Gold(I)-Catalysed Asymmetric Hydroamination of Alkenes: A Silver- and Solvent-Dependent Enantiodivergent Reaction

Ligand effects, solvent cooperation, and large kinetic solvent deuterium isotope effects in gold(I)-catalyzed intramolecular alkene hydroamination

Kinetic studies on the intramolecular hydroamination of protected variants of 2,2-diphenylpent-4-en-1-amine were carried out under a variety of conditions with cationic gold catalysts supported by phosphine ligands. The impact of ligand on gold, protecting group on nitrogen, and solvent and additive on reaction rates was determined. The most effective reactions utilized more Lewis basic ureas, and more electron-withdrawing phosphines. A DCM/alcohol cooperative effect was quantified, and a continuum of isotope effects was measured with low KIE's in the absence of deuterated alcoholic solvent, increasing to large solvent KIE's when comparing reactions in pure MeOH to those in pure MeOH-d4. The effects are interpreted both within the context of a classic gold π-activation/protodeauration mechanism and a general acid-catalyzed mechanism without intermediate gold alkyls.

Chiral NHC Ligands for Enantioselective Gold(I) Catalysis Under Aerobic Conditions: the Importance of Conformational Flexibility and Steric Hindrance of NHC Ligand on Reactivity

Gold(I) catalysis has been recognized as a valuable tool for the unique transformation of multiple carbon-carbon bonds. Enantioselective π-catalysis based on gold(I) complexes is, however, still underdeveloped due to lack of privileged ligands. Herein, we present an accessible method to a new family of stable yet catalytically active chiral NHC-Au(I)-Cl complexes. The key to preserving a simultaneous fine balance between reactivity and stability in this newly developed family appears to be sterically hindered, but conformationally flexible NHC ligands. These could be easily accessed on a multigram scale by merging sterically hindered anilines with commercially available amino alcohols and amines via a four-steps synthetic sequence without the need for chromatographic purification. Further investigations of the catalytic activity of NHC-Au-Cl complexes identified the OH functionality incorporated into the NHC core as crucial for the level of enantioselectivity as well as the TsO- anion responsible for the activation of NHC-Au(I)-Cl. Finally, NMR studies and X-ray investigations revealed for the first time that the widely accepted ion metathesis (NHC-Au-Cl to NHC-Au-OSO2 R) responsible for the activation of NHC-Au-Cl complexes does not take place (or it is very slow) in commonly used MeNO2 in contrast to DCM.

The Analysis of Two Distinct Strategies toward the Enantioselective Formal Total Synthesis of (+)-Gelsenicine

A full account of a formal enantioselective total synthesis of (+)-gelsenicine is described. Separate strategies based on catalytic cycloisomerization as the central step are considered. One plan involves chirality transfer from enantioenriched substrates, while the other employs asymmetric catalysis. The chirality transfer strategy is less effective, while in the latter, phosphoramidite- and bisphosphine-gold complexes are tested and ultimately provide a key intermediate in high enantiopurity in our Gelsemium alkaloid syntheses.

Activation of metal-involved halogen bonds and classical halogen bonds in gold(I) catalysis

In gold(I) catalysis, the activation of Au(I) chloride catalysts via chloride abstraction and noncovalent interactions has become a research focus in organometallic catalysis. In this work, taking halogen bond donors (C₄H₂INO₂, C₆F₅I, C₈H₉O₂I) as activators for a Au(I) chloride catalyst (Ph₃PAuCl), the mechanism of the cyclization reaction of propargylic amide was investigated. It was found that there are two activation modes as design principles to obtain the catalytically active species Ph₃PAu⁺: the halogen bond donors activate the Cl atoms of Ph₃PAuCl to form X-I⋯Cl (X = C, N) classical halogen bonds and activate the Au atoms of Ph₃PAuCl to form X-I⋯Au (X = C, N) metal-involved halogen bonds. For the two activation modes, the mechanism of the cyclization reaction of propargylic amide has pathways: the chloride abstraction process of the first step and the 5-exo/6-endo cyclization process of the second step. Both activation modes show good activity for the cyclization reaction with the activation ability of classical halogen bonds being slightly stronger than that of the metal-involved halogen bonds, which is consistent with the strength of the X-I⋯Cl halogen bonds being slightly stronger than that of the X-I⋯Au halogen bonds. Therefore, both metal-involved halogen bonds and classical halogen bonds have important development prospects for the activation of catalysts in gold(I) catalysis.

Origin of enantioselectivity reversal in Lewis acid-catalysed Michael additions relying on the same chiral source

Enantiodivergence is an important concept in asymmetric catalysis that enables access to both enantiomers of a product relying on the same chiral source as reagent. This strategy is particularly appealing as an alternate approach when only one enantiomer of the required chiral ligand is readily accessible but both enantiomers of the product are desired. Despite the potential significance, general catalytic methods to effectively reverse enantioselectivity by changing an achiral reaction parameter remain underdeveloped. Herein we report our studies focused on elucidating the origin of metal-controlled enantioselectivity reversal in Lewis acid-catalysed Michael additions. Rigorous experimental and computational investigations reveal that specific Lewis and Brønsted acid interactions between the substrate and ligand change depending on the ionic radius of the metal catalyst, and are key factors responsible for the observed enantiodivergence. This holds potential to further our understanding of and facilitate the design of future enantiodivergent transformations.

Enantiodivergence is an important concept in asymmetric catalysis that enables access to both enantiomers of a product relying on the same chiral source as reagent.

Computational study on N-triflylphosphoramide-catalyzed enantioselective hydroamination of alkenyl thiourea

The mechanism of the enantioselective intramolecular hydroamination of alkenyl thiourea catalyzed by chiral binaphthol N-triflylphosphoramide (NPTA) was investigated using density functional theory calculations. This study reveals the details of the hydrogen bonding mode between NPTA and the substrate and indicates the importance of the dual hydrogen binding properties of the thiourea moiety for the reactivity and stereoselectivity of the hydroamination.

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

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.

Nucleophile-assisted cyclization of β-propargylamino acrylic compounds catalyzed by gold(i): a rapid construction of multisubstituted tetrahydropyridines and their fused derivatives

A variety of medicinally important multisubstituted tetrahydropyridines, 4-aryl piperidines, isoquinolines and fused pyridines were prepared in 1–3 step sequences via nucleophile-assisted Au(i)-catalyzed cyclizations of easily available azaenynes.

Chiral N-heterocyclic Carbene Gold Complexes: Synthesis and Applications in Catalysis

N-Heterocyclic carbenes have found many applications in modern metal catalysis, due to the formation of stable metal complexes, and organocatalysis. Among a myriad of N-heterocyclic carbene metal complexes, gold complexes have gained a lot of attention due to their unique propensity for the activation of carbon-carbon multiple bonds, allowing many useful transformations of alkynes, allenes, and alkenes, inaccessible by other metal complexes. The present review summarizes synthetic efforts towards the preparation of chiral N-heterocyclic gold(I) complexes exhibiting C2 and C1 symmetry, as well as their applications in enantioselective catalysis. Finally, the emerging area of rare gold(III) complexes and their preliminary usage in asymmetric catalysis is also presented.

Deciphering the Origin of Enantioselectivity on the Cis-Cyclopropanation of Styrene with Enantiopure Di-chloro,Di-gold(I)-SEGPHOS Carbenoids Generated from Propargylic Esters

The stereoselective synthesis of cis-disubstituted cyclopropanes by the Au(I)/PPh₃-catalyzed cycloaddition of propargylic esters and styrene has been studied using density functional theory calculations. The computed mechanistic scheme involves the rate-limiting 1,2-rearrangement of the propargylic ester with the π-coordinated gold complex, followed by the (2 + 1)-cheletropic reaction of styrene with the alkenyl-Au(I) carbene intermediate to afford the cis-disubstituted cyclopropane derivative in a high cis/trans diastereomeric ratio. With a ( R)-di-chloro,di-gold-DTBM-SEGPHOS complex as the catalyst, computations are consistent with a rate-determining (2 + 1)-cheletropic reaction, in which facial discrimination is proposed to result from a combination of subtle steric and electronic effects in the SiRe facial approach transition structure, which favor the formation of the cis-cyclopropane diastereomer of 1 R,2 S absolute configuration, as experimentally observed.

Reversal of enantioselectivity in chiral metal complex-catalyzed asymmetric reactions

Asymmetric catalysis represents an efficient approach to prepare optically active compounds. Commonly, both enantiomers of a chiral catalyst are used to synthesize two enantiomers of a chiral compound, however, it is quite difficult to obtain the catalysts with opposite configurations in most cases. Thus, chemists pay much attention to look for new strategies. Enantiodivergent synthesis demonstrates cost effectiveness and practicability to solve this issue by tuning the reaction parameters with the use of ligands derived from a single chiral source. In 2003 and 2008, two reviews have commendably summarized the enantiodivergent reactions, and some representative examples were illustrated. In this review, reversal of enantioselectivity in metal complex-mediated asymmetric catalysis from 2008 to present was updated. Several factors of delivering enantiodivergence are introduced, including metal salts, ligands, additives, solvents, temperature and so on.

Gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes: towards functionalised azoles

Vinyl- and saturated azoles were synthesised through gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes and subsequent hydrogenation.

Well defined difluorogold(iii) complexes supported by N-ligands

Rare difluorogold(iii) complexes supported by N-ligands can be synthesized simply either from Au(i) precursors using XeF₂ or Au(iii) using KF.

Gold(III) NHC Complexes for Catalyzing Dihydroalkoxylation and Hydroamination Reactions

A gold(III) complex of an N-heterocyclic carbene based hemilabile ligand with two pendant pyrazole arms (1,3-bis((1H-pyrazol-3-yl)methyl)-2,3-dihydro-1H-imidazole, LH) was synthesized. Complex [LAu(III)Cl₃] is an excellent catalyst for promoting dihydroalkoxylation at room temperature, even catalyzing this reaction at 0 °C. [LAu(III)Cl₃] is one of the most efficient catalysts reported to date for the spirocyclization of alkynyl diols. Furthermore, [LAu(III)Cl₃] catalyzed intra- and intermolecular hydroamination reactions, achieving good to excellent conversions. [LAu(III)Cl₃] is a more efficient catalyst than a gold(I) analogue, [LAu(I)Cl]. The dependence of the quantity of weakly coordinating anion [BAr^F₄]^- ((3,5-trifluoromethyl)phenyl borate) present on catalysis efficiency was probed for the dihydroalkoxylation reaction. X-ray diffraction analysis of single crystals demonstrated the solid-state structure of gold complexes [LAu(III)Cl₃] and [LAu(I)Cl], which displayed the expected square-planar and linear coordination geometries, respectively.