Epoxide Formation by Indirect Electroreductive Coupling between Aldehydes or Ketones and Activatedgem-Dichloro Compounds

Abstract

Epoxides are prepared by indirect electroreductive coupling of carbonyl compounds (aldehydes or ketones) and activated gem-dichloro compounds. This process is more efficient with aryl ketones than with aryl aldehydes. Though yields are only moderate, this method offers the valuable advantage of avoiding the use of strong bases or peroxides.We previously reported that cyclopropanes can be formed in satisfactory yields by indirect electroreductive coupling of electrophilic olefins (i.e. a,b-unsaturated esters 1 or ketones 2 ) and activated a,a,a-trichloro or gem-dichloro compounds catalysed by complexes of copper and iron.With both a,b-unsaturated esters and ketones, the cyclocondensation on the C=C bond, leading to cyclopropanes, occurs regiospecifically, along with the formation of byproducts derived from the halo compound. With crotonaldehyde as the substrate, however, and in the presence of dichlorodiphenylmethane, two cyclic products are isolated: the cyclopropane 1a in 34% yield, along with 12% of the oxirane 1b derived from cyclocondensation on the carbonyl 2 (Scheme 1). Scheme 1It was therefore very interesting to determine the scope and limitations of this method in the formation of oxiranes from carbonyl compounds, as there are only a few methods for such cyclocondensation reactions. Indeed, oxiranes are mostly prepared by epoxidation of preformed C=C bonds. [3][4][5][6][7][8][9][10][11][12][13][14] As an alternative, the formation of oxiranes from carbonyls requires peculiar nucleophilic reagents like a-haloanions in the Darzens reaction, and in related reactions involving benzal bromides in the presence of BuLi, 15 or the fluorodesilylation of a-bromobenzyltrimethylsilane with CsF in DMF. 16 The thermal degradation of 2-halogenoalkoxytributyltin compounds also has similarities with the Darzens reaction. 17 The anions resulting from the treatment of N-tosylsulfilimines by NaH in THF 18 or by BuLi in DMSO-hexane 19 are scarcely reported in the literature. A third class of nucleophilic species includes sulfur ylides like the Corey reagent, 20,21 or reagents derived from sulfides and aryldiazomethanes in the presence of Rh 2 (OAc) 4 22 or from a-trimethylsilylbenzylsulfonium salts by desilylation with CsF in DMF. 23 Telluronium ylides can also be used. 24 Lastly, we can mention that b-hydroxy sulfides, obtained from carbonyls, can be transformed into oxiranes by alkylation at sulfur followed by treatment with a base. 25