Versatile electrochemically based preparation of unusual Grignard reagents containing electrophilic substituents

Upon electrochemical reduction of potassium perchlorate in dimethyl sulfoxide to potassium the strong dimsyl base (i.e. the conjugate base of dimethyl sulfoxide) is formed in the reaction of the alkali metal with the solvent. The electrolysis is performed in an undivided cell with a magnesium rod serving as sacrificial anode, thereby generating magnesium ions, which stabilize the generated dimsyl through ion-pair formation. In a second non-electrochemical step, Grignard type reagents R2Mg are obtained simply by letting the magnesium salt of dimsyl base deprotonate weakly acidic substrates (pKa < 26 in dimethyl sulfoxide) added to the solution. The advantage of generating these reagents through a deprotonation route rather than by reduction of halogen-substituted compounds as in the classical approach is that the substrates may contain electrophilic groups, such as halogen, carbonyl or cyano. The R2Mg reagents react with electrophiles akin to ordinary Grignard reagents as illustrated by the preparation of several substituted benzylidenefluorenes in nucleophilic addition reactions. The electrochemical properties of the products were investigated and a reasonable linear correlation of the first reduction potentials with the Hammett substituent coefficient σ− was found.