A Structure‐Based Activity Study of Highly Active Unsymmetrically Substituted NHC Gold(I) Catalysts

Abstract

Following the modular template synthesis using isonitriles,n ew unsymmetrically substituted five-membered saturated N-heterocyclic carbene (NHC)a nd N-heterocyclic oxo-carbene (NHOC) gold(I) complexes were prepared. With these species and already reported complexes,adetailed study concerning the catalytic activities of the complex classes available by the isonitrile routew as conducted. Thec atalytic properties of twelve different types of NHOCs, saturated and unsaturated NHC gold(I) pre-catalysts with different substituents,a sw ella s one representative of as ix-membered NHC and one N-acyclic carbene (NAC)g old(I) complex were analyzed by utilizing the phenol synthesisa satest reaction. Fort his reaction, the saturated NHC gold(I) complexes achieved higher conversions than the corresponding unsaturated NHCs andt he NHOC systems.W hile unsaturated NHC complexes show higher catalytic activity during the initial phase of the conversion, due to ah igher stability,h igher turnover numbers( TONs) were obtained for the corresponding saturated systems.Acyclopentadecyls ubstituenta tn itrogen turned out to be the privileged substituent for all of the unsymmetrical complexes. Furthermore,w ed etectedt hat light exclusion can significantly increase the catalytic activity of NHC gold(I) complexes for phenols ynthesis.

Unprecedented Use of NHC Gold (I) Complexes as Catalysts for the Selective Oxidation of Ethane to Acetic Acid

“…The observed trend regarding the structure of the central cycle is in accordance with the behavior found previously for other catalytic reactions of these complexes [ 27 ]. The higher electron density at the metal center of the saturated NHC ligand compared with the unsaturated one, in addition to a synergetic effect of π back-donation and σ donation of the saturated NHC ligand, leads to a more stable complex and this stability plays an important role in the catalytic properties of the gold(I) complexes, in particular for long reactions.…”

“…Five-membered saturated or unsaturated N-heterocyclic carbene (NHC) and unsymmetrical N-heterocyclic oxo-carbene (NHOC) gold(I) chloride complexes were synthesized [ 27 ]. Aiming to investigate structure-reactivity relationships, a set of cyclic carbene complexes was selected having on one nitrogen atom the 2,6-diisopropylphenyl group and on the other nitrogen atom the cyclohexyl or the cyclododecyl group ( Figure 1 ).…”

“…All solvents and reagents were obtained from commercial sources (Sigma-Aldrich, Munich, Germany) and were used without further purification. N-heterocyclic carbene (NHC) and N-heterocyclic oxo-carbene (NHOC) gold(I) complexes 1 – 4 were synthetized according to the literature [ 27 ]. Briefly, the strategic synthesis encompasses isonitrile-based routes as depicted in Scheme 2 .…”

Tuning the Gold(I)‐Carbon σ Bond in Gold‐Alkynyl Complexes through Structural Modifications of the NHC Ancillary Ligand: Effect on Spectroscopic Observables and Reactivity

“…Then, we introduced oxygen atoms in the backbone of the carbene (3 a-3 d): remarkably, similar NHCs have shown unique catalytic activity in some gold catalyzed reactions. [38] In 3 c and 3 d the size of the ring is also increased from five to six members. We also modified the backbone of the NHC by changing its unsaturation (4 and 5).…”

“…Starting from complex 1 as prototype, we replaced the hydrogen atoms on the N atoms of the NHC with alkyl ( 2 a , 2 c , 2 d ) and aryl ( 2 b , 2 e , 2 f ) groups. Then, we introduced oxygen atoms in the backbone of the carbene ( 3 a – 3 d ): remarkably, similar NHCs have shown unique catalytic activity in some gold catalyzed reactions [38] . In 3 c and 3 d the size of the ring is also increased from five to six members.…”