A sterically encumbered, nine-membered NHC-copper(i) complex was shown to be an effective catalyst for the chemical fixation of CO₂ by propargylic alcohols for the diastereoselective synthesis of α-alkylidene cyclic carbonates.

Nine‐ and ten‐membered N‐heterocyclic carbene (NHC) ligands have been developed and for the first time their gold(I) complexes were synthesized. The protonated NHC pro‐ligands 2 a–h were prepared by the reaction of readily available N,N′‐diarylformamidines with bis‐electrophilic building blocks, followed by anion exchange. In situ deprotonation of the tetrafluoroborates 2 a–h with tBuOK in the presence of AuCl(SMe₂) provided fast access to NHC‐gold(I) complexes 3–10. These new NHC‐gold(I) complexes show very good catalytic activity in a cycloisomerization reaction (0.1 mol % catalyst loading, up to 100 % conversion) and their solid‐state structures reveal high steric hindrance around the metal atom (%V_bur up to 53.0) which is caused by their expanded‐ring architecture.

A series of expanded‐ring NHC gold complexes of the formula (NaphtDHD−Ar)Au−X (NaphtDHD=4,5‐dihydro‐1H‐naphtho[1,8‐ef][1,3]diazocin‐3(2H)‐ylidene; Ar: Mes=2,4,6‐trimethylphenyl, Dipp=2,6‐diisopropylphenyl or Xyl=2,6‐dimethylphenyl; X=Cl, NCCH₃, NTf₂) have been synthesized, including the first gold(I) triflimidate complex (5) stabilized by an eight‐membered NHC ligand. The new organogold compounds have been characterized by mass spectrometry, IR spectroscopy, and ¹H and ¹³C NMR spectroscopy. The structural geometries of 3 b–c and 5 have been unequivocally established by crystallographic analysis revealing broad N‐C‐N angles (>121°) and high buried volume values (46–54%). The first catalytic studies were carried out on the cycloisomerization of 1,6‐enynes, obtaining full conversions (0.5 mol% catalyst loading) and excellent endo/exo selectivity (up to 99:1), and on the gold‐catalyzed phenol synthesis. Lastly, the (NaphtDHD‐Dipp)Au⁺ NTf₂⁻ species was subjected to a kinetic experiment in the cyclization of a N‐propargyl carboxamide to evaluate the efficiency of the pre‐formed catalyst (5) and the in situ activated gold complex (3 b+AgNTf₂). magnified image

Stabilized by a bulky N‐heterocyclic carbene [^BPDPr, 1,3‐bis(2,6‐diisopropylphenyl)‐1,3‐diazonine‐2‐ylidene] ligand, new silver carboxylate complexes of the form ^BPDPrAgO₂C‐R (R=Me, Ph) have been synthesized and fully characterized in solution and in the solid state and implemented as sole catalysts (base‐, additive‐, and, in some cases, solvent‐free) in the challenging fixation of carbon dioxide to unsubstituted propargylic derivatives for the synthesis of oxazolidinones and α‐methylene cyclic carbonates. Derived from X‐ray diffraction studies, the molecular geometry and the concept of buried volume were employed to describe the structural and steric features of these silver complexes. Their stability and efficiency as catalysts have been demonstrated by the synthesis of 29 carboxylation products (72–98 % yield) at low catalyst loadings (0.01–1.5 mol%). Characteristics are high turnover numbers (up to 9400), catalyst recyclability (up to 96 % yield after the 7th cycle with no decomposition of the silver complex), and the possibility to scale‐up the reaction.