Michael addition using dialkyldithiocarbamate and xanthate derivatives as organocatalysts

The Michael addition is an effective tool for carbon-carbon bond formation. Recently, our research group has developed the Michael addition of beta-nitrostyrenes in water using a salt of N,N-dialkyldithiocarbamate (DTC) as novel organocatalysts with a broad scope of substrates and high yields of Michael adducts. However, the reaction rates were slow; we hypothesized that the reaction rates were hindered by competitive hydrogen bonding between the DTC organocatalyst and water. Furthermore, DTC salts are prone to oxidize in the presence of oxygen. Hence, we herein investigated the Michael additions using the DTC salts as organocatalysts in organic solvents as a reaction medium. Xanthate salts are less prone to oxidization and have also been screened as organocatalysts for Michael additions, as they possess a retained nucleophilicity between sulfur atoms and structural similarity with DTC organocatalysts. The six DTC salts used in this study, i.e., sodium N,N-diethyldithiocarbamate (NaDTC-C2), sodium N,N-dihexyldithiocarbamate (NaDTC-C6), sodium N,N-didodecyldithiocarbamate (NaDTC-C12), diethylammonium N,N-diethyldithiocarbamate (AmDTC-C2), dihexylammonium N,N-dihexyldithiocarbamate (AmDTC-C6), didodecylammonium N,N-didodecyldithiocarbamate (AmDTC-C12), were conveniently prepared by condensation between N,N-dialkylamine and carbon disulfide in ethanol, and three xanthate organocatalysts, i.e., sodium ethyl xanthate (NaXanthate-C2), sodium hexyl xanthate (NaXanthate-C6) and sodium dodecyl xanthate (NaXanthate-C12) were prepared by the treatment of corresponding alcohols with carbon disulfide and sodium hydroxide in THF. The Michael additions between trans b-nitrostyrenes and 1,3-dicarbonyl compounds were carried out in nonpolar, protic, and aprotic polar organic solvents. The NaDTC-C2 demonstrated the highest catalytic activity in acetonitrile, providing 80% yield of the Michael adduct within 30 minutes, compared to 7 hours when carrying the reaction in water. The accelerated reaction rate is proposed to result from less hydrogen bonding between the reaction medium and the DTC organocatalyst. Similarly, the NaXanthate-C2 exhibited excellent catalytic activity in acetonitrile, affording 91% yield of Michael adduct within 2 hours. Furthermore, the S-methylation of the DTC and xanthate salts increased oxidative stability. Still, the catalytic activity of S-methylated DTC and xanthates was found to be lower for the Michael additions.