Thermally Reactive Phenylethynyl-Terminated Bis (benzylester) and Bis (amide) Monomers Based on Semi-Enzymatically Produced 6-Phenylethynyl Picolinic Acid
AIR FORCE RESEARCH LAB TYNDALL AFB FL
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The synthesis and isolation of 6-phenylethynyl picolinic acid PEPCA IUPAC name 6-2-phenylethynylpyridine-2-carboxylic acid was first demonstrated when Acinetobacter sp. strain F4 was used to biotransform diphenylacetylene to the ring-fission product that underwent facile ring closure to form PEPCA in the presence of ammonium ions. Here, the structure and properties of PEPCA were confirmed by comparison with those of PEPCA that was chemically synthesized. In the chemical route, commercially available 6-bromopicolinic acid was first converted to methyl 6- bromopicolinate using methyl iodide and 1,8-diazabicyclo5.4.0undec-7-ene DBU. The resulting methylester was coupled with phenylacetylene by the palladium-catalyzed reaction to yield methyl 6-phenylethynyl picolinate, which was then hydrolyzed by sodium carbonate to afford PEPCA. Both enzymatically and chemically synthesized PEPCA were used to prepare its thermally reactive bisbenzylester and bisamide derivatives. Thus, three phenylethynyl-terminated bisamide derivatives were synthesized by treating PEPCA with 1,3- phenylenediamine, 4,40-oxydianiline or 4,40-hexafluoroisopropylidenedianiline via dicyclohexylcarbodiimide DCC-mediated amidation. The bisbenzylester derivative was prepared from PEPCA and a,a0-dibromo-p-xylene. All the intermediates and final products were characterized by FT-IR, NMR, MS and elemental analysis. The thermal properties of PEPCA and the reactive derivatives were characterized by DSC and TGA. The exothermic peaks of three bisamide derivatives were at least 20 40 8C lower than typically reported for the phenylethynyl compounds. Lowered reaction temperatures observed for the thermally-induced free radical polymerization of phenylethynyl groups were attributed to the strong electron-withdrawing capability of the pyridine moiety. Bisamide derivatives exhibited excellent thermal stability after previously cured at 300 deg C for 30 min and 350 deg C for 30 min.
- Organic Chemistry