Lewis acid catalysis

How Lewis acids activate

Coordination of a Lewis acid (LA) to a carbonyl oxygen withdraws electron density, lowering the energy of the C=O π* orbital (the LUMO). A smaller HOMO–LUMO gap to the nucleophile or diene lowers the activation barrier. Common LAs: BF_3·OEt_2, AlCl_3, TiCl_4, SnCl_4, ZnCl_2, Cu(OTf)_2, Sc(OTf)_3, La(OTf)_3.

Diels–Alder reactions

With an α,β-unsaturated carbonyl dienophile, LA coordination:

• accelerates the reaction (lower LUMO);
• raises regioselectivity, by increasing the difference in LUMO coefficients at the two alkene carbons;
• raises endo selectivity, by strengthening secondary orbital interactions and polarisation.

Example: methyl acrylate + cyclopenta-1,3-diene gives a modest endo:exo ratio thermally, but with AlCl_3 the reaction is faster and strongly endo-selective.

Other classic reactions

• Mukaiyama aldol: a silyl enol ether adds to an aldehyde activated by TiCl_4 or BF_3; the enolate equivalent is too weak a nucleophile on its own.
• Friedel–Crafts acylation: AlCl_3 generates an acylium ion; because the ketone product binds AlCl_3 strongly, more than one equivalent is needed, a reminder that product inhibition can defeat catalysis.
• Chelation control: bidentate LAs (TiCl_4, MgBr_2) bind a carbonyl and a nearby alkoxy group, fixing conformation and reversing the facial selectivity predicted by the Felkin–Anh model.

Hard and soft

Hard, oxophilic LAs (B, Al, Ti, lanthanides) bind carbonyl oxygen; soft LAs (Ag(I), Au(I), Pt(II)) bind alkenes and alkynes. Choosing the right partner is HSAB in action.

Water-tolerant Lewis acids

Lanthanide and scandium triflates do not hydrolyse irreversibly in water: their hydrolysis constants and very fast water-exchange rates leave free coordination sites for the substrate. They catalyse aldol, Mannich and Michael reactions in aqueous media and can be recovered from the water layer and reused.

Cooperative and chiral catalysts

Chiral LAs (Cu–bis(oxazolines), oxazaborolidines, Ti–BINOL) control facial attack. Heterobimetallic BINOL complexes (for example La/Li) combine a Lewis acidic metal that activates the electrophile with a Brønsted basic metal alkoxide that deprotonates the nucleophile, pre-organising both partners for enantioselective nitroaldol and Michael reactions.

Hydroamination

Lanthanide metallocene alkyls catalyse intramolecular hydroamination of aminoalkenes: protonolysis forms Ln–NHR, the alkene inserts into Ln–N, and protonolysis by another amine releases the cyclic amine. Larger Ln^{3+} ions, with more open coordination spheres, are generally faster.

Lewis acid catalysis is old, but its frontiers are lively.

Quantifying Lewis acidity. No single scale works for every reaction. Gutmann–Beckett (the ^{31}P shift of Et_3PO), the Childs method (crotonaldehyde ^1H shifts), computed fluoride and hydride ion affinities and newer global-electrophilicity indices often rank acids differently, because they probe hard versus soft bases and include steric effects to different extents. Researchers increasingly report several measures.

Main-group and highly electrophilic acids. Silylium and borenium cations, perfluoroaryl boranes and Lewis superacids (defined as stronger than SbF_5 by fluoride ion affinity) activate C–F and Si–H bonds and catalyse hydrosilylation and Friedel–Crafts reactions. Frustrated Lewis pairs, in which steric bulk prevents acid–base adduct formation, cleave H_2 heterolytically and enable metal-free hydrogenation.

Hidden Brønsted acids. Many metal triflate and halide catalysts generate traces of TfOH or HCl through hydrolysis. Control experiments with hindered bases (2,6-di-tert-butylpyridine) and with the Brønsted acid alone are needed before claiming genuine Lewis acid catalysis.

Asymmetric and cooperative design. Confined Lewis acids and Lewis-acid-assisted Brønsted acids give high enantioselectivity with small substrates. Chiral counterions paired with achiral cationic Lewis acids (silylium, metal cations) transfer stereochemical information through ion pairing. Bifunctional catalysts combining a Lewis acid with a hydrogen-bond donor or a Lewis base mimic enzymes, though rational tuning of the two sites remains hard.

Lanthanides. Rare-earth catalysts for hydroamination and polymerisation (lactide ring opening) show strong metal-size effects. Their bonding is largely ionic, which makes stereocontrol difficult with metallocenes since the chiral information is poorly transmitted; non-Cp ligand designs (bis(phenolates), aminophenolates) are being developed to tighten the chiral pocket.

Sustainability. Recyclable, water-tolerant Lewis acids, supported catalysts and replacing stoichiometric AlCl_3 in Friedel–Crafts processes with zeolites or metal triflates remain industrially relevant goals.