Tri-p-methane rearrangement
is related to the well-known di-p-methane rearrangement (Zimmerman rearrangement).
This photoreaction is for compounds having an sp3 hybridized carbon atom substituted with three p systems. Thus, the rearrangement occurs for molecules having 3-vinyl-1,4-diene units and yields 3-vinyl cyclopentenes:
The reactants with three p groups attached to the central carbon mechanistically are capable of affording both di-p-methane and tri-p-methane photoproducts. In common with the di-p-methane rearrangement, bridging of two of the p-systems affords a cyclopropyldicarbinyl diradical intermediate that opens to an allyl carbinyl diradical. This diradical has the option of closing to a three-membered or a five-membered ring:
Three-ring di-p-methane photoproducts expand to the five-ring tri-p-methane products. Therefore, kinetic was solved to determine to what extent the reaction proceeds in a two-step versus direct formation of the five-ring product. It was found that the direct route is the major one.
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