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[7]Helicene to a Planar Hetero[8]Circulene Bodil Lousen,[A] Stephan K

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[7]Helicene to a Planar Hetero[8]Circulene Bodil Lousen,[A] Stephan K Communication Chemistry—A European Journal doi.org/10.1002/chem.201905339 & Non-Planar Aromatics Compressing aNon-Planar Aromatic Heterocyclic [7]Helicene to a Planar Hetero[8]Circulene Bodil Lousen,[a] Stephan K. Pedersen,[a] Pernille Bols,[a] Kasper H. Hansen,[a] MichelleR.Pedersen,[a] Ole Hammerich,[a] Sergey Bondarchuk,[b] Boris Minaev,[b] Glib V. Baryshnikov,[c] Hans gren,[c, d] and MichaelPittelkow*[a] cally chiral moleculeshave found vast applications in such di- Abstract: This work describes asyntheticapproachwhere verse fields as molecular recognition,catalysis anddetection or anon-planar aromatic heterocyclic [7]helicene is com- emissionofcircularlypolarized light.[2–5] Structurally related pressedtoyield ahetero[8]circulene containing an inner motifstothe [n]helicenes are the [n]circulenes. These struc- antiaromatic cyclooctatetraene (COT) core. This [8]circu- tures also consist of ortho-annulatedaromatic rings, but in- lene consists of four benzene rings and four heterocyclic stead forms acyclic structure, having acentral ring with n rings, and it is the first heterocyclic[8]circulenecontaining sides.Itisillustrative to view the all-benzene series of [n]circu- three differentheteroatoms. The synthetic pathway pro- lenes,which consistsofthe bowl-shaped carba[5]circulene ceeds via athe flattened dehydro-hetero[7]helicene, (corannulene), the planar carba[6]circulene(coronene) andthe which is partially ahelicene and partially acirculene:itis saddle-shaped[7]- and [8]circulenes. Hetero[8]circulenes incor- non-planar and helically chiral as helicenes, and contains a poratingfour five-membered heterocycles(furans,pyrroles, COT motif like [8]circulenes. The antiaromaticity of the thiophenes)are planar,[6,7] and these structures can be viewed COT core is confirmed by nucleusindependent chemical as containingantiaromaticcyclooctatetraene(COT) cores. shift (NICS) calculations. The planarizationfrom ahelically [n]Circulenes mayalso, instead of being viewed as two annu- p-conjugated[7]helicene to afully planar heterocyclic lenes connected by single bonds, be viewedas[n]radialenes, [8]circulene significantly altersthe spectroscopicproper- which are n-membered rings with n exocyclic double bonds. ties of the molecules.Post-functionalization of the [7]heli- The differencebetween being an [n]circuleneand an [n]radia- cenes and the [8]circulenes by oxygenation of the thio- lene, where each end of the exocyclic double bonds is con- phene rings to the corresponding thiophene-sulfones nected by an etheno-bridge or aheteroatom, is formally adis- allows an almost complete fluorescence emission cover- cussionofresonance structures. age of the visible region of the opticalspectrum (400– The first tetrahetero[8]circulenes were prepared by Erdtman 700 nm). and Hçgberg during their studies of the oligomerization of substituted benzoquinones 1.This yieldedthe tetraoxa[8]circu- lenes 2,via aLewis acid mediated tetramerization (Figure1 Helicenes are aromatic compounds in which angularly ortho- top).[8] annulated rings ultimately form astereogenic axis. Helicenes Inspired by this work we reported that 3,6-dihydroxycarba- exist as all-benzene molecules and as heterocyclic compounds, zoles 3,upon treatmentwith chloranil and boron trifluoride di- in which one or more benzene rings are replaced with hetero- ethyl etherate, can be dimerized into the corresponding diaza- cycles such as furans, thiophenesand pyrroles.[1] These intrinsi- dioxa[8]circulenes 4 which contain two pyrroles (Figure 1top). The introduction of four pyrroles was accomplished by Osuka [a] B. Lousen,Dr. S. K. Pedersen, P. Bols, K. H. Hansen, M. R. Pedersen, and co-workers by an oxidative “fold-in” fusion reaction of a Prof. O. Hammerich, Prof. M. Pittelkow Department of Chemistry,University of Copenhagen tetrapyrrole compound 5 providing tetraaza[8]circulene 6 [9] Universitetsparken5,2100 CopenhagenØ(Denmark) (Figure 1top). Recently also strategies for introductionof E-mail:[email protected] thiophene has been described.[10] [b] Dr.S.Bondarchuk, Prof. B. Minaev In this work, we describe how diazathia[7]helicene 7 can be Department of Chemistry and Nanomaterials Science converted into the intermediary dehydro[7]helicene 8 and di- Bohdan Khmelnytsky National University,18031 Cherkasy (Ukraine) azaoxathia[8]circulene 9 by asequential annulation protocol [c] Dr.G.V.Baryshnikov,Prof. H. gren DivisionofTheoretical Chemistryand Biology (Figure 1bottom). The dehydro[7]helicene resemblesboth the SchoolofEngineering Sciences in Chemistry, Biotechnology and Health hetero[8]circulene and the hetero[7]helicene by havingacyclo- KTH Royal Institute of Technology,10691, Stockholm (Sweden) octatetraene motif incorporated into anear helical structure. [d] Prof. H. gren The hetero[8]circulene 9 contains one thiophene, one furan College of Chemistryand Chemical Engineering and two pyrroles,and is the first compound in the hetero[8]cir- Henan University,Kaifeng,Henan475004 (P.R.China) culenefamily to include three different heterocycles.[11] Incor- Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: porationofthiopheneallowsfor convenientoxygenation and https://doi.org/10.1002/chem.201905339. the preparation of the closely related sulfones 7-SO2, 8-SO2 Chem. Eur.J.2020, 26,4935 –4940 4935 2020 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Communication Chemistry—A European Journal doi.org/10.1002/chem.201905339 Scheme1.Preparation of diazaoxathia[7]helicene (7). the 2,4’-coupled isomer (for more details see Supporting Infor- Figure 1. Top: Overview of previously reported strategies for synthesis of mation). Interestingly,noformation of furan by oxidative dehy- hetero[8]circulenes. Bottom: This work:Synthesisofahetero[8]circuleneby dration was observed, despite this being the major product for asequentialannulation strategy. 2,7-di-tert-butylanalogues of hydroxycarbazole 11.[6,14] To in- stall the annulating thiophene heterocycle, attention was turned to the thermal Newman–Kwart rearrangement reaction, and 9-SO2.Sequential annulation and oxygenation are found which has proven usefulinsynthesis of thia[n]helicenes from to be an efficient method for tuning the optical properties. biarylic 2,2’-Newman–Kwart substrates (bis-O-aryl-thiocarba- Both annulation and oxygenation cause abathochromic shift mates).[19] Heating bis-O-arylthiocarbamate (13)at3008Cfor in absorbance as wellasemission.Together, the compounds 25 minutes gave both the rearranged product (14)and the di- emissions cover almostthe entire visible spectrum. Computa- azathia[7]helicene (15). Underoptimized conditions, the [7]heli- tional experiments predict that the dehydro[7]helicenes 8 and cene 15 was isolated in 40%yield. Debenzylation of [7]heli- 8-SO2 have higherracemization barriers than the parent cene 15,using DMSO and KOtBu under an oxygen atmos- [15] [7]helicenes 7 and 7-SO2.NICS calculations showsignificant phere, provided thiadiaza[7]helicene (7)in97% yield. With paratropic and antiaromatic character in both the COT motifs the [7]helicene (7)inhand, its derivatization to avariety of oxi- are present in the dehydro[7]helicenes 8 and 8-SO2 and in the dation products was explored (Scheme 2). Treatment with [8]circulenes 9 and 9-SO2. BF3·OEt2 and chloranil gave the cyclodehydrogenated dehy- Thiadiaza[7]helicene 7 was synthesized in five steps from dro[7]helicene (8)quantitativelyinanintramolecular Scholl [6] [16,17] 3,6-dimethoxy-carbazole 10 (Scheme 1). Mono-demethylation type reaction. Demethylation using BCl3 and TBAI, before of 10 to 11 wasfollowed by dimerizationinto bicarbazolediol treatment with BF3·OEt2 and chloranil, severely changes the re- (BICOL, 12)in95% yield by an oxidative coupling reaction activity,enabling furan formation via an oxidative dehydration mediated by chloranil and BF3·OEt2,with complete regioselec- reaction, providing diazaoxathia[8]circulene (9). For further de- tivity for the 4,4’-coupled product. This high degree of regiose- rivatization,oxidation of the thiophene heterocycles into their lectivity is asignificant improvement to the previous reported corresponding sulfones was explored. Oxidation with m-CPBA, [12,13] synthesis of BICOL motifs. Omission of BF3·OEt2 did not gave the corresponding sulfones(7-SO2, 8-SO2 and 9-SO2, result in conversion, and increasing the temperature above Scheme 2). 08C, resulted in aloss of regioselectivity due to formationof Chem. Eur.J.2020, 26,4935 –4940 www.chemeurj.org 4936 2020 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Communication Chemistry—A European Journal doi.org/10.1002/chem.201905339 dramatically,asthe terminiofthe helicene are forced towards each other,which inherently causes some planarization of the molecules. Thisisreflected in the inner dihedrals of the com- pounds, which decrease from 15.98 to 6.68 when going from 7 to 8 and from 13.18 to 5.78 when going from 7-SO2 to 8-SO2. The methoxy substituents preclude complete planarization in the dehydro[7]helicenes (8 and 8-SO2), whereas the furans in- troducedin9 and 9-SO2 allows forplanarization of the [8]cir- culenes (9 and 9-SO2). The planarity of these heterocyclic [8]cir- culenes is consistent with what has previously been observed for related hetero[8]circulenes.[11] The optical properties of the series were investigated by UV/ Vis andfluorescencespectroscopy in THF (Figure 3). Looking first at the effect of planarization of the motif, it is observed that going from the [7]helicene (7)through dehydro[7]helicene (8)tothe planar [8]circulene
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