The stereochemistry of the Michael addition

Abstract

The nature of the products obtained in the addition of diethyl malonate to 4-t-butyl-1-cyanocyclohexene under various conditions of solvents, catalysts, and temperature, and their geometries have been determined. In protic solvents, and under conditions of kinetic control, the main product is diethyl cis-t-butyl-1-cyano-2-cyclohexylmalonate (XII), while under conditions of thermodynamic control the main product is the trans-isomer IX. In a non-protic solvent and in the absence of ethoxide ions, the main product is that of an “abnormal” Michael addition, ethyl 4-t-butyl(e)-2-carbethoxymethyl(a)-1- cyano(a)cyclohexanecarboxylate(e) (XXI). It is concluded that in protic solvent the solvated malonate anion is a relatively weak nucleophile and that product development control operates so that the intermediate with the equatorial malonate residue predominates. Protonation from the lease hindered side takes place under kinetic control. In a non-protic solvent the unsolvated malonate anion is a strong nucleophile and the transition states for addition will resemble the ground state. The axial intermediate rearranges rapidly and irreversibly to XXI so that in the absence of a proton donor this product accumulates. No unrearranged axial cyanomalonic ester was ever detected in these reactions. The mechanism ofthe “abnormal” Michael addition is discussed.