Recently, we have established that the electrochemical reduction of acyl halides in acetonitrile containing tetraalkylammonium salts can lead to the formation of aldehydes. Heptanoyl chloride is reduced to heptaldehyde at both carbon and mercury cathodes;' phenylacetaldehyde and hydroch~namaldehyde are produced by the electrolysis of phenylacetyl chloride and hydro&namoyl chloride, respectively, at mercury electrodes;l reduction of cyclohexanecarbonyl chloride at mercury affords cyclohexanecarboxaldehyde;3 and the reduction of trimethylacetyl chloride at either a carbon or mercury cathode generates trimethylacetaldehyde. 4 On the other hand, electrolytic reduction of phthaloyl dichloride produces 3&lorophthalide, phthalide, biphthalyl, and dihydrobiphthalide.5 These publications, which contain detailed i&rmation about experimental procedures, provide brief reviews of several previous papersG9 dealing with the electrochemistry of aromatic acyl halides. Because there have been no previous reports pertain@ to the electrolytic behavior of acid chlorides of aliphatic dicarboxylic acids, we describe here the results of a study of the reduction of glutaryl dichloride at mercury cathodes in acetonitrile comaining 0.1 M tetraethylammonimn perchlorate (TJZAP). We have found that 5-chlorovalerolactone andvalerolactone arise via reductive intramolecular cyclization of the starting material.
As shown in Figure , a cyclic voltammogram for reduction of glutaryl dichloride at a hanging mercury drop electrode exhibits three irreversible waves with peak potentials of -1.34, -1.43, and -1.75 V in acetonitrile containing 0.1 M TRAP at 100 mV s -l. A current spike appears in the cyclic voltammogram at -1.3g V which we attribute to an adsorption process; in fact, when cyclic voltammograms are recorded for reduction of the acyl halide at a glassy carbon disk, blockage of the electrode surface by an adsorbed species appears to be so effective that no current is observed on the second and all succeeding repetitive cathodic voltage scans. In addition, the current spike seen with a hanging mercury drop varies in prominence as the potential is cycled repetitively, which influences the distinctness with which the second main reduction wave for glutaryl dichloride can be seen; in some instances, the second wave is clearly delineated because the current spike is nearly absent. On the basis of our previous work concerning the electrochemical behavior of aliphatic acyl halides,lA4 we propose that the first step in the reduction of glutary~ dichloride is a one-electron process to yield a radical-anion which cyclizes intramolecularly to give the radical precursor of 5-chlorovalerolactone, whereas reductive cleavage of the carbon-chlorine bond of any electrochemically generated S-chlorovalerolactone to afford valerolactone is responsible for the second wave. We attribute the third wave for glutaryl dichloride to reduction of glutaric anhydride; a cyclic voltammogram for reduction of glutaric anhydride at a hanging mercury drop electrode reveals one irreversible wave at -1.77 V in acetonitrile containing 0.1 M TRAP at a scan