Valence isomers, perfluoroalkyl

The triazoline adducts from benzvalene (Scheme 21)162 and diphospha-benzvalene (Scheme 22) photolyze to yield novel tetracyclic aziridine ring systems165 that are valence isomers of azepines,162 whereas that from De-war thiophene (Scheme 20) gives a novel tricyclic aziridine that desulfurizes with triphenylphosphine to yield the trifluoromethylated Dewar pyrrole (Scheme 153).15 9,160 The stabilization of these strained molecules is attributed to the perfluoroalkyl effect.159... 

Still high in this system, but pyridine derivatives containing three or four perfluoroalkyl groups gave the first very stable valence isomers corresponding to structure 122 (Eqs. 99, 100). ... 

Perfluocoalkyl groups thermodynamically destabilize double bonds and small rings, but they can kineiically stabilize highly stramed molecules [75]. This remarkable perfluoroalkyl effect has made possible the isolation of stmctures that are uncommon m hydrocarbon chemistry, especially valence-bond isomers of aromatics and heteroaromatics such as 1, 2, and 3 [108],... 

Saturated, strained, small ring systems are uniquely stabilised by the introduction of perfluoroalkyl groups, as compared with the corresponding hydrocarbon derivatives, and this has allowed the study, for instance, of many long-lived valence-bond isomers... 

After a brief survey of the history of valence-bond isomers of aromatic compounds, new syntheses and the reactions of these isomers reported in the last decade are reviewed. In the second chapter, the valence-bond isomers of homoaromatic compounds, especially benzene derivatives, are described and in the third chapter those of heterocyclic compounds. Photoreactions of perfluoroalkylated aromatic compounds afford valence-bond isomers in high yields. These isomers are very stable and useful for the synthesis of highly strained compounds. Therefore, the emphasis is put on the chemistry of trifluoromethylated benzvalenes, Dewar thiophenes, and Dewar pyrroles. 

Similar results are reported for the perfluorodimethyl compound. The introduction of fluoro-substituents and particularly perfluoroalkyl groups thus imparts a notable increase in the ease of formation and stability of valence bond isomers of aromatic compounds, and the physical properties of those involved in the light-induced interconversion of perfluoro-1,3,5- and -1,2,4-trimethylbenzenes have also been discussed by the Manchester group.14 15 The energetics of the back reaction (i.e. valence-bond isomer to benzenoid compound) have been studied, and it is interesting to note that in the conversion of Dewar acetophenone into acetophenone, the location of the excitation starts off mr -like but ends as... 

---