Part per million levels of an anionic iron hydride complex catalyzes selective alkene isomerization via two-state reactivity

Abstract

The isomerization of terminal alkenes is an important and atom-economical process that accesses otherwise difficult-to-prepare internal alkenes via a precious metal (e.g., ruthenium and iridium)-catalyzed isomerization reaction. Using earth-abundant metal-based catalysts in this transformation, however, has only recently gained prominence and their selectivity and activity are not yet comparable with their precious metal counterparts. Here, we report that the anionic iron hydride complex [(PC NHC P)Fe(H)N 2 ] − ( 4 ) is highly active for the selective one-bond isomerization of terminal alkenes. Isomerization occurs at room temperature and provides the internal alkenes with good regio- and stereoselectivity ( E / Z ≥ 10:1). The maximum turn-over number (TON) of 4 is ≥160,000, and is better than the most active precious metal catalysts. Experimental and computational studies suggest an alkyl-type mechanism for alkene isomerization, where two-state reactivity is responsible for the exceptional reactivity of complex 4 . • PC NHC P pincer ligands stabilize iron-hydrides in various oxidation states • Only ppm levels of iron catalyst are needed for selective alkene isomerization • Oxidation and spin state are important catalyst activity descriptors Because of the growing efforts to increase sustainability of chemical processes in the molecular sciences, replacing precious metals catalyst with more environmentally friendly ones is of high interest. Because of the general low toxicity and high availability, iron has entered the spotlight as an excellent alternative to these precious metals and over the past years has become an archetypical example for enabling sustainable catalysis. Yet, despite the increasing interest in earth-abundant metals, most industrially relevant processes are still catalyzed by precious metals, which includes alkene isomerization. In this paper, we have developed a novel anionic iron hydride catalyst that is highly active for the selective one-bond isomerization of terminal alkenes. Isomerization occurs with good stereoselectivity ( E / Z ≥ 10:1) and with catalyst loadings as low as a few ppm, opening the door for applications in both academically and industrially relevant markets. The highly efficient one-bond isomerization of terminal alkenes is reported with an unusual anionic iron(0) hydride catalyst. The substrate scope includes a wide variety of functional groups (e.g., esters, ethers, halides, silanes, and boronic esters), while the isomerization reactions occur under ambient conditions with parts per million levels of the catalyst. The effect of oxidation and spin state of the iron hydride is discussed and a mechanistic study is provided that highlights the importance of two-state reactivity.