Di-7t-methane-rearrangement

The di-7r-methane rearrangement has been studied in a sufficient number of cases to develop some of the patterns regarding substituent effects. When the central sf carbon is unsubstituted, the di-7i-methane mechanism becomes less favorable. The case of 1,1,5,5-tetraphenyl-l,4-pentadiene is illustrative. Although one of the products has the expected structure for a product of the di-7t-methane rearrangement, labeling with deuterium proves that an alternative mechanism operates ... 

However, some substrates, generally rigid bicyclic molecules, (e.g., barrelene, which is converted to semibullvalene) give the di-7t-methane rearrangement only from triplet states. 

The di-7t-methane rearrangement of bicyclic systems is particularly extensive and fruitful in leading to products with three-membered rings. One typical example is that of naphthobarrelene which affords the corresponding naphthosemibullvalene (equation 11). 

A particularly intriguing set of reactions involving bicycling steps as well as a di-7t-methane rearrangement and a 1,4- (2,3) fragmentation is outlined in Scheme 1228. Here there is a superficial paradox. Thus, diradical D is formed in two ways. It formed from the benzo di-7i reactant (labeled Di-7t ) as well as the benzo bicyclic reactant labeled Bicyclic in Scheme 12. However, diradical D reacts differently depending on its mode of formation. 

DFT and ab initio calculations have shown that oq/S-unsaturated acyl radicals and a-ketenyl radicals are not resonance forms but are rapidly interconverting isomers separated by a low but not negligible energy barrier of <22 kJmol-1.64 Bi-cyclo[2.2.2]octenones (60) bearing an endo vinyl substituent have been shown to give diquinane products (61) on triplet-sensitized irradiation the reaction has been described as occuring via an oxa-di-7t-methane rearrangement. 

Dura, R.D. and Paquette, L.A. (2006) Ring contraction of bridgehead sultams by photoinduced di-7t-methane rearrangement. Journal of Organic Chemistry, 71 (6), 2456-2459. 

Zimmerman, H.E. and Cotter, B.R. (1974) Substituent effects and the di-7t-methane rearrangement. Mechanistic and exploratory organic photochemistry. Journal of the American Chemical Society, 96 (24), 7445-7453. 

Inspection of the triptych of Fig. 15 shows that the Sj state of the cyclopropyl-dicarbinyl diradical is necessarily formed when starting with the pentadiene, that is, in the Di-7t-Methane rearrangement. This is seen in two ways. In the MO triptych of Fig. 14, we see no HOMO-LUMO crossing in this wing of the triptych. In the state triptych of Fig. 15, the surfaces then do not approach one another in this wing. Hence there is no mechanism for interconversion to ground state enroute to the diradical from the St diene. 

A diversion of the general pathway in the photochemical di-7t-methane rearrangement has been found to lead to dihydrofurans. When one of the alkene moieties carries one or two carbonyl groups, this reaction pathway becomes dominant (Equation 66) <20020L1155>. 

Photochemical 27t-27t cycloaddition of benzo[l,2- 4,5-6 Jdithiophene and substituted acetylenes have been used to synthesize cyclobuta[6]thieno[2,3 /][I]benzothiophenes 103 <03JOC8258>. Both disubstituted acetylenes and diynes have been shown to undergo this type of 27t-27i photoaddition and moreover the addition was found to undergo via a syn addition. Photoirradiation of tris(2-benzo[6]thienylmethane) 104 in acetonitrile undergoes a di-7t-methane rearrangement to produce a cyclopropane, which upon further irradiation is easily converted to a secondary product, a thiopyran 105 <03TL751>. 

Dirnethyl-l,5-diphenylpenta-l,4-diyne (8) does not undergo a photochemical di-7t-methane rearrangement like that of the corresponding diene (9). However, the enyne 10 does give products of the di-7c-methane type, and the reaction is stereo-specific " , as can be seen from the reaction of 10 (equation 14) and of its cis isomer 11 (equation 15). 

Instead of the amide (350), the corresponding methyl imidate also could be used for the photoreaction. In this case, the labeling studies uniquely identified the process as a di-7t-methane rearrangement with initial vinyl-to-vinyl coupling " ... 

Naphthobarrelenes " and pyrazinobarrelenes afford mixtures of semibullvalene photoproducts, as both vinyl-vinyl and aryl-vinyl bridging occurs. Semibullvalenes can also be produced from the light-induced isomerization of benzobicyclo[4.2.0]octa-2,4,7-trienes. Their formation can be rationalized by a di-7t-methane rearrangement involving initial bridging between C5 and C7 of the two vinyl portions of the trienes. ... 

One of the first examples of the di-7t-methane rearrangement was found in the photochemical conversion of bicyclo[2.2.2]octa-2,5,7-triene (barrelene, 1) to tricyclo[5.1.0.0 ]octa-2,5-diene (semibullvalene, 2). ... 

Direct irradiation of 2,5-dihydrofuran (175) results in the formation of furan, tetrahydrofuran, and the isomeric oxiran (176). Certain 2,5-dihydrothiophen derivatives are similarly converted to the corresponding vinylthiirans, whereas 5,5-diaryl-2,5-dihydrofurans are reported to undergo di-7t-methane rearrangement. An unexpected transannular phototransformation has been observed on irradiation of the l,3-dioxolen-2-one (177)... 

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