Rhodium-Catalyzed Reductive Cleavage of Aryl Carbamates Using Isopropanol as a Reductant

Published as part of the Cluster C–O Activation

Abstract

Despite the widespread use of carbamates as a directing group in C–H bond-functionalization reactions, reductive removal of this directing group is not straightforward. Currently available methods are limited to nickel-catalyzed reactions using ⁱ PrMgX or hydrosilane as a reductant, leaving the functional group compatibility issue to be solved. Herein, we report rhodium-catalyzed reductive cleavage of aryl carbamates using ⁱPrOH as a milder reductant.

Key words

reductive cleavage - C–O activation - N-heterocyclic carbine - rhodium - isopropanol

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• 11 The reaction afforded an inseparable mixture of desired stilbene and 1,2-diphenylethane in a ratio of 82:18 (96% combined isolated yield).
• 12 [6-(Pyridin-3-yl)naphthalen-2-yl](3,4,5-trimethoxyphenyl)methanone (2l); Typical Procedure [RhCl(C₂H₄)₂]₂ (5.8 mg, 0.015 mmol), L2·HCl (21 mg, 0.060 mmol), KO ^t Bu (7.4 mg, 0.066 mmol), K₃PO₄ (64 mg, 0.60 mmol), and toluene (0.40 mL) were added to a 5 mL screw-capped vial in a glovebox filled with nitrogen, and the resulting mixture was stirred at 60 °C for 1 h. After 1 h, 1l (154 mg, 0.30 mmol), ⁱ PrOH (18 mg, 0.30 mmol) and toluene (0.60 mL) were added to the vial in the glove box. The vessel was stirred at 180 °C for 16 h followed by cooling to room temperature. The mixture was purified by flash column chromatography over silica gel (eluting with hexane/EtOAc, 100:1) to give 2l as a white solid (90 mg, 75%); R_f 0.33 (EtOAc); pale-orange oil. ¹H NMR (CDCl₃, 400 MHz): δ = 9.02 (br. s, 1 H), 8.68 (br. s, 1 H), 8.33 (s, 1 H), 8.13 (s, 1 H), 7.96–8.07 (m, 4 H), 7.81 (d, J = 8.2 Hz, 1 H), 7.46 (br. s, 1 H), 7.14 (s, 2 H), 3.97 (s, 3 H), 3.89 (s, 6 H). ¹³C NMR (CDCl₃, 100 MHz): δ = 195.5, 152.9, 149.0, 148.6, 142.2, 137.6, 136.0, 135.5, 135.3, 134.6, 132.7, 131.7, 131.1, 130.3, 128.5, 126.6, 126.1, 126.0, 123.7, 107.8, 61.0, 56. 3. IR (ATR): 2939 (w), 1715 (w), 1652 (w), 1626 (w), 1581 (m), 1503 (m), 1464 (m), 1413 (m), 1378 (w), 1328 (s), 1275 (w), 1234 (m), 1215 (m), 1171 (m), 1124 (s), 1001 (m), 912 (m), 823 (w), 801 (w), 750 (m), 726 (s), 645 (w) cm^–1; HRMS (EI): m/z calcd for C₂₅H₂₁NO₄: 399.1471; found: 399.1470.