Tetrabromo furan

An overcrowded PAH, 9,10,ll,20,21,22-hexaphenyltetrabenzo[a,c,I,n]pentacene (55), showed an interesting screw-type helicity (Fig. 15.21) [97]. An end-to-end twist of 144° was estimated from the X-ray structure of 55. Pentacene 55 was prepared by the reaction of l,3-diphenylphenanthro[9,10-c]furan 54 with the bisaryne equivalent generated from l,2,4,5-tetrabromo-3,6-diphenylbenzene in the presence of n-butyllithium, followed by deoxygenation of the double adduct with low-valent titanium. Pentacene 55 could be resolved by chromatography on a chiral support, but it racemized slowly at room temperature (t1/2 9 h at 25 °C). 

The thermal and photochemical [4 + 2] cycloadditions of o-quinones with olefinic and acetylenic dienophiles have been extensively reviewed4,5,200 and include their 4tt heterodiene Diels-Alder reactions with olefins,201-204 vinyl ethers,205 enamines,206 selected dienes,207-209 dipheny-lketenimines,210 ketenes,209,210 fulvenes,211 and selected heterocycles including furan,207-209,212 benzofuran,209,212,215 indoles,213 azepines,214 and 1,2-diazepines.214 The tetrahalo-substituted o-quinones, tetrachloro- and tetrabromo-o-quinone, generally participate in heterodiene [4 + 2] cycloadditions at an increased rate over the unsubstituted systems and generally provide higher overall yields of the Diels-Alder products.4,5 With simple olefins, the dienophile geometry is maintained in the course of the thermal [4 4- 2] cycloadditions [Eq. (52)],203,204... 

This modification has been used to synthesize the tropolones hinokitiol (2, from the adduct of tetrabromoacetone and 2-isopropylfurane) and a-thujaplicin (3, from the adduct of l,l,3,3-tetrabromo-4-methylpentane-2-one and furane). 

Tetrabromo-r,4-dibutyl-4-(dibromomethylene)spiro[furan-2(5, 6 -[3 oxabicyclo[3.2.0 heptane]-2, 5-dione, 9CI... 

The Hazards Substance Ordinance in Germany specifies the maximum level of chlorinated dibenzo-dioxins and furans that can be present in materials marketed in Germany. This has been extended to the brominated compounds. The two largest volume flame retardants decabromodiphenyl oxide and tetrabromo his-phenol A - are said to meet these requirements [1,2]. 

There are no regulations proposed or in effect anywhere around the world banning the use of brominated flame retardants. The proposed U Directive on the brominated diphenyl oxides has been withdrawn. Deca- and tetrabromo bis-phenol A as well as other brominated flame retardants meet the requirements of the German Ordinance regulating dioxin and furan content of products sold in Germany. The European search for a replacement for decabromo diphenyl oxide in HIPS has led to consideration of other bromoaromatics such as Saytex 8010 from Albemarle and a heat-stable chlorinated paraffin from ATOCH EM. The former product is more costly, and the latter, if sufficiently heat stable, lowers the heat distortion under load (HDUL) significantly. Neither approach has been fully accepted. 

Cycloheptatriene.—Synthesis. Details have been published on the preparation of specifically substituted cycloheptatrienes by the cycloadditon of cyclopropenes to thiophene-1,1-dioxides followed by expulsion of S02. Substituted tropones have been obtained from the furan adducts of bromo-oxyallyl cations e.g. treatment of l,l,3,3-tetrabromo-4-methylpentan-2-one with di-iron nonacarbonyl in the presence of furan gave adduct (141) which was converted into a-thujaplicin (142). Acid hydrolysis of the norbornyl ketal (143) followed by warming to 70 "C gave 7,7-dimethyl-cycloheptatriene. ... 

Early applications of iron cationic species resulted from the facile reaction of a, a -dibromo-(and tetrabromo) acetone with Fe2(CO)9 with formation of the cation (A). Such cationic complexes can be efficiently trapped by olefin with formation of five-membered ketones. The reaction with conjugated dienes affords a straightforward access to tropones. Furan is a particularly good acceptor of these cationic complexes whereas their addition to enamines constitutes a potentially useful pathway to prostaglandins (cyclopentenones being thus obtained). 

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