Facile and Efficient One-Pot Synthesis of Highly Functionalized Thieno[2,3-b]thiopyran-4-ones from β-Keto ε-Xanthyl Phosphonates

The reaction of potassium xanthates with five-membered cyclic carbonates: selectivity of the underlying cascade reactions and mechanistic insights

In this paper we describe the reaction between various potassium xanthates (potassium O-methyl xanthate, potassium O-isobutyl xanthate, xanthate functionalized MPEGs, etc.) and common five-membered cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC). The reaction was carried out under catalyst-free conditions. Intensive evolution of both CO2 and COS and the simultaneous formation of a rich precipitate were found to be the characteristic features of the studied reaction. It was determined that the precipitate consists of various alkoxides, including potassium ethane-1,2-bis(olate) (EC-based reaction) and potassium propane-1,2-bis(olate) (PC-based reaction) and alkoxide-terminated sulfides. It was also demonstrated that the resulting liquid phase (the mother liquor) contains polyalkylene sulfides whose number average molecular weight (M n) was found to be in the range 400-550 Da and 300-400 Da for EC- and PC-based oligomers, respectively. Further studies revealed that the distribution between major products varies considerably with variation in the parameters of the reaction. Thus, by applying reduced pressure conditions and a temperature equal to 90 °C, up to 95% selectivity towards the formation of alkoxide-terminated sulfide (e.g., potassium 1,1'-thiobis(propan-2-olate)) was achieved. On the other hand, selectivity towards alkoxide formation equal to 98% was achieved for the reactions carried out in the presence of water. As a general trend, it was also established that a shift in balance between products towards the formation of sulfur-containing products (sulfides) occurs with an appropriate increase in the temperature of the reaction. Based on the obtained experimental data supplemented with quantum-chemical calculations (NBO analysis and scanning of the potential energy surface), the mechanisms of the cascade reactions underlying the formation of key intermediates and final products were also proposed.