Visible-Light-Catalyzed Regioselective Arylcarboxylation of Allenes with CO₂

Abstract

A visible-light-catalyzed arylcarboxylation of allenes with CO₂ was developed using [Ir(ppy)₂(dtbbpy)]PF₆ (ppy = 2-phenylpyridine; dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) as a photocatalyst to synthesis β-aryl β,γ-unsaturated carboxylic acids. This multicomponent protocol proceeds in an atom-economical way with exclusive regioselectivity. Preliminary mechanistic experiments suggested that allylic carbanion species are the key intermediates.

Key words

photocatalysis - allenes - arylcarboxylation - regioselectivity - carbon dioxide

• References and Notes

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For selected examples and reviews of carboxylations using CO₂ as a C₁ source, see:
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• 10 Photocatalyzed Arylcarboxylation of Allenes with CO₂: General Procedure An oven-dried 10 mL Schlenk tube equipped with a magnetic stirrer bar was charged with the appropriate allene 1 (0.2 mmol, 1 equiv, if solid), DABCO (0.1 mmol, 0.5 equiv), K₂CO₃ (0.5 mmol, 2.5 equiv), HCOONa (0.4 mmol, 2 equiv), and Ir[(ppy)₂dtbbpy]PF₆ (5 mol%). The tube was sealed and degassed by three cycles of vacuum evacuation and subsequent backfilling with CO₂. Subsequently, the appropriate allene 1 (0.2 mmol, 1 equiv, if liquid), the appropriate aryl iodide 2 (0.4 mmol, 2 equiv), and anhyd DMSO (2 mL) were added. The tube was placed under a blue LED (λ = 450 nm, 20 W) and irradiated for 24 h at r.t. The mixture was then acidified with 2 N aq HCl (1 mL) and quenched with H₂O. It was then extracted with EtOAc (×3) and the combined organic layers were dried (MgSO₄) and concentrated under reduced pressure. The crude product was purified by column chromatography [silica gel, hexane–EtOAc (5:1 to 1:1)] or by preparative TLC (hexane–EtOAc, 1:1). 3,4,4-Triphenylbut-3-enoic acid (3a) Prepared by following the general procedure and purified by column chromatography [silica gel, hexane–EtOAc (3:1)] to give a white solid; yield: 27 mg (43%). ¹H NMR (400 MHz, CDCl₃): δ = 7.38–7.27 (m, 5 H), 7.19–7.10 (m, 5 H), 7.09–6.99 (m, 3 H), 6.96–6.88 (m, 2 H), 3.58 (s, 2 H). ¹³C NMR (101 MHz, CDCl₃): δ = 177.23, 143.75, 142.66, 142.21, 141.36, 131.80, 130.75, 129.80, 129.52, 128.58, 128.10, 127.64, 127.42, 126.87, 126.50, 41.57. HRMS (ESI): m/z [M + H]⁺ calcd for C₂₂H₁₉O₂: 315.1380; found: 315.1382.