DirectsynInsertion of Alkynes and Allenes into AuSi Bonds

Interrogating the anti-Insertion of Alkynes into Gold(III)

Alkyne hydrofunctionalizations are a powerful strategy to efficiently build up structural complexity. The selectivity of these reactions is typically governed by the interaction between the alkyne and a metal-hydride, which commonly proceeds via a well-understood syn-insertion mechanism. In contrast, anti-insertions are far less common, with proposed mechanisms often extrapolated from literature precedents rather than grounded in direct experimental evidence. While gold complexes rank among the most efficient catalysts for such transformations, the mechanistic understanding of the key alkyne insertion step remains incomplete. In this study, we demonstrate that stable gold(III)-hydrides, featuring a (P∧N∧C) ligand, undergo selective insertion of alkynes to yield the corresponding anti-Markovnikov Z-vinyl complexes. A combination of control experiments, kinetic studies, and computational analyses reveals a nonradical, bimolecular insertion process, in which water plays a pivotal role by accelerating the reaction and potentially stabilizing a highly reactive, T-shaped gold(I) intermediate. Notably, this is the first demonstration of the insertion of both activated and unactivated terminal and internal alkynes into a gold(III)-hydride complex.

Stepwise alkyne insertion in Au(i) acetylides: influence of the nuclearity

The reaction between NHC-supported (NHC = N-heterocyclic carbene) gold(i) trimethylsilylacetylide complexes with NHC gold(i) hydroxide species renders different symmetrical homobimetallic Au complexes. These compounds readily undergo migratory insertion of DMAD (dimethyl acetylenedicarboxylate) at 25 °C to give the corresponding bimetallic enyne products. On the contrary, monometallic analogues require much more forcing conditions (excess of DMAD and temperature ≥110 °C) to give the same transformation. Experimental and computational studies reveal that the second metal fragment is responsible for the enhanced nucleophilicity of the reactive carbon atom of the acetylide C[triple bond, length as m-dash]C bond, which initially leads to a more favorable interaction with DMAD in the rate-determining step of an unprecedented, stepwise mechanism where the lability of the Au-C bonds plays an instrumental role. The enhanced reactivity displayed by the bimetallic species was leveraged in the insertion of other substrates such as heterocumulenes.

Isolable T-Shaped Planar Silyl Anion

Silyl anions have garnered significant attention due to their synthetic abilities. However, previously reported silyl anions have been limited to either trigonal-pyramidal or trigonal-planar geometries, which confine them primarily as nucleophiles in substitution reactions. Herein, we report the isolation of the unprecedented T-shaped planar silyl anion salt 2 by employment of a geometrically constrained triamido pincer ligand. Theoretical calculations disclosed that the silicon centre in 2 possesses both a lone pair of electrons and an empty 3pz orbital. In addition to nucleophilic substitution reactions with Ph3PAuCl and W(CO)6, 2 readily undergoes oxidative additions with CO2 and 2,6-dimethylphenylisonitrile at room temperature. Furthermore, under mild conditions, compound 2 cleaves Csp2-H, Csp2-H, and H-H bonds in 1,2,4,5-tetrafluorobenzene, an intramolecular iPr group, and dihydrogen, representing the first examples of C-H and H-H activations mediated by a silyl anion, respectively. This work unveils new reactivity of silyl anions owing to the non-classical geometry and electronic structure.

Mechanistic Study of Alkyne Insertion into Cu-Al and Au-Al Bonds: A Paradigm Shift for Coinage Metal Chemistry

In this work, the mechanism of the insertion reaction of 3-hexyne into Cu-Al and Au-Al bonds in M-aluminyl (M = Cu, Au) complexes is computationally elucidated. The mechanism is found to be radical-like, with the Cu-Al and Au-Al bonds acting as nucleophiles toward the alkyne, and predicts a less efficient reactivity for the gold-aluminyl complex. The proposed mechanism well rationalizes the kinetic (or thermodynamic) control on the formation of the syn (or anti) insertion product into the Cu-Al bond (i.e., dimetallated alkene) which has been recently reported. A comparative analysis of the electronic structure reveals that the reduced reactivity at the gold site─usually showing higher efficiency than copper as a "standard" electrophile in alkyne activation─arises from a common feature, i.e., the highly stable 6s Au orbital. The relativistic lowering of the 6s orbital, making it more suitable for accepting electron density and thus enhancing the electrophilicity of gold complexes, in the gold-aluminyl system is responsible for a less nucleophilic Au-Al bond and, consequently, a less efficient alkyne insertion. These findings demonstrate that the unconventional electronic structure and the electron-sharing nature of the M-Al bond induce a paradigm shift in the properties of coinage metal complexes. In particular, the peculiar radical-like reactivity, previously shown also with carbon dioxide, suggests that these complexes might efficiently insert/activate other small molecules, opening new and unexplored paths for their reactivity.

Alkyne insertion into Cu-Al bonds and selective functionalization to form copper acyl compounds

We report on the insertion of alkynes into heterometallic M–M' bonds, producing (aluminylalkenyl)copper compounds which possess differential reactivity at the two derived M–C functions. Uniquely, this system is capable of...

Isomerization of a cis-(2-Borylalkenyl)Gold Complex via a Retro-1,2-Metalate Shift: Cleavage of a C-C/C-Si Bond trans to a C-Au Bond

This manuscript describes the first example of an alkyne insertion to the Au-B bond of a di(o-tolyl)borylgold complex to afford a cis-2-borylalkenylgold complex, and its isomerization to result in interchanging substituents on the alkenyl carbon atom and the boron atom. The former reaction is the first example of an alkyne insertion to a Au-B bond. In the latter reaction, the regiochemistry of the isomerized alkenylgold products varied depending on the substituents. DFT calculations revealed the formation of gold alkynylborates as a common intermediate via a "retro-1,2-metalate shift", which can be considered as an anti-β-carbon/silicon elimination, and identified a subsequent 1,2-metalate shift as the regiochemistry-determining step. Relative energies of the transition states to each isomer and natural-bond-orbital (NBO) analyses were used to clearly rationalize the regiochemistry of the products.

Reactivity of NHC/diphosphene-coordinated Au(I)-hydride

We report the reactivity of isolable Au(i)-hydride stabilized by an NHC-coordinated diphosphene towards substrates containing C-C and N-N multiple bonds (NHC = N-heterocyclcic carbene). Reactions with dimethyl acetylenedicarboxylate and azobenzene lead to a trans-addition of the Au(i)-H across the C-C triple bond and the N-N double bond, respectively. In contrast, the reaction with ethyl diazoacetate affords a gold(i)-hydrazonide as the 1,1-addition product to the terminal nitrogen atom. With phenyl acetylene, the corresponding Au(i)-alkynyl complex is obtained under the elimination of dihydrogen. Strikingly, diphosphene-containing Au(i)-hydride is more reactive - affording different products in some cases - than a related NHC-stabilized Au(i)-hydride without the mediating diphosphene moiety.

1,1-Phosphaboration of C[triple bond, length as m-dash]C and C[double bond, length as m-dash]C bonds at gold

The phosphine-borane iPr2P(o-C6H4)BFXyl2 (Fxyl = 3,5-(F3C)2C6H3) was found to react with gold(i) alkynyl and vinyl complexes via an original 1,1-phosphaboration process. Zwitterionic complexes resulting from Au to B transmetallation have been authenticated as key intermediates. X-ray diffraction analyses show that the alkynyl-borate moiety remains pendant while the vinyl-borate is side-on coordinated to gold. According to DFT calculations, the phosphaboration then proceeds in a trans stepwise manner via decoordination of the phosphine, followed by anti nucleophilic attack to the π-CC bond activated by gold. The boron center acts as a relay and tether for the organic group.

Coinage metal complexes of NHC-stabilized silyliumylidene ions

The first coinage metal complexes (M = Cu, Ag, Au) of an N-heterocyclic carbene-stabilized silyliumylidene ion are reported.

Gold Catalysis for Heterocyclic Chemistry: A Representative Case Study on Pyrone Natural Products

2-Pyrones and 4-pyrones are common structural motifs in bioactive natural products. However, traditional methods for their synthesis, which try to emulate the biosynthetic pathway of cyclization of a 1,3,5-tricarbonyl precursor, are often harsh and, therefore, not particularly suitable for applications to polyfunctionalized and/or sensitive target compounds. π-Acid catalysis, in contrast, has proved to be better for a systematic exploration of the pyrone estate. To this end, alkynes are used as stable ketone surrogates, which can be activated under exceedingly mild conditions due to the pronounced carbophilicity of [LAu]⁺ fragments (L=two electron donor ligand); attack of a tethered ester carbonyl group onto the transient alkyne-gold complex then forges the pyrone ring in a fully regiocontrolled manner.

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.

Coordination and insertion of alkenes and alkynes in AuIII complexes: nature of the intermediates from a computational perspective

Computational studies show how the stability and reactivity of gold(iii) alkene and alkyne complexes depend on the ancillary ligands.

Dramatic mechanistic switch in Sn/AuI group exchanges: transmetalation vs. oxidative addition

The mechanism of Ph/X exchange in reactions involving SnPhⁿBu₃ and [AuXL] complexes switches dramatically from the usual concerted mechanism involving Ar/X mixed bridges when X = Cl, to an unexpected oxidative addition/reductive elimination pathway via an Au^III intermediate when X = vinyl.

Gold-catalyzed bis(stannylation) of propiolates

Bis(stannylation) of propiolates with hexabutylditin is achieved under mild conditions using a cationic gold(i) catalyst.

Methyl-, Ethenyl-, and Ethynyl-Bridged Cationic Digold Complexes Stabilized by Coordination to a Bulky Terphenylphosphine Ligand

Reactions of the gold(I) triflimide complex [Au(NTf2 )(PMe2 Ar${{^{{\rm Dipp}{_{2}}}}}$)] (1) with the gold(I) hydrocarbyl species [AuR(PMe2 Ar${{^{{\rm Dipp}{_{2}}}}}$)] (2 a-2 c) enable the isolation of hydrocarbyl-bridged cationic digold complexes with the general composition [Au2 (μ-R)(PMe2 Ar${{^{{\rm Dipp}{_{2}}}}}$)2 ][NTf2 ], where Ar${{^{{\rm Dipp}{_{2}}}}}$=C6 H3 -2,6-(C6 H3 -2,6-iPr2 )2 and R=Me (3), CHCH2 (4), or CCH (5). Compound 3 is the first alkyl-bridged digold complex to be reported and features a symmetric [Au(μ-CH3 )Au](+) core. Complexes 4 and 5 are the first species of their kind that contain simple, unsubstituted vinyl and acetylide units, respectively. In the series of complexes 3-5, the bridging carbon atom systematically changes its hybridization from sp(3) to sp(2) and sp. Concomitant with this change, and owing to variations in the nature of the bonding within the [Au(μ-R)Au](+) unit, there is a gradual decrease in aurophilicity, that is, the strength of the Au⋅⋅⋅Au bonding interaction decreases. This change is illustrated by a monotonic increase in the Au-Au distance by approximately 0.3 Å from R=CH3 (2.71 Å) to CHCH2 (3.07 Å) and CCH (3.31 Å).

Enhanced π-backdonation from gold(I): isolation of original carbonyl and carbene complexes

The specific electronic properties of bent o-carborane diphosphine gold(I) fragments were exploited to obtain the first classical carbonyl complex of gold [(DPCb)AuCO](+) (ν(CO)=2143 cm(-1) ) and the diphenylcarbene complex [(DPCb)Au(CPh2 )](+) , which is stabilized by the gold fragment rather than the carbene substituents. These two complexes were characterized by spectroscopic and crystallographic means. The [(DPCb)Au](+) fragment plays a major role in their stability, as substantiated by DFT calculations. The bending induced by the diphosphine ligand substantially enhances π-backdonation and thereby allows the isolation of carbonyl and carbene complexes featuring significant π-bond character.

From understanding to prediction: gold- and platinum-based π-acid catalysis for target oriented synthesis

During the last century, conceptual advances in organometallic chemistry were often rapidly embraced by target oriented synthesis. Feedback provided by such preparative scrutiny has greatly benefitted method development; particularly prominent are examples from the entire cross coupling arena, as well as olefin metathesis. Seen against this backdrop, it is somewhat surprising that the explosive growth of research into π-acid catalysis has not yet yielded a matching number of implementations into the synthesis of structurally complex targets of biological significance. In contrast to the massive output of methodological and mechanistic investigations, few studies illustrate the strategic use of gold, silver, or platinum catalysis in late stages of such multistep endeavors. These elaborate and highly precious compounds demand utmost confidence in the reliability and robustness of the method to be applied. In this Account, we analyze the possible reasons for this imbalance, after a short summary of the conceptual basis of carbophilic activation of π-bonds with the aid of soft transition metal cations or complexes. We pinpoint mechanistic subtleties, which, at least in part, produce a great deal of structural diversity but can jeopardize predictive power. With the advances in the understanding of π-acid catalyzed processes in general, however, this uncertainty is gradually vanishing and the entire field is transitioning from comprehension to prediction. This is expected to foster advanced applications, while recent progress in asymmetric gold catalysis further improves the preparative significance. The presented work in this Account illustrates our own commitment to the field as well as our growing confidence in the maturity of platinum and gold catalysis. The carbophilic activation of π-bonds, particularly of alkynes, provides a method to manipulate functional groups that is orthogonal to traditional carbonyl chemistry. We illustrate this notion by presenting a new approach to hydroxypyrone derivatives that has enabled the total synthesis of the fragile polyunsaturated cyclophane derivative neurymenolide A. The synthesis of the pyrrole alkaloid streptorubin by an enyne cycloisomerization is equally instructive. In addition, different manifestations of transannular hydroxyl addition reactions across alkyne partners mark the late stages of our conquests of amphidinolide F, polycavernoside A, and spirastrellolide F. Together with a few model studies and a personal selection of recent highlights from other groups, these examples augur well for future applications of π-acid catalysts in the realm of target oriented synthesis.

Activation of aryl halides at gold(I): practical synthesis of (P,C) cyclometalated gold(III) complexes

Taking advantage of phosphine chelation, direct evidence for oxidative addition of Csp(2)-X bonds (X = I, Br) to a single gold atom is reported. NMR studies and DFT calculations provide insight into this unprecedented transformation, which gives straightforward access to stable (P,C) cyclometalated gold(III) complexes.

Intriguing mechanistic labyrinths in gold(I) catalysis

Many mechanistically intriguing reactions have been developed in the last decade using gold(I) as catalyst. Here we review the main mechanistic proposals in gold-catalysed activation of alkynes and allenes, in which this metal plays a central role by stabilising a variety of complex cationic intermediates.