Dewar benzene derivatives

The bicyclo[2.2.0]hexa-2,5-diene ring system is a valence isomer of the benzene ring and is often referred to as Dewar benzene. After many attempts to prepare Dewar benzene derivatives failed, a pessimistic opinion existed that all such efforts would be finitless because Dewar benzene would be so unstable as to immediately revert to benzene. Then, in 1962, van Tamelen and Pappas isolated a stable Dewar benzene derivative by photolysis of 1,2,4-tri(/-butyl)benzene. ... 

Fluorinated bicyclo[2 2 0]hex 5-en 2 ones prepared from Dewar benzene derivatives serve as synthons of cyclobutenes [269] Photochemical cleavage of fluonnated azacyclohexadiene in the gas phase also gives fluorinated cyclobutene [270] (equation 41)... 

There remains cyclobutadiene. It would appear that this elusive compound has finally been prepared. Pettit has reported that oxidation of cyclobutadiene-iron tricarbonyl98 with ceric ions releases a hydrocarbon which can be distilled out of the reaction vessel and thence made to react with dienophiles such as acetylenes to yield Dewar, benzene derivatives.99... 

Free 1,2,5-triphospha Dewar benzene derivative 95 is formed in two steps from 3 mol of 44a by oxidative decomplexation of the triphosphacyclohexa-l,4-diyl-2,5-diene ligand of the Hf complex 96 <1997CB1491>. An organometallic [2+1+1] cycloaddition reaction between the phosphinidene complex 97, 44a, and a coordinated CO gives access to the 1,2-diphosphacyclobutenone complex 98 (Scheme 31) <1998CEJ1917>. 

The Dewar benzene derivative l,8, 3,5-naphtho[5.2.2]propella-3,8,10-triene, Ci.sHu. also has Z = 4, and a crystallographic mirror plane passes through the central naphthalene C-C bond and the midpoints of the bonds C(8)-C(8 ), C(9)-C(9 ) and C(10)-C(10/). The measured bond lengths in the strained ring system and crystal structure are shown in Fig. 9.6.11. 

CYCLOHEXENONE. A remarkable large-scale preparation of a Dewar benzene derivative is illustrated by the preparation of HEXA-METHYL DEWAR BENZENE from dimethylacetylene. 

Interest in the photochemistry of the phthalimide systems has continued. The phthalimide derivatives (316) are phot ochemically reactive and on irradiation in acetone yields the cyclized products (317). The reaction involves hydrogen abstraction to yield the biradical (318) which subsequently bonds to afford the observed products. A recent study has examined the behaviour of the anion (319) in an attempt to reduce electron transfer processes. In t-butanol irradiation affords the solvent addition product (320) as the principal product presumably by a free radical path. Minor products (321) and (322) are also formed but are probably artefacts of the work-up procedure. Irradiation of (319) in methanol with added cyclohexene follows a different reaction path. In this system the reaction with methanol is minor while the dominant reaction is addition of the alkene to afford the adduct (323) in 20 % yield. The Dewar benzene derivative (324) is photocheraically unstable and irradiation affords t etramet hyl cyclobutadiene. ... 

The addition of the Lewis base (DM SO) to this betaine gave the tri-phospha-Dewar benzene derivatives via an unstable diphosphirene ring. 

Gleiter et al. have demonstrated that Dewar-benzene derivatives such as (118) undergo efficient conversion into prismanes on irradiation at X > 280 nm in ether solution. The reactions of these systems are substituent dependent and derivatives such as (119) are unreactive. 

Scheme 2.25. Electrocyclic rearrangements of substituted Dewar benzene derivatives and (Z)-l,3,5-hexatriene. All volumes are given in cm mol . ...

Another useful rearrangement is that of bicyclopropenyl compounds, which provides an attractive route to Dewar benzene derivatives as formulated in equations III and IV. The ratio of the products (9) and (10) formed in the latter reaction is highly dependent on the polarity of the solvent. 

A detailed spectroscopic and calorimetric study of the photorearrangements of (93) has shown that both zwitterionic and biradical species are involved in the isomerization.DCA-sensitized irradiation of the bicyclopropenyl compounds (94) results in conversion into Dewar benzene derivatives (95) and (96) and eventually into the corresponding benzene derivatives (97). The irradiation... 

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. ... 

The compound was reasonably stable, reverting to the aromatic starting material only on heating. Part of the stability of this Dewar benzene derivative could be attributed to steric factors. The t-butyl groups are farther apart in the Dewar benzene structure than in the aromatic structure. The unsubstituted Dewar benzene was prepared in 1963. 

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]. 

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