Umpolung reactivity in the direct esterification of carboxylic acids and alcohols catalyzed by sterically hindered and electron-deficient tri-tert-butylpyrimidinium B(ArF)4.

We introduce 2,4,6-tri-tert-butylpyrimidinium B(ArF)4 as a stable, crystalline organocatalyst for direct esterification of equimolar carboxylic acids and alcohols under mild conditions. The π-Lewis acidic character of the sterically hindered pyrimidinium cation allows direct esterification via Umpolung reactivity, activating alcohols as an electrophile and acids as nucleophiles.