π‐Extended Polyaromatic Hydrocarbons by Sustainable Alkyne Annulations through Double C−H/N−H Activation

Abstract The widespread applications of substituted diketopyrrolopyrroles (DPPs) call for the development of efficient methods for their modular assembly. Herein, we present a π‐expansion strategy for polyaromatic hydrocarbons (PAHs) by C−H activation in a sustainable fashion. Thus, twofold C−H/N‐H activations were accomplished by versatile ruthenium(II)carboxylate catalysis, providing step‐economical access to diversely decorated fluorogenic DPPs that was merged with late‐stage palladium‐catalyzed C−H arylation on the thus‐assembled DPP motif.

al condensationr eactions [4b, 15] or metal-catalyzed cross-coupling reactions with prefunctionalized substrates. [4a, 8b, 16] In comparison to the elegant efforts devoted to the modification of the DPP periphery by direct functionalization or direct arylation polymerization (DArP), [17] their de novo assembly leading to p-extendeds tructures continuous to be underdeveloped, with notable recent progress by Zumbusch [18] and Würthner. [13,19] Another possibility is represented by the introduction of an ethylene bridgeb etweent he nitrogen and the aromatic moiety,w hich leads to significantly p-expanded compounds. [20] These DPP derivativesp ossess sharp absorption and emission peaks, with very high molar absorption coefficients and unique fluorescenceq uantum yields, [20b] featuring after further modification at wo-photon absorption cross-section. [20a] Very recently, p-extended DPP moietiesw erei ncorporatedi ntoa lternating donor-acceptor copolymers resulting in av ariety of low band gap copolymers. [21] Despite of these indisputable advances, thus far,t hese DPPs were obtained by classical N-alkylation with bromoacetaldehyde diethyla cetal, along with electrophilic aromatic substitution. Notably,t his approach is hence limited to the introduction of ethylene junctions with only one substituent.
We initiated our studies by probing variousr eactionc onditions for the envisioned double CÀH/NÀHa ctivation of diketopyrrolopyrrole 1a with alkyne 2a (Table 1, for detailed information, see Tables S1 and S2 in the Supporting Information). [25] Thus, moisture-anda ir-stable ruthenium(II) complexes emerged as the catalysts of choice, with moste ffective catalysis accomplished by carboxylate assistance (Table 1, entries 1-4). [26] Given the rather poor solubility of substrate 1a [1, 4b] and the aromatic character of PAH 3aa,w en ext explored various solvents (entries 4-9), with o-xylene being superior.C ontrole xperiments confirmed the essential role of carboxylate-assisted ruthenium(II) catalysis (entries [10][11]. The particularly challenging character of the twofold CÀH/NÀHa ctivation was reflected by palladium and iridium catalysts failing short in delivering the desired product 3aa (entries13a nd 14). Likewise {Cp*Rh III }catalysis was significantly less effective than the cost-effective ruthenium(II) manifold(entries 15 and 16).
With the optimized catalysti nh and, we tested its versatility in the double CÀH/NÀHa ctivation of DPP 1a with av ariety of aryl-substituted alkynes 2 (Scheme 1). Thereby, p-extended PAHs were accessed from electron-richa sw ell as electron-deficient alkynes in an efficient manner,i ncluding sensitivet etrabromo DPP 3ag and thiophene-rich 3ah,w hich should prove instrumental for further modifications and applications of the DPPs 3.
The double ruthenium-catalyzed DPP CÀH/NÀHa ctivation was not restricted to unsubstituted, parent DPP 1a (Scheme 2). Indeed, the reaction also proceeded efficiently with alkyl-substituted derivative 1b as well as the aryl-modified-DPP 1c,w ith the isolated yield of annulation product 3ca being caused by the extremelyl ow solubility of substrate 1c.
The connectivity of the annulated product 3aa was unambiguously established by X-ray crystal diffraction analysis (Figure 2). The dihedral angle between the DPP unit and its adjacent thiophene unit was found with 6.48,c learly showing the importance of the molecular tethert oi nduce planarity,a s compared with the unbridged compound. [6c] Further, the individual molecules are arranged in al amellar packingm otif, [27] which appears to be stabilized by dispersive non-covalentC À H···p interaction between the CÀHb onds of the arene motif and the thiopheneu nit (2.82 ). The optical properties of the thus-obtained novel DPP PAHs 3, 5 and 7 were thereafter studied by detailed UV/Vis absorption and fluorescence spectroscopy ( Table 2). The unprecedented DPPs exhibited very intense absorption in the UV and visible region, with absorption maximab etween maxima between 600-680nmO Rm axima between 600-640nmf or the annulation products 3 and 5,w hichr esults in an intense blue to purple color.T he absorption maximum in all synthesized derivatives is bathochromically shiftedi nc omparison with the previously synthesized, unsubstituted compounds, [20b] whereas both Stoke shift and absorption coefficient are comparable. Interestingly,t he highest absorption coefficients were obtained for the p-tolyl-DPP derivative 3ca,w hile the largest Stokes shift was observed for thiophene-DPP derivative 3aa and 7.
To investigate further the electronic structure of the DPPs 3, we performed computational DFT studies for product 3aa at the B3LYP-D3(BJ)/6-311 + G(d,p) + SMD(o-Xylene)l evel of theory ( Figure 3). [25] Our calculations showed that the HOMO is delocalized over the DPPs core and the ethylene bridge, whereas the LUMO is evenly localized on all condensed rings. Moreover,o ur TD-DFT calculations, performeda tt he same level of theory,h ighlighted an optical gap (E opt )o f2 .02 eV.O ur computed absorption spectrum is in good qualitative agreement with the experimental data. [25] Finally,w eb ecameattracted by further late-stage functionalizationo fP AH 3am in terms of the introduction of two aryl motifs [17a] at the alpha positions of the thiophenes (Scheme 4). Thus, the desired assembly of octylphenyl-substituted DPP 7 was realized by palladium-catalyzed twofold CÀHa rylations. It is noteworthy that the CÀHa ctivation-based incorporation of two aromatic moieties drastically shifted both the absorption and the emission maximai nto the NIR region, which was mirrored by the green color of PAH 7.
Figure 2. X-ray structure of DPP 3aa.a)Molecular structure. b) Side view on the moleculars tructure, highlighting the planarity of the DPP core.c)Lamellar packing motif.Hydrogen atomsa re partially omitted for clarity. In summary,w eh ave devised an enabling strategy for the assembly of p-extended DPPs. Thus, ruthenium(II)-catalyzed double CÀH/NÀHa ctivation allowed for the synthesis of novel, diversely-decoratedD PP derivatives in as tep-economical manner.T he novel DPPs were fully characterized, including spectroscopy,X RD and DFT computation. The twofold alkyne annulation was furthermore merged with CÀHa rylations of the thus obtained p-extended PAHs to furnish DPPs with absorption and emission maximas hifted into the NIR region.O ur findings should prove invaluablef or applications to optoelectronics,material sciences and live cell imaging. [28]