Highly Efficient Catalysis of Retro‐Claisen Reactions: From a Quinone Derivative to Functionalized Imidazolium Salts

Abstract

A new and efficient method for the preparation of several imidazolium salts containing an ester group in the C4 position of the aromatic ring through a retro-Claisen reaction pathway between a quinone derivative and several alcohols is described. This new organic transformation proceeds in the absence of a catalyst, but it is greatly catalyzed by different Lewis acids, especially with AgOAc at a very low catalyst loading and in very short reaction times. The process takes place by the nucleophilic attack of the carbonyl groups by the alcohol functionality, thus promoting a double C-C bond cleavage and C-H and C-O bond formation. This reaction represents the first example of this type between a quinone derivative and alcohols.

Zirconium oxychloride hydrate: an efficient and reusable catalyst for retro-Claisen condensation of alcohols with 1,3-diketones

“…Significant progress was achieved when Yoichiro Kuninobu and Kazuhiko Takai reported the first example of indium triflate catalysed synthesis of esters via retro-Claisen condensation of 2,4-pentanedione with 2-phenylethanol. 19 Other metal-triflates 20 and Lewis acids based on Fe(III) 21,22 and Ag(I) 23 are found to be efficient catalysts for retro-Claisen type reactions. Zhang and co-workers discovered that the esterification of primary alcohols with 1,3-diketones can be carried out using 30 mol% t-BuONa in t-amyl alcohol.…”