Silver‐Free Au(I) Catalysis Enabled by Bifunctional Urea‐ and Squaramide‐Phosphine Ligands via H‐Bonding

Abstract

A library of gold(I) chloride complexes with phosphine ligands incorporating pendant (thio)urea and squaramide H‐bond donors was prepared with the aim of promoting chloride abstraction from Au(I) via H‐bonding. In the absence of silver additives, complexes bearing squaramides and trifluoromethylated aromatic ureas displayed good catalytic activity in the cyclization of N‐propargyl benzamides, as well as in a 1,6‐enyne cycloisomerization, a tandem cyclization‐indole addition reaction and the hydrohydrazination of phenylacetylene. Kinetic studies and DFT calculations indicate that the energetic span of the reaction is accounted by both the chloride abstraction step, facilitated by the bidentate H‐bond donor via an associative mechanism, and the subsequent cyclization step.

H-Bonded Counterion-Directed Enantioselective Au(I) Catalysis

“…Various bi- and tricyclic structures were accessed in good to excellent yield and enantioselectivity from 1,6-enynes, via 5- exo -dig and 6- endo -dig cyclizations with or without external nucleophiles. 14 To overcome the reactivity 8 and enantiocontrol 15 challenges described above, we envisioned to prepare gold(I) complexes with bifunctional phosphine ligands 16 incorporating dual H-bond donor groups 17 such as (thio)ureas 18 and combine them with BINOL-derived chiral anions 19 ( Scheme 2 ). The pendant H-bond donor would serve two purposes: (1) remove the chiral anionic ligand from the Au(I) coordination sphere, thus enabling substrate coordination and subsequent catalysis at the metal center; (2) place the chiral anion close to the substrate, ensuring good transmission of the stereochemical information.…”

H‐Bonded Counterion‐Directed Catalysis: Enantioselective Gold(I)‐Catalyzed Addition to 2‐Alkynyl Enones as a Case Study

“…[13] In order to screen these parameters systematically, we evaluated a library of phosphine Au(I) chloride complexes equipped with urea or squaramide groups on triphenylphosphine-(Au1-6) or John-Phos-type scaffolds (Au7-9), previously synthesized by our group (Figure 2A). [13,17] The complexes thus vary in terms of phosphine, H-bond donor and linker between the two. The chiral anion was introduced as a phosphoramidate metal salt (Figure 2B), with a countercation capable of abstracting the chloride from the Au(I) precatalyst, such as Ag(I), Cu(II) and Na(I).…”

Enantioselective Transition-Metal Catalysis via Anion Binding with Chiral Hydrogen-Bond Donors

“…Achiral HBDs have been applied as additives to enhance the reactivity of organometallic complexes, potentially by lowering the coordination strength of anionic ligands (28)(29)(30). Dual HBD motifs have been incorporated into ligand structures of gold(I) chloride or phosphate complexes to sequester the anions responsible for inhibition of the metal catalyst (31,32). In addition, chiral organic compounds bearing HBD components have been employed as co-catalysts in asymmetric transition-metal-catalyzed transformations (33)(34)(35)(36)(37)(38).…”