Cobalt-Catalyzed Chemoselective Transfer Hydrogenative Cyclization Cascade of Enone-Tethered Aldehydes

Abstract

The ligand-free Co-catalyzed chemoselective reductive cyclization cascade of enone-tethered aldehydes with i-PrOH as the environmentally benign hydrogen surrogate is developed by this study. Mechanistic studies disclosed that such a protocol is initiated by an ortho-enone-assisted Co­(I)-catalyzed reduction of the aldehyde functionality with i-PrOH. Meanwhile, the selectivity from the Michael–Aldol cycloreduction cascade to the oxa-Michael cascade is feasible and readily adjusted by the addition of steric Lewis bases, such as TEMPO and DABCO, delivering substituted 1H-indenes and dihydroisobenzofurans, respectively.

Synthesis of Bridged N,O‐Heterocyclic 1,6‐Epoxybenzo[c]azocines via Migration‐Annulation Cascade Reaction of α‐Imino Rhodium Carbenes

“…Therefore, the reaction time had to be shortened. The most effective dosage of rhodium catalyst was proved to be 2 mol% (entries [16][17] and the most efficient concentration was 0.1 M (entries [18][19]. Therefore, the optimized reaction conditions for the generation of 7 a were established as indicated in entry 14.…”

“…Additional references cited within the Supporting Information. [16][17][18][19][20][21][22][23][24] The Supporting Information contains general information, procedures, data and NMR spectra and X-ray data.…”

Brønsted acid catalyzed annulations of ketene dithioacetals: synthesis of 3-aryl coumarins and indenes

“…7 Among modern methods, the base-mediated multicomponent coupling of arynes, malonates and DMF, 8 NHC-catalysed synthesis of coumarins from salicylaldehydes and cinnamaldehydes, 9 oxidative hydrolysis of spirochroman-2,2′-1,3-dithianes, 10 palladium catalysed carbonylative annulation of salicylaldehydes with benzyl chloride, 11 and the base promoted annulation of salicylaldehydes with 1,1-dibromo-1-alkenes 12 are of special note (Scheme 1a). Modern methods toward indenes 13 include the Co-catalyzed C–H activation 14 and transfer hydrogenative cyclization approaches, 15 rhodium catalysed reactions of alkynes with 2-chloro-phenyl boronic acid, 16 and FeCl 3 mediated olefinic cyclization 17 (Scheme 1b). We hypothesized that upon activation by Brønsted acids, aldehydes would become sufficiently electrophilic to engage ketene dithioacetals 18 in a reaction sequence that would result in an annulation leading to either coumarins or indenes, depending on the precursor aldehyde (Scheme 1c).…”