Hexahelicene

1.Introduction

Hexahelicene (Phenanthro[3,4-c]phenanthren; [6]Helicene) is a molecule composed of six ortho- condensed benzene rings. The rings aren’t arraigned planar, because the terminal benzenes occupy the same room (the image above isn’t correct). They are constructed as a helix and so they get the name “Helicene”. The structure allows the molecule to do a left- or a right turn.

Another nomenclature for helicenes, which only consists of carbon-atoms, is all-benzene-helicene. In this case it is possible to name it by indicating the number of benzene rings in box brackets and ending with the structure description “Helicene”. To differentiate the enantiomers it is necessary to use the CIP-nomenclature

The two enantiomers of [6]Hexahelicene (1):

It is important to look at the helix axes to define chirality. If it turns to the left, away from the viewer, it is a M (minus) helix. If it turns to the right, it is a P (plus) helix. It is a special feature of helicenes to be chiral, without having a chirality center.

Author: Kerstin Bathon, Christin Hamm, Ruven Jilly, Benjamin Betz The orientation of [6]helicene could be shown schematic by using three levels (2).

X-ray crystallography illustrates an angel between the blue and the red level of 58.5°. In the molecule-core the six carbon- carbon bounds are 1.437+0.004 Å, in opposite to the carbon-carbon bounds, which are parallel and peripheral, they are 1.334+0.007 Å long.

2. Physical Dates

Molecular formula: C26H16 Molar mass: 328,413 g/mol Melting point: 230 + 1°C Density: 1.2715 + 0.0035 g/ml Crystal structure: Rhombic 25°C Amount of rotaion [α]D : 3640° (-) in CHCl3 3707° (+)

CAS-No.: 187-83-7

Author: Kerstin Bathon, Christin Hamm, Ruven Jilly, Benjamin Betz 3. Synthesis of hexahelicene

This paragraph shows us different synthesizes for preparing hexahelicene.

The first helicene was synthesized by Melvin S. Newman and Daniel Lednicer in 1955:

The condensation of 1-naphthaldehyde with Diethylmanolate afforded (2) in 70% yield. The yields of (3) by addition of the grignard reagent from 1-bromonaphthalene to (2) varied unpredictably between 35% and 56%. Reduction of (3) afforded pure (4) in 90% yield. Conversion of the latter to (6) was carried out in high over-all yield without purification of (5) or the corresponding dinitrile. Interestingly, the hydrolysis of the dinitrile by sulfuric in acetic acid was incomplete even after prolonged refluxing. However, alkine hydrolysis for one hour in ethylen glycol at reflux afforded the acid (6) in high yield. Cyclization of (6) to (7) bye anhydrous hydrogen fluoride proceeded in

Author: Kerstin Bathon, Christin Hamm, Ruven Jilly, Benjamin Betz 65-70% yield, the remaining product being a neutral glass. When pure (7) was dissolved in hydrogen fluoride it could be quantitatively recovered. The only fair yield obtained in this type of cyclization which ordinarily proceeds in high yield foreshadowed the increasing difficulties which were to be met in the synthesis because of the increasing steric strains gradually being introduced into the molecule. The ketoacid (7) was reduced to (8) in 87% yield by the Huang-Minlon procedure. There was found in this case, as well as in several others, that the long periods of heating often recommended are not necessary. The cyclization of the reduced acid (8) proved difficult. The ring closure of the acid chloride of (8) to (9) in 55% yield was finally accomplished by heating at 90° for one hour with stannic chloride in ODCB. The conversion of (9) to hexahelicene (11) was effected by several different routes, the best of which comprised reduction to the hexahydro derivative (10) followed by hydrogen transfer to benzene over 5% rhodium-on-alumina at 300°. The arise from (7), they must have the same stereochemistry unless isomerism occurred during the reaction of (7), a possibility that is unlikely.

Eleven years later, Bogaert-Verhoogen and Martin synthesized [6] helicene by using liquid potassium, as you see at the illustration.

The reagent could be made by an addition reaction from 2-(3- Phenanthro)-aceticacid and 1-bromnaphthalene-2-aldehyde in sodium acetate. In this case the yield is 44%

Today hexahelicene is synthesized with higher yield by an oxidation photocyclization of a stilbene- type in phenanthrene. This reaction was discovered by Wood and Mallory in 1964 and was the breakthrough for preparing [6] helicene in a important scale.

This illustration shows the photocyclization of hexahelicene:

Author: Kerstin Bathon, Christin Hamm, Ruven Jilly, Benjamin Betz In the year 2005 Shawn K. Collins, Alain Grandboi, Martin P. Vachon and Jolie Côté synthesized hexahelicene by using metathesis reaction for ring closure. In a simple way metathesis can be pictured like this:

A-B + C-D → A-D + C-B

It is necessary to use a catalytic transition-metal-complex to reach this reaction. Robert Grubbs developed an air stable Grubbs-Catalyst. This metathesis-reaction is an important tool for organic- chemistry.

The illustration shows the reaction to hexahelicene with the metathesis-reaction.

In CH2Cl2 under microwave irradiation 60min, 80% yield

in Benzene at 40°C, 24h, 70% yield

The rate is 100% (identified by NMR) with a yield of 80%/70%

Author: Kerstin Bathon, Christin Hamm, Ruven Jilly, Benjamin Betz 4. Dividing the racemates

A special optical active π-complexing-agent were synthesized to divide both racemates: TAPA (Tetranitrofluorenylidenaminooxypropionic acid). This molecule forms a charge-transfer-complex with electron-rich aromatic compounds.

The making of TAPA:

TAPA only builds charge-transfer-complexes with (+)-hexahelicene and stays in a solution of benzene/ethanol. The opposite is (-)-hexahelicene. It is indissoluble.

References:

- K. Peter C. Vollhardt, Organische Chemie; 1 Auflage; 1988; page 1189 - R. H. Martin, Angew. Chem. 1974, 86, 727; Angew. Chem.-Int. Edit. Engl. 1974, 13, 649 - http://www.uni- saarland.de/uploads/media/Aromaten_und_Heteroaromaten_WS_2008- 2009.pdf - http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/12/oc/vlu_organik/stereoch emie/weitere_chiralitaetselem.vlu/Page/vsc/de/ch/12/oc/stereochemie/helicale_ chiralitaet/helicale_chiralitaet.vscml.html - Journal; Hahn; ACCRA9; Acta Crystallographica; 11; 1958; 825; DOI: 10.1107/S0365110X58002310; ISSN: 0365-110X - Newman, M.S. &Lednicer, D. (1956). J. Amer.Chem. Soc. 78, 4765 - Angew. Chem. Int. Ed. 2006, 45, 2923 –2926

Author: Kerstin Bathon, Christin Hamm, Ruven Jilly, Benjamin Betz