A highly efficient [(NHC)Au(I)]-based (NHC = N-heterocyclic carbene) catalytic system for the hydration of an array of alkynes that operates under acid-free conditions and at very low catalyst loadings (typically 50-100 ppm and as low as 10 ppm) was developed. Terminal and internal alkynes possessing any combination of alkyl and aryl substituents (alkyl/H, aryl/H, alkyl/alkyl, alkyl/aryl, and aryl/aryl) were found suitable substrates in the present catalytic system.
The use of a versatile N-heterocyclic carbene (NHC) gold(I) hydroxide precatalyst, [Au(OH)(IPr)], (IPr=N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) permits the in situ generation of the [Au(IPr)](+) ion by simple addition of a Brønsted acid. This cationic entity is believed to be the active species in numerous catalytic reactions. (1)H NMR studies in several solvent media of the in situ generation of this [Au(IPr)](+) ion also reveal the formation of a dinuclear gold hydroxide intermediate [{Au(IPr)}(2)(μ-OH)], which is fully characterized and was tested in gold(I) catalysis.
Folding in the confine: Short fragments of tri‐ to hexapeptides have been encapsulated by a synthetic host in water and folded into their latent helical structures (see crystal structure). Crystallographic analysis of the complexes clearly reveals a mixed conformation of 310 and α helices, thereby illustrating the inherent helical propensity of very short peptide fragments.
The syntheses of the free carbene IPr* (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) and related gold complexes [Au(IPr*)Cl] (C1) and [Au(IPr*)(NTf2)] (C2) were achieved in high yields. The % V
Bur of IPr* for both gold complexes was measured, revealing IPr* as one of the bulkiest NHCs on gold complexes reported to date. In addition, the catalytic activity of C1 and C2 in several reactions, typically catalyzed by AuI complexes, was investigated. Examples include the tandem alkoxylation/lactonization of γ-hydroxy-α,β-acetylenic esters, the [3,3]-rearrangement of propargylic acetates leading to the formation of conjugated enones and substituted indenes, and the rearrangement of allylic acetates. These studies revealed a strong solvent effect on the catalytic activity with 1,2-dichloroethane as the solvent of choice. The screening of C1 and C2 demonstrated only slightly diminished activities in comparison to [Au(NHC)(L)] complexes bearing bulky ligands such as IPr and SIPr.
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