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Anti‐Aromatic Versus Induced Paratropicity ... -.:. Michael Pittelkow

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Anti‐Aromatic Versus Induced Paratropicity ... -.:. Michael Pittelkow Angewandte Forschungsartikel Chemie Deutsche Ausgabe:DOI:10.1002/ange.201913552 Circulenes Internationale Ausgabe:DOI:10.1002/anie.201913552 Anti-Aromatic versus Induced Paratropicity:Synthesis and InterrogationofaDihydro-diazatrioxa[9]circulene with aProton Placed Directly above the Central Ring Stephan K. Pedersen, Kristina Eriksen, Nataliya N. Karaush-Karmazin, Boris Minaev, Hans gren, Gleb V. Baryshnikov* und Michael Pittelkow* Abstract: We present ahigh-yielding intramolecular oxidative coupling within adiazadioxa[10]helicene to give adihydro- diazatrioxa[9]circulene.This is the first [n]circulene contain- ing more than eight ortho-annulated rings (n > 8). The single- crystal X-raystructure reveals atight columnar packing, with aproton from apendant naphthalene moiety centred directly abovethe central nine-membered ring. This distinct environ- ment induces asignificant magnetic deshielding effect on that particular proton as determined by 1HNMR spectroscopy. The origin of the deshielding effect was investigated computation- ally in terms of the NICS values.Itisestablished that the deshielding effect originates from an induced paratropic ring current from the seven aromatic rings of the [9]circulene structure,and is not due to the nine-membered ring being antiaromatic.UV/Vis spectroscopyreveals more efficient Figure 1. Simplified illustration of the influence of diatropic and para- conjugation in the prepared diazatrioxa[9]circulene compared tropic ring currents on the 1Hchemical shift inside and outside of to the parent helical azaoxa[10]helicenes,and DFT calcula- select aromatic, anti-aromatic, and non-aromatic rings. tions,including energy levels,confirm the experimental observations. benzene (1)while the 4n rule describes anti-aromatic systems Introduction such as planarized cyclooctatetraene (2;Figure 1).[4] Gener- ally speaking, aromatic compounds are chemically stable, Aromaticity and anti-aromaticity are phenomena that while anti-aromatic compounds are unstable.Studies of have received significant attention both from experimental planar, thus anti-aromatic,cyclooctatraene (2)are only and from theoretical chemists over the years.[1–3] Thefamous possible in the realm of computational chemistry.[5] In recent Hückel 4n + 2rule describes aromatic compounds such as years,several examples of chemically stable anti-aromatic molecules have been described, including arange of planar cyclooctatetraenes imbedded in [8]circulene structures (3), [*] Dr.S.K.Pedersen, K. Eriksen, Prof. M. Pittelkow nor-corrole structures,and indeno[1,2-b]fluorene struc- UniversityofCopenhagen tures.[6–10] Aprevalent strategy to stabilize anti-aromatic Department of Chemistry rings,within amolecule,istoanneal the anti-aromatic Universitetsparken 5, 2100 Copenhagen Ø(Denmark) [3,11] E-Mail:[email protected] segment with aromatic rings. This is the case for diazadioxa[8]circulene 3,where the eight aromatic rings form Dr.N.N.Karaush-Karmazin, Prof. B. Minaev [8] Department of Chemistry and Nanomaterials Science aplanar cyclooctatetraene motif (Figure 1). From acompu- Bohdan Khmelnytsky National University tational perspective,aconvenient determination of aroma- 18031 Cherkasy (Ukraine) ticity is anegative nuclear independent chemical shift (NICS) Prof. H. gren, Prof. G. V. Baryshnikov value due to the diatropic ring current, while anti-aromatic Division of Theoretical Chemistry and Biology rings have positive NICS values as aconsequence of apara- School of Engineering Sciences in Chemistry,Biotechnologyand tropic ring current.[12,13] Theinfluence of these ring currents Health, KTH Royal InstituteofTechnology on 1Hchemical shifts is illustrated for 1 and 2 in Figure 1. The 10691 Stockholm (Sweden) NICS predictions for molecule 3 are difficult to test exper- E-Mail:[email protected] imentally,by1HNMR spectroscopy,asitrequires the Prof. H. gren College of Chemistry and Chemical Engineering preparation of compounds with aproton placed directly Henan University,Kaifeng, Henan 475004 (P. R. China) above the aromatic or anti-aromatic ring under investiga- [14] Supportinginformation and the ORCID identification number(s) for tion. Theobtained NICS values are convoluted by the the author(s) of this article can be found under: influence of the aromatic ring(s) used to anneal and stabilize https://doi.org/10.1002/anie.201913552. the central postulated anti-aromatic ring that they form.[15–17] 5182 2020 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Angew.Chem. 2020, 132,5182 –5188 Angewandte Forschungsartikel Chemie In aspecific case,such as diazadioxa[8]circulene (3)and reported by the groups of Fuchter and Katz for the intra- related hetero[8]circulenes,the central eight-membered ring molecular rearrangement of carba[7]helicene and derivatives has experimentally been determined to have positive NICS thereof,forming the carba[6]circulene motifs.[20–22] It should values.[8,12,18] This can either be due to the presence of the be noted that thus far the biggest carba- and hetero- paratropic ring current of an anti-aromatic COT motif,orbe [n]circulenes reported are n = 8, and the properties and due to the influence of the diatropic ring currents of the structures of higher-order [n]circulenes have only been surrounding aromatic rings,that is,induced paratropicity. investigated theoretically.[6,23–25] Thedihydro[9]circulene is Herein, we describe ascenario where this exact problem is the first [n]circulene motif with n > 8. Formally,the motif is addressed, both experimentally and computationally.We adihydro-diazatrioxa[9]circulene as illustrated in 4,with rings report on the synthesis of adihydro[9]circulene derivative 2–8 being fully aromatic,and dihydrofuran rings 1and 9being (4), where the central non-aromatic nine-membered ring annulated via two sp3 carbon atoms.Our mechanistic fulfils two of the criteria for anti-aromaticity:positive proposal for the formation of 8,from 5 by treatment with calculated NICS values and adownfield shift of the resonance DDQ,ispresented in Scheme 2. Initial oxidation, mediated of aproton positioned above the centre of the ring. The dihydro[9]circulene derivative is prepared by ahigh-yielding intramolecular oxidative coupling within adiazadioxa- [10]helicene.The single-crystal X-ray structure reveals atight columnar packing, with the peri-proton from anaphthalene moiety centred directly above the central nine-membered ring. This distinct chemical environment induces asignificant downfield shift on the resonance of said proton as determined by 1HNMR spectroscopy,which is consistent with the positive NICS values determined computationally.Asthe central nine-membered ring of the dihydro[9]circulene is unambig- uously non-aromatic,the deshielding effect is due to the diatropic ring current of surrounding aromatic rings in the Scheme 2. Mechanistic proposal for the formation 8.Bonds formed dihydro[9]circulene structure,and not due to the central nine- during the reaction are shown in red. membered ring being antiaromatic. by DDQ,onthe hydroxy termini of 5 forms the quinoidal Results and Discussion species 9.Alternatively,aone-electron mechanism could also be aplausible pathway.[26,27] Conjugate addition from the 2- Previously we have reported on the DDQ-mediated position of the furan positioned directly beneath the furan oxidative hetero-coupling between diazadioxa[10]helicene 5 ring within the helicene framework gives oxocarbenium and 7-methoxy-2-naphthol (6)giving 7 (Scheme 1).[19] In this intermediate 10,which tautomerizes to phenol 11.Itshould work it is presented how,byconducting the reaction without be noted that in the crystal structure of 10,the carbon atoms naphthol 6,weaccess the intramolecular oxidative coupling between which the bond is formed are placed directly above product 8 in very high yield. Compound 8 contains a each other and are only 3.26 apart. Thenucleophilic phenol 7-methoxy-dihydronaphthalene orthogonally positioned termini of 11 can then undergo intramolecular furan forma- above a[9]circulene (4)system (highlighted in blue for tion, which upon deprotonation, likely by the monoanionic structure 8,see the box in Scheme 1). Peculiar reactivity hydroquinone of DDQ,forms the [9]circulene 8. within the aromatic skeleton of helicenes has also been Thestructure of 8 was unambiguously determined by single crystal X-ray diffraction (SCXRD), with the crystals readily obtained by slow evaporation from amixture of CH2Cl2 and EtOH. Focusing on 8,itisobserved that the two sp3 carbon atoms protrude the plane formed by the aromatic segment of the [9]circulene motif (Figure 2a). Thecentral nine-membered ring has adiameter of about 4 .Looking top down on 8,the naphthalene is positioned at aslight angle relative to the symmetry axis of the [9]circulene (Figure 2b). As aresult, 8 is chiral in the solid state,with the enantiomers consisting of the naphthalene motif tilted either to the left (as seen in Figure 2b)ortothe right. Theunit cell consists of both enantiomers,forming an overall achiral structure in the solid state (Figure 2c). Looking at the overall packing of 8,itforms columns consisting of alternating unit cells (Figure 2d). From the solid-state structure,itisobserved that the naphthalene is Scheme 1. Previously reported synthesis of [10]helicene 7 and the positioned in such amanner that the peri-proton of the herein reported synthesis of 8,containing [9]circulene motif 4. naphthalene (Ha,highlighted in green, Figure 2a), is posi- Angew.Chem. 2020, 132,5182 –5188 2020 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim www.angewandte.de
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