Carbon disulfide is a nonpolar liquid, similar to supercritical carbon dioxide. When a polar inorganic anion reacts with carbon disulfide, an organophilic quaternary ammonium phase-transfer catalyst often helps.
In the first reaction shown in the diagram, 50% aqueous cyanamide is reacted with 2 equiv 50% NaOH added at a rate to maintain the temperature under 10 C. This neutralization (no excess NaOH) forms sodium cyanamide, Na2CN2 (calcium cyanamide, CaCN2, is a commercial salt that is used to prepare cyanamide by hydrolysis). The quaternary ammonium chloride is then added followed by carbon disulfide in one portion. It is reasonable to assume that the quat cation pairs with the cyanamide anion and transfers it to the CS2 phase for the nucleophilic attack to form the disodium salt “disodium N-cyanodithioiminocarbonate”.
In Example 1 in the patent, methyl tributyl ammonium chloride (MTBAC) is used as the phase-transfer catalyst. MTBAC has 13 carbon atoms which is likely enough to be effective to transfer the cyanamide anion into the nonpolar carbon disulfide. Example 1 also uses 95 g of water which is about 1.5X the combined mass of the starting materials (neutral aqueous cyanamide and 50% NaOH). The reaction to form disodium N-cyanodithioiminocarbonate is stirred overnight at room temperature.
In Example 2, the inventors chose to use a more organophilic quat, Aliquat 336 with an average of 27 carbon atoms. Although the reason is not explained, one can assume that the 27-cabon quat may be better than the 13-carbon quat for transfer of an anion into the nonpolar carbon disulfide phase. Perhaps more importantly, the inventors also chose to greatly reduce the water content which is known to enhance reactivity solid-liquid PTC systems. The then followed the reaction by GC and determined that the reaction was complete in 7 hours when using Aliquat 336 and reduced water. This is consistent with the typical behavior of solid-liquid PTC I-Reactions that we teach in our 2-day course “Industrial Phase-Transfer Catalysis.”
In the second step, S-methlyation is performed using dimethyl sulfate to form “sodium methyl N-cyanodithioiminocarbonate.” The entire reaction mixture (both phases) is used in the second step that starts with a pH adjustment. Toluene is added and the mixture is cooled. Dimethyl sulfate us added at a rate to maintain the reaction mixture at a temperature below 15 C.
Since the phases were not separated after the first step, the phase-transfer catalyst is still present during the S-methylation step.
Again, the more organophilic quat of Aliquat 336 is expected to transfer more anion to the toluene phase than the less organophilic MTBA cation. In addition, the added water in the MTBAC system should suppress some of the transfer of the reacting anion into the toluene since MTBA quat salts are usually very water-soluble and preferably distribute into the aqueous phase if the aqueous phase has low ionic strength. In comparison, salts of Aliquat 336 are typically only slightly soluble in water. A comparison of the distribution of Aliquat 336 and MTBAC between water and various organic solvents was reported by Grinstein and Halpern 25 years ago as shown at http://www.phasetransfer.com/catsep.pdf.
The effects of both quat structure and water in this system are predictable based on the content we teach in our 2-day course “Industrial Phase-Transfer Catalysis.”
If you have not yet conducted the course “Industrial Phase-Transfer Catalysis” at your company, the time has come that you do so in 2022 or 2023 so you can develop higher performance PTC processes in shorter development times and stop wasting precious constrained process R&D resources. Now contact Marc Halpern of PTC Organics to schedule this extremely valuable PTC course in-house at your company.
