Letter pubs.acs.org/OrgLett
Skeletal Rearrangement of Twisted Polycyclic Aromatic Hydrocarbons under Scholl Reaction Conditions Shunpei Nobusue, Kazuya Fujita, and Yoshito Tobe*,† Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan S Supporting Information *
ABSTRACT: Treatment of a twisted polycyclic aromatic hydrocarbon containing cyclooctatetraene fused by two 9,9′-bifluorenylidene units under the Scholl reaction conditions (FeCl3 or 2,3-dichloro-5,6-dicyano1,4-benzoquinone and scandium trifluoromethanesulfonate) led to stepwise skeletal rearrangements to afford initially a hydrocarbon with a seven-membered ring and then tetrabenzo[a,d,j,m]coronene with all sixmembered rings. The course of the rearrangement was interpreted in terms of the acid-catalyzed isomerization of 9,9′-bifluorenylidene into dibenzo[g,p]chrysene moieties on the basis of theoretical investigations.
T
closely located hydrogen atoms attached to the peripheral benzene rings. In view of the successful transformation of the dibromo derivative of 9,9′-bifluorenylidene 9a to diindeno[1,2,3,4-defg;1′,2′,3′,4′-mnop]chrysene (10a) by flash vacuum pyrolysis6b and partial cyclization of the corresponding dichloride 9b to 10b under the Heck reaction conditions (Scheme 2a),6a we envisaged that cyclodehydrogenation of 11 at the peripheral benzene rings would give a novel [8]circulene congener 12 in which a planar COT is annelated with the surrounding five- and six-membered rings (Scheme 2b). In contrast to the nonplanar all-benzo-fused [8]circulene,7 12 is predicted to adopt a planar geometry due to the presence of five-membered rings. Compound 12 can also be regarded as tetrabenzo-fused congener of tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene (15a) whose stable derivative 15b was synthesized recently by us (Figure 1).8 Contrary to 15a with open-shell character, 12 should have a closed-shell electronic configuration. In our attempts at cyclodehydrogenation of 11 under the Scholl reaction conditions, we found skeletal rearrangement rather than cyclodehydrogenation took place to give initially polycyclic aromatic compound 13 having a seven-membered ring, which upon further reaction rearranged to all six-membered ring compound 14. We report herein the unexpected results obtained under the Scholl reaction conditions, structural, optical, and aggregation properties of product 13, and theoretical studies for the rearrangement. First, reaction with FeCl3 in nitromethane was examined,9 giving a single product that exhibited purple color in the solid state, isolated in 64% yield. However, the mass spectrum of the product showed a molecular ion m/z 500.2, indicating that the
he Scholl reaction is one of the most efficient methods to obtain fused polycyclic aromatic compounds via oxidative cyclodehydrogenations. A prototype of this method is the transformation of 1,1′-binaphthalene (1) to perylene (2) reported by Scholl more than a century ago (Scheme 1a).1 Various reagents or a combination of reagents such as FeCl3, MoCl5, Cu(OTf)2 + AlCl3, DDQ + Brønsted or Lewis acid, and [bis(trifluoroacetoxy)iodo]benzene (PIFA) + BF3·Et2O have been shown to be effective for this reaction.2 The high utility of the reaction has been demonstrated by the syntheses of many polycyclic frameworks of various geometries and sizes as exemplified by the transformation of hexaphenylbenzene (3) to hexabenzocoronene (4) (Scheme 1b).3 However, under the Scholl reaction conditions, unexpected reaction products were occasionally produced due to aryl migration (i.e., an o-terphenyl motif to m- and p-isomers), leading to cyclization at an undesired position or incomplete cyclization as illustrated in Scheme 1c,d for representative examples.4 For example, Durola and co-workers reported that migration of an aryl group followed by cyclodehydrogenation of 5 took place to give 6, a seven-membered ring-containing polycyclic aromatic hydrocarbon.4b Feng, Müllen, and co-workers recently reported that the reaction of 6,7,13,14-tetraarylbenzo[k]tetraphene 7 with DDQ and trifluoromethanesulfonic acid (TfOH) afforded 8 with five-membered rings via 1,2-aryl migrations.4c Despite many attempts toward a full understanding of the mechanism of the Scholl reaction, control over its selectivity remains unachieved. We previously reported the synthesis of twisted polycyclic aromatic hydrocarbon 11 containing cyclooctatetraene (COT) fused by two 9,9′-bifluorenylidene units.5 X-ray crystallographic analysis revealed that the COT ring in the center adopted a twisted conformation due to steric repulsion between the © 2017 American Chemical Society
Received: May 4, 2017 Published: June 6, 2017 3227
DOI: 10.1021/acs.orglett.7b01341 Org. Lett. 2017, 19, 3227−3230
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Organic Letters Scheme 1. (a) First Example of Cyclodehydrogenation Reaction Reported by Scholl. (b) Typical Example of Cyclodehydrogenation of 3 To Give 4. (c) Aryl Shift and Cyclodehydrogenation of 5 To Give 6. (d) Aryl Shift and Cyclodehydrogenation of 7 To Afford 8
Figure 1. Chemical structures of planar COT derivatives 15a,b.
indicated that a skeletal rearrangement of 11 took place to produce a less symmetric product. However, the structural determination of the product from the spectral data was difficult because of the presence of many quaternary carbon atoms. The structure of the product was therefore established by X-ray crystallographic analysis. A crystal of the product suitable for X-ray crystallographic analysis was obtained via vapor diffusion of methanol into an odichlorobenzene solution. The X-ray analysis revealed that it was benzo[p]fluoreno[1,2-g]indeno[1,2-c]chrysene-15,16-diylidene (13) with a seven-membered ring in the center.10 Compound 13 adopts a twisted conformation forming a saddlelike shape because of the steric repulsion of the closely located hydrogen atoms. There are three crystallographically independent molecules in the crystal unit cell, with two of them being disordered (Figure 2). Two crystallographically independent
Scheme 2. (a) Transformation of 9a,b to 10a,b. (b) Skeletal Rearrangement of Twisted COT 11 to 13 and 14
Figure 2. (a) Tilted view showing the twisting of ORTEP drawing of the nondisordered molecule of 13. (b) Top view of one of the disordered structures (molecule A) of 13. The two pairs of vertically and horizontally oriented carbon units indicate those belong to the other structure which is oriented reversely to the structure fully shown. (c) Molecular-packing structure of 13 in which four carbon atoms around the seven-membered ring in molecules A and C are disordered. Hydrogen atoms are omitted for clarity. Displacement ellipsoids are drawn at 50% probability level.
molecules of the same sense of chirality stack to each other so that the overlap of their π surfaces is maximized, whereas the third antipodal molecule locates nearly perpendicular to the above molecular pair. We noticed that the 1H NMR spectra of compound 13 showed concentration-dependent chemical shift changes, in spite of the nonplanar conformation (Figure S1). The chemical shifts of the two aromatic protons (Ha and Hb, numbering shown in Scheme 2b) showed upfield shifts (from 8.69 to 8.58 and 8.08 to 7.97 ppm, respectively) as the concentration increased from 0.465 to 5.58 mM at 30 °C. The upfield shift
elemental composition remained unchanged from that of 11. Additionally, the 1H and 13C NMR spectra showed 10 and 21 signals (Figure S5), respectively, whereas the desired product 12 should show 4 and 10 signals, respectively. These data 3228
DOI: 10.1021/acs.orglett.7b01341 Org. Lett. 2017, 19, 3227−3230
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Organic Letters
chlorobenzene at 120 °C gave the product of second isomerization, tetrabenzo[a,d,j,m]coronene 14, which has been known,18 in 49% yield. Treatment of 13 under the same reaction conditions furnished 14 (70%), confirming that 14 is derived from 13. As a control experiment, we tested the reaction of 11 with DDQ in the absence of Sc(OTf)3 at 80 °C; as a result, only a small amount of 13 was formed. The reaction of 11 with Sc(OTf)3 in the absence of DDQ did not give 13. These results suggested that the presence of an oxidizing agent is essential for the rearrangement. The rearrangement of 11 into 13 and 14 can be regarded as that of 9,9′-bifluorenylidene into dibenzo[g,p]chrysene, which was observed previously under thermal conditions (Scheme 3).19 Similar arrangements were also observed in other
indicates that 13 aggregates to adopt a face-to-face stacked geometry in solution.11 However, in contrast to the welldocumented aggregation behavior of planar polycyclic aromatic compounds bearing solubilizing alkyl side chains,12 little is known for curved aromatic compounds. For example, though coannulene is predicted theoretically to form a convex−concave dimer with a stabilization energy of 17.2 kcal/mol,13 there is no report on its aggregation in solution. Assuming a monomer− dimer equilibrium, the association constant for dimerization K2 was determined by curve fitting for protons Ha and Hb to be 24.1 ± 2.9 and 18.0 ± 1.4 M−1, respectively (for details of the estimation, see the Supporting Information).14 Though the association constant is not large compared to those of alkylsubstituted hexabenzocoronene derivatives, for instance,12 this represents a rare example for association of a curved polycyclic aromatic compound. In the saddle-like shape of 13, as observed in its crystal structure, overlap of π electrons can be attained. Though the calculated dipole moment of 13 (0.12 D) is not large, polarity of the hydrocarbon due to the unsymmetrical structure may contribute to the association of 13. The absorption spectrum and cyclic voltammogram of 13 are shown in Figure 3 and Figure S3 (Supporting Information),
Scheme 3. Rearrangement of 9,9-Bifluorenylidene to Dibenzo[g,p]chrysene
polycyclic aromatic hydrocarbons.20 Though the mechanism of the thermal rearrangement was explained in terms of initial addition of a radical species,20b it must be different from the present case because heating to 400 °C was necessary in the thermal reactions. The present reaction took place at room temperature initiated by an oxidizing reagent. In order to clarify the reason for the preferential rearrangement over dehydrogenation, we performed theoretical calculations to compare the activation barriers of two reaction pathways by the DFT method at the B3LYP/6-31G(d) level of theory. For optimization of the transition states and reaction intermediates, we start with carbocation A1 and C1 formed by direct protonation and radical cation B1 and D1 formed by one-electron oxidation, derived from 11 and 13 for the initial and second reactions (Scheme 4 and Scheme S1, respectively). Note that carbocations A1 and C1 can be generated from radical cations B1 and D1, respectively, by protonation and one-electron reduction.4c Since it is not possible to compare directly the stabilities of cations and radical cations, the relative energies of A1, B1, C1, and D1 are set to zero in each set of the
Figure 3. UV−vis absorption spectrum of 13. (Inset: vertically expanded spectrum for a long wavelength region).
respectively. Compound 13 did not exhibit fluorescence. The absorption spectrum showed a weak absorption band at the long wavelength region, extending to ca. 670 nm, which is assigned by TD-DFT calculations (B3LYP/6-31G* method with the implicit solvent model in CH2Cl2) to the HOMO− LUMO transition (603 nm, f = 0.090; Table S2). The distribution of HOMO and LUMO (Figure S4) indicates a weak charge transition nature of this band. In the cyclic voltammetry, compound 13 showed two reversible reduction waves at −1.59 and −2.08 V (vs Fc/Fc+). However, no reversible oxidative waves were observed, indicating the occurrence of chemical reactions by oxidation. It is worth mentioning that 13 has a [7]circulene-like framework, though one fused ring is missing,15 suggesting that this transformation may be utilized for the synthesis of [7]circulene. In addition, it belongs to the family of flexible, nonplanar polycyclic aromatic compounds with (a) sevenmembered ring(s), which attract substantial interest in view of their dynamic behavior16 and optoelectronic applications.17 The reaction of 11 was also examined with DDQ and scandium trifluoromethanesulfonate (Sc(OTf)3).9 When the reaction was conducted in toluene at 80 °C, the same product 13 was obtained in 34% yield. On the contrary, the reaction in
Scheme 4. Calculated Energies of Intermediates and Barriers for Isomerization and Cyclodehydrogenation Reaction Pathways for 11a
a
The calculated energies (kcal/mol) relative to A1 and B1 in each reaction are shown in parentheses.
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reactions, and we focus on the relative barriers of bond migration in the cation intermediates (A1 and C1) and the bond formation in the radical cation intermediates (B1 and D1). For the initial step, the barrier of bond migration (34.7 kcal/mol) is smaller than that of bond formation (44.4 kcal/ mol). This may suggest bond migration from cation intermediate A1 is a more favored pathway. Similarly, in the second step, the barrier of bond migration from C1 (30.7 kcal/ mol) is significantly smaller than that of bond formation in D1 (60.9 kcal/mol). These results suggest that skeletal rearrangements from carbocation intermediates A1 and C1, no matter whether they are formed direct protonation of 11 and 13 or via the corresponding radical cations B1 and D1, take place preferably over oxidative cyclodehydrogenation reactions in agreement with the experimental results. In conclusion, reaction of twisted COT congener 11 with FeCl3 gave 13 with a seven-membered ring in the center via a skeletal isomerization. Reaction of 11 with DDQ and Sc(OTf)3 also gave 13, whereas at higher temperature tetrabenzocoronene 14 was obtained via 2-fold isomerizations. Compound 13 adopts a twisted geometry to avoid intramolecular steric repulsions. Although weak, it exhibits aggregation behavior in solution via stacking of the π-conjugated framework. The theoretical calculations suggested that the activation barrier for skeletal rearrangements in cationic intermediates are lower than those of intramolecular bond formation for oxidative dehydrogenation reactions.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01341. Synthetic procedures, estimation of association constant, theoretical calculations, cyclic voltammogram, NMR spectra (PDF) Crystal data for 13 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yoshito Tobe: 0000-0002-1795-5829 Present Address †
(Y.T.) The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Nos. JP25620032 and JP15H02164. REFERENCES
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