Pyrene-Based Diarynes as Precursors for Twisted Fused Polycyclic Aromatic Hydrocarbons: A Comparison of Two Routes

Abstract

Two bench-stable and readily accessible pyrene-based diaryne precursors based on triflate as well as TMS triflate motifs are introduced and compared in their [4+2]-Diels–Alder reactions with tetracyclone to give an oligophenyl-substituted dibenzo[e,l]pyrene in both cases. By single-crystal X-ray analysis, this twistacene showed helical chirality and an end-to-end contortion of 49.6° due to steric repulsion.

Key words

polycyclic aromatic hydrocarbons - twistacene - Diels–Alder reaction - pyrenes - arynes

Introduction

Although a large portfolio of synthetic methods has already been developed for the synthesis of larger fused polyaromatic hydrocarbons (PAHs),[1] the [4+2] cycloaddition of (multifunctional) arynes with aryl-substituted cyclopentadienones under subsequent thermal CO extrusion is still one of the most frequently used and reliable methods to quickly provide PAH scaffolds in high yields.[2] [3] [4] Such structures can be further used, for example, for cyclodehydrogenative fusion of rings to synthesize larger 2D or 3D structures.[4] [5]

In this respect, pyrene derivatives are excellent molecular precursors to build up larger PAHs.[3] [5] [6] For the above-mentioned approach (the [4+2] cycloaddition), pyrene derivatives can either act as precursors for arynes as dienophiles or as dienes. The latter was frequently used for the synthesis of PAHs, for example, dibenzo[e,l]pyrenes (path A in [Scheme 1]),[7] despite the fact that pyrene biscyclopentadienones are not very stable under ambient conditions and therefore difficult to purify and handle.[3] [5] [8] To the best of our knowledge, the approach with inverse electronic demand on the pyrene scaffold in the reaction with cyclopentadienones (path B in [Scheme 1]) has not been reported till date. There are a few examples where the in situ generation of pyrene-based bis-arynes has been described in the cycloaddition to furans,[9] [10] benzofurans,[11] or arylacetonitrils.[12] In all these cases, either pyrene dibromides[9] [12] or tetrabromides[10] [11] [13] have been used as molecular precursors, which were transformed into the arynes with non-nucleophilic bases, or n-BuLi. Similar to aryl bromides, aryl triflates[14] can be transformed into arynes by non-nucleophilic bases, or, more elegant, ortho-TMS triflates[15] as bench stable precursors that are in situ transformed to arynes by fluoride anions.

Scheme 1 Comparison of two synthetic approaches involving pyrene biscyclopentadienones (path A) or pyrene diarynes (path B) exemplarily shown for the synthesis of dibenzo[e,l]pyrenes.

Here we describe two routes to access pyrene-based diaryne precursors as bench-stable compounds for the synthesis of larger PAHs, such as twistarenes by [4+2] cycloadditions.[16] [17]

Results and Discussion

The synthesis of both aryne precursors 3 and 6 started from 4,9-diborylated pyrene 1, which can be readily synthesized via literature-known procedures on gram scale.[18] Base-mediated (NaOH_aq) oxidation using H₂O₂ gave the corresponding pyrene diol 2 in 80% yield after recrystallization from a chloroform/n-heptane mixture ([Scheme 2]). The condensation with trifluoromethanesulfonic anhydride (Tf₂O; 2.4 equiv) under standard conditions [NEt₃ (4 equiv), CH₂Cl₂] gave pyrene bistriflate 3 in 68% yield ([Scheme 2]). Diol 2 and bistriflate 3 have been fully characterized by common analytical methods (see the [Supporting Information, SI]). Additionally, the structure of bistriflate 3 was proven by single-crystal X-ray analysis (SCXRD; [Scheme 2], top right).

Scheme 2 Left: synthesis of the pyrene-based diaryne precursors 3 and 6 from bisborylated pyrene 1. Right: single-crystal X-ray structures of 3 (top) and 6 (bottom) as thermal ellipsoids at the 50% probability level. Carbon: grey; hydrogen: white; oxygen: red; sulfur: yellow; fluorine: lime, silicon: light blue. HMDS: hexamethyldisilazane.

To synthesize bis-TMS triflate 6, pyrene diol 2 was selectively ortho-brominated using NBS and ⁱPr₂NH₂ to give dibromo dihydroxy pyrene 4 in 96% yield.[19] Using hexamethyldisilazane, 4 was transformed in 84% yield to the double TMS ether 5 ([Scheme 2]). Subsequently, 5 was converted under Sila-Fries[20] conditions (1. nBuLi; 2. Tf₂O) under careful control of the reaction temperature (−100 °C to −80 °C) to the TMS triflate 6 and isolated in 54% yield. Pyrenes 4, 5, and 6 have been fully characterized (see [SI]) and the structures of 5 (see [SI]) and 6 ([Scheme 2], right, bottom) were additionally proven by SCXRD analyses.

The in situ generation of pyrene diarynes from 3 and 6 was investigated in the Diels–Alder reaction with tetracyclone 7 to obtain PAH 8 ([Scheme 3]), whose dibenzo[e,l]pyrene core structure was till now only accessible via path A with dodecyl chains as discussed in [Scheme 1].[7] Different bases for the deprotonation of bistriflate 3 were tested to generate the aryne in situ and react with tetracyclone 7 to give 8. Neither KO ^t Bu in different solvents (THF, Et₂O, Ph₂O) in a wide temperature range (0 °C to 180 °C) nor n-BuLi gave the twisted PAH 8. Treatment of 3 with lithium hexamethyldisilazane as a strong non-nucleophilic base for 21 h at −78 °C to rt followed by thermal treatment at 150 °C for 3 h (for details, see [SI]) resulted in 8, which was isolated in 40% yield after column chromatography ([Scheme 3]). Besides characterization by ¹H and ¹³C NMR spectroscopy, a molecular ion peak at m/z = 1022.575 (calcd. for C₈₀H₆₂: 1022.485) for [8]⁺ was clearly detected by MALDI-TOF MS (see [SI]). As mentioned above, TMS triflate 6 was also used as an aryne precursor, which was generated by CsF in THF at 80 °C to give nearly the same yield of 8 (43%), again after thermal treatment.

Scheme 3 Synthesis of PAH 8 by the Diels–Alder reaction of aryne precursors 3 and 6, respectively with tetracyclone 7.

By slow evaporation of an n-hexane/CHCl₃ solution of 8, crystals of suitable quality for single-crystal X-ray diffraction have been obtained ([Figure 1]).

Figure 1 Single-crystal X-ray structure of dibenzo[e,l]pyrene 8. a) Thermal ellipsoid plot shown at a probability level of 50% (only the M-enantiomer is shown exemplarily). b) Side view of the helical M-enantiomer. c) Side view of the helical P-enantiomer. d) Cutout from an enantiopure layer (P-enantiomer) found in the crystal packing. e) View along the ab-plane of the crystal packing. Structures b) to e) are depicted as stick models. Carbon: grey; hydrogen: white.

PAH 8 crystallizes in the orthorhombic space group Fddd with Z = 8 and approximately 24 molecules of disordered chloroform within the one-dimensional channels along the ab-plane formed by PAH 8, which had to be removed by the SQUEEZE routine function of Platon.[21] Because of the eight phenyl groups of the aromatic backbone of 8, the dibenzo[e,l]pyrene core structure is contorted by 49.6° (considering the outer edges, see [Figure 1b, c]), creating a helical chirality. This twist is noticeably smaller than that for the structurally related dodecaphenyltetracene (97°)[17] due to the stiffening of the tetracene backbone by the annulated benzene rings. Within the racemic crystal structure of 8, enantiopure sheets can be found ([Figure 1d]) with dispersion interactions between the peripheral phenyl group and the tert-butyl groups as main interactions ([Figure 1e]). Dibenzo[e,l]pyrene 8 was furthermore investigated using UV-vis spectroscopy and the colorless compound shows an absorption maximum at λ _abs = 309 nm (log ε = 4.71). Upon excitation (λ _ex = 309 nm), a blue fluorescence with λ _em = 412 nm and a resulting considerably large Stokes shift of  = 8090 cm⁻¹ were observed (see [SI]).

Conclusions

Two routes to twisted PAH 8 via different pyrene-based aryne precursors were compared. While for the bistriflate 3 low temperatures and a strong non-nucleophilic base (LHDMS) were necessary to generate the desired diaryne, the bis-TMS triflate 6 was transferred to the bis-aryne using CsF as a fluoride ion source at 80 °C. In both cases, the Diels–Alder reaction with tetracyclone gave twisted phenyl-substituted dibenzo[e,l]pyrene 8 in comparable yields of 40% and 43%. Since bistriflate 3 is synthesized with two steps less than 6, this route is preferred to generate a valuable pyrene-based aryne in situ. Currently we are exploiting both precursors in the broader sense for PAH synthesis.

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Supporting Information

Supporting information for this article is available online at https://doi.org/10.1055/s-0040-1721851.

• References and Notes

• 1a Wu D, Ge H, Liu SH, Yin J. RSC Adv. 2013; 3: 22727
• 1b Narita A, Wang X.-Y, Feng X, Müllen K. Chem. Soc. Rev. 2015; 44: 6616
• 1c Majewski MA, Stępień M. Angew. Chem. Int. Ed. 2019; 58: 86
• 1d Rüdiger EC, Müller M, Freudenberg J, Bunz UH. F. Org. Mater. 2019; 01: 001
• 2a Simpson CD, Mattersteig G, Martin K, Gherghel L, Bauer RE, Räder HJ, Müllen K. J. Am. Chem. Soc. 2004; 126: 3139
• 2b Smyth N, Van Engen D, Pascal RA. J. Org. Chem. 1990; 55: 1937
• 3 Chen W, Li X, Long G, Li Y, Ganguly R, Zhang M, Aratani N, Yamada H, Liu M, Zhang Q. Angew. Chem. Int. Ed. 2018; 57: 13555
• 4 Dötz F, Brand JD, Ito S, Gherghel L, Müllen K. J. Am. Chem. Soc. 2000; 122: 7707
• 5a Byun Y, Coskun A. Chem. Mater. 2015; 27: 2576
• 5b Byun Y, Cho M, Kim D, Jung Y, Coskun A. Macromolecules 2017; 50: 523
• 6a Wang L, Han Y, Zhang J, Li X, Liu X, Xiao J. Org. Lett. 2020; 22: 261
• 6b Baumgärtner K, Rominger F, Mastalerz M. Eur. J. Org. Chem. 2019; 4891
• 6c Baumgärtner K, Meza Chincha AL, Dreuw A, Rominger F, Mastalerz M. Angew. Chem. Int. Ed. 2016; 55: 15594
• 7 Wasserfallen D, Kastler M, Pisula W, Hofer WA, Fogel Y, Wang Z, Müllen K. J. Am. Chem. Soc. 2006; 128: 1334
• 8 Baumgärtner K, Kirschbaum T, Krutzek F, Dreuw A, Rominger F, Mastalerz M. Chem. Eur. J. 2017; 23: 17817
• 9 Moursounidis J, Wege D. Aust. J. Chem. 1988; 41: 235
• 10a Itami K, Segawa Y, Watanabe K, Cheung KY, Watanabe K, Segawa Y, Itami K. ChemRxiv 2020; , preprint; https://doi.org/10.26434/chemrxiv.12324353.v2
• 10b Franz D, Robbins SJ, Boeré RT, Dibble PW. J. Org. Chem. 2009; 74: 7544
• 11 Wang J, Miao Q. Org. Lett. 2019; 21: 10120
• 12 Han W, Tran J, Zhang H, Jeffrey S, Swartling D, Ford GP, Biehl E. Synthesis 1995; 827
• 13 Han Y, Dong S, Shao J, Fan W, Chi C. Angew. Chem. Int. Ed. 2020; DOI: 10.1002/anie.202012651.
• 14a Wickham PP, Hazen KH, Guo H, Jones G, Reuter KH, Scott WJ. J. Org. Chem. 1991; 56: 2045
• 14b Truong T, Mesgar M, Le KK. A, Daugulis O. J. Am. Soc. Chem. 2014; 136: 8568
• 14c Reuter KH, Scott WJ. J. Org. Chem. 1993; 58: 4722
• 14d Pun SH, Wang Y, Chu M, Chan CK, Li Y, Liu Z, Miao Q. J. Am. Chem. Soc. 2019; 141: 9680
• 14e Mesgar M, Nguyen-Le J, Daugulis O. Chem. Commun. 2019; 55: 9467
• 15 Pérez D, Peña D, Guitián E. Eur. J. Org. Chem. 2013; 5981
• 16a Pascal Jr RA. Chem. Rev. 2006; 106: 4809
• 16b Xiao J, Liu S, Liu Y, Ji L, Liu X, Zhang H, Sun X, Zhang Q. Chem. Asian J. 2012; 7: 561
• 16c Xiao J, Duong HM, Liu Y, Shi W, Ji L, Li G, Li S, Liu X.-W, Ma J, Wudl F, Zhang Q. Angew. Chem. Int. Ed. 2012; 51: 6094
• 16d Xiao J, Divayana Y, Zhang Q, Doung HM, Zhang H, Boey F, Sun XW, Wudl F. J. Mater. Chem. 2010; 20: 8167
• 16e Walters RS, Kraml CM, Byrne N, Ho DM, Qin Q, Coughlin FJ, Bernhard S, Pascal Jr RA. J. Am. Chem. Soc. 2008; 130: 16435
• 16f Qiao X, Padula MA, Ho DM, Vogelaar NJ, Schutt CE, Pascal RA. J. Am. Chem. Soc. 1996; 118: 741
• 16g Qiao X, Ho DM, Pascal Jr RA. Angew. Chem. Int. Ed. Engl. 1997; 36: 1531
• 16h Lu J, Ho DM, Vogelaar NJ, Kraml CM, Bernhard S, Byrne N, Kim LR, Pascal Jr RA. J. Am. Chem. Soc. 2006; 128: 17043
• 16i Duong HM, Bendikov M, Steiger D, Zhang Q, Sonmez G, Yamada J, Wudl F. Org. Lett. 2003; 5: 4433
• 16j Clevenger RG, Kumar B, Menuey EM, Kilway KV. Chem. Eur. J. 2018; 24: 3113
• 17 Xiao Y, Mague JT, Schmehl RH, Haque FM, Pascal Jr RA. Angew. Chem. Int. Ed. 2019; 58: 2831
• 18 Ji L, Krummenacher I, Friedrich A, Lorbach A, Haehnel M, Edkins K, Braunschweig H, Marder TB. J. Org. Chem. 2018; 83: 3599
• 19 Compound 4 can be isolated in 96% yield in sufficient purity to be used in further synthetic steps. To obtain an analytical pure sample, purification by column chromatography has to be taken into account, accompanied by a material loss and an isolated yield of 31% (see the SI)
• 20 Korb M, Lang H. Chem. Soc. Rev. 2019; 48: 2829
• 21a van der Sluis P, Spek AL. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990; 46: 194
• 21b Spek A. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009; 65: 148

Publikationsverlauf

Eingereicht: 20. Oktober 2020

Angenommen: 12. November 2020

Artikel online veröffentlicht:
23. Dezember 2020