Reaction of Dicarbonates with Carboxylic Acids Catalyzed by Weak Lewis Acids: General Method for the Synthesis of Anhydrides and Esters

Abstract

The reaction between carboxylic acids (RCOOH) and dialkyl dicarbonates [(R¹OCO)₂O], in the presence of a weak Lewis acid such as magnesium chloride and the corresponding alcohol (R¹OH) as the solvent, leads to the esters RCOOR¹ in excellent yields. The mechanism involves a double addition of the acid to the dicarbonate, affording a carboxylic anhydride [(RCO)₂O], R¹OH and carbon dioxide. The esters arise from the attack of the alcohols on the anhydrides. Exploiting the lesser reactivity of tert-butyl alcohol in comparison with other alcohols, a clean synthesis of both carboxylic anhydrides and esters has been set up. In the former reaction, an acid/Boc₂O molecular ratio of 2:1 leads to the anhydride in good to excellent yields, depending on the stability of the resulting anhydride to the usual workup conditions. In the latter reaction, stoichiometric mixtures of the acid and Boc₂O are allowed to react with a twofold excess of a primary alcohol, secondary alcohol or phenol (R²OH) to give the corresponding esters (RCOOR²). Purification of the products is particularly easy since all byproducts are volatile or water soluble. A very easy chromatography is required only in the case of nonvolatile alcohols. A broad variety of sensitive functional groups is tolerated on both the acid and the alcohol, in particular a high chemoselectivity is observed. In fact, no transesterification processes occur with the acid-sensitive acetoxy group and methyl esters.

Key words

synthetic methods - anhydrides - esters - Lewis acids - magnesium salts

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4

Organic carbonates, for example, find employment as fuel additives, lubricating oils, herbicides, pesticides, plastics and solvents, and for medicinal and biological applications.

6

Ref. 5, p 281.

13

By comparison, Gooßen obtained methyl 3-phenylpropan­-oate(3ac) in 93% yield after 16 hours at room temperature by mixing acid 1a, Moc₂O (2c) and Mg(ClO₄)₂ in nitromethane (4 mL) in the ratio 1:1.3:0.01, respectively.

15

In a blank run, ethanol and dicarbonate 2a were allowed to react in the presence of 10 mol% of magnesium chloride at room temperature and, after 48 hours, no appreciable amount of carbonate was detected.

19

In contrast to magnesium perchlorate, which has a high activity for esterification (see ref. 9), magnesium chloride is unable to catalyze esterification between acid 1a and alcohol 4a in reaction times comparable with those reported in Table [2] .