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Synlett 2017; 28(19): 2609-2613
DOI: 10.1055/s-0036-1590937
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© Georg Thieme Verlag Stuttgart · New York

Synthesis of Fluorine-Containing Tetraarylanthracenes via Ruthenium-Catalyzed C–O or C–F Arylation and their Crystal Structures

Akiko Izumoto
a   Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan   Email: kakiuchi@chem.keio.ac.jp
,
Hikaru Kondo
a   Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan   Email: kakiuchi@chem.keio.ac.jp
,
Takuya Kochi
a   Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan   Email: kakiuchi@chem.keio.ac.jp
,
Fumitoshi Kakiuchi*
a   Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan   Email: kakiuchi@chem.keio.ac.jp
b   JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
› Author Affiliations

This work was supported in part by JSPS KAKENHI Grant Numbers JP15H05839 in Middle Molecular Strategy, and CREST and ACT-C (Grant Number JPMJCR12Y8) from the Japan Science and Technology Agency (JST), Japan. H.K. gratefully acknowledges the Japan Society for the Promotion of Science (JSPS) for a Research Fellowship for Young Scientists (JP16J02904). T.K. is also grateful for support by JSPS KAKENHI Grant Number 16H01040 (Precisely Designed Catalysts with Customized Scaffolding).
Further Information

Publication History

Received: 02 August 2017

Accepted after revision: 26 September 2017

Publication Date:
08 November 2017 (online)

 
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Published as part of the Cluster C–O Activation

Abstract

Tetraarylanthracenes containing several fluoro groups were synthesized using the ruthenium-catalyzed C–O or C–F arylation with arylboronates and their structural and spectroscopic studies were conducted. The RuH2(CO)(PPh3)3-catalyzed C–O arylation of aromatic ketones was found to be effective for the introduction of aryl groups containing multiple fluoro groups. Anthracenes possessing fluorinated aryl groups were prepared in two steps from 1,4,5,8-tetramethoxyanthraquinone by C–O arylation and reduction of the carbonyl groups. A tetraphenylanthracene containing a fluorinated anthracene moiety was also prepared using C–F phenylation of octafluoroanthraquinone. Single-crystal X-ray diffraction analysis showed that the positions of fluoro groups on the tetraarylanthracenes lead to notable difference in the crystal packing structures. The larger difference between the tetraarylanthracenes was observed in the fluorescence spectra in the solid state than those in chloroform.


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Key words

C–O bond cleavage - C–F bond cleavage - ruthenium catalyst - tetraarylanthracenes - fluoroarenes - crystal structures - fluorescence spectra - arylboronates

Polycyclic aromatic hydrocarbons (PAHs) have received much attention due to their intriguing structures and properties as well as their potential utility as organic semiconductors.[1] The electronic properties of the PAHs are highly dependent on the molecular structures, but when controlling the properties in the solid state, it is also important that the molecules adopt appropriate packing structures. Introduction of substituents onto the PAH cores have been examined by many researchers to tune both the electronic properties and the packing structures in crystals.[1] Replacement of hydrogen atoms on the PAH cores with fluorine atoms is one of the strategies used to drastically change both characters.[2] For example, perfluoropentacene shows n-type semiconducting properties, while pentacene itself is known as an excellent p-type semiconductor.[2e] In addition, both molecules adopt herringbone structures in crystals, but the angle between two molecules are different by ca. 40°. Our group has been developing ruthenium-catalyzed functionalization reactions of aromatic ketones via cleavage of unreactive bonds[3] such as C–H,[4] C–O,[5] C–N,[6] C–F[7] bonds and has been applying these reactions to the convenient syntheses of PAH derivatives such as substituted acenes.[8] During these studies, we envisioned that introduction of aryl groups possessing multiple fluoro groups onto the acene core would force the acenes to adopt different packing structures and would show altered properties in the solid state. However, the introduction of aryl groups containing multiple fluoro groups has not been studied in detail for our ruthenium-catalyzed functionalization via unreactive bond cleavage.

Here we describe the synthesis of tetraarylanthracene derivatives bearing multiple fluoro groups via ruthenium-catalyzed C–O or C–F bond cleavage. Introduction of various fluorine-containing aryl groups to simple aromatic ketones was also examined using the ruthenium-catalyzed arylation via cleavage of unreactive bonds to study the reactivities of the corresponding arylboronate substrates. X-ray diffraction analyses of the crystals of the tetraarylanthracene derivatives showed that these molecules indeed adopt different types of crystal structures depending on the position of the fluorine atoms.

The use of fluorine-containing arylboronates in the ruthenium-catalyzed C–H arylation was first examined, because the C–H tetraarylation of anthraquinone was employed in our previous synthesis of a tetraarylanthracene.[8a] When the reaction of α-tetralone (1) with phenylboronate 2a was performed using 5 mol% of RuH2(CO)(PPh3)3 in pinacolone at 125 °C for 20 h, phenylation product 3a was obtained in 93% yield as expected (Table [1], entry 1). The use of 4-fluorophenylboronate 2b decreased the product yield to 72% (Table [1], entry 2). The reaction with di- and trifluorophenylboronates 2c–e provided arylation products 3c–e in lower yields (Table [1], entries 3–5), but 2,3,4,5-tetrafluorophenylboronate 2f and pentafluorophenylboronate 2g did not give any arylation products. Therefore, it is suggested that introduction of fluoro groups on the aromatic ring of arylboronates 2 decreases the efficiency of the C–H arylation.

Table 1 Ruthenium-Catalyzed C–H Arylation of α-Tetralone (1) with Phenylboronate 2a or Fluorine-Containing Arylboronates 2b–g a

Entry

2

Ar

3

Isolated yield (%)

1

2a

Ph

3a

93

2

2b

4-FC6H4

3b

72

3

2c

3,4-F2C6H3

3c

68

4

2d

3,5-F2C6H3

3d

45

5

2e

3,4,5-F3C6H2

3e

46

6

2f

2,3,4,5-F4C6H

3f

Not detected

7

2g

F5C6

3g

Not detected

a Reaction conditions: 1 (0.5 mmol), 2 (0.75 mmol), RuH2(CO)(PPh3)3 (0.025 mmol), pinacolone (0.5 mL), 125 °C, 20 h.

The C–H arylation of anthraquinone (4) was attempted using arylboronates 2c and 2e, which have multiple fluorine atoms and can be used to arylate 1, but failed to give the desired tetraarylation products 5c and 5e (Equation 1).

Zoom Image
Equation 1 Attempted C–H tetraarylation of anthraquinone (4)

Then we turned our attention to the arylation via cleavage of aromatic C–O bonds in aryl ethers. To test the feasibility of the use of fluorine-containing arylboronates in this reaction, diarylation of 2′,6′-dimethoxyacetophenone (6) was examined, and various arylboronates were found to be applicable to this reaction (Table [2]). The reactions with 2a as well as mono- and difluorophenylboronates 2b–d gave products 7a–d in high yields (>85%, Table [2], entry 1–4). C–O diarylation product 7e was also obtained in 62% yield using trifluorophenylboronate 2e (Table [2], entry 5). The use of tetrafluorophenylboronate 2f was also possible for the C–O arylation to give 7f in 19% yield (Table [2], entry 6), but the reaction with 2g did not give the arylation product (Table [2], entry 7).

Table 2 Ruthenium-Catalyzed C–O Diarylation of 6 with Phenyoboronate 2a or Fluorine-Containing Arylboronates 2b–g a

Entry

2

7

Ar

GC yield (%)

1

2a

7a

Ph

94

2

2b

7b

4-FC6H4

86

3

2c

7c

3,4-F2C6H3

85

4

2d

7d

3,5-F2C6H3

87

5

2e

7e

3,4,5-F3C6H2

62

6

2f

7f

2,3,4,5-F4C6H

19

7

2g

7g

F5C6

Not detected

a Reaction conditions: 6 (0.5 mmol), 2 (1.5 mmol), RuH2(CO)(PPh3)3 (0.025 mmol), toluene (0.5 mL), 125 °C, 20 h.

Table 3 Ruthenium-Catalyzed C–O Tetraarylation of 8 with Fluorine-Containing Arylboronatesa

Entry

2

Solvent

Temp (°C)

5

Ar

Isolated yield (%)

1

2c

toluene

125

5c

3,4-F2C6H3

56

2

2c

p-xylene

150

5c

3,4-F2C6H3

76

3

2b

p-xylene

150

5b

4-FC6H4

70

4

2d

p-xylene

150

5d

3,5-F2C6H3

40

5

2e

p-xylene

150

5e

3,4,5-F3C6H2

71

6

2f

p-xylene

150

5f

2,3,4,5-F4C6H

Not detected

a Reaction conditions: 8 (0.125-0.5 mmol), 2 (10 equiv), RuH2(CO)(PPh3)3 (20 mol %), solvent (1 mL per 0.125 mmol of 8), 20 h.

The C–O arylation was then applied to the tetraarylation of 1,4,5,8-tetramethoxyanthraquinone (8) (Table [3]). The reaction of 8 with 10 equiv of 2c was first examined in toluene at 125 °C, and tetrarylanthraquinone 5c was obtained in 56% yield (Table [3], entry 1),[9] and the product yield was increased to 76% by running the reaction in p-xylene at 150 °C (Table [3], entry 2).[10] The reactions with 2b, 2d, and 2e were also conducted in p-xylene at 150 °C, and the corresponding products 5b, 5d, and 5e were obtained in 70%, 40%, and 71% yields, respectively (Table [3], entries 3–5). The use of 2f was also attempted but did not provide tetraarylation product 5f (Table [3], entry 6).

The C–F phenylation of octafluoroanthraquinone (9) was also investigated to prepare a tetraphenylanthraquinone possessing fluoro groups on the anthraquinone core. When the reaction of 9 with 10 equiv of 2a was performed in the presence of 20 mol% of RuH2(CO)(PPh3)3 in p-xylene at 150 °C, tetraphenylation product 5h was obtained in 35% yield (Equation 2).

Zoom Image
Equation 2 C–F tetraphenylation of octafluoroanthraquinone (9)

Conversion of tetraarylanthraquinones 5 into anthracene derivative 10 was then examined (Table [4]). The reduction of 5c–e with hydroiodic acid in acetic acid at 140 °C in a sealed tube provided anthracene derivative 10c–e in 88%, 50%, and 95% yields, respectively (Table [4], entries 1–3), but the reaction of 5b failed to provide the corresponding product 10b (Table [4], entry 4). The reaction of tetrafluoroanthraquinone derivative 5h also proceeded to give anthracene derivative 10h in 96% yield (Table [4], entry 5).

Table 4 Reduction of Anthraquinones 5 to Anthracene Derivatives10 a

Entry

5

Ar

X

10

Isolated yield (%)

1

5c

3,4-F2C6H3

H

10c

88

2

5d

3,5-F2C6H3

H

10d

50

3

5e

3,4,5-F3C6H2

H

10e

95

4

5b

4-FC6H4

H

10b

Not detected

5

5h

Ph

F

10h

96

a Reaction conditions: 5 (0.015–0.05 mmol), aq HI(10 equiv), AcOH (0.5–1 mL), 140 °C, 6 days.

Zoom Image
Figure 1 Crystal structures of tetraarylanthracenes. (a) Molecular structure of 10d. (b) Packing structure of 10d. (c) Molecular structure of 10e. (d) Packing structure of 10e. (e) Molecular structure of 10h. (f) Packing structure of 10h.

With several tetraarylanthracenes 10 in hand, single-crystal X-ray diffraction analyses were performed. Recrystallization of 10d, 10e, and 10h provided single crystals suitable for X-ray analysis, but those of 10c could not be obtained. Crystal structures of 10d, 10e, and 10h are shown in Figure [1].[11] The molecular structures of tetraarylanthracenes 10d, 10e, and 10h are all similar (Figure [1 a, c, e]), but adopt different packing structures, despite the small difference in size of hydrogen and fluorine atoms (Figure [1, b, d, f]). In the crystal of 10d, the anthracene cores of the molecules do not stack together, and each anthracene core is surrounded by 3,5-difluorophenyl groups of other molecules (Figure [1, b]). However, partial stackings of anthracene cores were observed in the crystals of 10e and 10h. In these crystals, anthracene molecules form pairs, and two anthracene cores in each pair aligned in a parallel fashion. The distances between the anthracene planes in the pairs of 10e and 10h are 3.90 Å and 3.58 Å, respectively. A difference in the packing structures of 10e and 10h was observed for the distance between the pairs of stacked anthracene cores. While the anthracene pairs of 10e are close to each other with the edge-to-edge distance of 3.25 Å, each pair is far apart in the crystal of 10h.

UV-vis absorption and fluorescence studies were conducted for tetraarylanthracenes 10c–e,h. UV-vis absorption spectra of anthracenes 10c–e having di- or trifluorophenyl groups in chloroform showed similar patterns of peaks to those of tetraphenylanthracene (TPA), and the absorption maxima were observed at 388–390 nm, which are close to that of TPA (394 nm, Figure [2]). A similar pattern of peaks as those of 10c–e were observed for 2,3,6,7-tetrafluoroanthracene derivative 10h, but were blue-shifted with the maximum located at 373 nm. Fluorescence spectra of the tetraarylanthracenes in chloroform also exhibited similar patterns of peaks to those of TPA, and the emission maxima were observed at 435–438 nm, which are again close to that of TPA (441 nm, Figure [3]). In contrast, larger differences in the maxima were found in the fluorescence spectra of 10c–e in the solid state (Figure [4]). Particularly, the emission maxima of 10d and 10e were separated by 9 nm in the solid state (465 nm and 456 nm, respectively), while the emission maxima of these two molecules in chloroform are almost identical (435 nm) and much lower than those in the solid state. In addition, a larger red shift was observed for TPA in the solid state (500 nm). Therefore, the differences in the packing structures appear to cause some effects on the fluorescence property of these molecules in the solid state.[12]

Zoom Image
Figure 2 UV-vis absorption spectra of tetraarylanthracenes in chloroform (1 × 10–4 M)
Zoom Image
Figure 3 Fluorescence spectra of tetraarylanthracenes in chloroform (1 × 10–7M)
Zoom Image
Figure 4 Fluorescence spectra of tetraarylanthracenes in the solid state

In summary, tetraarylanthracenes containing several fluoro groups were synthesized using the ruthenium-catalyzed C–O or C–F arylation with arylboronates. Investigation of the RuH2(CO)(PPh3)3-catalyzed arylation methods showed that C–O arylation was effective for the introduction of aryl groups containing multiple fluoro groups. The synthesis of anthracenes possessing fluorinated aryl groups were achieved in two steps from anthraquinone 8 by C–O arylation, followed by reduction with hydroiodic acid in acetic acid. X-ray structural analyses of 10 showed that the positions of fluoro groups lead to significant difference in the crystal-packing structures. The larger difference between 10d and 10e was observed in the fluorescence spectra in the solid state than those in chloroform indicates and may be caused by the difference in the packing structures.


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Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/s-0036-1590937.
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Zoom Image
Equation 1 Attempted C–H tetraarylation of anthraquinone (4)
Zoom Image
Equation 2 C–F tetraphenylation of octafluoroanthraquinone (9)
Zoom Image
Figure 1 Crystal structures of tetraarylanthracenes. (a) Molecular structure of 10d. (b) Packing structure of 10d. (c) Molecular structure of 10e. (d) Packing structure of 10e. (e) Molecular structure of 10h. (f) Packing structure of 10h.
Zoom Image
Figure 2 UV-vis absorption spectra of tetraarylanthracenes in chloroform (1 × 10–4 M)
Zoom Image
Figure 3 Fluorescence spectra of tetraarylanthracenes in chloroform (1 × 10–7M)
Zoom Image
Figure 4 Fluorescence spectra of tetraarylanthracenes in the solid state