The synthesis of fluorinated endcaps: Part 1. the effect of C-7 fluorination on the base-catalyzed monohydrolysis of 5-norbornenyl-2,3-diesters

In an attempt to improve the thermo-oxidative stability of aerospace polyimides, we have explored the preparation of 7-fluorinated nadic endcaps. The latter are 5-norbonenyl acid esters in a 2-endo,3-endo configuration, which is required for imidization. The most obvious precursor is the corresponding diester and, hence, we carried out a preliminary study of the saponification of the symmetric dimethyl esters of various 7-mono- and 7,7-disubstituted nadic acids (5-norbornenyl-2-endo,3-endo-dicarboxylic acids). These included the 7-fluoro (7c) and 7,7-difluoro (7e) derivatives, as well as the 7,7-diprotio (7a), 7-hydroxy (7b) and 7-oxo (7d) analogs for the purpose of comparison. Contrary to previous reports, and depending on the substitution pattern and the basic conditions used, various amounts of hydrolysis and configuration inversion were observed giving various yields of 2-endo,3-endo monoacid monoesters 8, 2-endo,3-exo monoacids monoester 13, 2-endo,3-endo diacids 6, and/or 2-endo,3-exo diacids 15. Interestingly, the inversion in configuration always occurred at C₃-i.e., on the carbon α to the remaining ester-and the 2-exo,3-endo acid ester 14 was not observed. Overall, the order of increasing configuration inversion is dependant on the C₇- substituent(s) with H₂ < OH < F < F₂ < CO. This product distribution can be rationalized based primarily on electronic considerations stemming from the C₇-substituent(s), which contribute(s) to the stability of the enolate formed at C₃. These results require a novel synthetic approach to 7-fluorinated nadic endcaps.