Dibenzo[a,e]pentalenes with Low-Lying LUMO Energy Levels as

two benzene rings are fused to the pentalene core, dibenzo[a,e]pentalene (DBP, 1 in ... transistors (OFETs), as shown for several diaceno[a,e]pentalen...
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Article Cite This: J. Org. Chem. 2018, 83, 656−663

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Dibenzo[a,e]pentalenes with Low-Lying LUMO Energy Levels as Potential n‑Type Materials David C. Grenz,† Maximilian Schmidt,† Daniel Kratzert,‡ and Birgit Esser*,† †

Institute for Organic Chemistry and ‡Institute for Inorganic and Analytical Chemistry, University of Freiburg, Albertstraße 21, 79104 Freiburg, Germany S Supporting Information *

ABSTRACT: Ambipolar organic semiconductors are of high interest for organic field-effect transistors. For n-type conduction, low LUMO energies are required. Dibenzo[a,e]pentalenes (DBPs) are promising compounds; however, few derivatives exist with energetically low-lying LUMO levels. Here, we present DBP derivatives with LUMO energies down to −3.73 eV and small bandgaps down to 1.63 eV determined through cyclic voltammetry, UV/vis absorption spectroscopy, and TDDFT calculations. Single-crystal X-ray diffraction analysis revealed a 1D π-stacking mode. The addition of arylalkynyl substituents at the fivemembered rings in a facile and versatile synthetic route allowed for tuning of the band gaps and LUMO energies. The synthetic route can easily be modified to access a variety of DBP derivatives. The LUMO energies of the DBP derivatives presented herein make them attractive for an application in n-type or ambipolar field-effect transistors.

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INTRODUCTION Polycyclic-conjugated hydrocarbons containing fused five- and six-membered rings have attracted interest for a long time,1 and many new derivatives have been synthesized in the past few years.2−11 These compounds are so-called nonalternant hydrocarbons.12 They have been discussed in the context of antiaromaticity because counting π-electrons in such linearly fused systems that contain an even number of five-membered rings results in a multiple of 4n, indicating potentially antiaromatic character.4,5,8,11 True antiaromatic character has been observed for pentalene, an unstable and highly reactive species, which could only be isolated when kinetically stabilized through sterically demanding substituents.13,14 When two benzene rings are fused to the pentalene core, dibenzo[a,e]pentalene (DBP, 1 in Figure 1) is formed, where the aromatic character of the six-membered rings prevails, resulting in a thermodynamically and kinetically stable compound.13 The presence of five-membered rings in polycyclic-conjugated hydrocarbons leads to a lowering of the LUMO energy relative to acenes composed solely of six-membered rings and, consequently, a small band gap, as observed for many compounds such as fullerenes,15 pentalene derivatives,2−5 indenofluorenes,6,16 and others.9 This shift in orbital energies can be explained by the nonalternant nature of the π-systems.12 Because of their low bandgap, DBP derivatives show amphoteric redox behavior. This makes them particularly attractive for applications in organic field-effect transistors (OFETs), as shown for several diaceno[a,e]pentalenes.17−21 When the LUMO energy is lowered further, DBP derivatives have the potential to be used as n-type materials.21 Compared © 2017 American Chemical Society

to their p-type analogues, many fewer examples of n-type materials for OFETs exist, and hence the development of new materials is of high interest.22,23 We have recently shown that the HOMO and LUMO energy levels in DBP can be tuned through modification of substituents at the 2,7-positions with the strongest effect on the HOMO levels (2 in Figure 1).24 In the LUMO of DBP, the largest coefficients are found on the C5 and C10 positions;25 hence, it is expected that substituents here have the highest influence on the LUMO energy levels. We herein present the synthesis, optoelectronic properties, and TDDFT calculations on eight DBP derivatives 3, where LUMO energies were varied between −3.36 and −3.73 eV. Electronically modulating substituents were introduced at the 5,10positions via alkynyl bridges to ensure coplanarity and unhindered conjugation.

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RESULTS AND DISCUSSION In the past ten years, many new synthetic strategies of DBP derivatives have been reported.26−34 In the original route by Brand,1 the DBP core is generated by addition of Grignard reagents to diphenyl succindandione followed by a 2-fold elimination of water. This has the advantage of allowing for a late-stage functionalization of the DBP core and provides facile access to differently substituted derivatives. The synthesis of 3a−h proceeded through bisethynyl-substituted diol 4, which is a versatile DBP precursor for further functionalization (Scheme 1). Compound 4 was accessed in two steps from Received: October 10, 2017 Published: December 12, 2017 656

DOI: 10.1021/acs.joc.7b02250 J. Org. Chem. 2018, 83, 656−663

Article

The Journal of Organic Chemistry

Figure 1. Dibenzo[a,e]pentalene (DBP, 1), DBP derivatives 2,24 previously investigated, and donor/acceptor-functionalized DBPs 3a−h presented herein.

our case mesityl), and the nucleophilic addition of ethynyl magnesium bromide to 7 can also be performed on a large scale. For interpreting the UV/vis absorption spectra and electrochemical measurements, TDDFT calculations were performed on DBP derivatives 3a−h. For the calculations, all mesityl groups in 3a−h were replaced by methyl groups denoted with an asterisk (3a*−h*). In a previous study on 2,7-bisarylalkynylsubstituted DBPs 2, the TPSSH38,39 functional best reproduced experimental orbital energies and the B3LYP40,41 functional gave the best absorption spectra compared to experimental results (see the Supporting Information (SI) for more details).24 Hence, these functionals were used in this study for DBPs 3a*−h* in combination with D342,43 and the def2TZVP44 basis set. The amphoteric redox behavior of DBP derivatives 3a−h became apparent in the cyclic voltammograms (Figure 2 and Table 1), where each featured two reduction waves and one oxidation wave. Only for derivatives 3f and 3g with electronwithdrawing substituents was no oxidation observed in the electrochemical window of the solvent (more CV measure-

Scheme 1. Synthesis of DBP Derivatives 3a−h

dibromodiphenyl succindandione 5, which was first subjected to Suzuki−Miyaura coupling with mesityl boronic acid pinacol ester to introduce solubility-enhancing mesityl substituents at the 2,7-positions, providing diketone 7 in 82% yield. The cerium trichloride-promoted35 nucleophilic addition of ethynyl magnesium bromide to 7 yielded diol 4. Electronically modulating aryl groups were attached to 4 using Sonogashira coupling reactions with the corresponding aryl iodides to furnish diols 8a−h followed by acid-catalyzed water elimination yielding DBP derivatives 3a−h. Using dehydrated p-TsOH in toluene allowed for performing the final water elimination step under mild conditions at room temperature. DBP derivatives 3a−h were dark red to purple-colored solids, indicating their low band gap. 5,10-Bis(arylalkynyl)-substituted DBPs have also been previously reported by Otera and co-workers.36 Their route, however, started from highly strained 5,6,11,12tetradehydrodibenzo[a,e]cyclooctene, which is challenging to synthesize.37 Our synthetic route to 3a−h is facile and versatile because diketone 5 as starting material can be accessed in gramscale quantities from inexpensive starting materials and reagents.24 Cross-coupling reactions allow for a substitution of the bromine atoms in 5 by any aromatic or other group (in

Figure 2. Cyclic voltammograms of 3a−h (1 mM in CH2Cl2, 0.1 M nBu4NPF6, 0.1 V s−1 scan rate, glassy carbon electrode for 3b−f, Pt electrode for 3a, 3g, and 3h).45 657

DOI: 10.1021/acs.joc.7b02250 J. Org. Chem. 2018, 83, 656−663

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The Journal of Organic Chemistry Table 1. Electrochemical and Optical Data for DBPs 3a−h

a

compound

E1/2 Red‑2/V

E1/2 Red‑1/V

E1/2 Ox/V

Eg (el. chem.)/eVa

Eg (opt)/eVb

3a 3b 3c 3d 3e 3f 3g 3h

−2.05 −1.88 −1.76 −1.82 −1.60 −1.56 −1.65 −1.54

−1.52 −1.43 −1.34 −1.36 −1.20 −1.21 −1.23 −1.14

0.22 0.79c 0.93 0.92 1.05 1.03

1.64 2.02 2.09 2.09 2.02 2.06 2.00 2.07

1.63 1.82 1.88 1.91 1.84 1.90 1.86 1.83

1.23c

b

From the onset of the reduction/oxidation peaks. From the onset of the longest wavelength absorption band. cAnodic peak potential.

strongly. As had been observed for derivatives 2, the energetic order of the HOMOs and LUMOs of 3a−h corresponds well with the Hammett constants σ of the substituents R: the σpara constants amount to −0.83 for NMe2 (3a), −0.27 for OMe (3b), 0 for H (3c), 0.23 for Cl (3d), 0.43 for CF3 (σmeta, 3e), 0.66 for CN (3f), and 0.78 for NO2 (3g) (a Hammett substitution constant for five fluorine substituents as in 3h is not available).47 The LUMOs in DBPs 3a*−h* are distributed over the DBP core and the arylalkynyl substituents, whereas for 3c*−h* the HOMOs are mostly localized on the DBP core (see representative orbital representations for 3h* in Figure 4).

ments can be found in the SI). HOMO and LUMO energies were obtained from the onset of the first oxidation/reduction peak vs Fc/Fc+, assuming an ionization energy of 4.8 eV for ferrocene,46 and compared to calculated orbital energies (TPSSH-D3/def2-TZVP, Figure 3). DBP derivatives 3a−h showed LUMO energies between −3.36 and −3.73 eV with the latter being in the range of values reported for typical n-type OFET materials.22

Figure 3. Energy diagram of experimental HOMO/LUMO energies of 3a−h (from CV measurements in solution determined from the onsets of the first reduction/oxidation peaks vs Fc/Fc+ assuming an ionization energy of 4.8 eV for ferrocene46) and calculated HOMO/ LUMO energies of 3a*−h* (TPSSH-D3/def2-TZVP).

The LUMO energy levels of 3a−h lie lower than those of 2,7-substituted DBP derivatives 2, where values between −3.18 and −3.34 eV were measured.24 This is in accordance with our assumption that attaching substituents at the 5,10-positions most strongly affects the LUMO energies. With values between −5.00 and −5.78 eV, the HOMO energy levels of 3a−h lie in a range comparable to that measured for 2 (−5.17 to −5.77 eV). The calculated orbital energies of DBP derivatives 3e*−h* with electron-withdrawing substituents were found relatively close in energy to the experimental ones, whereas those for 3a*−d* with electron-neutral or -rich substituents deviated more

Figure 4. Molecular orbitals of 3a* (left) and 3h* (right) (TPSSHD3/def2-TZVP).

Exceptions are the NMe2- and OMe-substituted derivatives 3a* and 3b*, where the HOMO and HOMO−1 are switched in energetic order compared to the other DBPs. Here, the HOMO is spread out over the arylalkynyl substituents likely due to the electron-donating nature of the NMe2 and OMe substituents. As a result, the HOMO has bg symmetry, which 658

DOI: 10.1021/acs.joc.7b02250 J. Org. Chem. 2018, 83, 656−663

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The Journal of Organic Chemistry leads to an allowed HOMO−LUMO transition (bg → au) visible in the absorption spectrum of 3a and 3b, as will be discussed later. The absorption spectra of DBPs 3a−h demonstrate the low bandgap of these compounds (Figure 5). According to

details on TDDFT calculations). Compared to the 2,7substituted DBP derivatives 2 described previously, the absorption range in 3a−h is wider, spanning the range from 260 to 790 nm compared to 270−680 nm in 2.24 None of the DBP derivatives 3a−h showed fluorescence, which had also been observed for 2,7-substituted DBP derivatives 2.24 For the structurally related indeno[1,2b]fluorenes, the lack of emissive properties was rationalized through transient absorption spectroscopy measurements that showed that the excited state lifetimes were shorter than the time scale at which fluorescence is usually observed.49 Although challenging to crystallize, single crystals suitable for X-ray diffraction were obtained from chloro- (3d, Figure 6) and methoxy-substituted DBP 3b (see the Supporting Information). With bond lengths between 1.397 and 1.436 Å, the C−C bonds in the six-membered rings of the DBP unit are fairly equally distributed and indicate aromatic conjugation within these rings. In the five-membered rings, on the other hand, significant bond length alternation is observed: With 1.521(3) Å, the C9′−C16 bond can be classified as a single bond, whereas the lengths of bonds C10−C11 and C10−C10′ (1.496(4) Å, 1.476(5) Å) indicate partial conjugation as does the distance of 1.392(3) Å for the C9−C10 bond. These bond lengths indicate that significant conjugation is present between the double bonds in the pentalene units and the aryl-alkyne units. As can be seen in Figure 6b, the molecules of 3d pack in a 1D-π-stacking pattern, where the distance between the DBP cores in two stacked layers amounts to 4.7 Å (4.8 Å for 3b, see the Supporting Information) with a shift of 7.1 Å parallel to the rings’ mean plane. The packing is stabilized by π−π interactions between the chlorobenzene units, which align in a parallel fashion, spaced out by 3.51 Å (Figure 6c, 2.39 Å for 3b). The dihedral angle between the mesityl groups and the DBP core amounts to 66°, whereas that between the chlorobenzene units and the DBP core is 81°.

Figure 5. Absorption spectra of 3a−h in CH2Cl2 solution (inset: spectra plotted on a logarithmic scale).

Laporte’s rule, the HOMO−LUMO transition is symmetry forbidden in a centrosymmetric molecule when both orbitals are of au symmetry.48 This is the case for derivatives 3c−h, whereas in 3a and 3b, the HOMO has bg symmetry (see Figure 4). Hence, the HOMO → LUMO transitions (S0 to S1) were only visible for 3a and 3b with NMe2 and OMe substituents at 565 and 574 nm, respectively, with extinction coefficients of log ε = 5.0 and 3.6 (see logarithmic scale in the inset in Figure 5). For 3c−h, the lowest visible absorption bands from 494−512 nm correspond to the HOMO−1 → LUMO transitions, according to TDDFT calculations, with log ε values of 4.2−4.7. The other absorption bands represent multiple transitions involving energetically lower or higher lying orbitals (see SI for

Figure 6. Molecular structure (a) and packing diagrams (b, c) of 3d (hydrogen atoms are omitted for clarity, thermal ellipsoids (in a) are shown at 50% probability level).50 659

DOI: 10.1021/acs.joc.7b02250 J. Org. Chem. 2018, 83, 656−663

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mL). The combined organic layers were dried (MgSO4) and filtered, and the solvent was evaporated under reduced pressure. Column chromatography (silica gel, cyclohexane/EtOAc: 8/1 to 5/1) afforded compound 7 (787 mg, 1.67 mmol, 82%) as a white solid. Rf = 0.60 (cyclohexane/EtOAc: 4/1); mp 277 °C; 1H NMR (400 MHz, CDCl3) δ 7.99 (d, J = 7.8 Hz, 2H), 7.54 (d, J = 1.7 Hz, 2H), 7.49 (dd, J = 7.8, 1.7 Hz, 2H), 6.94 (s, 2H), 6.92 (s, 2H), 4.50 (s, 2H), 2.32 (s, 6H), 1.98 (s, 6H), 1.92 (s, 6H); 13C NMR (101 MHz, CDCl3) δ 201.9, 148.4, 142.6, 137.7, 137.4, 136.0, 135.8, 135.5, 128.4, 128.4, 126.7, 125.6, 52.9, 27.1, 21.2, 21.0; HRMS (APCI+) m/z calcd for C34H31O2+ 471.2319 [M + H]+, found 471.2318. 5,10-Diethynyl-2,7-dimesityl-4b,5,9b,10-tetrahydroindeno[2,1-a]indene-5,10-diol (4). Commercially available anhydrous CeCl3 (786 mg, 3.19 mmol) was dried at 130 °C for 4 h in vacuo. After cooling to rt, anhydrous THF (20 mL) was added, and the suspension was stirred overnight in a glovebox. The white suspension was cooled to −20 °C, and ethynylmagnesium bromide solution (0.5 M in THF, 1.60 mL, 3.19 mmol) was added. The suspension was stirred at −20 °C for 4 h. Diketone 7 (485 mg, 1.03 mmol) was added, and the resulting mixture was stirred at −20 °C for 6 h. The reaction mixture was allowed to warm to rt and stirred for another 18 h. Aq AcOH (10% v/v, 20 mL) was added, and the aqueous layer was extracted with Et2O (3 × 40 mL). The combined organic layers were dried (MgSO4) and filtered, and the solvent was removed under reduced pressure. Column chromatography (silica gel, cyclohexane/ EtOAc: 10/1 to 4/1) afforded diol 4 (424 mg, 811 μmol, 79%) as a white solid. Rf = 0.45 (cyclohexane/EtOAc: 4/1); mp 274 °C (dec.); 1 H NMR (300 MHz, CDCl3) δ 7.54 (d, J = 7.7 Hz, 2H), 7.36 (d, J = 1.6 Hz, 2H), 7.17 (dd, J = 7.7, 1.6 Hz, 2H), 6.88 (s, 4H), 4.40 (s, 2H), 2.78 (s, 2H), 2.63 (s, 2H), 2.26 (s, 6H), 1.97 (s, 6H), 1.95 (s, 6H); 13C NMR (101 MHz, CDCl3) δ 147.7, 142.3, 138.5, 136.8, 136.4, 136.1, 135.7, 131.3, 128.2, 128.1, 125.9, 125.6, 84.9, 75.0, 73.4, 62.0, 21.0, 20.9, 20.8; HRMS (ESI+) m/z calcd for C38H34O2Na+ 545.2451 [M + Na]+, found 545.2449. General Procedure A for Sonogashira Coupling Reactions of 4 with Solid Iodobenzene Derivatives. Diol 4, Pd(PPh3)2Cl2, CuI, and the respective iodobenzene derivative were dried under vacuum for 1 h. A degassed and anhydrous mixture of toluene and NEt3 was added, and the reaction mixture was stirred at rt for the indicated time. Sat. aq NH4Cl was added, and the aqueous layer was extracted with Et2O. The combined organic layers were dried (MgSO4) and filtered, and the solvent was removed under reduced pressure. The residue was purified by column chromatography to afford the respective product. General Procedure B for Sonogashira Coupling Reactions of 4 with Liquid Iodobenzene Derivatives. Diol 4 and Pd(PPh3)2Cl2 were dried under vacuum for 15 h. CuI and a degassed and anhydrous mixture of toluene and NEt3 were added followed by the respective iodobenzene derivative. The mixture was stirred at rt for the indicated time. Sat. aq NH4Cl was added, and the aqueous layer was extracted with Et2O. The combined organic layers were dried (MgSO4) and filtered, and the solvent was removed under reduced pressure. The residue was purified by column chromatography to afford the respective product. General Procedure C for Sonogashira Coupling and Water Elimination to DBP Derivatives 3. Diol 4, Pd(PPh3)2Cl2, CuI, and the respective iodobenzene derivative were dried under vacuum for 1 h. A degassed and anhydrous mixture of toluene and NEt3 was added, and the mixture was stirred at rt for the indicated time. Sat. aq NH4Cl was added, and the aqueous layer was extracted with Et2O. The combined organic layers were dried (MgSO4) and filtered, and the solvent was removed under reduced pressure. The crude product was dissolved in anhydrous and degassed toluene, and anhydrous p-TsOH was added. The mixture was stirred for the indicated time in a glovebox. H2O was added, and the aqueous layer was extracted with CH2Cl2. The combined organic layers were washed with brine, dried (MgSO4), and filtered, and the solvent was removed under reduced pressure. The residue was purified by column chromatography. General Procedure D for Water Elimination from Diols 8. The respective diol was dissolved in anhydrous toluene, and anhydrous p-TsOH was added. The mixture was stirred for the indicated time.

CONCLUSIONS In conclusion, we have presented facile and versatile access to 5,10-bis(arylalkyne)-substituted dibenzo[a,e]pentalenes 3 with mesityl groups in the 2,7-positions. Electrochemical, optical, and theoretical investigations of eight derivatives 3a−h showed that the HOMO/LUMO energy levels can be tuned and that low-lying LUMO energies down to −3.73 eV and small bandgaps down to 1.63 eV (optical band gap) can be achieved. X-ray crystallographic analysis of derivative 3d revealed 1D-πstacking in the solid state. These values render DBP derivatives 3a−h of interest as ambipolar or n-type materials for organic field-effect transistors.

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EXPERIMENTAL SECTION

Chemicals were purchased from Acros-Organics, Alfa-Aesar, Merck, Roth, Sigma-Aldrich, ChemPur TCI-Europe, or VWR and used directly without further purification unless otherwise noted. Experiments with moisture- or oxygen-sensitive substances were carried out in dried glassware, heated under vacuum (10−3 mbar), under an argon atmosphere using standard Schlenk techniques. Anhydrous solvents (THF, toluene, CH2Cl2) were obtained from an M. Braun solvent purification system (MB-SPS-800) and stored over 3 Å molecular sieves for a minimum duration of 48 h before use. Other anhydrous solvents were obtained by drying over activated molecular sieves (3 Å) for several days. Diethyl ether, ethyl acetate, and cyclohexane were purchased in technical grade and purified via distillation using a rotary evaporator. Other solvents were purchased and used in analytical or HPLC grade. 1H, 13C{1H}, and 19F NMR spectra were recorded at 300 K on the following spectrometers: Bruker Avance III HD [300.1 MHz (1H resonance), 282.4 MHz (19F resonance)], Bruker Avance III HD [500.3 MHz (1H resonance), 125.8 MHz (13C resonance)], and at 303 K on a Bruker Avance II [400.1 MHz (1H resonance), 101.6 MHz (13C resonance)]. 1H NMR spectra were referenced to tetramethylsilane (δ = 0.00 ppm) or the residual proton resonance of the respective solvent: CDCl3 δ = 7.26 ppm; CD2Cl2 δ = 5.32 ppm; DMSO-d6 δ = 2.50 ppm.51 13C NMR spectra were referenced to the carbon resonance of the deuterated solvent: CDCl3 δ = 77.16 ppm; CH2Cl2 δ = 53.84 ppm; DMSO-d6 δ = 39.52 ppm.51 19F NMR spectra were referenced to tetramethylsilane following the IUPAC recommendations.52 NMR spectra were analyzed using MestReNova software. Analysis followed first order, and the following abbreviations for multiplets were used: singlet (s), broad singlet (br), doublet (d), triplet (t), quartet (q), multiplet (m), and combinations thereof, i.e., doublet of doublets (dd). Coupling constants (J) are given in hertz [Hz]. Electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) mass spectra were recorded with a Thermo Fisher Scientific Inc. Exactive or LCQ Advantage with an orbitrab analyzer. Only the structurally significant and strongest fragmentation peaks are reported. UV/vis absorption spectra were measured on a PerkinElmer LAMBDA 950 UV/vis spectrophotometer. Melting points were measured using an MPM-HV2 from Schorpp-Gerätetechnik. Silica gel-layered alumina plates (Merck, Silica Gel 60 F254) were used for TLC, and compounds were detected using UV light (λ = 366 and 254 nm) and/or with a KMnO4 staining solution (3.0 g KMnO4, 20 g K2CO3, 5.0 mL 5.0% NaOH, 300 mL H2O). For flash column chromatography, silica gel (Machery-Nagel 60, 40−63 μm) was used as the stationary phase. Eluents are described in the respective synthetic procedures. Synthesis of 2,7-Dimesityl-4b,9b-dihydroindeno[2,1-a]indene-5,10-dione (7). Pd(PPh3)4 (118 mg, 102 μmol), diketone 5 (800 mg, 2.04 mmol), and 2-mesityl-4,4,5,5-tetramethyl-1,3,2dioxaborolane (6 (for synthesis, see SI), 1.11 g, 4.49 mmol) were dissolved in degassed THF (40 mL) and degassed aq Na2CO3 (2 M, 40 mL). The resulting suspension was stirred at 80 °C for 10 h. Additional portions of Pd(PPh3)4 (71.0 mg, 61.4 μmol) and 2-mesityl4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6, 502 mg, 2.04 mmol) were added, and the reaction mixture was stirred at 80 °C for 15 h. After cooling to rt, the reaction mixture was extracted with CH2Cl2 (3 × 50 660

DOI: 10.1021/acs.joc.7b02250 J. Org. Chem. 2018, 83, 656−663

Article

The Journal of Organic Chemistry

CD2Cl2) δ = 150.2, 149.2, 143.0, 139.1, 137.3, 136.4, 136.0, 133.9, 131.9, 129.7, 129.5, 128.6, 123.9, 123.2, 121.7, 120.3, 102.6, 85.5, 21.3, 21.0; HRMS (APCI+) m/z calcd for C50H39+ 639.3046 [M + H]+, found 639.3043; UV/vis (CH2Cl2) λabs (ε) = 284 (70,087), 302 (63,764), 468 (16,213), 498 (18,225) nm. 5,10-Bis((4-chlorophenyl)ethynyl)-2,7-dimesityl-4b,5,9b,10tetrahydroindeno[2,1-a]indene-5,10-diol (8d). General procedure A was followed using diol 4 (93.0 mg, 178 μmol), PdCl2(PPh3)2 (10.0 mg, 14.3 μmol), CuI (4.70 mg, 24.7 μmol), 1-chloro-4iodobenzene (130 mg, 543 μmol), toluene (10 mL), and NEt3 (3 mL). The reaction mixture was stirred at rt for 14 h and at 60 °C for 7 h. Column chromatography (silica gel, cyclohexane/EtOAc: 10/1 to 4/1) afforded 8d (65.9 mg, 88.6 mmol, 50%) as a white solid. Rf = 0.70 (cyclohexane/EtOAc: 4/1); mp 218 °C (dec); 1H NMR (300 MHz, CDCl3) δ 7.66 (d, J = 7.7 Hz, 2H), 7.50 (d, J = 1.5 Hz, 2H), 7.45−7.42 (m, 4H), 7.32−7.30 (m, 4H), 7.26 (dd, J = 7.7, 1.5 Hz, 2H), 6.96 (s, 4H), 4.58 (s, 2H), 2.99 (s, 2H), 2.33 (s, 6H), 2.05 (s, 6H), 2.05 (s, 6H); 13C NMR (126 MHz, CDCl3) δ 148.2, 142.4, 138.6, 137.0, 136.6, 136.4, 135.9, 134.9, 133.2, 131.4, 128.9, 128.4, 128.3, 126.2, 125.8, 121.0, 91.2, 84.4, 75.8, 62.5, 27.1, 21.2, 21.1. 5,10-Bis((4-chlorophenyl)ethynyl)-2,7-dimesitylindeno[2,1a]indene (3d). General procedure D was followed using diol 8d (65.9 mg, 88.6 μmol), anhydrous p-TsOH (20.0 mg, 116 μmol), and toluene (10 mL). The reaction mixture was stirred at rt for 1 h. Column chromatography (silica gel, cyclohexane/CH2Cl2: 2/1 to 0/1) afforded 3d (48.4 mg, 68.4 μmol, 76%) as a dark red solid. Rf = 0.72 (cyclohexane/EtOAc: 5/1); mp >350 °C; 1H NMR (300 MHz, CD2Cl2) δ 7.62−7.58 (m, 4H), 7.47 (dd, J = 7.5, 0.5 Hz, 2H), 7.41− 7.37 (m, 6H), 6.96−6.95 (m, 4H), 6.80 (dd, J = 7.5, 1.5 Hz, 2H), 2.32 (s, 6H), 2.12 (s, 12H); 13C NMR (101 MHz, CD2Cl2) δ 150.0, 149.4, 142.8, 139.2, 137.3, 133.9, 132.7, 132.1, 130.0, 129.6, 129.5, 129.2, 128.6, 123.9, 123.2, 120.6, 103.9, 84.7, 21.3, 21.0; HRMS (APCI+) m/z calcd for C50H37Cl2+ 707.2267 [M + H]+, found 707.2263; UV/vis (CH2Cl2) λabs (ε) = 286 (56,117), 320 (26,720), 470 (25,946), 503 (28,790) nm. 5,10-Bis((3,5-bis(trifluoromethyl)phenyl)ethynyl)-2,7-dimesityl-4b,5,9b,10-tetrahydroindeno[2,1-a]indene-5,10-diol (8e). General procedure A was followed using diol 4 (50.0 mg, 96.0 μmol), PdCl2(PPh3)2 (5.60 mg, 8.00 μmol), CuI (2.28 mg, 8.00 μmol), 1iodo-3,5-bis(trifluoromethyl)benzene (130 mg, 384 μmol), toluene (3 mL), and NEt3 (2 mL). The reaction mixture was stirred at rt for 18 h. Column chromatography (silica gel, cyclohexane/EtOAc: 10/1 to 2/ 1) afforded 8e (77.2 mg, 81.5 μmol, 85%) as a colorless solid. Rf = 0.48 (cyclohexane/EtOAc: 5/1); 1H NMR (500 MHz, CDCl3) δ 7.95 (s, 4H), 7.85 (s, 2H), 7.67 (d, J = 7.7 Hz, 2H), 7.50 (d, J = 1.5 Hz, 2H), 7.31 (dd, J = 7.7, 1.5 Hz, 2H), 6.97 (s, 4H), 4.63 (s, 2H), 2.99 (s, 2H), 2.33 (s, 6H), 2.06 (s, 6H), 2.05 (s, 6H); 13C NMR (126 MHz, CDCl3) δ 147.5, 142.6, 138.2, 137.0, 136.1, 135.7, 132.1 (q, J = 33.8 Hz), 131.7, 128.3, 128.2, 126.1, 125.7, 124.6, 122.9 (q, J = 273.2 Hz), 93.6, 82.4, 75.6, 62.5, 21.0, 20.9, 20.9; 19F NMR (282 MHz, CDCl3) δ −63.1; HRMS (ESI+) m/z calcd for C54H38O2F12Na+ 969.2572 [M + Na]+, found 969.2571. 5,10-Bis((3,5-bis(trifluoromethyl)phenyl)ethynyl)-2,7dimesitylindeno[2,1-a]indene (3e). General procedure D was followed using diol 8e (58.3 mg, 62.0 μmol), anhydrous p-TsOH (23.1 mg, 134 μmol), and toluene (10 mL). The reaction mixture was stirred at rt for 2.5 h. Column chromatography (silica gel, cyclohexane/ CH2Cl2: 25/1 to 5/1) afforded 3e (50.7 mg, 55.7 μmol, 90%) as a red solid. Rf = 0.60 (cyclohexane/CH2Cl2: 1/1); mp >350 °C; 1H NMR (500 MHz, CD2Cl2) δ 7.98 (s, 4H), 7.86 (s, 2H), 7.41 (d, J = 7.4 Hz, 2H), 6.98 (s, 4H), 6.90 (d, J = 1.4 Hz, 2H), 6.84 (dd, J = 7.4, 1.4 Hz, 2H), 2.35 (s, 6H), 2.13 (s, 12H); 13C NMR (126 MHz, CDCl3) δ 151.1, 148.5, 142.9, 138.3, 137.1, 135.9, 132.4 (q, J = 34.1 Hz), 131.7, 131.0, 129.3, 128.2, 123.9, 123.4, 123.2, 122.8 (q, J = 273.0 Hz), 119.3, 99.0, 86.7, 21.0, 20.7; 19F NMR (282 MHz, CDCl3) δ −63.1; HRMS (APCI+) m/z calcd for C54H34F12+ 910.2474 [M]+, found 910.2474; UV/vis (CH2Cl2) λabs (ε) = 286 (112,007), 338 (61,768), 468 (43,308), 494 (45,733) nm. 4,4′-((2,7-Dimesitylindeno[2,1-a]indene-5,10-diyl)bis(ethyne-2,1-diyl))dibenzonitrile (3f). General procedure C was

H2O was added, and the aqueous layer was extracted with CH2Cl2. The combined organic layers were washed with brine, dried (MgSO4), and filtered, and the solvent was removed under reduced pressure. The residue was purified by flash column chromatography. 5,10-Bis((4-(dimethylamino)phenyl)ethynyl)-2,7-dimesityl4b,5,9b,10-tetrahydroindeno[2,1-a]indene-5,10-diol (8a). General procedure A was followed using diol 4 (60.0 mg, 115 μmol), PdCl2(PPh3)2 (3.20 mg, 4.60 μmol), CuI (0.9 mg, 4.60 μmol), 4-iodoN,N-dimethylaniline (for synthesis see, SI, 85.0 mg, 345 μmol), toluene (4 mL), and NEt3 (4 mL). The reaction mixture was stirred for 15 h at rt. Column chromatography (silica gel, cyclohexane/ EtOAc: 20/1 to 1/1) afforded compound 8a (31.0 mg, 40.7 μmol, 35%) as a beige solid. Rf = 0.42 (cyclohexane/EtOAc: 4/1); 1H NMR (300 MHz, CDCl3) δ 7.60 (d, J = 7.7 Hz, 2H), 7.44 (d, J = 1.6 Hz, 2H), 7.33−7.28 (m, 4H), 7.15 (dd, J = 7.7, 1.6 Hz, 2H), 6.88 (s, 4H), 6.58−6.53 (m, 4H), 4.50 (s, 2H), 2.94 (s, 2H), 2.90 (s, 12H), 2.26 (s, 6H), 1.99 (s, 6H), 1.98 (s, 6H). 4,4′-((2,7-Dimesitylindeno[2,1-a]indene-5,10-diyl)bis(ethyne-2,1-diyl))bis(N,N-dimethylaniline) (3a). General procedure D was followed using diol 8a (38.6 mg, 48.8 μmol), anhydrous pTsOH (12.1 mg, 70.3 μmol), and toluene (7 mL). The mixture was stirred at rt for 4 h. Column chromatography (silica gel, cyclohexane/ CH2Cl2: 3/2 to 0/1) afforded 3a (14.8 mg, 20.4 μmol, 42%) as a dark purple solid. Rf = 0.76 (cyclohexane/CH2Cl2: 1/1); mp >350 °C; 1H NMR (300 MHz, CDCl3) δ 7.48−7.43 (m, 6H), 6.98 (s, 4H), 6.95 (d, J = 1.5 Hz, 2H), 6.77 (dd, J = 7.4, 1.5 Hz, 2H), 6.68−6.63 (m, 4H), 3.02 (s, 12H), 2.36 (s, 6H), 2.14 (s, 12H); HRMS (APCI+) m/z calcd for C54H49N2+ 725.3890 [M + H]+, found 725.3892; UV/vis (CH2Cl2) λ abs (ε) = 295 (176,024), 365 (24,525), 420 (27,809), 565 (103,669) nm. 2,7-Dimesityl-5,10-bis((4-methoxyphenyl)ethynyl)indeno[2,1-a]indene (3b). General procedure C was followed using diol 4 (50.0 mg, 96 μmol), 4-iodoanisole (67.0 mg, 287 μmol), PdCl2(PPh3)2 (3.40 mg, 4.80 μmol), CuI (0.90 mg, 4.80 μmol), NEt3 (0.5 mL), and toluene (2.0 mL). The reaction mixture was stirred for 18 h. After workup, the crude product was dissolved in toluene (7.0 mL), and anhydrous p-TsOH (0.5 M in toluene, 1.0 mL) was added. The dark red solution was stirred for 3 h. Column chromatography (silica gel, cyclohexane/EtOAc/NEt3: 100/0/3 to 100/25/3) afforded 3b (32.3 mg, 46.0 μmol, 48%) as a red solid. Rf = 0.51 (cyclohexane/EtOAc: 4/1); 1H NMR (400 MHz, CDCl3) δ 7.53−7.49 (m, 4H), 7.44 (d, J = 7.5 Hz, 2H), 6.97 (s, 4H), 6.93 (d, J = 1.5 Hz, 2H), 6.92−6.87 (m, 4H), 6.78 (dd, J = 7.5, 1.5 Hz, 2H), 3.84 (s, 6H), 2.35 (s, 6H), 2.13 (s, 12H); 13C NMR (101 MHz, CDCl3) δ 160.5, 149.0, 148.7, 141.6, 139.0, 136.8, 136.2, 133.8, 132.0, 128.7, 128.2, 123.2, 122.5, 120.1, 115.0, 114.3, 103.6, 83.7, 55.5, 21.1, 20.8; HRMS (APCI+) m/z calcd for C52H43O2+ 699.3258 [M + H]+, found 699.3259; UV/vis (CH2Cl2) λabs (ε) = 285 (78,630), 377 (18,555), 480 (33,761), 512 (39,546) nm. 2,7-Dimesityl-5,10-bis(phenylethynyl)-4b,5,9b,10tetrahydroindeno[2,1-a]indene-5,10-diol (8c). General procedure A was followed using diol 4 (60.0 mg, 115 μmol), PdCl2(PPh3)2 (8.10 mg, 11.5 μmol), CuI (2.20 mg, 11.5 μmol), iodobenzene (110 mg, 544 μmol), toluene (5 mL), and NEt3 (1 mL). The mixture was stirred for 17 h at rt. Column chromatography (silica gel, cyclohexane/ EtOAc: 5/1 to 4/1) afforded 8c (61.6 mg, 91.3 mmol, 79%) as a yellow solid. Rf = 0.54 (cyclohexane/EtOAc: 4/1); 1H NMR (300 MHz, CDCl3) δ 7.68 (d, J = 7.6 Hz, 2H), 7.53−7.49 (m, 6H), 7.35− 7.33 (m, 6H), 7.36 (dd, J = 7.6, 1.6 Hz, 2H), 6.95 (s, 4H), 4.60 (s, 2H), 3.01 (s, 2H), 2.33 (s, 6H), 2.06 (s, 12H); HRMS (APCI+) m/z calcd for C50H39+ 639.3046 [M + H]+, found 639.3042. 2,7-Dimesityl-5,10-bis(phenylethynyl)indeno[2,1-a]indene (3c). General procedure D was followed using crude diol 8c (61.6 mg, 91.3 μmol), anhydrous p-TsOH (20.0 mg, 116 μmol), and toluene (10 mL). The mixture was stirred at rt for 5 h. Recrystallization from MeCN afforded 3c (50.0 mg, 78.3 μmol, 86%) as a dark red solid. Rf = 0.72 (cyclohexane/EtOAc: 5/1); mp >350 °C; 1H NMR (300 MHz, CDCl3) δ 7.52−7.48 (m, 4H), 7.41 (d, J = 7.4 Hz, 2H), 7.37−7.32 (m, 4H), 7.26 (s, 2H), 6.97 (s, 4H), 6.90 (d, J = 1.4 Hz, 2H), 6.78 (dd, J = 7.4, 1.4 Hz, 2H), 2.34 (s, 6H), 2.12 (s, 12H); 13C NMR (126 MHz, 661

DOI: 10.1021/acs.joc.7b02250 J. Org. Chem. 2018, 83, 656−663

Article

The Journal of Organic Chemistry followed using diol 4 (82.0 mg, 157 μmol), 4-iodobenzonitrile (180 mg, 784 μmol), PdCl2(PPh3)2 (8.80 mg, 12.6 μmol), CuI (2.40 mg, 12.6 μmol), NEt3 (0.3 mL), and toluene (7.0 mL). The reaction mixture was stirred for 2 h. After workup and filtration through a silica plug, the crude product was dissolved in toluene (5.0 mL), and pTsOH (34.0 mg, 197 μmol) was added. The dark red solution was stirred at rt for 3 h. Two consecutive column chromatography steps (silica gel, cyclohexane/EtOAc: 1/0 to 0/1 and cyclohexane/CH2Cl2: 1/2 to 0/1) afforded 3f (54.4 mg, 79.0 μmol, 50%) as a red solid. Rf = 0.51 (cyclohexane/EtOAc: 4/1); mp >350 °C; 1H NMR (300 MHz, CDCl3) δ 7.69−7.63 (m, 8H), 7.40 (d, J = 7.4 Hz, 2H), 6.98 (s, 4H), 6.89 (d, J = 1.4 Hz, 2H), 6.80 (dd, J = 7.4, 1.4 Hz, 2H), 2.35 (s, 6H), 2.13 (s, 12H); 13C NMR (126 MHz, CDCl3) δ 151.0, 148.7, 142.8, 138.5, 137.2, 136.1, 132.6, 132.3, 131.3, 129.3, 128.3, 127.4, 123.5, 123.2, 119.7, 118.4, 112.6, 101.0, 87.9, 21.2, 20.8; HRMS (APCI+) m/z calcd for C52H40N3+ 706.3217 [M + NH4]+, found 706.3215; UV/vis (CH2Cl2) λabs (ε) = 292 (71,386), 347 (29,580), 474 (27,434), 506 (28,588) nm. 2,7-Dimesityl-5,10-bis((4-nitrophenyl)ethynyl)-4b,5,9b,10tetrahydroindeno[2,1-a]indene-5,10-diol (8g). General procedure A was followed using diol 4 (81.2 mg, 155 μmol), PdCl2(PPh3)2 (9.20 mg, 13.1 μmol), CuI (2.00 mg, 15.8 μmol), 1-iodo-4nitrobenzene (155 mg, 622 μmol), toluene (12.0 mL), and NEt3 (3.0 mL). The reaction mixture was stirred at rt for 40 h. Column chromatography (silica gel, cyclohexane/EtOAc: 8/1) afforded 8g (83.0 mg, 109 μmol, 70%) as a brown solid. Rf = 0.51 (cyclohexane/ EtOAc: 3/1); mp 189 °C (dec); 1H NMR (300 MHz, CDCl3) δ 8.25−8.21 (m, 4H), 7.69−7.65 (m, 6H), 7.51 (d, J = 1.6 Hz, 2H), 7.31 (dd, J = 7.7, 1.6 Hz, 2H), 6.97 (s, 4H), 4.64 (s, 2H), 3.07 (s, 2H), 2.34 (s, 6H), 2.06 (s, 12H); 13C NMR (101 MHz, CDCl3) δ 147.7, 147.5, 142.6, 138.3, 137.0, 136.2, 136.1, 135.7, 132.6, 131.6, 129.2, 128.3, 128.2, 126.1, 125.7, 123.6, 95.2, 83.5, 75.7, 62.3, 21.0, 20.9, 20.9; HRMS (ESI+) m/z calcd for C50H40O6N2Na+ 787.2779 [M + Na]+, found 787.2777. 2,7-Dimesityl-5,10-bis((4-nitrophenyl)ethynyl)indeno[2,1-a]indene (3g). General procedure D was followed using crude diol 8g (48.4 mg, 62.3 μmol), anhydrous p-TsOH (42.2 mg, 245 μmol), and toluene (13 mL). The reaction mixture was stirred at rt for 3.5 h. Column chromatography (silica gel, cyclohexane/CH2Cl2: 1/1 to 0/1) afforded 3g (41.3 mg, 56.7 μmol, 91%) as a red solid. Rf = 0.40 (cyclohexane/CH2Cl2: 1/1); 1H NMR (300 MHz, CDCl3) δ 8.27− 8.22 (m, 4H), 7.73−7.69 (m, 4H), 7.50 (d, J = 7.4 Hz, 2H), 6.98 (s, 4H), 6.90 (d, J = 1.4 Hz, 2H), 6.82 (dd, J = 7.4, 1.4 Hz, 2H), 2.35 (s, 6H), 2.13 (s, 12H); HRMS (APCI+) m/z calcd for C50H40O4N3+ 746.3013 [M+NH4]+, found 746.3008; UV/vis (CH2Cl2) λabs (ε) = 300 (42,251), 365 (21,941), 483 (17,505), 512 (17,907) nm. 2,7-Dimesityl-5,10-bis((per fluorophenyl)ethynyl)4b,5,9b,10-tetrahydroindeno[2,1-a]indene-5,10-diol (8h). General procedure A was followed using diol 4 (50.0 mg, 96.0 μmol), PdCl2(PPh3)2 (5.60 mg, 8.00 μmol), CuI (2.28 mg, 8.00 μmol), pentafluoroiodobenzene (113 mg, 384 μmol), toluene (3 mL), and NEt3 (2 mL). The reaction mixture was stirred at rt for 18 h. Column chromatography (silica gel, cyclohexane/EtOAc: 10/1 to 6/1) afforded 8h (26.7 mg, 31.2 μmol, 33%) as a colorless solid. Rf = 0.40 (cyclohexane/EtOAc: 5/1); 1H NMR (300 MHz, CDCl3) δ 7.65 (d, J = 7.7 Hz, 2H), 7.50 (d, J = 1.5 Hz, 2H), 7.30 (dd, J = 7.7, 1.5 Hz, 2H), 6.97 (s, 4H), 4.63 (s, 2H), 3.02 (s, 2H), 2.34 (s, 6H), 2.05 (s, 12H); 13C NMR (126 MHz, CDCl3) δ 148.6−148.4 (m), 147.2, 146.6−146.4 (m), 142.7, 141.1−140.9 (m), 138.8−138.5 (m), 138.2, 136.9, 136.7−136.5 (m), 136.1, 136.0, 135.8, 131.7, 128.2, 128.2, 126.1, 125.7, 102.6, 75.7, 69.5, 62.3, 21.0, 20.9, 20.8; 19F NMR (282 MHz, THF-d8) δ −(138.1−138.2) (m), −(154.7−154.8) (m), −(164.0−164.2) (m); HRMS (APCI + ) m/z calcd for C50H32O2F10Na+ 877.2135 [M + Na]+, found 877.2133. 2,7-Dimesityl-5,10-bis((perfluorophenyl)ethynyl)indeno[2,1a]indene (3h). General procedure D was followed using diol 8h (21.2 mg, 25.0 μmol), anhydrous p-TsOH (23.1 mg, 134 μmol), and toluene (20 mL). The reaction mixture was stirred at rt for 2.5 h. Column chromatography (silica gel, cyclohexane/CH2Cl2: 8/1 to 0/1) afforded 3h (18.4 mg, 22.4 μmol, 90%) as a red solid. Rf = 0.55 (cyclohexane/

CH2Cl2: 4/1); mp >350 °C; 1H NMR (500 MHz, CD2Cl2) δ 7.41 (d, J = 7.4 Hz, 2H), 6.96 (s, 4H), 6.87 (d, J = 1.4 Hz, 2H), 6.79 (dd, J = 7.4, 1.4 Hz, 2H), 2.34 (s, 6H), 2.12 (s, 12H); 19F NMR (282 MHz, THF-d8) δ −(134.6−134.7) (m), −(150.2−150.4) (m), −(160.9− 161.1) (m); HRMS (APCI+) m/z calcd for C50H28F10+ 818.2037 [M]+, found 818.2040; UV/vis (CH2Cl2) λabs (ε) = 284 (44,813), 468 (15,574), 500 (16,531) nm. Electrochemical Measurements. Cyclic voltammograms (CVs) were measured inside a glovebox using a PGSTAT128N by Metrohm Autolab. A glassy carbon disc electrode (2 mm diameter) or a platinum disc electrode (2 mm diameter) was used as the working electrode; a platinum rod was used as counter electrode, and a Ag/ AgNO3-electrode containing a silver wire immersed in an inner chamber filled with 1 M AgNO3 and 0.1 M n-Bu4NPF6 in anhydrous CH3CN was used as reference electrode. The analyte solution contained 10 mL of solvent (anhydrous CH2Cl2) with 0.1 M nBu4NPF6 and the specified analyte concentration. The ferrocene/ ferrocenium redox couple was used as internal reference. HOMO and LUMO levels were calculated using the following equations: ELUMO (eV) = −(Ei,Fc + xRed) (with Ei,Fc = ionization energy of ferrocene = 4.8 eV;46 xRed = onset of the first reduction peak, calibrated vs Fc/Fc+ in eV), EHOMO (eV) = −(Ei,Fc + xOx) (with xOx = onset of the first oxidation peak, calibrated vs Fc/Fc+ in eV). DFT Calculations. TDDFT calculations were performed with the TURBOMOLE v7.0 program package.53 The resolution-of-identity (RI, RIJDX for SP)54,55 approximation for the Coulomb integrals was used in all DFT calculations employing matching auxiliary basis set def2-XVP/J.56 Further, the D3 dispersion correction scheme42,43 with the Becke−Johnson damping function was applied.57−59 The geometries of 3a*−h* were optimized without symmetry restrictions at the PBE60-D3/def2-SVP61 level followed by harmonic vibrational frequency analyses to confirm minima as stationary points. The structures were then reoptimized using TPSS38-D3/def2-TZVP.44 Vertical excitation energies were calculated using TDDFT using the TPSSH38,39 and B3LYP40,41 functionals with the def2-TZVP basis set. For TDDFT calculations, geometries were optimized with symmetry restriction within the C2h point group at the functional-D3/def2-TZVP level of theory. Vertical excitation energies were then calculated using TDDFT at the same level of theory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02250. X-ray crystallography for compound 3 (CIF) Additional synthetic procedures, CVs, NMR spectra, energies, and Cartesian coordinates of calculated structures, results of TDDFT calculations, and X-ray crystallographic data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Birgit Esser: 0000-0002-2430-1380 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

Generous support by the German Research Foundation (Emmy Noether grant ES 361/2-1 and grant No. INST 40/ 467-1 FUGG) and the state of Baden-Württemberg through bwHPC is gratefully acknowledged. 662

DOI: 10.1021/acs.joc.7b02250 J. Org. Chem. 2018, 83, 656−663

Article

The Journal of Organic Chemistry

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DOI: 10.1021/acs.joc.7b02250 J. Org. Chem. 2018, 83, 656−663