Catalysis / Homogeneous catalysis

Gold catalysis

Gold(I) and gold(III) as soft pi-acids that activate alkynes and allenes towards nucleophilic attack.

PhD

In a nutshell

Gold is famous for being unreactive, yet small amounts of gold complexes are remarkably good catalysts. Gold(I) ions behave as soft, carbon-loving Lewis acids: they grab the electron cloud of an alkyne or allene and make it attractive to nucleophiles such as alcohols, amines, water or even other alkenes. This allows rings and complex frameworks to be built in one step under mild conditions, often in air. A simple example is the addition of water to an alkyne to give a ketone, which once required toxic mercury salts and now works with a gold catalyst. Much of gold's special behaviour comes from relativistic effects: its electrons move so fast near the heavy nucleus that its orbitals change size, altering its chemistry in ways lighter metals do not show.

Why gold?

Relativistic contraction of the 6s orbital and expansion of the 5d orbitals make gold the most electronegative metal and give Au(I) a low-lying LUMO. Au(I) prefers linear two-coordination, L–Au–X, so catalytic species are cationic [\mathrm{LAu}]^+ generated by halide abstraction with Ag(I) salts or by using weakly coordinating anions (NTf_2^-, SbF_6^-). Gold(I) back-donates weakly into π* orbitals, so a coordinated alkyne carries partial positive charge on carbon. Carbene-like resonance forms exist but are minor contributors for simple alkyne complexes.

The basic cycle

1. π-Activation: the alkyne binds [\mathrm{LAu}]^+, slipping towards η^1. 2. Anti nucleophilic attack (outer sphere), giving a trans-vinylgold species. 3. Protodeauration: protonolysis of the Au–C bond releases the product and regenerates [\mathrm{LAu}]^+.

Isolable vinylgold intermediates have confirmed the anti addition.

Ligand design follows the turnover-limiting step

Electron-poor ligands (phosphites, cationic phosphines) increase Lewis acidity and speed alkyne activation; electron-rich ligands (bulky dialkylbiarylphosphines, NHCs) make the Au–C bond more basic and speed protodeauration. Kinetic studies show that strong nucleophiles with reactive alkynes are often limited by protodeauration, whereas weak nucleophiles with less reactive alkynes are limited by activation, so the best ligand depends on the substrate. Catalyst decomposition, often to Au(0) or inactive gem-diaurated species, typically forces loadings of 1–5 mol%; counterion and additive effects (the "silver effect") must be controlled.

Characteristic reactions

Hydration, hydroalkoxylation and hydroamination of alkynes and allenes.
Enyne cycloisomerisations, proceeding through cyclopropyl gold carbene-like intermediates that rearrange or are trapped.
Propargyl ester 1,2- and 1,3-acyloxy migrations generating vinyl gold carbenoids.
Oxidative alkyne functionalisation with N-oxides as O-transfer agents, giving α-oxo gold carbenes.

Enantioselective catalysis

Because the linear geometry puts the ligand far from the substrate, chiral bis(gold) complexes with axially chiral bisphosphines, bulky phosphoramidites and chiral counterion strategies (chiral phosphate anions) are used.

Au(I)/Au(III) redox catalysis

Oxidative addition to Au(I) is normally difficult, so redox cycles have required strong oxidants (Selectfluor, hypervalent iodine) or photoredox co-catalysis. Hemilabile P,N ligands that bend Au(I) have enabled oxidative addition of aryl iodides, opening cross-coupling chemistry distinct from Pd.

Key points

Relativistic effects make Au(I) a soft, carbophilic π-acid with a low-lying LUMO.
Au(I) catalysts are linear, cationic [\mathrm{LAu}]^+ species.
The cycle is π-activation, anti nucleophilic attack and protodeauration.
Electron-poor ligands speed activation; electron-rich ligands speed protodeauration.
Au(I)/Au(III) redox catalysis needs strong oxidants or specially designed ligands.

Check yourself

5 questions

1What is the preferred geometry of gold(I) complexes?
Au(I), d^{10}, strongly prefers linear L–Au–X coordination, which shapes ligand design.
2Why are Ag(I) salts often added to LAuCl precatalysts?
AgCl precipitates, leaving cationic gold with a weakly coordinating anion that can bind the alkyne.
3Nucleophilic attack on a gold-activated alkyne usually occurs how?
The nucleophile attacks the face opposite the metal, giving trans-vinylgold intermediates.
4Protodeauration is turnover-limiting in a gold-catalysed hydroalkoxylation. Which ligand change is most likely to speed the reaction?
More electron-rich ligands make the Au–C bond more basic and easier to protonate.
5Why is enantioselective gold(I) catalysis challenging?
With only two coordination sites opposite each other, the ligand's chirality is remote from the reaction centre, hence bis(gold) and chiral counterion strategies.

Recent preprints on this topic

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Matched by keyword from ChemRxiv via Crossref. Not peer-reviewed; read critically.