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Heterocoupling of Different Aryl Nitrenes to Produce Asymmetric Azoarenes Using Iron–Alkoxide Catalysis and Investigation of the Cis–Trans Isomerism of Selected Bulky Asymmetric Azoarenes

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posted on 2021-10-19, 20:06 authored by Duleeka Wannipurage, Sudheer S. Kurup, Stanislav Groysman
Heterocoupling of different aryl nitrenes (originating in organoazides) to produce asymmetric azoarenes using two different iron–alkoxide catalysts is reported. Fe­(OCtBu2(3,5-Ph2C6H3))2(THF)2 was previously shown to catalyze the homocoupling of a variety of aryl nitrenes. While bulky nitrenes featuring ortho substituents were coupled more efficiently, coupling of the less bulky meta- and para-substituted aryl nitrenes was also demonstrated. In contrast, the iron­(II) complex of a chelating bis(alkoxide) ligand, Fe­[OO]Ph(THF)2, was previously shown to efficiently couple nonbulky aryl nitrenes lacking substituents in ortho positions. In the present work, we demonstrate that the combination of two different nitrenes (10 equiv overall, 5 equiv each) with Fe­(OCtBu2(3,5-Ph2C6H3))2(THF)2 (10 mol %) produced a statistical or close to statistical distribution (25:25:50 for the two homocoupled products and the heterocoupled product, respectively) for various combinations containing one or two ortho alkyl substituents at one nitrene and a single ortho alkyl group at another. Surprisingly, the combination of Fe­[OO]Ph(THF)2 with two different nonbulky organoazides was found to primarily catalyze the homocoupling of the resulting aryl nitrenes (21–49%), with a smaller proportion (∼8–15%) of asymmetric product formation. Six different heterocoupled products featuring one or two alkyl groups in the ortho positions were isolated as a mixture of cis and trans isomers at room temperature and characterized by NMR spectroscopy, UV–vis spectroscopy, and high-resolution mass spectrometry. Following their isolation, cis–trans isomerism in these species was investigated. Heating the cis–trans mixture to 60 °C produced the trans isomer cleanly, while shining UV light on the cis–trans mixture significantly increased the amount of the cis isomer (up to 90%). The cis isomer was found to be relatively stable, exhibiting t1/2 values of approximately 10 days at room temperature.

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