Quat bromides are often used as a source of organic-soluble bromide to catalytically open epoxides for the purpose of attack by alcohols. In the reaction shown in the diagram, the quat bromide is used to catalyze the ring opening of an epoxide for the purpose of esterification. This esterification reaction with an epoxide catalyzed by TBAB is not commonly seen in the literature.
We wonder about the mechanism of this esterification. One possibility is that bromide of tetrabutylammonium bromide (TBAB) attacks the epoxide to open the ring and form a 1-bromo-2-alkoxy anion (bromohydrin anion). The basic alkoxy anion could deprotonate the acidic carboxylic acid of the 2,2-bishydroxylmethyl propionic acid to form the substituted propionate anion. This carboxylate anion could displace the bromide to form the ester and regenerate the free bromide anion for another catalytic cycle. The high temperature of 120 deg C is likely sufficient to induce a successful attack of the carboxylate anion on the C-Br. During the time that the bromide is covalently bound to the carbon, the quat would be free to pair with the carboxylate to perform a classical PTC esterification.
Is this a plausible mechanism, especially in the absence of a base? Do you have any better ideas of how this reaction is catalyzed by TBAB?
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!).
