Bicalicene

Bicalicene is polycyclic hydrocarbon with chemical formula C₁₆H₈, composed of two cyclopentadiene and two cyclopropene rings linked into a larger eight-membered ring.^[1] There are two isomers: cis-bicalicene and trans-bicalicene. It is a dimer of calicene.

trans-Bicalicene

Names
IUPAC names (4aZ,8aZ)-Dicyclopenta[a,e]dicyclopropa[c,g][8]annulene Dicyclopenta[a,e]dicyclopropa[c,g]cyclooctene
Identifiers
CAS Number	73091-52-8 Y
3D model (JSmol)	Interactive image
ChemSpider	4423214
PubChem CID	5257656
UNII	494N9YP93C Y
InChI InChI=1S/C16H8/c1-3-9-10(4-1)14-8-16(14)12-6-2-5-11(12)15-7-13(9)15/h1-8H/b13-9-,14-10-,15-11-,16-12- Key: FVDGMBFXSFSQOU-NONDJZLBSA-N InChI=1/C16H8/c1-3-9-10(4-1)14-8-16(14)12-6-2-5-11(12)15-7-13(9)15/h1-8H/b13-9-,14-10-,15-11-,16-12- Key: FVDGMBFXSFSQOU-NONDJZLBBM
SMILES C1=CC2=C3C=C3C4=CC=CC4=C5C=C5C2=C1
Properties
Chemical formula	C16H8
Molar mass	200.240 g·mol−1
Melting point	125–130 °C (dec)[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

cis-Bicalicene

Names
IUPAC name Dicyclopenta[a,c]dicyclopropa[e,g]cyclooctene
Identifiers
CAS Number	122605-73-6
3D model (JSmol)	Interactive image
PubChem CID	101619525
InChI InChI=1S/C16H8/c1-3-9-10-4-2-6-12(10)14-8-16(14)15-7-13(15)11(9)5-1/h1-8H Key: MIGVZWUAYKWALJ-UHFFFAOYSA-N
SMILES C1=CC2=C3C=C3C4=CC4=C5C=CC=C5C2=C1
Properties
Chemical formula	C16H8
Molar mass	200.240 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Synthesis

Bicalicene is prepared by treatment of 1,2-bis(tert-butylthio)-3,3-dichlorocyclopropene with cyclopentadiene anion, followed by desulfurizing stannylation with tributyltin hydride, and then treatment with silica gel.^[1]

Properties

The four aromatic rings of trans-bicalicene

trans-Bicalicene is polycyclic aromatic hydrocarbon, which is unusual for a 16 π electron ring system. Viewed as a unified ring structure, Hückel's rule predicts it would be anti-aromatic (4n π electrons). Instead, however, the structure has a dominant partially-delocalized charge-separated structure consisting of four independently-aromatic (4n+2 π electron) rings: two as cyclopropenyl cations (two π electrons each) and two as cyclopentadienyl anions (six π electrons each).^[2]

cis-Bicalicene, by contrast, is an antiaromatic hydrocarbon.^[3] A resonance structure with four aromatic rings, analogous to the one that makes the trans isomer stable, would suffer from destabilizing charge effects, and other resonance structures have 4n rather than 4n+2 π electrons in at least one ring.

References

1. Yonedo S, Shibata M, Kida S, Yoshida Zi, Kai Y, Miki K, Kasai N (January 1984). "A Novel Aromatic Hydrocarbon with 16 π-Electron Periphery: "Cyclic Bicalicene"". Angewandte Chemie International Edition in English. 23 (1): 63–64. doi:10.1002/anie.198400631.
2. Oziminski WP, Palusia M (2013). "Capturing the elusive aromaticity of bicalicene". Physical Chemistry Chemical Physics. 15 (9): 3286–3293. Bibcode:2013PCCP...15.3286O. doi:10.1039/C2CP43426A. PMID 23358331.
3. Jelena Đurđević Nikolić; Ivan Gutman (2016). "A Comparative Study of the Two Isomers of Bicalicene". Polycyclic Aromatic Compounds. 38: 1–7. doi:10.1080/10406638.2016.1140659. S2CID 99213944.