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Here, we present a protocol to synthesize a complex organic compound comprised of three nonplanar polyaromatic units, assembled easily with reasonable yields.
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.
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 area16-19 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 dispersion forces.29-39
The chemistry of the above mentioned nonplanar polyaromatic compounds is similar to that described for totally planar molecules, but it is sometimes difficult to find suitable conditions to achieve desired selectivities and yields.40 In this work we present the synthesis of a molecule (7) having three polyaromatic units in a few steps with good yields by applying easy and typical techniques found in every research laboratory. The molecule is of great importance because it can adopt a pincer-like conformation to establish good interactions with C6037 in solution; and it may open a research line as a potential receptor for higher chiral fullerenes thanks to the helicene linker, which is a chiral molecule due to the existence of a stereogenic axis.41-45 However, only racemic helicene will be used in this work.
At this point, the only limitation to synthesize these receptors is the preparation of helicenes and corannulenes, since they are not commercially available. But, according to new methods published elsewhere46-48 they can be obtained in suitable amounts in a reasonable short period of time.
1. Functionalization of 2,15-Dimethylhexahelicene
Figure 1. 1H-NMR spectra (500 MHz, CDCl3) of 2,15-dimethylhelicene (top) and an aliquot taken after 2 hr. New signals, corresponding to -CH2-, are depicted in a red circle (bottom). Please click here to view a larger version of this figure.
Figure 2: 1H-NMR spectra (500 MHz, CDCl3) of 4b (top) and an aliquot taken after 3 hr (bottom). Note the changes in the aliphatic region. Please click here to view a larger version of this figure.
2. Functionalization of Corannulene
Figure 3: 1H-NMR spectra (500 MHz, CDCl3) of 5a (top) and 6a (bottom). -CCH singlet is depicted in a red circle. Please click here to view a larger version of this figure.
3. Final Assembly by Click Chemistry
Figure 4: 1H-NMR spectra (500 MHz, CDCl3) of 5b (top), 6a (middle) and an aliquot taken after 2 days (bottom). Note the disappearance of -CCH signal in the crude. Please click here to view a larger version of this figure.
Corannulene (3a) and 2,15-dimethylhexahelicene (3b) could be prepared following current methods46-48 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.
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
The authors have nothing to disclose.
This work was funded by the Spanish Ministerio de Economìa y Competitividad (CTQ 2013-41067-P). H.B. acknowledge with thanks a MEC-FPI grant.
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 40-60mm. |
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. |
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