Contra-Thermodynamic Positional Isomerization of Olefins

Kuo Zhao and Robert R. Knowles*

*Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States, Email: rknowlesprinceton.edu

K. Zhao, R. R. Knowles, J. Am. Chem. Soc., 2022, 144, 137-144.

DOI: 10.1021/jacs.1c11681

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Abstract

A stepwise PCET activation of thermodynamically stable olefins is mediated by an excited-state oxidant and a Brønsted base to afford an allylic radical that is captured by a Cr(II) cocatalyst to furnish an allylchromium(III) intermediate. In situ protodemetalation of this allylchromium complex by methanol is highly regioselective and affords an isomerized and less thermodynamically stable alkene product.

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proposed mechanism

General Procedure

An oven-dried screw cap 2-dram vial outfitted with a PTFE/silicone septum was charged with the relevant olefin substrate (0.500 mmol, 1.00 equiv.), 9-mesityl-3,6-di-tert-butyl-10-phenylacridinium tetrafluoroborate (0.00500 mmol, 1.00 mol%, 2.87 mg), CrCl₃ (0.0500 mmol, 10.0 mol%, 7.92 mg), and 4,4′-di-tert-butyl-2,2′-dipyridyl (0.0750 mmol, 15.0 mol%, 20.1 mg). The 2-dram vial was brought into a glove box under an atmosphere of nitrogen, where a solvent premix (2.50 mL) consisting of CH₂Cl₂ (15.0 mL) and MeOH (0.600 mL) were added in the glovebox. The 2-dram vial was then sealed with PTFE, electric tape and removed from the glovebox, and sonicated for 10 minutes. The vial was placed on a stir plate approximately 2 cm away from Kessil H150B LED lamps, and the reaction solution was allowed to stir under blue light irradiation (~450 nm) for 18 h. The average temperature of the reaction setup was 40°C with cooling fans running constantly; variations of up to ±5°C were observed. After 18 h, the irradiation was ceased, and the crude mixtures were concentrated and purified by flash column chromatography. All preparativescale reactions were run in duplicates, and the reported yields are the average yields of the two runs.

Key Words

isomerization of alkenes, terminal olefins, photochemistry

ID: J48-Y2022