An Ion‐Responsive Pincer‐Crown Ether Catalyst System for Rapid and Switchable Olefin Isomerization

• Published in: Angewandte Chemie International Edition Vol. 56; no. 20; pp. 5498 - 5502
• Main Authors: Kita, Matthew R., Miller, Alexander J. M.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 08.05.2017
• Edition: International ed. in English
• Subjects: Alkali metals; Binding; Catalysts; Chlorides; Coordination compounds; Crown ethers; Ethers; hemilability; homogeneous catalysis; Iridium; Isomerization; olefin isomerization; Salts; Sodium; Sodium salts; Substrates; switchable catalysis; iridium; hemilability; olefin isomerization; switchable catalysis; homogeneous catalysis
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.201701006

Rapid, selective, and highly controllable iridium‐catalyzed allylbenzene isomerization is described, enabled by tunable hemilability based on alkali metal cation binding with a macrocyclic “pincer‐crown ether” ligand. An inactive chloride‐ligated complex can be activated by halide ion with sodium salts, with the resulting catalyst [κ5‐(15c5NCOPiPr)Ir(H)]+ exhibiting modest activity. Addition of Li+ provides a further boost in activity, with up to 1000‐fold rate enhancement. Ethers and chloride salts dampen or turn off reactivity, leading to three distinct catalyst states with activity spanning several orders of magnitude. Mechanistic studies suggest that the large rate enhancement and high degree of tunability stem from control over substrate binding. Multi‐stage cati‐ON booster: Complete control over activity is achieved for olefin isomerization with the system shown. Mechanistic studies suggest that cations help control substrate binding through interaction with the hemilabile macrocycle of “pincer‐crown ether” ligands. Acceleration using an appropriate cation and deceleration using ether or chloride sources provide exquisite control over the catalytic activity.