Photocatalytic Decarboxylative Cyclization of 1,6-Enynes and N-Methacryloyl-2-phenylbenzimidazoles by Potassium Iodide/ Triphenylphosphine Catalysis

Abstract

We describe a photocatalytic decarboxylative cyclization of 1,6-enynes or N-methacryloyl-2-phenylbenzimidazoles for efficient and ecofriendly syntheses of lactams or benzimidazo[2,1-a]isoquinoline-6(5H)-ones, respectively, by the use of KI/PPh₃ as a redox catalyst system. In this method, N-hydroxyphthalimide esters were used as alkyl-radical precursors to achieve this decarboxylative cyclization without any dyes, transition-metal catalysts, oxidants, or bases.

Keywords

green chemistry - photocatalysis - transition-metal free - cyclization - lactams - benzimidazoquinolinones

Publication History

Received: 04 August 2022

Accepted after revision: 19 September 2022

Accepted Manuscript online:
19 September 2022

Article published online:
21 November 2022

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• 15 Compounds 3a–y; General Procedure A Schlenk tube was charged with the appropriate 1,6-enyne 1 (0.2 mmol), NHPI ester 2 (2 equiv), PPh₃ (20 mol%), and KI (1.5 equiv), and the tube was evacuated and filled with N₂ three times. DMF (2 mL) was added from a gastight syringe under N₂. The mixture in the tube was then stirred at r.t. under 5 W blue LED light for 18 h until the starting material was completely consumed (TLC or GC/MS). The mixture was then extracted with EtOAc (×3), and the organic layers were combined and concentrated under reduced pressure. The residue was purified by column chromatography [silica gel, hexane–EtOAc (15:1)]. 3-(Cyclohexylmethyl)-3-methyl-4-methylene-1-phenylpyrrolidin-2-one (3a) Colorless oil; yield: 0.0470 g (83%). ¹H NMR (400 MHz, CDCl₃): δ = 7.70–7.68 (m, 2 H), 7.41–7.37 (m, 2 H), 7.18–7.14 (m, 1 H), 5.23 (t, J = 2.4 Hz, 1 H), 5.09 (t, J = 2.8 Hz, 1 H), 4.47–4.43 (m, 2 H), 1.86–1.81 (m, 1 H), 1.61–1.52 (m, 8 H), 1.30 (s, 3 H), 1.12–1.07 (m, 2 H), 0.95–0.88 (m, 2 H). ¹³C NMR (101 MHz, CDCl₃): δ = 177.6, 146.0, 139.2, 129.0, 124.6, 119.9, 107.9, 52.0, 49.3, 46.6, 35.1, 34.7, 33.6, 26.9, 26.4, 26.3, 26.2. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₉H₂₆NO: 284.2009; found: 284.2004.
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