Hexafluorobenzene Photochemistry:  Wellspring of Fluorocarbon Structures

Abstract

Ultraviolet irradiation of hexafluorobenzene in the vapor phase gives exclusively its Dewar valence isomer, from which a wealth of highly fluorinated compounds has been derived. Photoexcited in solution, hexafluorobenzene is capable of cycloaddition with a variety of unsaturated molecules. Like the Dewar benzene, resulting adducts have served as intermediates en route to many new fluorocarbons and fluorocarbon derivatives.

Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations

“…Here it should be pointed out that the reactions often involve intermediates, and the connection between the electronic state and formation of the COT vs the semibullvalene isomer is not always clear-cut. For example, octafluorobarrelene 137 photorearranges exclusively to octafluorocyclooctatetraene 138 both upon direct and sensitized irradiation (Scheme , reaction C), , suggesting that also gain in T 1 state aromaticity can function as a driver for the rearrangement. On the other hand, the napthobarrelene photochemistry was found to be dependent on the connectivity because 1,2-napthobarrelene gave the corresponding 1,2-napthocyclooctatetraene as major product (76%) upon direct irradiation while the 2,3-napthobarrelene lead to 2,3-napthosemibullvalene upon both direct and sensitized irradiation .…”

Role of the Perfluoro Effect in the Selective Photochemical Isomerization of Hexafluorobenzene

“…4 The 4π electrocyclic ring-closing reaction is known for many highly fluorinated cyclic polyenes, many of which result from cycloadditions with HFB. [6][7][8][9] HFB itself undergoes the same ringclosing reaction, 10 photochemically isomerizing to hexafluoro-Dewar-benzene (Dewar-HFB) chemoselectively. The hexafluoro-benzvalene or -fulvene isomers are not observed, but are the major products of benzene photochemistry (Figure 1a).…”

“…The 4π electrocyclic ring-closing reaction is known for many highly fluorinated cyclic polyenes, many of which result from cycloadditions with HFB. − HFB itself undergoes the same ring-closing reaction, photochemically isomerizing to hexafluoro-Dewar-benzene (Dewar-HFB) chemoselectively. The hexafluoro-benzvalene or -fulvene isomers are not observed, but are the major products of benzene photochemistry (Figure a). , HFB also shows significantly different photophysical behavior than benzene; its absorption spectrum has a broad, featureless absorption band and a substantial (100 nm) Stokes shift in the emission band, even in the gas phase .…”

Photochemical mechanistic study of hexafluorobenzene involving the low-lying states

“…The synthetic photochemistry of hexafluorobenzene (HFB) originated in 1966, 1,2 which provided an excellent starting point for new synthetic pathways, leading to a great variety of fluorinated compounds. 3 The photolysis of HFB in the liquid or gas phase in the range of 212–265 nm has been found to afford only a Dewar-form isomer, 1,4 whereas the excitation of HFB in an Ar matrix at 193 nm affords both the benzvalene and Dewar forms. 5 Except for these two well-known isomers, very little is known about other potential isomeric forms, such as the prismane or fulvene isomers.…”