Letter pubs.acs.org/OrgLett
Direct Synthesis of Large-Scale Ortho-Iodinated Perylene Diimides: Key Precursors for Functional Dyes Jiajun Wu, Dezhi He, Li Zhang, Yudong Liu, Xiaogang Mo, Jianbin Lin,* and Hui-jun Zhang* Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China S Supporting Information *
ABSTRACT: A highly regioselective tetra-ortho-iodination reaction of perylene diimides (PDIs) has been developed, which could be conducted on a multigram scale (>10 g), featuring a column chromatography-free workup and purification. The orthoiodinated-PDIs serve as key intermediates for the preparation of a variety of ortho-functionalized PDIs and PDI-based conjugated polymers.
A
electrophilic halogenation of perylene dianhydrides at baypositions followed by subsequent nucleophilic exchange reactions. In fact, 1,6(7)-dibromo and 1,6,7,12-tetrachloro PDIs were frequently employed as key intermediates for the preparation of the corresponding amino-, (het)aryl-, cyano-, alkynyl-, aryloxy-, and thiol-substituted PDI derivatives.11 Likewise, ortho-halogen-substituted PDIs should also be one class of key building blocks in PDI chemistry.9b,c,12 However, only one excellent, but tedious, procedure for the synthesis of ortho-halogen substituted PDIs based on ruthenium-catalyzed C−H borylation of PDIs followed by copper-mediated boronate exchange has been reported recently.9b,c In this context, we envisaged developing a more straightforward and simple route toward ortho-halogen-substituted PDIs and employing them as significant precursors for the preparation of various ortho-functionalized PDIs, taking advantage of the versatile reactivity of sp2 C−X (X = Cl, Br, or I) bonds in palladium-catalyzed cross-coupling reactions and SNAr-type halogen displacements (Scheme 1).
s a class of important organic chromophores, perylene diimides (PDIs) have received great attention from both academia and industry owing to their unique properties, such as low cost, robustness, large optical absorption range, high fluorescence quantum yields, and excellent n-type semiconductivity.1 To achieve desirable solubility, photophysical, and electronic properties for specific applications in organic material devices, chemical modification of PDIs are critical. Fine tuning of the substituents on the imide nitrogen and at the 1,6,7,12(bay)-positions of PDIs are two commonly used synthetic strategies. The former mainly has an important effect on the solubility and self-assembly of PDIs, and the latter has a pronounced influence on the electro-optical properties of new dyes. However, the steric repulsion between bay-substituents causes a geometric distortion of the perylene core, which disturbs the intermolecular π−π stacking.2 Moreover, the incorporation of four substituents at the same time into the bay-positions of PDIs is challenging.3 In recent years, functionalization of PDIs at 2,5,8,11(ortho)-positions without introducing the core twisting has become a more compelling strategy.4 In particular, a significant improvement in photovoltaic performance for ortho-substituted PDIs relative to baysubstituted has been demonstrated by recent research.5 In addition, undistorted PDI monomers involving reactive groups at ortho-positions are perfect substrates for the construction of PDI-based conjugated polymers.6 At this time, synthetic pathways toward ortho-functionalized PDIs are limited to transition-metal-catalyzed direct ortho-alkylation,7 -arylation,8 -borylation,9 and -olefination10 of PDIs, which display considerable substrate dependence and suffer from low efficiency. More importantly, these methods are not suitable for multigram-scale synthesis, which is crucial for highly efficient diversification of PDIs. Therefore, the development of more general and practical synthetic methods toward orthofunctionalized PDIs is required. It is known that various bayfunctionalized PDIs could be synthesized through large-batch © 2017 American Chemical Society
Scheme 1. Functionalization of PDIs from the OrthoHalogenated PDIs
Received: August 31, 2017 Published: September 28, 2017 5438
DOI: 10.1021/acs.orglett.7b02718 Org. Lett. 2017, 19, 5438−5441
Letter
Organic Letters
Table 1. Rhodium-Catalyzed Ortho-Tetraiodination of PDIs
Within the past few years, complementary to the traditional electrophilic halogenation of arenes, transition-metal-catalyzed ortho-selective oxidative sp2 C−H halogenation has become a powerful and practical tool for the synthesis of aromatic halides.13 This approach eradicated the problems involved in traditional halogenation methods, such as poor regioselectivity, over-halogenation, and harsh reaction conditions. Among the various transition-metal (Pd, Rh, Ru, Cu, Co, etc.) catalysts used in these transformations, Cp*Rh exhibited superior catalytic activity, which is even suitable for the halogenation of electron-poor arenes.14 Herein, we report on the first efficient Cp*Rh-catalyzed direct synthesis of a series of orthoiodinated PDIs via carbonyl-directed ortho C−H halogenations of electron-deficient PDIs and their applications as valuable building blocks for functional dyes. We initiated our study through performing the reaction of PDI 1a with N-bromosuccinimide (NBS) in the presence of 5 mol % of [Cp*RhCl2]2, 20 mol % of AgSbF6, and 2.2 equiv of Cu(OAc)2 in DCE at 100 °C for 48 h. Unexpectedly, the formation of both ortho- and bay-monobrominated PDIs was observed (see the Supporting Information (SI)). However, the low regioselectivity of this halogenation reaction was overcome simply by changing NBS to N-iodosuccinimide (NIS). Under similar reaction conditions using NIS as halogenating reagent, only ortho-iodionated PDIs were formed, albeit including mono-, di-, and triiodination products. The perfect regioselectivity of this iodination process may be ascribed to the lower reactivity of NIS and the steric encumbrance of bulky iodine substituents in the bay regions.15 Delightfully, treatment of PDI 1a with 8 equiv of NIS in the presence of 5 mol % of [Cp*RhCl2]2, 20 mol % AgSbF6, and 4 equiv of Cu(OAc)2 in DCE at 80 °C for 48 h afforded 2,5,8,11-tetraiodinated PDI 2a in 11% yield (Table 1, entry 1). By increasing the amount of AgSbF6 to 60 mol % and 80 mol %, the yield of 2a could be improved dramatically to 46% and 56% (entries 2 and 3). However, further increasing the amount of AgSbF6 to 100 mol % led to the formation of 2a in relatively lower yield (51%, entry 4).16 Furthermore, a control experiment showed that no reaction took place in the absence of [Cp*RhCl2]2 (entry 5). Notably, the addition of Cu(OAc)2 is also crucial for the success of this transformation. When the reaction was performed in the absence of Cu(OAc)2, no desired product was observed (entry 6). Using other copper salts such as CuCl2, CuBr2, CuO, Cu(OTf)2, and Cu(acac)2 instead of Cu(OAc)2 could not afford the desired product (Table S3). Thereafter, the yield of the tetraiodination product 2a was increased from 56% to 68% by increasing the catalyst loading (entry 7). Extending the reaction time to 72 and 96 h could further improve the yield (75% and 82%, entries 8 and 9). Gratifyingly, gram-scale reactions worked also smoothly and afforded the desired product in good yields (entries 10−13). Notably, performing the large-scale reaction of 1a with addition of only 1 equiv of Cu(OAc)2 at lower catalyst loading still led to the formation of 2a in 87% yield (entry 12). Further enlargement of the reaction scale to 10 mmol was also realized, and 10.5 g of the desired product 2a was isolated only through filtration and recrystallization of reaction mixture (entry 13, see the SI). Subsequently, the iodination reactions of PDI 1b were also conducted (entries 14 and 15). Although 1b has lower solubilities than 1a, the desired tetraiodinated PDI 2b was also isolated in good yields (84% for 1 mmol and 71% for 5 mmol).
entry
PDI (mmol)
1 2 3 4 5 6c 7 8 9 10 11 12d 13d 14d 15d
1a (0.05) 1a (0.05) 1a (0.05) 1a (0.05) 1a (0.05) 1a (0.05) 1a (0.05) 1a (0.05) 1a (0.05) 1a (3) 1a (3) 1a (5) 1a (10) 1b (1) 1b (5)
[Cp*RhCl2]2 (mol %) 5 5 5 5 5 7.5 7.5 7.5 7.5 7.5 5 5 7.5 5
AgSbF6 (mol %)
time (h)
yieldb (%)
20 60 80 100 80 80 90 90 90 90 90 80 80 90 80
48 48 48 48 48 48 48 72 96 96 120 120 120 96 120
11 46 56 51 NR 0 68 75 82 79 89 87e,f 83e,g 84 71e,h
a
Reaction conditions: 1 (0.05 mmol), NIS (0.4 mmol), [Cp*RhCl2]2, AgSbF6, Cu(OAc)2 (4 equiv), DCE (1.0 mL) at 80 °C under Ar for 48−120 h. bIsolated yield of 2a. cWithout Cu(OAc)2. dCu(OAc)2 (1 equiv). eColumn chromatography-free workup; see the SI. f5.5 g of 2a. g 10.5 g of 2a. h3.7 g of 2b.
Through a slow diffusion of hexane into a chloroform solution of 2b, a single crystal suitable for X-ray analysis was obtained. The structure of 2b reveals the highly planar structure of the perylene core (Figure 1a) and that PDI 2b stacks in a slip-stacked geometry by π−π stacking due to the steric interactions of the branched imide substituents (Figure 1b). This is different from the unsubstituted PDI 1b molecules, which are arranged in stacks with their long axes tilted in relation to the column grow direction.17 The enhanced stacking ability of tetra-ortho-iodinated PDIs can explain the great tendency of precipitation during the reaction, which is also advantageous for the column chromatography-free purification of PDIs 2 through recrystallization. More importantly, it has been demonstrated that employing slip-stacked PDI acceptors is a useful strategy for developing high-performance nonfullerene acceptors in organic photovoltaics.18
Figure 1. Molecular structure of PDI 2b in the solid state: (a) top view and (b) side view (hydrogen atoms have been omitted for clarity). 5439
DOI: 10.1021/acs.orglett.7b02718 Org. Lett. 2017, 19, 5438−5441
Letter
Organic Letters Considering the wide applications of mono-, di-, and trihalogen-substituted PDIs in the synthesis of various functional molecules such as A−D−A (acceptor−donor−acceptor) molecules5 and ring-fused PDI dimers,19 partial iodination at the ortho-positions of PDIs was also investigated. First, the preparation of both ortho-monoiodinated PDI 2a-I1 and diiodinated PDI 2a-I2 can be achieved by decreasing the amount of NIS, AgSbF6, and Cu(OAc)2 added in the reaction (Scheme 2). Treatment of 1a with 1.2 equiv of NIS in the
Scheme 3. Derivatization of PDI 2a
Scheme 2. Mono-, Di-, and Tri-ortho-iodination of PDI 1a
For the construction of conjugated networks, different from bay-functionalized PDI monomers which may result in strong twisting of the perylene core and steric hindrance issues, the tetra-ortho-functionalized PDIs with perfect planarity will be ideal quadrilateral building units. Therefore, two kinds of conjugated microporous polymers (CMPs 7 and 8) were synthesized through the Sonogashira coupling of 2a with 1,4diethynylbenzene and 4,4′-diethynylbiphenyl, respectively (Scheme 4). After careful purification via Soxhlet extraction Scheme 4. Synthesis of PDI-Containing Conjugated Microporous Polymers
presence of 5 mol % of [Cp*RhCl2]2, 35 mol % of AgSbF6, and 0.5 equiv of Cu(OAc)2 provided 2a-I1 in 50% yield. Under similar reaction conditions, employing 3.0 equiv of NIS gave the corresponding product 2a-I2 as a mixture of 2,5-, 2,8-, and 2,11-isomers (overall yield: 45%). Only one of them was isolated in an isomerically pure form through chromatography (see the SI). Second, conducting the iodination reaction in hexafluoroisopropanol (HFIP), a privileged solvent in C−H activation processes, instead of DCE led to the formation of ortho-triiodinated PDI 2a-I3 as a major product in 62% yield. To prove the significance of ortho-iodinated PDIs in the synthesis of various ortho-functionalized PDIs, the nucleophilic substitution reactions of 2a with aniline, phenol, and thiophenol, as well as the cross-coupling reactions with CuCN, terminal alkynes, and arylboronic acids, were performed, which all afforded the desired products in good yields (Scheme 3). Later, the influences of different substituents at the ortho-positions of PDIs on the electronic, photophysical, and redox properties of these new chromophores were also investigated by UV−vis spectrophotometry, fluorescence spectroscopy (Figure S1), and cyclic voltammetry (Figure S2). Although it seems that the ortho-substituents have little influence on the absorption maximum, the fluorescence and HOMO/LUMO levels of PDI derivatives can be easily tuned. In addition, rather small alterations of the ortho-substituents could result in extremely different optical features (such as PDIs 3a−c, Table S11).
and drying to constant weight, two CMPs were obtained, both as black powders. Because of their bad solubilities in common organic solvents, CMPs 7 and 8 cannot be characterized with most of the traditional instrumentations. However, FTIR spectral and elemental analyses provide evidence for the chemical structures of these CMPs. A new vibration band appeared at 2200 cm−1, which was assignable to the internal alkynes. Also, the strong CO signal can be found (Figure S4). Elemental analysis results of CMPs 7 and 8 further support the structures proposed (Table S4), and the deviation between the theoretical and experimental values is normal for crosslinked conjugated systems. Notably, the CMPs contain higher levels of nitrogen than theoretical values, possibly indicating trapped triethylamine (Et3N) or residual DMF, despite extensive Soxhlet extraction.20 The morphology of the solid state of CMPs was investigated by scanning electron 5440
DOI: 10.1021/acs.orglett.7b02718 Org. Lett. 2017, 19, 5438−5441
Organic Letters
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microscopy (SEM). SEM images (Figure S5) displayed the two CMPs both adopt plate-shaped aggregates on the submicrometer scale. Diffuse reflectance UV−vis measurements of the solid state of these CMPs were carried out to extract reliable optical absorption profiles. Compared to tetraphenylacetylene PDI 5b, the absorption spectrum with redshift attributed to the donor− acceptor “push-pull” effect.6b We also observed the formation of a new absorption feature with maxima at 730 and 1040 nm (Figure S6), in good agreement with data for the formation of the PDI radical anion (PDI•−)21 in the presence of trapped Et3N as electron donor.22 Considering that all of the workup and measurement procedures are carried out in air, it is clear that the radical anions formed are stable in air. Therefore, these two CMPs will be invaluable for a wide range of applications.23 In conclusion, iodine was for the first time directly incorporated into four ortho-positions of PDIs via a multigram scale and column chromatography-free protocol. This is a highly efficient procedure, especially concerning the average yield for per C−I formation. Taking advantage of the versatile reactivities of sp2 C−I bond, ortho-iodinated PDIs represent a new generation of key building blocks which may largely broaden the research scope of PDI chemistry. We have preliminarily synthesized a range of ortho-functionalized PDIs and PDI-based CMPs starting from tetra-ortho-iodinated PDI 2a, which have demonstrated several unique optical and electrochemical properties. Further investigation on the applications of ortho-I-PDIs is currently underway in our group.
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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.orglett.7b02718.
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Letter
Reaction condition screening tables, experimental procedures, full characterization data (PDF)
AUTHOR INFORMATION
Corresponding Authors
* E-mail:
[email protected]. * E-mail:
[email protected]. ORCID
Jianbin Lin: 0000-0002-0064-3079 Hui-jun Zhang: 0000-0001-9567-3010 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the Natural Science Foundation of China (Nos. 21572188 and 21302159), Fundamental Research Funds for the Central Universities (No. 20720160049), NFFTBS (No. J1310024), and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21521004). We also thank Prof. APHJ (Albert) Schenning (The Eindhoven University of Technology) and Prof. Pierre H. Dixneuf (Université de Rennes 1) for useful discussions. 5441
DOI: 10.1021/acs.orglett.7b02718 Org. Lett. 2017, 19, 5438−5441
