Elucidating the Origins of Reactivity and Selectivity of PHOX–Ni Aryne Complexes

Abstract

Metal aryne complexes are stable, isolable counterparts to free aryne intermediates that have access to a wider range of reactivity which has been explored both stoichiometrically and catalytically, allowing for nucleophile/electrophile, dielectrophilic, and even dinucleophilic difunctionalizations. However, the directing group approach used to gain regiocontrol in free aryne reactions described in the Aryne Distortion Model (ADM) has no effect on their metal-bound counterparts—as such, there are no previous reports of approaches to difunctionalize aryne complexes regioselectively. By leveraging the steric and electronic effects of unsymmetrical, bidentate P, N-donor ligands and combining them with the ability of late transition metals to stabilize arynes, we have developed a system using PHOX–Ni (phosphinooxazoline) aryne complexes to introduce ligand control for the regioselective synthesis and difunctionalization of aryne complexes. A combination of spectroscopic, crystallographic, and computational studies have led to the development of the Metal Aryne Reactivity/Selectivity (MAR/S) Model. Aryne complex difunctionalizations can be rendered regioselective by leveraging the synergistic effects of ligand- and aryne substituent-induced distortion. Similarly to the ADM, greater degrees of aryne angle distortion are correlated with higher regioselectivities in aryne formation. Aryne binding selectivities of greater than 86:14 were found to yield regioconvergent difunctionalizations. The observed selectivities are complementary to the ADM, where the smaller internal aryne angle is the site of electrophilic attack. Furthermore, in order to gain further insight into the reactivity of these complexes, the first experimental electronic structure study of aryne complexes via UV/Vis spectroscopy and cyclic voltammetry were conducted. These studies provide insight into the redox and photophysical properties of CyPHOX–Ni aryne complexes, as well as a framework for assigning the formal oxidation state of Ni benzyne and heteroaryne complexes to rationalize reactivity differences. These findings have implications in the development of new oxidative reactivity of Ni benzyne complexes, aryne complex photochemistry, and the development of regioselective Ni-catalyzed aryne reactions.