Photolysis cyclobutadiene

NMR, 3, 542 oxidation, 3, 546 phosphorescence, 3, 543 photoelectron spectra, 3, 542 photolysis, 3, 549 reactions, 3, 543-555 with alkenes, 3, 50 with alkynes, 3, 50 with IH-azepines, 3, 552 with azirines, 3, 554 with cyclobutadiene, 3, 551 with cyclopropenes, 3, 550 with dimethylbicyclopropenyl, 3, 551 with heterocyclic transition metal complexes, 7, 28 29... 

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

Our next experience with a carbene was associated with the attempt to get 1,2-diphenylcyclobutadiene by photolysis of diphenylcyclopropenyldiazo-methane17. It was found that the (diphenylcyclopropenyl)carbene splits mainly into diphenylacetylene and acetylene and rearranges only to a very small extent to the wanted cyclobutadiene. 

Figures. Calculated IR spectrum a 1 1 mixture of [1,2- H2]-and [1,4- H2]-cyclobutadiene and the spectra observed by Chapman from the photolysis of either [5,6- H2]-2-pyrone or [3,6-H2]-2-pyrone. The open bars show additional weak experimental lines observed for the mixture by Michl. ...

Photolysis of 2-oxetanones gives decarboxylative cleavage to alkenes, similar to pyrolysis, but subsequent photoaddition reactions of the alkene product may lead to complex reaction mixtures. A very useful example of 2-oxetanone photolysis is that of 5-oxabicyclo[2.2.0]oct-2-en-6-one, the photoisomer of a-pyrone when it was irradiated in a argon matrix at 80 K, carbon dioxide and cyclobutadiene were formed (equation 7) (73JA1337). 

An interesting dimer derived from tetrakis(trifluoromethyl)cyclobutadiene 40 is obtained by photolysis of an ozonide precursor. The diene is stable at — 196°C but forms dimer 41 upon... 

This type of isomerization is much more common in carbonyl-containing rings. A well-known example is the generation of cyclobutadiene by photolysis of pyran-2-one with the loss of CO2. l-Methyl-2-pyridone 24 (XY = MeNC=0, Z = CH) gives 25 (XY = MeNC=0, Z = CH) l,3-oxazin-6-ones 24 (XY = 0C=0, Z = N) form the corresponding bicycles, which can eliminate CO2, and the l,2,3-benzotriazin-4-ones similarly give the corresponding benzazetones. 

Neutral (cyclobutadiene)iron complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate, and with the nitrosonium cation. Cationic nitrosyl complexes (e.g. 210) undergo ligand substitution by treatment with phosphines. Photolysis of (tetraphenylcyclobutadiene)Fe(CO)3 in THF at -40 °C is reported to give the novel bimetallic complex (214), which reacts with carbon monoxide (140 atm, 80 °C) to regenerate the starting material.An X-ray diffraction analysis of (214 R = Ph, R = t-Bu) reveals a very short Fe-Fe distance of 2.117 A. 

Photolysis of substituted (cyclobutadiene)Fe(CO)3 complexes in the presence of alkynes affords substituted benzenes (Scheme 60). From the substituent patterns of the products, it is clear that this reaction cannot be accounted for by generation of the free hgand. The proposed mechanism involves initial loss of a carbon monoxide ligand and coordination of the alkyne. Insertion of the alkyne predominantly into the... 

Delduc et al. have reported that the photolysis of S-alkyl and -acyl xanthates provides a useful source of free alkyl and acyl radicals. Photolysis of the imide (30) gives tetramethyl-cyclobutadiene (Kashima et al.). 

Photoreaction of a-pyridones give [2 + 2] dimers. The photolysis of N-methyl-a-pyridone in a dilute solution gives a bicyclic isomer, which has a mass spectrum showing m/e 52, perhaps due to cyclobutadiene (Scheme 30).64... 

The bicyclic isomer of l,2-dihydropyridazine-l,2-dicarboxylate was obtained by photolysis.107 It could be a precursor for cyclobutadiene it... 

Allenes can also be synthesized from diazocyclopropanes by chemical or photochemical [l,2,l]-elimination of nitrogen. In the thermolysis of 28 to give 30 the carbene-intermediate 29 could be trapped 18), and in the low-temperature photolysis of 31 the triplet carbene 32 could be detected by EPR-spectroscopy 17). 32 is longlived in a polycrystalline matrix and rearranges to 33 (28 %) at a temperature of —154 °C 19>. Numerous applications are included in Ref. 20). Especially noteworthy are the syntheses of stable cyclobutadienes by Masamune (90%)21) and Regitz (67%)22). 

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 attempted elimination of five-membered heterocycles from the adducts formal from phenyl azide or diazomethane and hexakis(trifluoromethyl)benzvalene was unsuccessful. The photolysis of the ozonide of the benzvalene at room temperature gives the dimer of tetrakis(trifluoromethyl)cyclobutadiene. When the irradiation is carried out in a matrix at —196 °C, the reaction mixture was coloured yellow. The... 

The adduct generated from the benzvalene and butadiene was brominated and dehydrobrominated to a cyclohexadiene which seems to be a suitable precursor of tetrahedrane. However, its photolysis yields (2+2)intramolecular cycloadducts and its thermolysis gives only a cyclobutadiene dimer (99)99). 

Corey and Streith found that the photolysis of pyrone and pyridone yields bicyclo[2.2.0]hexane analogs 133). They suggested that the product has a structure formed by formal addition of carbon dioxide to cyclobutadiene (126). 

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