We typically see the use of hexaalkyl guanidinium salts for high-temperature PTC applications performed a 120 deg C and higher. The reaction shown in the diagram was performed at 75 deg C and uses tetraethyl dibutyl guanidinium chloride (TEDBG Cl). Why?
Initially, the inventors used the classical tetrabutylammonium bromide (TBAB) for this S-alkylation and the reaction indeed did work with 91.5% monomer content after workup which degraded slightly to 90.7% monomer content after 3 months of storage. However, there was residual TBAB in the product that decomposed to result in 0.54% tributylamine in the product and that impurity constitutes a significant disadvantage.
In contrast, when the more stable tetraethyl dibutyl guanidinium chloride (TEDBG Cl) was used, there was obviously no residual tributylamine contaminating the product, bis(triethoxysilylpropyl)disulfide, and only 6 ppm residual TEDBG Cl was present in the product. The monomer content was 94.7% immediately after workup and degradation after 3 months of storage was minimal to 94.4% monomer in the product.
In our 2-day course “Industrial Phase-Transfer Catalysis,” we teach a session on separating the phase-transfer catalyst from the product and we show certain cases for minimizing waste while minimizing residual phase-transfer catalyst in the product.
About Marc Halpern
Dr. Halpern is founder and president of PTC Organics, Inc., the only company dedicated exclusively to developing low-cost high-performance green chemistry processes for the manufacture of organic chemicals using Phase Transfer Catalysis. Dr. Halpern has innovated PTC breakthroughs for pharmaceuticals, agrochemicals, petrochemicals, monomers, polymers, flavors & fragrances, dyes & pigments and solvents. Dr. Halpern has provided PTC services on-site at more than 260 industrial process R&D departments in 37 countries and has helped chemical companies save > $200 million. Dr. Halpern co-authored five books including the best-selling “Phase-Transfer Catalysis: Fundamentals, Applications and Industrial Perspectives” and has presented the 2-day course “Practical Phase-Transfer Catalysis” at 50 locations in the US, Europe and Asia.
Dr. Halpern founded the journal “Industrial Phase-Transfer Catalysis” and “The PTC Tip of the Month” enjoyed by 2,100 qualified subscribers, now beyond 130 issues. In 2014, Dr. Halpern is celebrating his 30th year in the chemical industry, including serving as a process chemist at Dow Chemical, a supervisor of process chemistry at ICI, Director of R&D at Sybron Chemicals and founder and president of PTC Organics Inc. (15 years) and PTC Communications Inc. (20 years). Dr. Halpern also co-founded PTC Interface Inc. in 1989 and PTC Value Recovery Inc. in 1999. His academic breakthroughs include the PTC pKa Guidelines, the q-value for quat accessibility and he has achieved industrial PTC breakthroughs for a dozen strong base reactions as well as esterifications, transesterifications, epoxidations and chloromethylations plus contributed to more than 100 other industrial PTC process development projects.
Dr. Halpern has dedicated his adult life to his family and to phase-transfer catalysis (in that order!).
