Synthesis of axially chiral gold complexes and their applications in asymmetric catalyses

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.

Mechanism of the Transmetalation of Organosilanes to Gold

Density functional theory (DFT) calculations were carried out to study the reaction mechanism of the first transmetalation of organosilanes to gold as a cheap fluoride‐free process. The versatile gold(I) complex [Au(OH)(IPr)] permits very straightforward access to a series of aryl‐, vinyl‐, and alkylgold silanolates by reaction with the appropriate silane reagent. These silanolate compounds are key intermediates in a fluoride‐free process that results in the net transmetalation of organosilanes to gold, rather than the classic activation of silanes as silicates using external fluoride sources. However, here we propose that the gold silanolate is not the active species (as proposed during experimental studies) but is, in fact, a resting state during the transmetalation process, as a concerted step is preferred.

Origins of unique gold-catalysed chemo- and site-selective C-H functionalization of phenols with diazo compounds

In past decade, gold revealed more and more unique properties in carbene chemistry. It was disclosed in our recent communication (J. Am. Chem. Soc. 2014, 136, 6904) that gold carbenes have unprecedented chemo- and site-selectivity and ligand effect toward the functionalization of C-H bonds in phenols. In this full article, we report a comprehensively combined theoretical and experimental study on the mechanism of the insertion of gold carbenes into C-H and O-H bonds in phenol. It significantly revealed that the ligands have an important effect on C-H insertion and the reaction proceeds through a pathway involving the formation of an enolate-like intermediate. Moreover, two water molecules serving as a proton shuttle are believed to be the key issue for achieving chemoselective C-H functionalization, which is strongly supported by the DFT calculations and control experiments. It is the first time that a clear explanation is given about the prominent catalysis of gold carbenes toward C-H functionalization based on a theoretical and experimental study.

Competing amination and C–H arylation pathways in Pd/xantphos-catalyzed transformations of binaphthyl triflates: switchable routes to chiral amines and helicene derivatives

A Pd(OAc)₂/xantphos catalyst system can be tuned to promote either amination or C–H arylation of hindered binaphthyl 2-triflates, with xantphos's hemilability playing a key role.

When Phosphosugars Meet Gold: Synthesis and Catalytic Activities of Phostones and Polyhydroxylated Phosphonite Au(I) Complexes

The synthesis and characterization of P-chiral phosphonite-, phosphonate- and thiophosphonate-Au(I) complexes are reported. These novel ligands for Au(I) are based on glycomimetic phosphorus scaffolds, obtained from the chiral pool. The catalytic activities of these complexes are shown in the cyclization of allenols and the hydroamination of 2-(2-propynyl)aniline combined with an organocatalyzed reduction to the corresponding 2-phenyl tetrahydroquinoline. All described gold complexes present excellent catalytic activities.

Enantioselective gold-catalyzed intermolecular [2+2] versus [4+2]-cycloadditions of 3-styrylindoles with N-allenamides: observation of interesting substituent effects

The cycloaddition mode ([2+2] vs. [4+2]) can be unexpectedly switched by the simple modification of the N-substituent of the 3-styrylindoles.

A highly enantioselective [2+2] versus a [4+2]-cycloaddition of 3-styrylindoles to N-allenamides catalyzed by identical gold(i)/chiral phosphoramidite complexes is presented, which provides facile access to synthetically valuable, optically active substituted cyclobutanes and tetrahydrocarbazoles. The cycloaddition mode unexpectedly depends on the electronic nature of the N-substituent 3-styrylindoles, the origin of which could be well rationalized using DFT calculations and experimental results. To the best of our knowledge, the present work represents the first example of such an impressive substituent effect in tuning the reaction mode with high chemo-, regio- and enantioselectivity in asymmetric gold catalysis.

Gold(i)-catalyzed asymmetric [3 + 2]-cycloadditions of γ-1-ethoxyethoxy-propiolates and aldehydes

A gold-catalyzed asymmetric intermolecular [3 + 2]-cycloaddition of γ-1-ethoxyethoxy-propiolate and aldehydes by using the chiral ligand 5a or 5b is developed, which provides facile access to highly substituted 2,5-dihydrofurans in up to 84% yield with up to 97% ee.

Asymmetric gold-catalyzed lactonizations in water at room temperature

Asymmetric gold-catalyzed hydrocarboxylations are reported that show broad substrate scope. The hydrophobic effect associated with in situ-formed aqueous nanomicelles gives good to excellent ee's of product lactones. In-flask product isolation, along with the recycling of the catalyst and the reaction medium, are combined to arrive at an especially environmentally friendly process.