We appreciate the input of expert process chemist, Peter Wuts, who pointed out that there is a better alternative explanation for the mode of action of the phase-transfer catalyst in the PTC-Suzuki reaction reported last month at http://phasetransfercatalysis.com/ptc_tip/ptc-suzuki-for-polymers/. Following is Dr. Wuts’ input.
“It is generally well accepted that boronates, in order to transfer to the Pd complex, require a nucleophile like water to add to the empty orbital on boron. It seems that these substrates are so “greasy” that the only way to accomplish this is with PTC and maybe the greasier and bigger catalyst is more effective. I don’t believe the quats have any influence on the palladium. The first thing that needs to happen in these reactions is the reduction of Pd(II) to LnPd(0). In this case it is probably the phosphine ligand (Ln) which is used in excess that reduces the Pd(II). Once this is accomplished, Pd(0) does oxidative addition with the bromide and then the Ar group from the boronate-nucleophile complex is transferred and then reductive elimination returns the Pd(0) which cannot react with a quat. The cycle starts over until all bromide is consumed. The [Q+ PdCl3-] complex is not relevant to the case at hand because we need to get into the Pd(0) manifold for the Suzuki to work.”
We would like to add that it may be possible that the role of the catalyst may be to deliver the required water to the reaction site by hydrogen bonding to the counteranion of the organophilic quat cation.
Peter Wuts of Wuts Chemistry Consulting (http://www.petergmwuts.com/) is one of the leading process chemists in the industry and is the author of the authoritative book “Protecting Groups: Effects on Chemical Reactivity.”
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!).
