Stable enols from Grignard addition to 1,2-diesters: serendipity rules

The temperature-dependent formation of a remarkably stable enol from the reaction of EtMgBr with a 1,2-diester was accidentally discovered. This compound was spectroscopically characterized (1H and 13C NMR, IR), and both methyl carbonate and trimethylsilyl ether derivatives were prepared. A mechanism for the selective formation of the stable enol ester and its corresponding keto form was suggested, and the kinetic stability of the enol was also documented. The generality of the observation of such a stable enol ester was demonstrated with the use of other Grignard reagents, and also other 1,2-diesters. The reaction of EtMgBr with a series of 1,2-amide esters also produced stable enol amides. The remarkable stability of the enol esters was attributed to the steric hindrance present in the aryl ester moiety of these compounds, and further studies will address the origin of this effect.