C(sp³)–H Activation via Dehydrogenation of Cyclic and Heterocyclic Alkanes by Single-Site Iridium Pincer Ligated Complexes

Abstract

The direct dehydroaromatization of C(sp³)–H alkanes may seem conceptually simple but in fact is a challenging transformation. Industrially practiced methods utilize energy intensive processes operating at high pressures and temperatures due to the requirement of such conditions to overcome the endergonic and unreactive nature of alkanes. Chapter 1 briefly discusses early and recent achievements in the field of alkanes dehydrogenation by Ir pincer ligated complexes. While there has been great advancement in the dehydrogenation transformation recently, the direct dehydroaromatization of heterocyclic substrates generating functionalized aromatics is significantly underdeveloped. In Chapter 2, we successfully extended the applicability of Ir catalyzed dehydrogenation systems using pincer ligated complexes on a diverse collection of heterocyclic alkanes with functionalities known to be strongly coordinating and poorly compatible with (PCP)–Ir type catalysts. Carbo- and heteroarenes containing oxygen and nitrogen can be synthesized in moderate to excellent yields up to 99%, and the reaction tolerates functional groups such as bromides and fluorides. In Chapter 3, we demonstrate the efficient disproportionation of cycloalkenes to the corresponding arenes and cycloalkanes with up to 100% conversion, which has been a long-standing challenge in the field of pincer-ligated Ir-catalyzed dehydrogenation studies. For example, 1-cyclohexene was disproportionated to benzene and cyclohexane and 1-4-vinyl-1-cyclohexene was disproportionated to ethylbenzene and ethylcyclohexane. We also demonstrate that a key mechanistic feature of our system is a lack of catalyst inhibition by arenes. In addition, our method is advantageous to previous reports as no sacrificial olefin is used, thereby circumventing the requirement for exogeneous hydrogen acceptors. Our studies presented in Chapter 2 and Chapter 3 provides a novel and a complementary pathway to access important aromatic building blocks and may help create alternative routes to complex molecules via late stage dehydrogenation without the need of stoichiometric oxidants.