[584-08-7] · CK2O3 · Potassium Carbonate · (MW 138.21)
Physical Data: mp 891 °C; d 2.43 g cm-3; hygroscopic until 16.4% water is absorbed; pH of saturated aqueous solution is 11.6.
Solubility: 1.12 g mL-1 in water at 20 °C; practically insol alcohol.
Form Supplied in: white solid; widely available. Use in form supplied for best results.
Purification: crystallize from water 70 °C; dry over H2SO4.
Handling, Storage, and Precautions: keep in a tightly sealed container; avoid inhalation.
The mildly basic nature of this reagent makes it ideal for the selective deprotonation of organic acids. This property makes K2CO3 a useful reagent for the alkylation of phenols. The procedure2 for phenol alkylation involves dissolving the phenol and alkylating agent in acetone with excess K2CO3 added as an insoluble component (eq 1). The reaction is run at reflux. Workup consists of removal of solvent and addition of water. The product is isolated by either extraction or filtration. Popular alternative solvents are DMF3 and MeCN.4 If the aqueous solution needs to be neutralized, it is convenient to remove excess K2CO3 via filtration before solvent removal and subsequent water addition.
Sulfides can also be alkylated using K2CO3 in DMF, as demonstrated in the shell closure reaction used to capture guests within linked anisyl moieties.5
Ester hydrolysis using K2CO3 in MeOH was found to give clean conversion to the allylic alcohol (eq 2).6
When amides that contain a,b-amino alcohols undergo diazotization followed by treatment with aqueous K2CO3,7 they are cleanly converted to the epoxide (eq 3), while the b-amino ester gives the corresponding alcohol (eq 4).
The enolate of b-keto esters can be alkylated in a procedure developed by Claisen8 that is analogous to the alkylation of phenols. The b-keto ester and alkylating agent are combined with K2CO3 in acetone and the reaction heated to reflux. The reaction is useful for the C-alkylation of substrates. In the case of ethyl 2-oxocyclopentanecarboxylate9 the reaction gave good yields and high selectivity for C-alkylation with unobstructed alkylating agents (eq 5).
The reagent also shows good selectivity in the monoalkylation of b-dicarbonyl compounds. The reaction of 2-chloro-2-methylproponal with malonic esters in THF or ether gives good yield of the monoalkylated malonic ester (eq 6).10
Aldehydes can be reacted with 1,3-dicarbonyl compounds in THF to give a,b-unsaturated carbonyl compounds with good (Z/E) selectivity (eq 7).11
The reagent is also useful for the generation of intramolecular Michael addition products with excellent stereoselectivity (eq 8).12
Wittig reactions can be done either under heterogeneous (water-1,4-dioxane) conditions13 or more conveniently using 18-crown-6 as a phase-transfer catalyst (see Potassium Carbonate-18-Crown-6).14 The phase-transfer conditions give alkenes in good yields with a preference for the (E)-isomer (eq 9). The procedure involves the combination of alkylphosphonium salt, K2CO3, aldehyde, and 18-Crown-6 in either CH2Cl2 or THF. The solvent is removed, product extracted with petroleum ether, and filtered through silica gel.
b-Ketosulfonium salts react with K2CO3 to generate the corresponding ylide. a-Methylthio ketones are reacted with allylic halides to afford sulfonium salts that are reacted with aqueous K2CO3 to generate a sulfur ylide15 that can undergo [2,3]-sigmatropic rearrangement (eq 10).
Kurt D. Deshayes
Bowling Green State University, OH, USA