Sumanene

Sumanene is a polycyclic aromatic hydrocarbon and of scientific interest because the molecule can be considered a fragment of buckminsterfullerene. Suman means "sunflower" in both Hindi and Sanskrit.^[1] The core of the arene is a benzene ring and the periphery consists of alternating benzene rings (3) and cyclopentadiene rings (3). Unlike fullerene, sumanene has benzyl positions which are available for organic reactions.

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Organic synthesis

The structure of Sumanene can be inferred from oxidation of 1,5,9-trimethyltriphenylene but the first practical synthesis starts from norbornadiene.^[2] Norbornadiene is converted into a stannane by action of n-butyllithium, dibromoethane and tributyltinchloride. A Ullmann reaction of this stannane with CuTC affords the benzene core. The methylene groups created in this conversion then migrate in a tandem ring-opening metathesis and ring-closing metathesis by the Grubbs' catalyst. The final structure is obtained by oxidation by DDQ.

Properties

Sumanene is a bowl shaped molecule with a bowl depth of 1.18 angstroms (118 picometers).^[3] The 6 hub carbon atoms are pyramidalized by 9° and the molecule displays considerable bond alternation (138.1 to 143.1 pm). Sumanene also experiences bowl-to-bowl inversion with an inversion barrier of 19.6 kcal/mol (82 kJ/mol) at 140 °C which is much higher than that found for its corannulene cousin. Like any benzylic proton, the sumanene protons can be abstracted by a strong base such as t-butyl lithium to form the sumanene mono carbanion. This strong nucleophile can react with an electrophile such as trimethylsilylchloride to the trimethylsilyl derivative.

References

1. ^ Towards the design of tricyclopenta [def, jkl, pqr] triphenylene (sumanene): a bowl-shaped hydrocarbon featuring a structural motif present in C₆₀ (buckminsterfullerene) Goverdhan Mehta, Shailesh R. Shahk and K. Ravikumarc Journal of the Chemical Society, Chemical Communications, 1993, (12), 1006 - 1008 Abstract
2. ^ A Synthesis of Sumanene, a Fullerene Fragment Hidehiro Sakurai, Taro Daiko, and Toshikazu Hirao Science, Vol 301, Issue 5641, 1878 , 26 September 2003 Abstract
3. ^ Structural Elucidation of Sumanene and Generation of Its Benzylic Anions Hidehiro Sakurai, Taro Daiko, Hiroyuki Sakane, Toru Amaya, and Toshikazu Hirao J. Am. Chem. Soc., 127 (33), 11580 -11581, 2005 Abstract