Solvent-Free Synthesis of α-Cyanophosphonates from β-Nitrostyrenes using a Deep Eutectic Solvent

Abstract

New α-Cyanophosphonates, which are useful reagents for Horner-Wittig reaction, were synthesized in a solvent-free condition, using choline chloride-zinc chloride deep eutectic solvent (DES) as a catalyst. This work is only the second report on the synthesis of these compounds. In the previous report, diethyltrimethylsilylphosphite was used as a reagent and TiCl4 as a catalyst while in this study, both employed reagent and catalyst (triphenyl phosphine and choline chloride-zinc chloride DES) are cheaper, more available and less harmful than the previous work. Moreover, the reaction involves an interesting cascade reaction between β-nitrostyrenes and two equivalents of triphenylphosphite led to the desired product as a new synthetic route. These compounds can be used in the pharmaceutical and agricultural industries, in addition to their synthetic applications in the preparation of α,β-unsaturated nitriles. The reactions were completed using 20 mol% of DES at 80 °C in 6 hours. 10 different β-nitrostyrenes were synthesized in 55-87% yield after purification. β-nitrostyrenes containing electron-donating groups showed higher yields. The reaction was failed when aliphatic and heteroaromatic nitroalkenes and β-nitrostyrenes with electron-withdrawing substituent were employed. Finally, three plausible mechanistic routes were proposed for the reaction, starting from the nucleophilic addition of triphenylphosphite to α-carbon, nitrogen and oxygen atoms.