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 (NaOHaq) 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.
