Development of a Catalytic Protocol for Alkane Dehydrogenation by C–H Activation at Iridium(III)

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PROJECT SUMMARY Inexpensive and abundant, unactivated alkanes are ideal precursors for the production of commodity chemicals. Common starting materials in industrial processes and ubiquitous motifs in complex molecules, alkenes can be produced from more readily available alkanes through transition-metal-catalyzed dehydrogenation. While heterogeneous catalysis has proven useful in industrial dehydrogenation of low-carbon alkanes, applications in chemical synthesis have been limited due to high temperatures, susceptibility to catalyst-deactivating coking, and low selectivity with longer-chain alkane substrates. In contrast, homogeneous catalytic reactions generally proceed under milder conditions and can be effective in the dehydrogenation of higher alkanes. Extensive research has revealed pincer-ligated Ir compounds to be the optimal catalysts, and recent studies have implicated Ir(III) complexes as promising candidates capable of circumventing many limitations of established Ir(I) systems. This application describes the development of a novel protocol for catalytic alkane dehydrogenation by pincer- ligated Ir(III) complexes using the most economical oxidant available, molecular oxygen. The research strategy builds on the previously demonstrated capabilities of a (NCN)Ir(III) complex for mediating alkane dehydrogenation and for regenerating under aerobic conditions. To date, catalysis has remained elusive due to the instability of the Ir(III) complex in the presence of oxygen at the high temperatures required for dehydrogenation. Computational studies suggest that ligation of the Ir(III) metal center with electron-poor pincer ligand frameworks will lower the energy barrier to dehydrogenation. Based on the conclusions drawn from these experimental and computational investigations, the central hypothesis of this research plan is that catalytic dehydrogenation can be realized by reducing the reaction temperature through strategic modification of the pincer ligand supporting the Ir(III) metal center. The specific aims of this application are: 1) synthesis of novel Ir(III) pincer complexes for alkane dehydrogenation, 2) Ir(III)-catalyzed alkane dehydrogenation under aerobic conditions, and 3) dehydrogenation of functionalized organic substrates. The development of a method for Ir(III)-catalyzed alkane dehydrogenation under aerobic conditions will contribute a novel olefin preparation strategy invulnerable to many limitations of current processes while using an environmentally friendly oxidant. The results of this project will greatly impact the preparation of olefin- containing commodity chemicals and complex molecules relevant to human health or intermediates thereto.