IPaulHet – spatially-defined, wingtip-flexible, N,C-chelating oxazole and thiazole donor N-heterocyclic carbene ligands

Abstract

N-Heterocyclic carbenes (NHCs) are among the most versatile ligands in transition metal catalysis with their steric and electronic properties playing a critical role in governing reactivity and selectivity. In this area, the wingtip unsymmetrical IPaul ligand introduced by Nelson and co-workers offers a unique balance of steric bulk and flexibility characterized by spatially-defined steric features. Herein, we report a new class of IPaul-based ligands bearing benzoxazole and benzothiazole donor wingtips. The synthesis, application, and structural and electronic characterization are described. These ligands retain the defining ‘bulky yet flexible’ profile of IPaul, while enabling precise control over the catalytic pocket geometry through N-heteroaryl wingtip substitution. We present their coordination chemistry with Ag(I), Pd(II), Rh(I), and Se as well as catalytic studies in cross-coupling and hydrosilylation catalyzed by Ag, Pd, and Rh complexes. By combining the steric asymmetry of IPaul with the chelating flexibility of N-azole donors, this ligand class provides stabilization of reactive metal centers and is well-suited for diverse catalytic applications. We anticipate that the combination of steric flexibility with N,C-chelation in versatile N-heterocyclic carbenes will be of broad interest across organometallic, inorganic and catalytic chemistry.