Dewar benzene derivative, synthesis

Sulphur.—Desulphurizations. Pyrolyses of cyclic bis-sulphones can be used to obtain large-ring cyclophanes (e.g. Scheme 24)/ Sulpholenes undergo pyrolysis or LiAlH4-promoted SO2 elimination to give dienes, and this approach has been used in the synthesis of azulenes. A related reaction gives Dewar-benzene derivatives. ... 

Tsuji had success in the synthesis of [62]CPPA using Dewar benzenes as masked aromatic rings which can be irreversibly aromatized by irradiation in the final synthetic step (Fig. 15) [30, 33]. Tsuji s approach consists of the synthesis of Dewar benzene derivative 24 by [2+2] cycloaddition of 1,2-dichloroethylene to dimethylacetylenedicarboxylate. This adduct was then reduced and protected as the cyclohexyl ketal 26, which is resistant to photoisomerization. Elimination afforded a mix of dichlorides 27 and 28 which were easily separated by chromatography [29]. 

Gleiter and Treptow reported the synthesis of the first doubly bridged prismane derivatives 46a (n = 5) and 46b (n = 6). ° Irradiation (500 W high-pressure Hg lamp) of the Dewar benzene derivatives 45a (n = 5) at 250 nm in diethyl ether under Ar yields only polymeric material because of the instability of the initially formed [5]paracyclophane 47a, whereas irradiation at 280 nm gives the prismane 46a (15%, 72 h). When 45b (n = 6) was irradiated at 250 nm, rapid isomerization to the [6]paracyclophane 47b (100%, 1 h) took place increasing the wavelength to 280 nm led to the prismane 46b (30%, 24 h). 

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. 

The synthesis of tri-/-butyl-lP,32, 5A -triphosphinine arose from studies of the behavior of t-butylphosphaethyne (206) in the coordination sphere of transition metals. The first indication of the transition metal template catalyzed trimerization of (206) was the isolation of the vanadium-complexed 1,3,5-triphospha-Dewar benzene (207) <87AG(E)908>. The formation of the tetra-phosphabarrelene complexed to zirconium (208) implied the intermediate formation of the com-plexed tri-(-butyltriphosphinine derivative. The free barrelene could be obtained by oxidative decomplexation using C2CI6 <95AG(E)8i>. 

Soon after the first synthesis, a notable landmark, the second route to la, was achieved by Jones and Bickelhaupt they reported that the thermal valence isomerization of Dewar benzene type isomer of la, [6.2.2]propelladiene (8a), proceeded smoothly, giving la quantitatively (Scheme 2) [4]. This reaction provided an entirely new general synthetic method for [n]paracyclophanes. However, the overall sequence of reactions was not enough to give a large quantity of la at that time, because the precursor 8a was prepared as a minor product of silver-(I)-catalyzed isomerization of the bicyclopropenyl derivative 7. 

The synthesis of oxide 2 is quite complex and involves six steps.Two other perfluorinated derivatives of F-oxepine, compounds 5 and 6, were made by thermal isomerization of bicyclic compounds endo-3 and endo- and exo-A isomers of, prepared by reaction of Dewar-F-benzene with CF2(OF)2 under UV irradiation in 2% and 6% yield, respectively. At elevated temperature, both 3 and 4 undergo irreversible isomerization affording 5 and 6, respectively (Fig. 10.2). " ... 

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