The synthesis of fluorinated endcaps: Part 2. Preserving the endo,endo configuration in the monohydrolysis of 7-fluorinated nadic diesters

Fluorination should impart greater thermal-oxidative stability to the versatile norbonenyl system. Anti-7-Fluoronadic and 7,7-difluoro nadic acid esters (5-norbornenyl-2-endo,3-endo-acid esters) 5c and 5d were synthesized as precursors for a variety of uses including the manufacture of high temperature aerospace polyimides (7c and 7d). Acid ester 5c and 5d cannot be simply prepared by the saponification of symmetrical diesters 3c and 3d because of concomitant epimerization. We have developed an alternative approach, involving the preparation and monohydrolysis of the mixed t-butyl methyl diesters 8c and 8d – while maintaining the cis-endo,endo configuration around the C2 and C3 linkage. Selective ester hydrolysis of the mixed esters mediated by formic acid produced the desired acid ester 5c and 5d.

Graphical abstract7-Fluorinated 5-norbornenyl acid esters 5c,d were prepared from mixed t-butyl methyl diesters 8c,d – while maintaining the cis-endo,endo configuration around the C2 and C3 linkage.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Fluorination of norbornene endcaps should improve thermal stability of polyimides. ► Require facile approach to 7-fluoro and 7,7-difluoro-5-norbornenly-2,3-acidesters. ► Differentiation between the two carboxyls must precede fluorination. ► Mixed t-butyl methyl diesters were prepared without configuration loss. ► Selective t-butyl ester hydrolysis is mediated by formic acid.