Organocatalytic Aerobic Oxidative Cleavage of Cyclic 1,2-Diketones

Abstract

The first organocatalytic aerobic oxidative cleavage of cyclic 1,2-diketones is reported. The reaction occurs in either aqueous or alcoholic media and is promoted by a simple N-heterocyclic carbene catalyst derived from a 1,2,4-triazolium ion. No strong oxidants are required. The application of the process in a one-pot synthesis of a cyclic anhydride is also possible.

Key words

oxidation - biaryls - carboxylic acids - catalysis - esters

• References and Notes

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Cooperative transition-metal-ion catalysis with air as the oxidant
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• 15 General Procedure for the Esterification of Cyclic DiketonesTo a 25 mL vial equipped with a magnetic stirring bar was charged the triazolium precatalyst 4 (18 mg, 0.07 mmol, 15 mol%). Dry THF (1.25 mL) and deionised H₂O (62.5 μL) were added. DBU (170.0 μL, 1.10 mmol, 220 mol%) was added, and the solution was stirred for 2 min. The aromatic diketone (0.50 mmol) was then added. The vessel was sealed with a plastic lid perforated by 4 holes (ca. 2 mm in diameter). After stirring for 20 h at r.t., MgSO₄ (1.40 mmol, 170 mg) and 5.0 equiv of MeI were added, and the resulting mixture was stirred for 12 h. The solvent was then removed in vacuo, and the resulting residue was subjected to flash chromatography to yield the diester product.Dimethyl Biphenyl-2,2′-dicarboxylate (8)Prepared according to the general procedure using phenanthrene-9,10-dione (7, 104.11 mg, 0.50 mmol). Purified via flash chromatography (n-hexane–EtOAc, 9:1), 110 mg (82%) as a pale yellow solid; mp 73–75 °C (lit.¹⁶ 74–75 °C). ¹H NMR (400 MHz, CDCl₃): δ = 7.98 (d, J = 7.6 Hz, 2 H), 7.51 (app. t, 2 H), 7.40 (app. t, 2 H), 7.18 (d, J = 7.6 Hz, 2 H), 3.59 (s, 6 H). HRMS: (ESI⁺): m/z calcd for C₁₆H₁₅O₄: 271.0970; found: 271.0984.Dimethyl 4,4′-Dinitrobiphenyl-2,2′-dicarboxylate (16)Prepared according to the general procedure using 2,7-dinitro-9,10-phenanthrenequinone (10, 149.10 mg, 0.50 mmol). Purified via flash chromatography (n-hexane–EtOAc, 9:1), 144 mg (80%) as a white solid; mp 178–180 °C (lit.¹⁷ 182–183 °C). ¹H NMR (400 MHz, CDCl₃): δ = 8.94 (d, J = 2.3 Hz, 2 H), 8.45 (dd, J = 2.3, 8.5 Hz, 2 H), 7.37 (d, J = 8.5 Hz, 2 H), 3.76 (s, 6 H).Dimethyl Naphthalene-1,8-dicarboxylate (19)Prepared according to the general procedure using 13 (91.02 mg, 0.50 mmol). Purified via flash chromatography (n-hexane–EtOAc, 9:1), 80 mg (66%) as a pale yellow solid; mp 100–104 °C (lit.¹⁸ 102–103 °C). ¹H NMR (400 MHz, CDCl₃): δ = 8.00–7.97 (m, 4 H), 7.53 (app t, 2 H), 3.90 (s, 6 H). HRMS: (ESI⁺): m/z calcd for C₁₄H₁₃O₄: 245.0814; found: 245.0817.
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