The Au(I)-Catalyzed Intramolecular Hydroarylation of Terminal Alkynes Under Mild Conditions: Application to the Synthesis of 2H-Chromenes, Coumarins, Benzofurans, and Dihydroquinolines

Abstract

Operationally simple Au(I)-catalyzed intramolecular hydroarylation (IMHA) reactions of terminal alkynes that proceed in high yield and under very mild conditions are described. These processes involve low catalyst loadings, mild reaction temperatures, and short reaction times, require no cocatalysts or additives, and allow for the generation of a number of important heterocyclic motifs from readily accessible starting materials.

Similarities and Differences between the “Relativistic” Triad Gold, Platinum, and Mercury in Catalysis

“…[110] The corresponding heterocycles 154 are formed in high yields when the appropriate Pt, Au, or Hg species is used ( Table 2, compare entries [35][36][37][38][39]. [73][74][75] The catalyst of choice seems to depend on the substrate, since Au and Hg catalysts only work for terminal alkynes while Pt catalysts present a wider scope.…”

“…However, an alternative reaction pathway was found when using Au catalysts in benzofurane derivatives. [74] Intermediate 156 leads to the final products 154 after proto-deauration but, if rearrangement occurs instead, allene intermediate 157 is formed and oxo-attack leads finally to 158. This last step could also be metal catalyzed.…”

Ph3PAuNTf2 as a Superior Catalyst for the Selective Synthesis of 2H‐Chromenes: Application to the Concise Synthesis of Benzopyran Natural Products

“…[4] This reaction is known to proceed under extreme thermal conditions [5] or microwave irradiation [6] via intermediate formation of labile ortho-allenyl phenols, resulting from a [3,3] Claisen rearrangement (Scheme 1). Apart from several milder versions of this reaction using metal-catalyzed procedures (e.g., Pd, Pt, Hg, Ag, etc.…”

“…), [7] it was also found that Au I catalyzes the cyclization of aryl propargyl ethers. [4,[8][9][10] The initial report by the group of Echavarren [8] made use of precatalyst I (Scheme 2), which requires stoichiometric amount of silver salts to abstract a chloride ion and to generate the catalytically active Ph 3 PAu + species in situ. Surprisingly, dimeric byproducts that arise from a Ag + -catalyzed pathway are also formed in various yields.…”

“…Surprisingly, dimeric byproducts that arise from a Ag + -catalyzed pathway are also formed in various yields. More re- [ cently, Banwell [4] used catalyst II (Scheme 2), which is far more active than I, and it does not require in situ activation by AgX. Yet, the product selectivity is peculiar with the majority of substrates, affording primarily 2H-chromenes in good to moderate yields, whereas other substrates formed mixtures of 2H-chromenes and 2-methylbenzofurans.…”