Configurations and chemistry of the perfluorotricyclo[4.2.0.02,5]octa-3,7-dienes

Abstract

Irradiation at 2537 Á of perfluorocyclooctatetraene yielded antiand syn-perfluorotricyclo[4.2.0.0y]octa-3,7-dienes la and lb, respectively, which reverted quantitatively to the tetraene at 150 °C. Bromine addition occurred exclusively in the exo, suprafacial fashion with both dienes, but only the anti isomer added 2 mol of bromine. The latter fact, together with analysis of the 19F NMR spectra of the bromine adducts, constitutes convincing evidence for the configurational assignments for la and lb. Perfluorobenzene underwent [2 + 2] photocycloaddition with chlorotrifluoroethylene, and the bicyclic adducts suffered photochemical electrocyclization to a stereoisomeric mixture of 7-chlorotricyclo[4.2.0.02,5]oct-3-enes. Analysis of the 19F NMR spectra of the major isomers permitted their assignment as exo,anti and endo.anti. Reduction of the former gave la, thus confirming the diene's configuration. An explanation based on "second-order" orbital symmetry effects is suggested for the syn/anti stereoselectivity found in the thermal and photochemical electrocyclic processes described above.

Novel Keto−Enol Systems: Cyclobutane Derivatives

“…Now the fluorine at C 3 which resonated at Φ −121.8 in 10 appears as a subsplit doublet at −186.0 ppm. The doublet splitting of ∼50 Hz is caused by the geminal hydrogen, and further splitting of this signal results from through-space interaction with the endo fluorine at C 5 . Thus, the hydrogen at C 3 must be on the exo face of the molecule.…”

“…This resonance was assigned to the fluorine geminal to the tert -butyl group. The large size of this coupling constant is probably a consequence of the bulkiness of the tert -butyl group, which (crowded by the bridgehead fluorine at C 4 ) forces the geminal fluorine into close proximity to the endo fluorine at C 5 . Formation of the C-alkylation product can be eliminated by carrying out the enol ether cleavage in the presence of several equivalents of bibenzyl.…”

Electron in a cube: Synthesis and characterization of perfluorocubane as an electron acceptor

“…Several per-substituted cubanes have been synthesized through the [2+2] photocycloaddition of the corresponding octa-substituted syn -tricyclo[4.2.0.0 2,5 ]octa-3,7-dienes ( 19 , 20 ). Although it is feasible to conceive the synthesis of 1 from syn -octafluorotricycloocta-3,7-diene, its conversion to 1 has not yet been reported ( 21 , 22 ). Against this background, the introduction of multiple fluorine atoms through radical C–H fluorination using fluorine gas might seem the most promising strategy to synthesize 1 .…”

Hexafluorobenzene Photochemistry: Wellspring of Fluorocarbon Structures

“…The two isomers are easily distinguished by their behavior toward bromine. 52 In each case, addition of the halogen occurs exclusively in exo, cis fashion for steric reasons; the difference is that even at -20 °C the anti isomer starts to add a second equivalent of bromine to give 59 before reaction of the first is complete, whereas the syn stops abruptly after a single equivalent has added at room temperature. Dibromide 60 eschews further bromination because of the severe crowding which tetrabromide 61 would experience in the endo cavity of the molecule.…”

“…As noted earlier, photosensitized isomerization of 6 yields anti - 7 accompanied by a small amount of syn - 7 . The two isomers are easily distinguished by their behavior toward bromine . In each case, addition of the halogen occurs exclusively in exo, cis fashion for steric reasons; the difference is that even at −20 °C the anti isomer starts to add a second equivalent of bromine to give 59 before reaction of the first is complete, whereas the syn stops abruptly after a single equivalent has added at room temperature.…”