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

Heterocoupling of different aryl nitrenes (originating in organoazides) to produce asymmetric azoarenes using two different iron–alkoxide catalysts is reported. Fe­(OC^tBu₂(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(OC^tBu₂(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 t_1/2 values of approximately 10 days at room temperature.