Di-pi-methane rearrangement

The di-pi-methane rearrangement is a photochemical reaction of a molecular entity that contains two π-systems separated by a saturated carbon atom (a 1,4-diene or an allyl-substituted aromatic ring), to form an ene- (or aryl-) substituted cyclopropane. The rearrangement reaction formally amounts to a 1,2 shift of one ene group (in the diene) or the aryl group (in the allyl-aromatic analog) and bond formation between the lateral carbons of the non-migrating moiety. The di-pi-methane rearrangement is a photochemical reaction of a molecular entity that contains two π-systems separated by a saturated carbon atom (a 1,4-diene or an allyl-substituted aromatic ring), to form an ene- (or aryl-) substituted cyclopropane. The rearrangement reaction formally amounts to a 1,2 shift of one ene group (in the diene) or the aryl group (in the allyl-aromatic analog) and bond formation between the lateral carbons of the non-migrating moiety. This rearrangement was originally encountered in the photolysis of barrelene to give semibullvalene. Once the mechanism was recognized as general by Zimmerman in 1967, it was clear that the structural requirement was having two pi groups attached to an sp3-hybridized carbon, and then a variety of further examples was obtained. One was the photolysis of the Mariano Compound, 3,3-methyl-1,1,5,5-tetraphenyl-1,4-pentadiene. Another was the reaction of the Pratt diene