Diethyl Phosphite Promoted Electrochemical Oxidation of Tetrahydroisoquinolines to 3,4-Dihydroisoquinolin-1(2H)-ones

Abstract

A diethyl phosphite mediated electrochemical oxidation strategy for the synthesis of 3,4-dihydroisoquinolin-1(2H)-ones from tetrahydroisoquinolines under mild conditions has been developed. This protocol provides an environmentally friendly and simple way for the construction of C=O bonds in an undivided cell unit.

Key words

electrochemistry - oxidation - diethyl phosphite - tetrahydroisoquinoline - C=O bond formation

• References and Notes

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• 11 2-Phenyl-3,4-dihydroisoquinolin-1(2H)-one (4a); Typical Procedure A 10 mL distillation flask equipped with a magnetic stirring bar was charged with diethyl phosphite (42 mg, 0.3 mmol), wet CH₂Cl₂ (5.0 mL), 2-phenyl-1,2,3,4-tetrahydroisoquinoline 1a (52 mg, 0.25 mmol), and Et₄NOTs (151 mg, 0.5 mmol). The resulting suspension was stirred until complete dissolution was achieved. The flask equipped with graphite rod anode (d = 5 mm) and Pt plate cathode (0.5×0.5 cm). The reaction mixture was stirred and electrolyzed at a constant current of 5 mA at rt for 9 h. The reaction mixture was diluted with CH₂Cl₂ (15 mL), washed successively with water (10 mL) and brine (10 mL), dried with Na₂SO₄, and concentrated in vacuo. Purification by flash column chromatography (silica gel, petroleum ether–ethyl acetate 20:1) afforded the desired product 4a 48 mg (86%) as a white solid; mp 119–120 °C. ¹H NMR (300 MHz, CDCl₃): δ = 8.19–8.14 (m, 1 H), 7.51–7.34 (m, 6 H), 7.29–7.21 (m, 2 H), 4.04–3.97 (m, 2 H), 3.15 (t, J = 6.6 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 162.8, 143.1, 138.3, 132.0, 129.7, 128.9, 128.7, 127.1, 126.9, 126.2, 125.3, 49.4, 28.6.