Di-re-methane rearrangement

The di-7t-methane rearrangement has been studied in a sufficient number of cases to develop some of the patterns regarding substituent effects. When the central sp carbon is unsubstituted, the di-7t-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-re-methane rearrangement, labeling with deuterium proves that an alternative mechanism operates ... 

In a number of compounds there exists the possible choice between an oxa-di-7t-methane and a di-7r-methane rearrangement. Both modes of reaction are found. With 15c [Eq. (7)] only the oxa-di-n-methane product 17 c is obtained and not the corresponding di-Ti-methane product. 20) On sensitized irradiation of 42 with R=H, 44a is formed, which is the 1,2-acyl shift product and not 44b and/or 44c which are the benzo-vinyl-methane rearrangement products. 44) On the other hand, 45 gives only 46, a di-ir-methane reaction in which one of the participating double bonds is part of an a,P-enone, and none of the oxa-di-re-methane or benzo-vinyl methane products 47 and 48, respectively, are formed.46). In the same way 49 only gives 50, and not the oxa-di-Tc-methane or the divinylmethane product. 45) in the reaction of 51 to 52 it has not yet been established whether an oxa-di-Tc- or a di-7i-methane pathway is taken. 46)... 

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