"Sigmatropic shiftamers" are polymers that contain local "defects" - single or double bonds - which propagate via sigmatropic rearrangements. An example of a [3,3]-shiftamer is shown above in which a pair of parallel pi-bonds propagates via sequential boatlike [3,3]-shifts (Cope rearrangements). Since this process is predicted to occur with a low activation barrier (~15 kcal/mol) this system is expected to be fluxional at room temperature. Our search for additional [3,3]-shiftamers also led to the discovery of a new class of compounds called "sigma-polyacenes".

We have also explored additional shiftamers based on [1,5], [1,7], and [1,8] shifts of hydrogen. Several examples are shown above. We have also been interested in the relationships of transition structures for these sorts of [1,n] sigmatropic shifts to transition structures for electrocyclic reactions and to stable 3-center 2-electron carbocations.

Tantillo, D. J.; Hoffmann, R. Angew. Chem. Int. Ed. 2002, 41, 1033-1036: "Sigmatropic Shiftamers: Fluxionality in Broken Ladderane Polymers"

Tantillo, D. J.; Hoffmann, R. J. Am. Chem. Soc. 2002, 124, 6836-6837: "Helicoid Shiftamers"

Tantillo, D. J.; Hoffmann, R. Helv. Chim. Acta 2003, 86, 3525-3532: "Helicoid Shiftamers for the Transport of pi-Clumps and Charges," part of a special issue in honor of Duilio Arigoni.

Tantillo, D. J.; Hoffmann, R. Eur. J. Org. Chem. 2004, 273-280: "Hydrogen Migration Over Organic Tapes: [1,5] Sigmatropic Shiftamers"

Tantillo, D. J.; Hoffmann, R.; Houk, K. N.; Warner, P. M.; Brown, E. C.; Henze, D. K. J. Am. Chem. Soc. 2004, 126, 4256-4263: "Extended Barbaralanes - Sigmatropic Shiftamers or sigma-Polyacenes?"

Tantillo, D. J.; Hoffmann, R. Acc. Chem. Res. 2006, 39, 477-486: "Snakes and Ladders. The Sigmatropic Shiftamer Concept"

 

Dean J. Tantillo and Roald Hoffmann: "Sigmatropic Shiftamers." Lecture presented by Roald Hoffmann at the 228th ACS National Meeting, Philadelphia, PA, August 22-26, 2004; paper ORGN 11.

 

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