Synthesis of Cyclobutadienoid-Fused Phenazines with Strongly

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Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Synthesis of Cyclobutadienoid-Fused Phenazines with Strongly Modulated Degrees of Antiaromaticity Yew Chin Teo, Zexin Jin, and Yan Xia* Department of Chemistry, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: The streamlined synthesis of a series of regioisomeric azaacene analogues containing fused phenazine and antiaromatic cyclobutadienoids (CBDs), using a catalytic arene−oxanorbornene annulation, followed by aromatization is reported. Controlling the fusion patterns allowed strong modulation of local antiaromaticity. Enhancing antiaromaticity in these regioisomeric azaacenes led to stabilized LUMO, reduced band gap, and quenched fluorescence. This synthetic strategy provides a facile means to fuse CBDs with variable degrees of antiaromaticity onto N-heteroarenes to tune their optoelectronic properties.

N

works revealed that the regioconnectivity of a fused antiaromatic moiety in large PCHs can strongly affect the degree of antiaromaticity and, thus, their optoelectronic properties.26−28 Therefore, it is interesting to investigate how the varied degree of antiaromaticity in azaarenes can be used to tune their optical and electronic properties. Intrigued by these questions, we sought to use the CANAL strategy to synthesize a new class of regioisomeric CBD-containing azaacene analogues using electron-deficient phenazine as an exemplary unit. Aiming to fuse CBD onto the 1,2- or 2,3-bond of phenazine, we first attempted a CANAL reaction between bromophenazine 1a or 1b and 1,4-dimethylbenzooxanorbornadiene 2a in the presence of 5 mol % of Pd(OAc)2 with Johnphos ligand and 1 equiv of Cs2CO3 in THF at 130 °C, which were the optimized conditions for the nonheterocyclic aromatic substrates.25 Although the reaction with 1a provided the annulation product in 60% yield, only the starting materials were observed for the reaction with 1b. We postulated that the reaction may have been inhibited by the potential coordination of the palladium complex after migratory insertion of oxanorbornene by the phenazine nitrogen in close proximity. Interestingly, when 1,4-dioxane was used as the solvent at 150 °C, exclusive regioselective annulation was observed at the 3- and 2-positions of phenazine for 1a and 1b, respectively, leading to the “linear” and “bent” fusion selectively in about 70% yield (Scheme 1a). Both the linear and bent CANAL products can be aromatized readily using HCl in iPrOH/CHCl3 at 80 °C to yield the CBD-containing azaacenes 3 and 4 in >80% yield. Using CANAL between oNBE and 2,7-, 1,8-, and 1,6-dibromophenazines (1c−e), we also regioselectively synthesized longer phenazine-centered PCHs containing two CBDs as linear, bent, and angular regioisomers 5−7, respectively, under similar conditions (Scheme 1b). Dixylyl

-heteroarenes have attracted significant attention due to their potential applications in organic electronics.1−8 Introducing nitrogens to conjugation increases their electron affinity and stabilizes their frontier molecular orbitals (FMOs). In particular, several azaacenes have been reported to have good electron mobility as potential n-type materials.9−15 These azaacenes have been predominantly synthesized by condensation between o-diaminoarenes and ortho-quinonones or Pd-catalyzed aryl amination, followed by oxidation. Among the available methods for tuning the structures and properties of azaacenes, incorporating four-membered rings has been recently sought as a novel approach to extend the conjugation length without compromising the stability.16−20 The conjugated four-membered rings also elicit antiaromaticity arising from cyclobutadienoid (CBD), thus providing a path to tune the energy levels of the FMOs. CBD is highly unstable, but its fusion with aromatic motifs can result in stable conjugated π-systems. The most notable examples of such π-systems are the so-called [N]phenylenes developed by Vollhardt and co-workers,21−24 where CBD and benzenoid are alternatively fused with the smallest member being biphenylene. Recently, Bunz and coworkers synthesized stable azaphenylenes via condensation reactions between aromatic diamines and biphenylene-2,3dione.16,18 Miao and co-workers also reported a series of cyclobutadiquinoxalines and new azaphenylene structures.17,19,20 These recent studies shed light on the intriguing effects of incorporating CBD in azacenes. However, the previous synthetic approaches can only access linearly fused azaphenylenes, and the antiaromaticity of CBDs in all these previous examples is relatively weak. Recently, we have developed a versatile and modular synthetic strategy to produce diverse polycyclic conjugated hydrocarbons (PCHs) containing CBDs, via a catalytic arene−NBE annulation (CANAL) between aryl bromides and oxanorbornenes (oNBE), followed by aromatization.25,26 We envisioned facile access to CBD-containing azaarenes via this synthetic strategy. Recent © XXXX American Chemical Society

Received: April 14, 2018

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DOI: 10.1021/acs.orglett.8b01190 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

Whereas 5 was fluorescent with a quantum yield (QY) of 0.58, 6 was strongly quenched with a low QY of 0.07, and 7 was nonemissive (Table 1). Cyclic voltammetry (CV) in dichloromethane showed that, within the solvent window, 5 exhibited one reversible reduction wave, whereas both 6 and 7 exhibited two reversible reduction waves (Figure S3). The first reduction potentials became progressively less negative, going from 5 (−1.74 V) to 6 (−1.55 V) to 7 (−1.35 V), corresponding to a more stabilized LUMO. The HOMO level, calculated using the optical band gap, was slightly increased by 0.13 eV from 5 to 7. This concurrent increase in HOMO and decrease in LUMO level led to a reduction in band gap by 0.52 eV going from linear to angular isomers (Table 1). When exposed to light at ambient conditions, 5 exhibited robust stability with no degradation after 10 days, whereas 6 and 7 underwent gradual decomposition with a half-life of 6 days and 10 h, respectively (Figure S4). Because 3 and 4 represent the elemental structural units in larger PCHs 5−7, we sought to interrogate their paratropicity, bonding structures, and redox reactivity in depth to understand the influence of antiaromaticity. We first performed NICS(1)π,ZZ calculations to investigate the degree of paratropicity in 3 and 4.29−32 The four-membered rings in both 3 and 4 showed positive NICS(1)π,ZZ values of 8.3 and 22.1, which indicated the presence of paratropicity (Figure 2). The significantly more positive NICS value in 4 suggested stronger paratropicity of CBD in the bent fusion. This observation corroborates our recent findings in regioisomeric [3]naphthylenes, where the antiaromaticity of CBD was increased when it was fused onto the shorter 1,2-bond of naphthalene rather than its 2,3-bond.26 This was attributed to the forced double bond character into the frame of fused CBD, and a similar rationale can be drawn in the case of phenazine based on the resonance structures drawn using Clar’s sextet rule (Figure S6). Additionally, NICS calculations indicated dearomatization for benzenoids fused with CBD generally, reflected by the less negative values for CBD-fused benzenoids compared to those that are not directly fused with CBD. This dearomatization or forced bond localization has been commonly observed in linearly fused phenylene-containing structures to reduce the antiaromaticity of CBD.16−20,24,25,33 An intriguing exception came from the benzenoid in the phenazine with a bent fusion pattern, which showed increased diatropicity, as is evident from the more negative NICS value of −26.3 in 4. This can be rationalized by decreased bond alternation to minimize the forced increasing antiaromaticity in CBD. NICS calculations on the larger azaacene analogues 5−7 revealed the same trend of modulating local antiaromaticity and aromaticity depending on the fusion pattern of the structural subunits (Figure S8). 1H NMR spectroscopy of the compounds are also in agreement with the NICS results. A greater upfield shift of the arene protons in the central phenazine ring of 5 compared to that of 7 (Figure S5) can be attributed to the increased diatropicity of the benzenoid in the phenazine with a bent fusion. Thus, it is now evident that as the degree of local antiaromaticity progressively increases from 5 to 7, their optical band gaps shrink as a result of increasing HOMO energy and decreasing LUMO energy. The effect of antiaromaticity should be reflected in the bond alternation. To investigate their bond lengths, single crystals of 3 and 4 were grown by slow vapor diffusion of hexanes into CHCl3 and were analyzed by X-ray diffraction. Compound 3 adopted a planar conformation in the crystal structure, whereas 4 exhibited a small dihedral angle of 7.9° along the angularly fused CBD (Figure 3). In both compounds, the E ring fused to the CBD displayed strong π-bond localization with short bonds around

Scheme 1. Synthesis of CBD-Containing Azaacene Analoguesa

a

All yields are isolated yields.

groups were installed on oNBE to ensure the solubility of the final products. Interestingly, these regioisomers showed distinct absorption and fluorescence profiles. Focusing on compounds 5−7 with extended conjugation, we noticed that the color of their dilute solutions changed from greenish yellow for 5 to light brown for 7. Under 365 nm light irradiation, the fluorescence astonishingly changed from bright greenish-blue for 5 to dim pink for 6 to almost unnoticeable emission for 7 (Figure 1, inset). The linear

Figure 1. UV−vis absorption spectra of 5−7 in CHCl3. Inserted photographs are the dilute solutions of 5−7 under room light and 365 nm UV irradiation.

isomer 5 showed intense absorption with λmax = 503 nm and λonset = 523 nm, whereas both the bent and angular isomers, 6 and 7, showed broad and red-shifted absorption with λonset = 576 and 670 nm, respectively (Figure 1). Time-dependent density functional theory (TD-DFT) calculations revealed that the weak absorption tail of 6 and 7 corresponded to the symmetryforbidden HOMO−LUMO transition (Tables 1 and S2). B

DOI: 10.1021/acs.orglett.8b01190 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Electrochemical, Optical, and Computational Data of 5−7 electrochemicala 1

E 5 6 7

red

(V)

−1.74 −1.55 −1.35

E2red

(V)

−1.72 −1.53

opticalb

calculationc

LUMOCV (eV)

HOMO (eV)

λmax (nm)

λonset (nm)

λem,max (nm)

QY

Egap (eV)

Egap,TDDFT (eV)

fd

−3.36 −3.55 −3.75

−5.73 −5.70 −5.60

503 487 501

523 576 670

508 666 497

0.58 0.07 N/A

2.37 2.15 1.85

2.57 2.05 1.67

1.83 0.025 0.016

Reduction potentials (Ered) measured by CV in DCM containing 0.1 M nBu4NPF6 at a scan rate of 100 mV s−1 and Fc+/Fc as an external standard. Energy levels in eV were calculated using the equation LUMO = −(5.10 + E1red), HOMO = LUMO − Egap,opt. bOptical spectra were obtained in CHCl3 solution at 298 K; Egap was estimated from the absorption edge (λonset). cTD-DFT calculations (B3LYP/6-311+G*) using model compounds. d Oscillator strength. a

bond character, which is expected to increase the antiaromaticity of CBD, as suggested by the NICS calculation. Bunz and co-workers investigated the interesting hydrogenation of azatetracene to dihydroazatetracene, converting the aromatic pyrazine to formally antiaromatic dihydropyrazine.36 They observed an increased HOMO−LUMO gap upon hydrogenation. As for CBD-fused azaacenes containing different degrees of antiaromaticity, we were curious whether the pyrazine ring or the partially dearomatized rings fused with CBD would be reduced first and the properties of the products upon hydrogenation. We investigated the hydrogenation of simpler structures 3 and 4 for the ease of analysis. Compounds 3 and 4 underwent facile hydrogenation under 1 atm H2 at room temperature using Pd/C as the catalyst to form the N,N′-dihydro compounds 3-H2 and 4-H2. Interestingly, the reduction occurred on the pyrazine rings instead of the dearomatized benzenoids fused with CBD. Reduction of linear 3 to 3-H2 resulted in quenching of blue fluorescence and a red shift of 28 nm in the maximum absorption (Figure 4). This is opposite to the observation for azatetracene, which blue-shifted upon reduction due to the partial disruption of conjugation by the dihydropyr-

Figure 2. NICS(1)π,ZZ calculations on 3 and 4 (GIAO-B3LYP/6311+G*).

Figure 3. X-ray crystal structures with both top and side views of (a) 3 and (b) 4.

1.35 Å radial to the CBD and long bonds around 1.44 Å. This is consistent with the bond localization observed in phenylenecontaining structures to minimize the antiaromaticity of CBD.16−20,24,25,33 The innate bond alternation present in phenazine was also perturbed by CBD fusion. In the linear fusion of phenazine, the C ring in 3 showed strong bond alternation similar to the E ring with a relatively long bond of 1.46 Å fused with CBD. The harmonic oscillator model of aromaticity (HOMA)34 is commonly used to quantify bond alternation. HOMA values of 3 and 4 were calculated and tabulated in Table S3. The HOMA number for the C ring in 3 (0.242) is lower than that in innate phenazine (0.627),35 reflecting a high degree of bond alternation. In contrast, in the bent fusion, the C ring in 4 exhibited less bond alternation than native phenazine, as quantified by its slightly larger HOMA number (0.657) than that for phenazine (0.627). Specifically, the bond of the C ring in 4 fused with CBD was 1.39 Å long, thus exhibiting certain double

Figure 4. (a) Reduction of 3 and 4 and reoxidation with photographs of the reaction mixtures shown upon irradiation with 365 nm UV light before and after reduction. The red numbers show the calculated NICS(1)π,ZZ values of CBD. (b) UV−vis absorption spectra of 3, 3-H2, 4, and 4-H2. C

DOI: 10.1021/acs.orglett.8b01190 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters azine subunit.36 (TD)DFT calculations on 3 and 3-H2 revealed that both the HOMO and LUMO levels were raised and the band gap was reduced upon hydrogenation (Tables S1 and S2). The broad absorption of hydrogenated species may be attributed to intramolecular charge transfer from the electron-rich dihydropyrazine unit (Figure S2b). NICS calculation on 3-H2 showed a more positive NICS value for the CBD and less negative NICS values for the benzenoids fused with the CBD (Figure S8) compared to 3, suggesting stronger paratropicity of CBD and greater dearomatization of fused benzenoids upon hydrogenation. In contrast, reduction of the bent 4 to 4-H2 resulted in rapid turn-on of green fluorescence and a blue shift in maximum absorption (Figure 4). (TD)DFT calculations revealed that both the HOMO and LUMO levels were raised, but the band gap was increased upon hydrogenation. Comparison of NICS values for 4 and 4-H2 suggested a reduction of paratropicity for CBD upon hydrogenation (Figure S8). The dihydropyrazine subunit may partially break the conjugation to enlarge the band gap, as suggested by Bunz,36 but in our CBD-fused azaacenes, the band gap is strongly affected by the change in the degree of CBD antiaromaticity upon hydrogenation of the pyrazine ring. Furthermore, both 3-H2 and 4-H2 were rapidly oxidized back to 3 and 4 under air, and this redox interconversion was readily reversible at room temperature. In conclusion, we report the facile synthesis of a new series of regioisomeric CBD-fused azaacene analogues, via a regioselective CANAL reaction, followed by aromatization. The fusion pattern of phenazine with CBD strongly influenced the degree of local antiaromaticity. The bent fusion gave rise to greater antiaromaticity than the linear fusion, leading to stabilized LUMO, reduced band gap, and strongly quenched fluorescence. The pyrazine ring of these CBD-fused azaacene analogues underwent hydrogenation to generate the N,N-dihydrophenazine counterparts, which led to significant changes in the optical properties and band gaps, again influenced by the different degrees of CBD antiaromaticity in the reduced and oxidized forms. We expect the CANAL synthetic strategy to be applicable to various other N-heteroarenes to introduce fused CBDs with variable antiaromaticity as a means of tuning the optoelectronic properties and redox reactivities of extended hetero-PCHs.

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Zexin Jin: 0000-0002-4971-3656 Yan Xia: 0000-0002-5298-748X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Cottrell Scholar Award (24067) from the Research Corporation for Science Advancement. Y.C.T. is supported by an A*STAR graduate fellowship. We thank Prof. C.E.D. Chidsey (Stanford University) for the use of cyclic voltammetry. Single-crystal X-ray diffraction experiments were performed at beamline 11.3.1 at the Advanced Light Source (ALS). The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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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.8b01190. Experimental details, 1H and 13C NMR spectra, UV−vis and CV characterizations, and calculations (PDF) Accession Codes

CCDC 1586460 and 1586464 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yew Chin Teo: 0000-0002-5630-4367 D

DOI: 10.1021/acs.orglett.8b01190 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.8b01190 Org. Lett. XXXX, XXX, XXX−XXX