Cyclobutadienes furans

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

It would be useful if triple bonds could be similarly epoxidized to give oxirenes. However, oxirenes are not stable compounds.Two of them have been trapped in solid argon matrices at very low temperatures, but they decayed on warming to 35 Oxirenes probably form in the reaction, but react further before they can be isolated. Note that oxirenes bear the same relationship to cyclobutadiene that furan does to benzene and may therefore be expected to be antiaromatic (see p. 58). 

Intramolecular cycloadditions between cyclobutadiene and an oxygen-tethered unactivated alkene (alkyne) offers an attractive route to benzo[c]furans (Scheme 26, <96JA9196>). 

This, the ozonide of cyclobutadiene (and also a likely peroxide of furan), may be stored in liquid nitrogen. It can explode with considerable force on warming. 

A very elegant formation of heptafulvenes from triafulvenes was found by Gompper294 utilizing the push-pull stabilized cyclobutadiene 511 in the case of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the 4-acyl group caused additional ring closure yielding cyclohepta(b)furan 514 ... 

Even fusion with four-membered rings has been achieved, at least with the smaller of the two furan projection angles as in the cyclobutadiene equivalent (32) (72JA8950) the presumably more strained (and antiaromatic ) isomer (33) remains unknown. 

Polymercuration frequently occurs. The Hg atom is not large [the non-bonded radius is 1.50-1.73 A (150-173 pm), compared, e.g., with 1.95 A (195 pm) for Br], and its electronic effect is weak. Permercuration of aromatics is possible, and not all compounds require drastic conditions, such as fusion with reactive Hg salts . Furan , pyrrole , thiophene and cyclobutadiene iron tricarbonyl are mercurated under mild circumstances. For the monomercuration of the more reactive aromatics, the extent of reaction must be limited, e.g., by using the mild HgCl2 in the presence of Na02CCH3 to buffer the solution. 

The photoisomer of a-pyrone was synthesized in order to prepare a cyclobutadiene.64 It was pyrophoric in air at room temperature and its mass spectrum was similar to that of furan. Flash thermolysis was reported to give many compounds, as shown in Scheme 26.65 No evidence for the formation of cyclobutadiene was obtained, but carbon dioxide and acetylene products seem to support its intervention. 

The photolysis of tetrafluorocyclobutenedicarboxylie anhydride obtained from hexafluoro Dewar benzene yields tetrafluorocyclobutadiene which is trapped as an adduct with furan (68) 67). This is another useful method for cyclobutadiene. 

The Diels-Alder reaction of hexakis(trifluoromethyl)benzvalene with furan had been reported 63) but not its stereochemistry. We have examined this reaction and found that it proceeds smoothly with many 1.3-dienes if the steric requirement of the olefin is not large in the transition state where exo-attack occurs as confirmed by X-ray analysis (97)96). Therefore, 2,3-dimethylbutadiene reacts smoothly with hexakis(trifluoromethyl)benzvalene whereas 1,3-pentadiene reacts very slowly and 2,4-hexadiene does not at all. The cycloaddition to cyclobutadiene gives a mixture of endo- and exo-adducts (98)97) ... 

The former assignment of the structures of the cyclobutadiene adducts of hexakis-(trifluoromethyl)benzvalene by Warrener was corrected by the X ray analysis of the bromination product of the exo-adduct98). The reaction of the adduct, formed from hexakis(trifluoromethyl)benzvalene and furan, with bromine yields a ew-adduct, probably because of the high steric requirement of the trifluoromethyl groups on the bridge-head. 

Allene Ethylene Butadiene Cyclobutadiene Furan Pyrrole Thiophene Stilbene Benzoquinone Vinyl bromide sym-Dibromoethylene (cis-trans mix.) Tribromoethylene Trichloroethylene Tetrachl or oethylene... 

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