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Π‐Extended Diaza[7]Helicenes by Hybridization of Naphthalene

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Π‐Extended Diaza[7]Helicenes by Hybridization of Naphthalene Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202003402 & Organic chemistry |HotPaper| p-Extended Diaza[7]helicenes by Hybridization of Naphthalene Diimides and Hexa-peri-hexabenzocoronenes Carolin Dusold,[a] Dmitry I. Sharapa,[b] Frank Hampel,[a] and Andreas Hirsch*[a] Abstract: The synthesis of an unprecedented, p-extended mation of the diaza[7]helicene moiety in the final Scholl oxi- hexabenzocorene (HBC)-based diaza[7]helicene is presented. dation is favoured, affording the symmetric p-extended heli- The target compound was synthesized by an ortho-fusion of cene as the major product as apair of enantiomers. The sep- two naphthalene diimide (NDI) units to aHBC-skeleton. A aration of the enantiomers was successfully accomplished combination of Diels–Alder and Scholl-type oxidation reac- by HPLC involving achiral stationary phase. The absolute tions involving asymmetric di-NDI-tolane precursor were configuration of the enantiomers was assigned by compari- crucial for the very selectiveformation of the helical super- son of circular dichroismspectrawith quantum mechanical structure via ahexaphenyl-benzene (HPB) derivative. The for- calculations. Introduction can notably tune optical, electronical and supramolecular properties.[10] Elongation of the spiral arrangementofthe con- The development of nonplanar PAHs is currently an emerging jugated p-systemrepresents the most common way to pre- field in synthetic organic chemistry.[1] Among the most impor- pare higher helicene derivatives.[11] In contrasttothat, the ex- tant starting points for this progress is the discoveryofthe pansion of the conjugatedsystem on the outer rim is still a parenthelicenes.[2] Sincethen, research groups all over the challenging issue. Recently,alot of progress has been made world further elaborated the investigation and understanding on the lateral extension of helicenes,[10a,12] including p-extend- of helicene chemistry,with great success.[3] Outstandingchi- ed helicenes based on perylenediimide,[13] pyrene[14] and hexa- roptical properties,[4] such as inducing the emission of circularly peri-hexabenzocoronene(HBC).[15] Especially the use of HBC polarizedlight, large optical rotation values and pronounced subunits enables the design of new „superhelicenes“,[15a] repre- circulardichroism bands make them very attractive for applica- senting new proto-types in the field of chiralnanographene. tions in nonlinearoptics,[5] asymmetric synthesis,[6] chiroptical Such helicallytwisted graphenes can be considered as promis- switches,[7] and organic semiconductors.[8] Next to the classical ing candidates in the field of chiralelectronics,[16] not only due carbohelicenes, heterohelicenes,which include one or more to their outstanding chiroptical properties, but also due to an heteroaromatic rings such as pyridine, pyrrole, and thiophene enhanced p–p stacking relative to the parenthelicene, which are receiving growing interest and it was realized that their is crucial to achieve efficient charge carriertransportation,[17] property combinationscan even surpass those of the conven- and an improved solubility compared to their planar ana- tional all-carbon analogues.[9] Furthermore, the p-extension of logues.[18] In this contribution, we will expand the concept of the basic helicene skeleton is of considerable interestsince it HBC-based helicenes by introducing additional aromaticsubu- nits to furtherextend the conjugated p-system. We have se- [a] C. Dusold, Dr.F.Hampel,Prof. Dr.A.Hirsch lected naphthalene diimides (NDIs)aspromising candidates Department of Chemistry and Pharmacy since they feature desirable chemical and (opto)electronical Friedrich-Alexander University Erlangen-Nürnberg [19] Nikolaus-Fiebiger-Straße 10, 91058 Erlangen (Germany) properties themselves. As aconsequence, we accomplished E-mail:[email protected] an ortho-fusion of two NDI units to an HBC-skeleton, generat- [b] Dr.D.I.Sharapa ing an unprecedented, p-extended HBC-based heterohelicene. Institute of Catalysis Research and Technology The high degree of pre-organization of the diaza[7]helicene Karlsruhe Institute of Technology subunit results in the formation of achiral compound with Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germa- ny) two NDI arms lying on top of each other (Figure 1). Supporting Information and the ORCID identification number(s) for the au- This helical superstructure combines not only the advantag- thor(s) of this article can be found under: es of lateral p-extended helicenes with those of azahelicenes, https://doi.org/10.1002/chem.202003402. at the same time, the fusion of two NDI arms enables the gen- 2020 The Authors. Published by Wiley-VCH GmbH. This is an open access eration of an extensive conjugated p-surface all over the mole- article under the terms of Creative Commons Attribution NonCommercial- cule. Photophysicalinvestigations were carried out and the NoDerivs License, which permits use and distribution in any medium, pro- vided the originalwork is properly cited, the use is non-commercial and no corresponding enantiomers were separated in order to deter- modifications or adaptations are made. mine their chiroptical properties. Chem. Eur.J.2021, 27,2332 –2341 2332 2020 The Authors. Published by Wiley-VCH GmbH Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202003402 were obtained, and the quality of the X-ray analysiswas suffi- cient for unambiguous structure assignment (see Supporting Information S2 for furtherinformation). Thus, asecure assign- ment of the orange fraction to the 5 isomer is possible as de- picted in Figure 2. The boc-protected NDI-tolane 6 synthesised by Sonogashira reaction was obtained in apure form in good yields of 82% (Scheme 2). The boc-protecting group was removed using [21] Figure 1. General structure of achiral, p-extended diaza[7]helicene based on TFA to afford the free diamino functionality.Due to the sen- [22] HBC andNDI. sitivitytowards oxidation of such aminotolane compounds, the crude product 7 was used directly in the next condensa- tion reaction with 8 without further purification or characteri- Results and Discussion sation to generate di-NDI-tolane precursors 1 and 9 with an overall yield of 56%. The new fused naphthalene benzimida- Our strategy to synthesise the p-extended HBC-based diaza[7]- zole moiety is once again formed as two constitutional iso- helicene HBC 3 relies on the combination of standard Diels– mers. One theone hand, asymmetrical compound 1 is formed Alder and Scholl-type reactions (Scheme 1). in which the introduced naphthalene monoimide is connected The key intermediate of our synthetic concept is the tolane to the tolane in abinding pattern similar to the already exist- derivative 1 containing two symmetrically fused NDI units. The ing naphthalene-benzimidazole unit, highlighted in blue in symmetric constitution of the corresponding di-NDI-hexaphen- Scheme 2. On the other hand, an asymmetrictolane is formed yl-benzene (HPB) (2)iscrucial to generate the desired 7-helical when the naphthalenemonoimide is fused to the tolane in an moiety in the final HBC. The synthetic sequence started with alternative binding pattern, highlighted in red. The asymmetric the preparation of the symmetric precursor 1 as illustratedin tolane 9 is not asuitableprecursor for the targeted p-extend- Scheme 2. ed HBC-based diaza[7]helicene HBC 3,since planarization of The first step was the palladium-catalysed Sonogashira the corresponding asymmetrical HPB 10 would not resultin cross-coupling reactionbetween tert-butyloxycarbonyl (boc)- the desired 7-helical structure in the end. Thesymmetrical di- protectedalkyne 4 and 4-bromo-1,2-naphthaleneimidebenzi- NDI tolane 1 with both NDI units fused to it through the blue midazole (5). The requiredboc-protectedalkyne buildingblock binding pattern is essential for the formation of the helical su- 4 was prepared according to literature procedures,[20] starting perstructure in the final compound.Unfortunately,isomers 1 with the protection of diamino-bromobenzene (11)toenable and 9 could not be separated at this stage.Hence, the fused the successful performance of the subsequentSonogashira di-NDI-tolane was reactedasamixtureofisomersinthe subse- coupling reactions, followed by deprotection of the TMS-group quent Diels–Alder reactionwith tetrakis-(tert-butyl)tetracyclo- (see Supporting Information S1 forsynthetic details). The ne[15a, 23] at 2208Ctogenerate the corresponding di-NDI HPBs 2 second buildingblock NDI-compound 5 was prepared via con- densation of naphthalene monoimide (8)with diamino-bromo- benzene (11)(Scheme 3). During this reaction, two constitu- tional isomers are formed, 5 as well as 5-bromo-1,2-naphthale- neimidebenzimidazole (12). The two compounds can easily be separated by column chromatography,isolatingthe first yellow fraction in 42%yield, and the second orange fractionin36% yield. Althoughthe NMR and opticalspectra of both com- pounds differ significantly as shown in Scheme 3(for further information see SupportingInformation S2), clear structural as- signment was not possible at this point. However,crystalsof the orange fraction suitable for X-ray diffraction analysiswere grown by slow evaporation of asolutionofthe compound in Figure 2. Crystalstructure of 4-bromo-1,2-naphthaleneimidebenzimidazole dichloromethane (DCM) at room temperature. Small needles (5). Scheme1.Synthetic concept to generate 7-helical HBC 3. Chem. Eur.J.2021, 27,2332 –2341 www.chemeurj.org 2333 2020 The Authors. Published by Wiley-VCH GmbH
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