Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

Summary

Here, we present a protocol to synthesize a complex organic compound comprised of three nonplanar polyaromatic units, assembled easily with reasonable yields.

Abstract

The main purpose of this video is to show 6 reaction steps of a convergent synthesis and prepare a complex molecule containing up to three nonplanar polyaromatic units, which are two corannulene moieties and a racemic hexahelicene linking them. The compound described in this work is a good host for fullerenes. Several common organic reactions, such as free-radical reactions, C-C coupling or click chemistry, are employed demonstrating the versatility of functionalization that this compound can accept. All of these reactions work for planar aromatic molecules. With subtle modifications, it is possible to achieve similar results for nonplanar polyaromatic compounds.

Introduction

Due to their special geometry, corannulene and helicenes are molecules that can adopt a structure far from planarity and give rise to interesting properties.^1-15 In the last few years, the search of molecular receptors for carbon nanotubes and fullerenes is a very active area^16-19 due, mainly, to their potential applications as materials for organic solar cells, transistors, sensors and other devices.^20-28 The excellent complementarity in shape between corannulene and a fullerene have attracted the attention of several researchers with the aim of designing molecular receptors capable of establishing supramolecular association by disper....

Protocol

1. Functionalization of 2,15-Dimethylhexahelicene

1. Dibromination of 2,15-dimethylhexahelicene
   1. Weigh 0.356 g (1.0 mmol) of 2,15-dimethylhexahelicene, 0.374 g (2.1 mmol) of freshly recrystallized N-bromosuccinimide (NBS) and 24 mg (0.07 mmol) of benzoyl peroxide (BPO) (70% wt with 30% of water as stabilizer). Place all solids in a 100 ml Schlenk flask with a magnetic stir bar. Put under nitrogen atmosphere by three cycles of gas evacuation followed by refilling with inert gas in the Schlenk line.
   2. Add 21 ml of carbon tetrachloride (CCl₄). Degas the solution by the same evacuation/refi....

Results

Corannulene (3a) and 2,15-dimethylhexahelicene (3b) could be prepared following current methods^46-48 in a straightforward fashion with very good yields (Figure 5). Both share a common molecule, 2,7-dimethylnaphthalene, as the starting material, giving rise to a divergent to convergent synthesis of the final molecule.

Discussion

Final compound 7 has been prepared after 6 steps from nonplanar polyaromatic precursors 3a and 3b with moderate to very good yields at each reaction. The main limitation observed in this route was the bromination of both nonplanar polyaromatic compounds. However, in the case of compound 4a, an important amount of free corannulene can be recovered for further uses. The synthesis of 4

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2,15-Dimethylhexahelicene	N/A	N/A	Prepared according to reference 5b,c in the main text.
Corannulene	N/A	N/A	Prepared according to reference 5a in the main text.
N-Bromosuccinimide (NBS)	Sigma Aldrich	B8.125-5	ReagentPlus®, 99%. Recrystallized from hot water.
Benzoyl peroxide (BPO)	Sigma Aldrich	B-2030	~70% (titration). 30% water as stabilizer.
Sodium azide	Sigma Aldrich	S2002	ReagentPlus®, ≥99.5%.
Gold(III) chloride Hydrate	Sigma Aldrich	50778	puriss. p.a., ACS reagent, ≥49% Au basis.
Ethynyltrimethylsilane	Sigma Aldrich	218170	98%
[PdCl2(dppf)]	N/A	N/A	Prepared according to reference 6 in the main text.
CuI	N/A	N/A	Prepared according to reference 7 in the main text.
KF	Sigma Aldrich	307599	99%, spray-dried.
(+)-Sodium L-ascorbate	Fluka	11140	BioXtra, ≥99.0% (NT).
Copper(II) Sulphate 5-hydrate	Panreac	131270	For analysis.
Carbon tetrachloride (CCl4)	Fluka	87030	For IR spectroscopy, ≥99.9%.
Dichloromethane (DCM)	Fisher Scientific	D/1852/25	Analytical reagent grade. Distilled prior to use.
Hexane	Fisher Scientific	H/0355/25	Analytical reagent grade. Distilled prior to use.
Ethyl acetate	Scharlau	AC0145025S	Reagent grade. Distilled prior to use.
Tetrahydrofuran (THF)	Fisher Scientific	T/0701/25	Analytical reagent grade. Distilled prior to use.
1,2-Dichloroethane (DCE)	Sigma Aldrich	D6,156-3	ReagentPlus®, 99%.
Methanol (MeOH)	VWR	20847.36	AnalaR NORMAPUR.
Triethyl amine (NEt3)	Sigma Aldrich	T0886	≥99%
Silica gel	Acros	360050010	Particle size 40-60 mm.
Sand - low iron	Fisher Scientific	S/0360/63	General purpose grade.
TLC Silica gel 60 F254	Merck	1.05554.0001
Monowave 300 (Microwave reactor)	Anton Para
Sonicator	Grupo Selecta	3000513	6 Litres.