Diels–Alder reactions of polyfluorocyclohexa-1,3-dienes. Part III. Addition of alkynes, ethylene, and allene to 2H,3H-hexafluorocyclohexa-1,3-diene and reaction of some Diels–Alder adducts of octafluorocyclohexa-1,3-diene with lithium aluminium hydride

Abstract

2H,3H-Hexafluorocyclohexa-1,3-diene (10), hexafluoro-2,3-dimethylcyclohexa-1,3-diene (7), and perfluorocyclohexa-1,3-diene (14) react with ethylene by 1,4-addition to give 2,3-disubstituted 1,4,5,5,6,6-hexafluorobicyclo[2,2,2]oct-2-enes (3), (6), and (5) in good yield. Dienes (10) and (14) react with allene to give polyfluoro-5-methylenebicyclo[2,2,2]oct-2-enes (9) and (15). Diene (10) reacts with alkynes to give 2,3-disubstituted 1,4,7,7,8,8-hexafluorobicyclo[2,2,2]octa-2,5-dienes together with poly-addition products. Some of the Diels–Alder adducts of (10) may be more conveniently prepared by reaction of the analogous adducts of (14) with lithium aluminium hydride; however, where trifluoromethyl occurs as a vinylic substituent the products of this reaction are polyfluoro-5-methylenebicyclo[2,2,2]oct-2-enes. Pyrolysis of alkyne adducts (11) and (18) results in the elimination of tetrafluoroethylene to give the expected aromatic product, but pyrolysis of ethylene adducts (3), (5), and (6) results in the elimination of ethylene rather than tetrafluoroethylene and the formation of mixtures of isomeric polyfluorocyclohexa-1,3-dienes.