Cesium Carbonate

Technical Specifications

Product Description & Scientific Applications

Cesium Carbonate (Cs₂CO₃) is an inorganic carbonate base. The carbonate is a mild base (pKa ≈ 10.3) that deprotonates O–H, N–H, S–H, and acidic C–H bonds without the harshness of alkoxide, amide or hydride bases, while the large, polarisable Cs⁺ cation improves reactivity through weaker ion pairing and better substrate-salt solubility in polar aprotic media (DMF, DMSO, NMP, acetonitrile). This "cesium effect" underpins its use across O-, N-, S-, and C-functionalisation.

O-alkylation ("cesium effect"). For Williamson-type O-alkylation of alcohols and phenols, the loose Cs⁺ ion pair makes the alkoxide or phenoxide reactive, with O/C selectivity and elimination substrate-dependent; TBAI is added when halide reactivity is limiting. Gives unsymmetric aryl alkyl ethers, dialkyl ethers, and cyclic ethers milder than NaH.

Palladium cross-coupling base. A common screening base for Suzuki–Miyaura, Buchwald–Hartwig amination, and selected Sonogashira and Hiyama systems, supporting deprotonation or transmetalation without strongly basic or nucleophilic conditions, limiting boronic-acid decomposition, α-epimerisation, and functional-group degradation.

N-alkylation. For primary amines it can give high mono-N-alkylation selectivity (substrate- and electrophile-dependent), and it deprotonates acidic N–H heterocycles — indoles, pyrazoles, imidazoles, triazoles, tetrazoles — for N-substitution with alkyl halides or sulfonates, milder than NaH or KOtBu. Regioselectivity on tautomerisable rings is substrate-dependent.

Carboxylate ester synthesis. It forms cesium carboxylates in situ — weakly ion-paired nucleophiles that alkylate with primary, benzylic, allylic, or secondary halides or sulfonates to esters, useful for acids sensitive to Fischer or acid-chloride conditions (peptides, glycosides, acid-labile protecting groups). High dilution favours macrocyclisation of seco-acids over polymerisation.

Nucleophilic aromatic substitution. With electron-deficient aryl halides, chloropyridines, and fluoropyrimidines it deprotonates the nucleophile while sparing sensitive groups, giving diaryl ethers, aryl amines, and aryl thioethers; non-activated substrates need a copper or palladium catalyst.

S-alkylation. Thiols and thiophenols are S-alkylated to thioethers under mild carbonate conditions where alkoxide or hydride bases would compromise sensitive substrates.

C-alkylation. Activated C–H substrates — malonates, β-ketoesters, and 1,3-dicarbonyls — are α-alkylated where carbonate basicity suffices and stronger bases are not required.

CO₂ utilisation and C1 transfer. It acts as base, promoter, or carbonate/C1 partner for carbonate, carbamate, urethane, cyclic-carbamate, and oxazolidinone formation.

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