Iron-Catalyzed Oxidative Decarboxylation of Oxamic Acids: A Safe and Efficient Photochemical Route to Urethanes

Abstract

This study presents a facile method for synthesizing urethanes through the photocatalyzed oxidative decarboxylation of oxamic acids. The process involves the formation of an isocyanate in situ from an oxamic acid under blue-light irradiation (427 nm) in the presence of ferrocene as a photocatalyst, 2-picolinic acid as a ligand, and potassium bromate as an oxidant. The one-pot procedure effectively avoids the need for separation, purification, and storage of carcinogenic isocyanates, making it a safer and more practical method for obtaining target urethanes from easily accessible starting materials.

Key words

urethanes - oxamic acids - iron catalysis - ferrocene - photocatalysis - decarboxylation

Publication History

Received: 23 May 2023

Accepted after revision: 17 July 2023

Accepted Manuscript online:
17 July 2023

Article published online:
07 September 2023

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• 22 Carbamates 2a–v; General Procedure A 10 mL vial equipped with a Teflon septum and a magnetic stirrer bar was charged with the appropriate oxamic acid 1 (1.0 equiv, 0.5 mmol), ferrocene (2.5 mol%, 0.0125 mmol), 2-picolinic acid (5.0 mol%, 0.0250 mmol), and KBrO₃ (2 equiv, 1.0 mmol). The vial was then degassed with N₂ for about 10 min using an outlet needle before distilled dried DCE (5 mL) and the appropriate alcohol ROH (2.0 equiv, 1.0 mmol) were added. The purge process was repeated by sparging with N₂ for 5 min, and then the reaction vial was irradiated with a Kessil lamp (40 W, 427 nm LEDs) at a distance of 5 cm for 24 h. When the reaction was complete, the crude product was collected by evaporation of the mixture and purified by flash chromatography (silica gel, usually EtOAc–hexane). Ethyl (4-Methylbenzyl)carbamate (2a) White solid; yield: 85 mg (87%); mp 55–57 °C (EtOAc–hexane, 10:90); R_f = 0.28 (EtOAc–hexane, 10:90). FTIR (KBr): 3352, 2924, 1948, 1708, 1451, 1248, 1132, 1036, 797, 469 cm^–1. ¹H NMR (300 MHz, CDCl₃): δ = 7.15 (t, J = 6.9 Hz, 4 H), 4.94 (s, 1 H), 4.32 (d, J = 5.9 Hz, 2 H), 4.14 (q, J = 7.1 Hz, 2 H), 2.33 (s, 3 H), 1.25 (t, J = 7.1 Hz, 3 H). ¹³C NMR (76 MHz, CDCl₃): δ = 156.8, 137.2, 135.7, 129.4, 127.6, 61.0, 44.9, 21.2, 14.8. HRMS (ESI): m/z [M + Na]⁺ calcd for C₁₁H₁₅NNaO₂: 216.09950; found: 216.09890.