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, via the ring-opening of a donor–acceptor cyclopropane (1). Subsequent transmetalation of 2 or 3 by Pd(COD)Cl₂ (COD = cyclooctadiene) results in [(κ^2-C-benzoxCAAC)PdCl₂] (4), and Cl-abstraction by AgSbF₆ affords the dicationic [(κ^2-C-benzoxCAAC)Pd(μ-Cl)]₂[SbF₆]₂ (5). Both 4 and 5 are precatalysts for Buchwald–Hartwig C–N cross-coupling, in which the cationic 5 performs consistently better than the neutral 4. The ligating influence of ^C-benzoxCAAC in this catalysis is compared with a known example in the literature, a similarly C,N-bidentate but more rigid ^C-imineCAAC, and a monodenate ^Et2CAAC using 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.