Metal‐Catalyzed [1,2]‐Alkyl Shift in Allenyl Ketones: Synthesis of Multisubstituted Furans

Abstract

Sogar kondensierte Furane sind durch die Cycloisomerisierung substituierter Allenylketone zugänglich. Schlüsselschritt der Kaskadenreaktion ist die [1,2]‐Wanderung von Alkyl‐ oder Arylgruppen in Allenylketonen. Die einfache Reaktion in Gegenwart kationischer Komplexe sowie die Eignung für die Cycloisomerisierung unsymmetrisch substituierter Allene sprechen überzeugend für einen elektrophilen Mechanismus dieser Umwandlung.

Exclusive 1,2‐Aryl Shift in Platinum(II) Chloride‐Catalyzed Cyclization of 1‐(Indol‐2‐yl)‐2,3‐allenols

“…Interestingly, PtCl 4 catalyzed this transformation affording 6a in 43% yield (entry 3, Table 1). Then, it was observed that the reaction of 5a with PtCl 2 (5 mol%) in toluene afforded carbazole 6a in a surprising 86% NMR yield as the only product indicating the smooth 1,2‐methyl migration (entry 4, Table 1), which is different from the previous report 16f. Next, several solvents were tested for the PtCl 2 ‐catalyzed reaction of 5a at room temperature: xylenes was also effective affording 6a in 74% NMR yield (entry 5, Table 1); other solvents failed to afford better yields (entries 6–10, Table 1).…”

Gold‐Catalyzed Cyclization of 1‐(Indol‐3‐yl)‐3‐alkyn‐1‐ols: Facile Synthesis of Diversified Carbazoles

“…[18] Subsequent 1,2-alkyl migration gives intermediate III and protonation of the hydroxyl group affords intermediate IV, followed by elimination of H 2 O to generate cyclovinylic gold carbene intermediate V. [19] A 1,2-migaration to the adjacent carbenoid center leads to ring-expanded intermediate VI, and then elimination of LAu + forms carbazole 2 in a subsequent step. [20] When the reaction is performed in an alcohol or toluene, the cyclo- To examine the necessity of the cyclopropyl substitution, we prepared various 2,2-disubstituted 1-(indol-3-yl)-3-alkyn-1ols, 1 m-1 s, that do not contain a cyclopropyl moiety. [21] Screening of the reaction conditions revealed that…”

Gold(I)‐Catalyzed Tandem Reactions Initiated by 1,2‐Indole Migrations

“…[4] In this context, highly interesting processes would be the 1,2-migration of propargylic alkyl or aryl groups, as they would imply the rupture and formation of carboncarbon bonds (Scheme 1). [5] However, as far as we know, these reactions have not been reported and herein we present the first examples. In particular, we describe new gold-catalyzed tandem reactions of propargylic indoles which are initiated by an unprecedented 1,2-indole migration.…”

“…These starting materials differ from alkynes 1 in that they do not bear an aryl substituent at the propargylic position, and so the subsequent formal CÀH insertion reaction is not possible. When alkynes 11 were treated with 5 mol % of [AuNTf 2 -(Ph 3 P)] in methylene chloride at reflux, the new 2-indenylindole derivatives 12 were obtained exclusively in high yields ( Table 3, entries [1][2][3][4][5][6]. Surprisingly, in these reactions the phenyl group that participates in the formation of the indene skeleton is that at the terminal position of the triple bond.…”