Rhodium-Catalyzed Intramolecular Acylation of 2-(Indol-1-yl)-benzoic Acids under Redox-Neutral Conditions

Abstract

We developed a novel access to indoloindolone by a rhodium-catalyzed intramolecular acylation of 2-(indol-1-yl)benzoic acids. This reaction proceeds via the in situ formation of a mixed anhydride under redox-neutral reaction conditions. Preliminary mechanistic investigations revealed that the in situ formed mixed anhydride participates in the C–H activation step, which is facilitated by a RhI catalyst. The utility of this reaction was demonstrated by a large-scale reaction.

Key words

rhodium - intramolecular acylation - redox-neutral conditions - indoloindolones - C–H activation

Publikationsverlauf

Eingereicht: 28. März 2023

Angenommen nach Revision: 08. Mai 2023

Accepted Manuscript online:
08. Mai 2023

Artikel online veröffentlicht:
14. Juni 2023

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• 15 Typical Experimental Procedure for the Rhodium-Catalyzed Intramolecular Acylation of 2-(Indol-1-yl)benzoic Acid (1) To an oven-dried test tube equipped with a stirring bar were added 2-(indol-1-yl)benzoic acid (1, 0.3 mmol), [RhCl(CO)₂]₂ (5.8 mg, 0.015 mmol), NaI (13.5 mg, 0.09 mmol), and 1,4-dioxane (0.6 mL). Subsequently, Piv₂O (83.8 mg, 0.45 mmol) was injected to the solution via a syringe, and the tube was sealed with a PTFE cap. The reaction mixture was stirred at 150 °C for 18 h. After cooling to room temperature, the reaction mixture was filtered through a pad of silica gel (hexane/EtOAc = 1:1). The filtrate was concentrated under reduced pressure, and the residue was purified by preparative thin-layer chromatography (dichloromethane) or flush column chromatography (dichloromethane/hexane = 1:1) to give the product 2. 4-Methyl-10H-indolo[1,2-a]indol-10-one (2f) The title compound was obtained as an orange solid (43.5 mg, 62%); purified by preparative TLC (Et₂O/toluene = 1/19); mp 174.0–175.0 °C. ¹H NMR (500 MHz, CDCl₃): δ = 7.78 (d, J = 8.6 Hz, 1 H), 7.64 (dd, J = 7.4, 1.1 Hz, 1 H), 7.47–7.42 (m, 2 H), 7.17–7.12 (m, 2 H), 7.06 (t, J = 7.4 Hz, 1 H), 7.01 (t, J = 7.7 Hz, 1 H), 2.91 (s, 3 H). ¹³C NMR (125 MHz, CDCl₃): δ = 181.4, 145.3, 138.8, 135.3, 135.2, 134.7, 130.3, 129.4, 124.9, 124.4, 123.8, 123.6, 111.2, 111.2, 108.1, 22.2. IR (neat): 3027, 2970, 2323, 1741, 1686, 1614, 1560, 1472, 1446, 1376, 1217, 1133, 928, 756 cm^–1. HRMS (ESI-TOF): m/z calcd for C₁₆H₁₂NO⁺ [M + H]⁺: 234.0913; found: 234.0905. 8-Methyl-10H-indolo[1,2-a]indol-10-one (20) The title compound was obtained as an orange solid (61.0 mg, 87%); purified by preparative TLC (dichloromethane); mp 143.5–144.5 °C. ¹H NMR (500 MHz, CDCl₃): δ = 7.55 (d, J = 8.0 Hz, 1 H), 7.36–7.29 (m, 3 H), 7.17 (d, J = 8.0 Hz, 1 H), 7.08–7.02 (m, 2 H), 6.99 (s, 1 H), 2.25 (s, 3 H). ¹³C NMR (125 MHz, CDCl₃): δ = 181.6, 143.2, 135.8, 135.7, 134.0, 133.5, 132.3, 129.3, 127.8, 125.3, 124.8, 121.6, 111.1, 110.8, 107.3, 20.7. IR (neat): 2919, 1687, 1623, 1539, 1488, 1439, 1388, 1321,1234, 1033, 800, 780, 740, 518 cm^–1. HRMS (ESI-TOF): m/z calcd for C₁₆H₁₂NO⁺ [M + H]⁺: 234.0913; found: 234.0924.