an acid additive and a specific type of carbocatalyst was employed under slightly modified reaction conditions (Scheme 17 ). 6 COT synthesis was suggested to proceed through a radical cation mechanism, while the migration pathway goes through an arenium cation mechanism (i.e., the Scholl-mechanism). 48 Molecular quinone model compounds AQ and PQ were unable to mediate the reaction, and the plain AC also performed poorly.…”