Pyrrolidine Carboxaldehyde Hydrohalide Catalyzed Halogenations of Aliphatic Aldehydes

Abstract

Efficient methods for the direct chlorination or bromination of aliphatic aldehydes to their corresponding α,α-dihalo aldehydes in high yield and purity are unknown. Many approaches have suffered from one or more of the following, formation of byproducts, aldol condensations incomplete conversions and losses through tedious workup procedures. Early investigations have shown that the vapor phase chlorination of aliphatic ketones has produced good yields of 2-chloro ketones,⁽¹⁾ while the vapor phase chlorination of aliphatic aldehydes has resulted in the formation of acid chlorides.⁽²⁾ One of the first successful acid catalyzed chlorinations of propionaldehyde was reported by Dick⁽³⁾ who obtained 50–60% yields of 2,2-dichloropropionaldehyde accompanied by aldol condensations and polyhalogenated trioxanes. The reaction was carried out in standardized aqueous mineral acid solutions and the product was isolated by means of a laborious azeotropic distillation. The use of sulfuryl chloride in the presence of a catalytic amount of diphenyl sulfide for the chlorination of aldehydes was shown by Dalman⁽⁴⁾ to give 60–70% yields of 2-chloro and 2,2-dichlorinated aldehydes. Clean separations i.e. products free of sulfuryl chloride could only be obtained by distillation with a spinning band column at high reflux ratios.