Organophotocatalytic Radical–Polar Cross-Coupling of Styrylboronic Acids and Redox-Active Esters

Abstract

We report the development of a radical–polar cross-coupling reaction using styrylboronic acids and redox-active esters under organophotoredox catalysis. The reaction proceeds through a formal polarity-mismatched radical addition. The use of an organic photocatalyst permitted very low loadings of the electron-shuttle additive and accelerated reaction times compared with established processes. The scope of the reaction was explored, and the utility of the products is demonstrated.

Key words

esters - styrylboronic acids - photocatalysis - radical–polar cross-coupling

Publication History

Received: 01 September 2023

Accepted: 21 September 2023

Accepted Manuscript online:
21 September 2023

Article published online:
23 October 2023

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• 19 Alkenes 3–34; General Procedure An oven-dried photoreactor vial equipped with a Teflon-coated stirrer bar was charged with the appropriate NHPI ester (200 μmol, 1.0 equiv) and styrylboronic acid (400 μmol, 2.0 equiv), together with 4CzIPN (1.6 mg, 2.0 μmol, 1 mol%) and Ph₃N (1.0 mg, 4.0 μmol, 2 mol%). The vial was then sealed, purged by using N₂–vacuum cycles (×3), and backfilled with N₂. Degassed dry DMSO-d ₆ (2.0 mL, 0.1 M) was then added from a syringe. The cap was wrapped with Parafilm, and the mixture was stirred under blue LEDs at RT (~20 °C) for 1 h. The mixture was then partitioned between Et₂O (5 mL) and brine (5 mL), and the organics were extracted with Et₂O (2 × 10 mL). The organic phases were combined, washed with brine (15 mL), dried (Na₂SO₄), filtered, and concentrated under vacuum. The crude residue was purified by flash chromatography (silica gel; hexane, hexane–EtOAc, or hexane–Et₂O). [(E)-2-Cyclohexylvinyl]benzene (3) Prepared according to the general procedure from 1,3-dioxoisoindolin-2-yl cyclohexanecarboxylate (2; 54.7 mg, 200 μmol, 1.0 equiv), [(E)-2-phenylvinyl]boronic acid (1; 59.2 mg, 400 μmol, 2.0 equiv), 4CzIPN (1.6 mg, 2.0 μmol, 1 mol%), and Ph₂N (1.0 mg, 4.0 μmol, 2 mol%) in DMSO-d ₆ (2 mL, 0.1 M). The crude residue (95% ¹H NMR yield) was purified by flash chromatography (silica gel, hexane) to give a colorless oil; yield: 31.8 mg (85%, E/Z > 20:1). ¹H NMR (500 MHz, CDCl₃): δ = 7.37–7.33 (m, 2 H), 7.31–7.27 (m, 2 H), 7.21–7.16 (m, 1 H), 6.35 (d, J = 16.01 Hz, 1 H), 6.18 (dd, J = 15.98, 6.96 Hz, 1 H), 2.18–2.08 (m, 1 H), 1.86–1.74 (m, 4 H), 1.72–1.65 (m, 1 H), 1.39–1.25 (m, 2 H), 1.25–1.14 (m, 3 H). ¹³C NMR (126 MHz, CDCl₃): δ = 138.2, 137.0, 128.6, 127.3, 126.9, 126.1, 41.3, 33.1, 26.3, 26.2.