Lewis Acid Mediated Vinylogous Additions of Enol Nucleophiles into an α,β-Unsaturated Platinum Carbene

Abstract

A variety of substituted indoles and benzofurans are accessed via a platinum catalyzed annulation and vinylogous addition of enol nucleophiles. Several β-dicarbonyl compounds participate in the reaction, as do α-nitro and α-cyano carbonyl species. Subjecting the indole products to acidic conditions results in the formation of fused heterocycles.

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

“…This tandem reaction was also attempted using β-dicarbonyl compounds as nucleophiles in the preparation of 2-substituted indoles 257 (Scheme 206). 232 The reaction generated α,β-unsaturated carbene intermediates via an intramolecular nucleophilic addition into alkynes bearing propargyl ethers, followed by vinylogous nucleophilic additions using enol as the nucleophile.…”

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans

“…Moreover, Ferreira's group investigated on the use of platinum catalysis to generate α,β-unsaturated carbene intermediates via an intramolecular nucleophilic addition into alkynes bearing propargylic ethers. These carbenes have been demonstrated to undergo cycloadditions, hydrogen migrations, and vinylogous nucleophilic additions [65], where β-diketones, ketoesters, and ketoamides all successfully added onto the platinum carbene intermediate (Scheme 9).…”

“…Molecules 2020, 24, x 8 of 54 vinylogous nucleophilic additions [65], where β-diketones, ketoesters, and ketoamides all successfully added onto the platinum carbene intermediate (Scheme 9).…”

Recent Progress in the Transition Metal Catalyzed Synthesis of Indoles

“…Propargyl ethers undergo nucleophilic attack with ether extrusion by some platinum catalyst. In the presence of a Lewis acid enhancing the platinum reactivity, 4-(2-aminoaryl)propargyl ethers originated indole α,β-unsaturated carbene intermediates, which in turn allowed vinylogous addition of enol nucleophiles (Scheme 76) [145]. The enol nucleophiles was used in large excess (5 equiv), because lower amounts generated indole in lower yield after longer reaction times.…”