A Benzoxazolyl-Linked Cyclic(alkyl)(amino)carbene (CAAC): the N- Sidearm Negatively Influences the Buchwald-Hartwig Coupling

Abstract

Additional donor-functionalized cyclic(alkyl)(amino)carbenes (CAACs) are rare but can be valuable as chelating ligands. We introduce a flexible bidentate CAAC with a benzoxazol-2-ylmethyl sidearm (C-benzoxCAAC), first as its (CuCl)2 (2) and AgCl (3) complexes by ring-opening a donor-acceptor cyclopropane (1). A following transmetalation of 2 or 3 by Pd(COD)Cl2 (COD = cyclooctadiene) results [(k2-C-benzoxCAAC)PdCl2] (4) and a Cl-abstraction thereof by AgSbF6 affords the dicationic [(k2-C-benzoxCAAC)Pd(m-Cl)]2[SbF6]2 (5). Both 4 and 5 are precatalysts for Buchwald-Hartwig C-N cross-coupling, where the cationic 5 performs consistently better than the neutral 4. The ligating influence of C-benzoxCAAC in this catalysis is compared with a literature-known and similarly C,N-bidentate but more rigid C-imineCAAC and the monodenate Et2CAAC by both experiments and DFT analysis. Apparently, the N-sidearms induce more robustness but inhibit the catalysis by hindering the reductive elimination step. A subtle difference between the more flexible C-benzoxCAAC and the more rigid C-benzoxCAAC is also envisaged.