Valence-bond isomer chemistry. Part VIII. Cycloaddition reactions of derivatives of hexafluorobicyclo[2.2.0]hexa-2,5-diene, and of hexakis(trifluoromethyl)benzvalene

Abstract

Certain polyfluorobicyclo[2.2.0]hexa-2,5-dienes and related bicyclo[2.2.0]hex-2-enes are active dienophiles in the Diels–Alder reaction, and 1,3-dipolarophiles, at ambient temperatures. Furan reacts with perfluoro-1,2- and -1,3-dimethyl- and 1,2,5- and 1,3,5-trimethyl-bicyclo[2.2.0]hexa-2,5-dienes, 2H-, 2-methyl-, and 2-methoxy-pentafluorobicyclo[2.2.0]hexa-2,5-dienes, and exo-5H,exo-6H-, exo-5,exo-6- and exo-5,endo-6-dichloro-, and exo-5,exo-6-dibromo-hexafluorobicyclo[2.2.0]hex-2-enes, to form 1 : 1 adducts; the last compound also forms adducts with cyclopenta-1,3-diene and with buta-1,3-diene. 2H-Pentafluorobicyclo[2.2.0]hexa-2,5-diene forms a 2 : 1 adduct also. The apparent order of reactivity of the various double bonds is CF:C·CF₃ CF:CF > CH:CF CF:C·CH₃, CF:C·OCH₃.

Phenyl azide forms 1 : 1 adducts with perfluoro-1,3-dimethyl- and -1,3,5-trimethyl-bicyclo[2.2.0]hexa-2,5-dienes, by addition to the CF:F·CF₃ double bonds, and 2,4,6-trimethylbenzonitrile oxide forms adducts with the 5,6-dichloro- and exo-5H,exo-6H-hexafluorobicyclo[2.2.0]hex-2-enes.

Hexakis(trifluoromethyl)tricyclo[3.1.0.0^2,6]hex-3-ene very readily forms adducts with cyclopenta-1,3-diene, pyrrole, and phenyl azide, whereas hexakis(trifluoromethyl)bicyclo[2.2.0]hexa-2,5-diene is unreactive.

The variation in reactivity is ascribed to the release of various amounts of ring-strain during reaction, modified by the intervention of steric factors. Crowded endo-fluorine atoms have large ⁴J values (28–52 Hz).