Gold‐Photoredox‐Cocatalyzed Tandem Oxycyclization/Coupling Sequence of Allenols and Diazonium Salts with Visible Light Mediation

Abstract

Ther oom temperature radical cycloetherification/arylation cascade of allenols andd iazonium salts hasb een accomplished via ac ombination of gold and photoredox catalysis to provide 2,3,4-trisubstituted-2,5-dihydrofurans.T he functionalized oxacycle formation sequence is chemo-andr egioselective for the cycloetherification and for the position that bears the aryl moiety after the cross-coupling. Mechanistici nvestigationsr evealed that this transforma-tion proceeds through an initial oxidation of gold(I) to ap henyl gold(III) complex, which,u pon coordination to the allene,c atalyzes its cyclization and leads to the coupling product after ar eductive elimination regenerating Au(I).Scheme 5. Gold-photoredox-cocatalyzed reaction of allenols 5a-e with aryldiazonium salts 2.C ontrolled synthesis of oxaspirooxindoles 6aa-6ee.Scheme6.Gold-photoredox-cocatalyzed reaction of allenols 7 with aryldiazonium salts 2.C ontrolled synthesis of oxaspiro-b-lactams 8.Scheme9.Alternative mechanistice xplanation for the goldphotoredox-cocatalyzed preparation of oxacycles 3, 6, 8,a nd 10 from allenols 1, 5, 7,and 9 and diazonium salts 2.Scheme 10. Oxidationo ft he original Au(I) complex by addition of aP hr adical and single electron transfer from Ru(bipy) 3 . [3] Theb arrier for the reductive eliminationoni ntermediate VI is low (2.2 kcal mol À1 )a nd explains the presence of the aryl-phospine couplingproduct amongt he reaction products.

Light‐Induced Mechanistic Divergence in Gold(I) Catalysis: Revisiting the Reactivity of Diazonium Salts

“…If the reaction is carried out in the absence of a light source, the N 2 ‐unit from the diazonium salt is retained and azobenzofurans are formed in moderate to high yields. Stoichiometric experiments demonstrate that these products are accessible from the same vinyl Au I intermediate and lead us to propose that the often suggested photochemical oxidative addition–––– either occurs after the formation of a vinyl Au I species or may even not involve oxidation state changes.…”

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

“…Reductive elimination would then recover the Au(I) species to the catalytic cycle and explain the formation of the observed 4-aryl-dihydrofurans 432 after deprotonation. 296 , 297 …”

Solvent-Controlled Switching of Cycloisomerization to Transposition in the Ag/Cu-Promoted Reaction of Terminal α-Allenols

“…Dihydrofurans are one of the most studied oxacycles [19][20][21][22][23] which can be found in different applications such as pharmaceuticals, flavor and fragrance compounds [17].…”