Visible-Light Photoredox-Catalyzed Decarboxylative α-tert-Butylation of C(sp³)–H Bonds of N-Aryltetrahydroisoquinolines with Pivalic Acid under Transition-Metal-Free Conditions

Abstract

A transition-metal-free visible-light photoredox-catalyzed decarboxylative alkylation of the benzylic C(sp³)–H bonds of N-aryltetrahydroisoquinolines is reported. The method tolerates various functional groups and proceeds smoothly without requiring a stoichiometric oxidant. A preliminary mechanistic study indicated that a radical process is involved in the reaction.

Key words

photoredox catalysis - decarboxylation - alkylation - tetrahydroisoquinolines - carboxylic acids - organocatalysis

Publication History

Received: 21 February 2021

Accepted after revision: 22 March 2021

Accepted Manuscript online:
22 March 2021

Article published online:
12 April 2021

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• 18 1-(tert-Butyl)-2-phenyl-1,2,3,4-tetrahydroisoquinoline (3a); Typical ProcedureA tubular vessel containing a stirrer bar was charged with 1a (0.2 mmol), pivalic acid (2a; 0.5 mmol), [Acr⁺-Mes]ClO₄ (3.0 mol%), Na₂HPO₄·12 H₂O (0.4 mmol), acetone (1.5 mL), and H₂O (1.5 mL). The mixture was stirred and irradiated by a blue LED (450–455 nm, 1.5 W) for 12 h in air at rt. When the reaction was complete, the mixture was diluted with H₂O and extracted with EtOAc. The organic layers were combined, dried (Na₂SO₄), filtered, and concentrated. The crude product was purified by flash chromatography [silica gel, PE–EtOAc (20:1 to 9:1)] to give a white solid; yield: 36.09 mg (68%); mp 78.5–79.0 °C.¹H NMR (600 MHz, CDCl₃): δ = 7.21–7.19 (m, 2 H), 7.15 (s, 2 H), 7.12 (s, 2 H), 6.92 (d, J = 7.8 Hz, 2 H), 6.68–6.66 (m, 1 H), 4.67 (s, 1 H), 3.88–3.86 (m, 1 H), 3.56–3.51 (m, 1 H), 3.07–3.05 (m, 1 H), 3.00–2.97 (m, 1 H), 1.02 (s, 9 H). ¹³C NMR (150 MHz, CDCl₃): δ = 151.2, 137.0, 135.4, 129.0, 128.7, 128.3, 126.5, 125.0, 116.6, 114.2, 66.0, 44.0, 39.2, 29.2, 27.3. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₉H₂₄N: 266.1903; found: 266.1905.
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