Di-7t-methane

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 photoreactions of four-membered nitrogen containing heterocycles have been less well investigated and meaningful results are confined to certain azetidines and azetidinones. The photochemically induced rearrangement of azetines 148 to 1,4-diazepines 149 has been rationalized in terms of a process equivalent to a di-7t-methane rearrangement116 a more... 

Another aspect of the reaction is the stereochemistry. There are three points of interest— C-1, C-3 and C-5 in a 1,4-pentadiene type di-7t-methane reactant. If C-5 is taken as the carbon of that vinyl group which remains in the photoproduct, that stereochemistry—cis or trans—is retained32. The reaction stereochemistry at C-3 (the methane carbon ) has been shown to be inversion of configuration33-34 (equations 9a and 9b). The situation at C-l is... 

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. 

The di-7T-methane reaction is fairly general with molecules which contain two double bonds separated by two single bonds of the same carbon atom (Figure 4.38). 

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. 

Janz, K.M. and Scheffer, J.R. (1999) The use of ionic chiral sensitizers in the crystalline state application to the di-7t-methane photorearrangement of a benzonorbornadiene derivative. Tetrahedron Letters, 40 (50), 8725-8728. 

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. 

Zimmerman, H.E. and Factor, R.E. (1981) Mechanistic and exploratory organic photochemistry. 126. Di-7t-methane hypersurfaces and reactivity multiplicity and regioselectivity relationship between the di-7t-methane and bicycle rearrangements. Tetrahedron, 37 (Suppl. 9), 125-141. 

Within the area of SET-promoted di-7t-methane reactions, recent studies have shown that irradiation of 1-aza-1,4-dienes, such as 32, and the 1,4-diene 34, using AW-dimethylaniline (DMA) as electron-donor sensitizer, leads to production of the corresponding cyclopropane derivatives 33 and 35 resulting from 1-ADPM and DPM rearrangements, respectively, in reactions that take place via radical-anion intermediates (Sch. 11) [25]. 

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>. 

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