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Synlett 2013; 24(17): 2297-2301
DOI: 10.1055/s-0033-1339710
letter
© Georg Thieme Verlag Stuttgart · New York

Synthesis of Phenanthrenes by Cationic Chromium(III) Porphyrin-Catalyzed Dehydration Cycloaromatization

Ryota Wakabayashi
a   Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan   Fax: +81(75)3832438   Email: kurahashi.takuya.2c@kyoto-u.ac.jp   Email: matsubara.seijiro.2e@kyoto-u.ac.jp
,
Takuya Kurahashi*
a   Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan   Fax: +81(75)3832438   Email: kurahashi.takuya.2c@kyoto-u.ac.jp   Email: matsubara.seijiro.2e@kyoto-u.ac.jp
b   JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan
,
Seijiro Matsubara*
b   JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan
› Author Affiliations
Further Information

Publication History

Received: 26 July 2013

Accepted after revision: 12 August 2013

Publication Date:
23 September 2013 (online)

 
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Abstract

Readily available biphenyl derivatives with ortho oxirane moiety react in the presence of cationic chromiun(III) porphyrin catalyst to afford phenanthrenes. The reaction is considered to be triggered by activation of the oxirane moiety through coordination to the Lewis acidic cationic chromium to give aldehyde via 1,2-hydride shift, which reacts with arene through intramolecular electrophilic aromatic substitution and subsequent dehydration. The reaction allows constructing a variety of polycyclic aromatic and heteroaromatic compounds.


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Key words

arene - chromium - cyclization - porphyrin - transition metals

Phenanthrene is one of the most ubiquitous structural component in organic functional materials such as organic photoconduction and organic electroluminescence.[1] Thus, the development of synthetic strategies for their preparation is of great significance.[2] [3] [4] Herein, we report the cationic chromium porphyrin catalyzed reaction of biphenyl derivatives having ortho oxirane moiety to afford phenanthrenes via dehydrative cycloaromatization. The transformation represents rare examples of Lewis acid catalyzed reactions, which may open the way to divergent synthesis of polycyclic aromatic compounds.[2a,3e]

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Scheme 1 Strategy for the synthesis of phenanthrenes with dehydrative cycloaromatization

Recently, we reported that cationic metalloporphyrins could perform as Lewis acid and catalyze various reactions such as the formal hetero-Diels–Alder reaction and cycloisomerization.[5] [6] In pursuit of our investigation on the unique character of metalloporphyrin as Lewis acid catalyst, we presumed that biphenyl derivatives 1 having ortho oxirane moiety might be transformed into phenanthrenes 2 with the cationic metalloporphyrin catalyst via dehydrative cycloaromatization (Scheme [1]). Of note, the substrates 1 are readily available and can be prepared in two steps, which consists of (1) palladium-catalyzed cross-coupling, for example, the reaction of ortho-halobenzaldehyde and phenylboronic acid; (2) Corey–Chaykovsky reaction.

Table 1 Dehydrative Cycloaromatization of 1a to Afford 2a a

Entry

Catalyst

Solvent

Yield (%)b

 1

[Cr(TPP)]SbF6

DCE

92

 2

[Mn(TPP)]SbF6

DCE

<1

 3

[Fe(TPP)]SbF6

DCE

85

 4

[Co(TPP)]SbF6

DCE

66

 5

[Cr(TPP)]BF4

DCE

<1

 6

[Cr(TPP)]OTf

DCE

<1

 7

[Cr(TPP)]Cl

DCE

<1

 8

AgSbF6

DCE

14

 9

[Cr(TPP)]SbF6

toluene

46

10

[Cr(TPP)]SbF6

1,4-dioxane

 8

11

[Cr(TPP)]SbF6

MeCN

 6

12

[Cr(TPP)]SbF6

MeNO2

<1

13

[Cr(TPP)]SbF6

DCE

18c

14

[Cr(TPP)]SbF6

DCE

<1d

a Reaction conditions: catalyst (2.5 mol%), 1a (0.3 mmol), solvent (3 mL), 100 °C, 2 h.

b Yields were determined by NMR spectroscopy.

c Reaction temperature: 60 °C.

d Reaction temperature: 25 °C.

In our initial investigation of the reaction, 5,10,15,20-tetraphenylporphyrinatochromium(III) hexafluoroantimonate [Cr(TPP)]SbF6 was prepared by treating [Cr(TPP)]Cl with AgSbF6 in dichloromethane at ambient temperature for six hours under argon atmosphere.[7] It was found that the reaction of 2-[(1,1′-biphenyl)-2-yl]oxirane (1a) in the presence of cationic chromium porphyrin catalyst [Cr(TPP)]SbF6 (2.5 mol%) in 1,2-dichloroethane at 100 °C for two hours afforded phenanthrene (2a) in 92% yield (Table [1], entry 1). The use of cationic manganese porphyrin catalyst [Mn(TPP)]SbF6 (2.5 mol%) in place of [Cr(TPP)]SbF6 did not provide 2a (Table [1], entry 2), while the use of [Fe(TPP)]SbF6 or [Co(TPP)]SbF6 also afforded 2a in 85% and 66% yields, respectively (Table [1], entries 3 and 4). With the examination of the effects of the counteranion of the chromium porphyrin catalyst, phenanthrene (2a) was obtained in excellent yield only when the chromium porphyrin catalyst had an SbF6 – counteranion; the reaction was retarded when counteranions such as BF4 –, TfO–, and Cl– were introduced (Table [1], entries 5–7). The use of AgSbF6 in place of the cationic chromium porphyrin catalyst resulted in the formation of 2a in 14% yield (Table [1], entry 8). In other solvents such as toluene, 1,4-dioxane, MeCN, and nitromethane the yields were even lower (Table [1], entries 9–12). The yield of 2a decreased when the reaction was performed at lower reaction temperature (60 °C and 25 °C, Table [1], entries 13 and 14).

Table 2 Dehydrative Cycloaromatization of 1 to Afford 2 [9] with Cationic Chromium(III) Porphyrin Catalysta

Entry

1

Substrate

2

Product

Yield (%)b

1

1b

2b

95

2

1c

2c

75c

3

1d

2d

90d

4

1e

2e

75e

5

1f

2f

93

6

1g

2g

73

7

1h

2h

96

8

1i

2i

95

a Reactions were carried out in the presence of [Cr(TPP)]SbF6 catalyst (2.5 mol%), 1 (0.3 mmol) in DCE (3 mL) at 100 °C for 2 h unless otherwise noted.

b Isolated yields are given.

c [Cr(TPP)]SbF6 catalyst (15 mol%).

d Regioisomeric ratio: 94:6.

e Regioisomeric ratio: 79:21.

The scope of the reaction was briefly examined, and the results are summarized in Table [2]. The reaction of biphenylyloxirane 1b possessing a para-methyl substituent afforded the correspondingly substituted phenanthrene 2b in 95% yield (Table [2], entry 1). When a biphenylyloxirane 1c with a methyl substituent at the ortho position was used, the reaction was retarded probably due to steric repulsive effect and afforded 2c in trace amounts. However, phenanthrene 2c was obtained in 75% yield by increasing the amount of catalyst to 15 mol% (Table [2], entry 2). Substrates with a methyl group at the meta position, biphenylyloxirane 1d, reacted to afford phenanthrene 2d in 90% yield with a regioisomeric ratio of 94:6 (Table [2], entry 3), while biphenylyloxirane 1e possessing a methoxy group at the same position reacted to produce 2e in 75% yield with a regioisomeric ratio of 79:21 (Table [2], entry 4). Biphenylyloxirane 1f with a methoxy substituent afforded phenanthrene 2f in 93% yield (Table [2], entry 5). The reaction of 2-[2-(thiophen-2-yl)phenyl]oxirane (1g) in the presence of chromium porphyrin catalyst afforded naphthothiophene (2g) in 73% yield (Table [2], entry 6). The chromium porphyrin catalyst was also found to be effective for the reaction of disubstituted oxiranes such as 1h and 1i to provide the correspondingly substituted phenanthrenes 2h and 2i in 96% and 95% yields, respectively (Table [2], entries 7 and 8).

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Scheme 2 Construction of polycyclic aromatic compounds with dehydrative cycloaromatization with cationic chromium(III) porphyrin catalyst

To demonstrate the scope of this cycloaromatization, we next examined the reaction to construct polycyclic aromatic molecular frameworks. As shown in Scheme [2], naphthalenylphenyloxirane (1j) reacted in the presence of cationic chromium porphyrin catalyst (15 mol%) to furnish benzo[c]phenanthrene (2j) in moderate yield (68%). On the other hand, when 2-[(1,1′-binaphthalen)-2-yl]oxirane (1k) was treated under the reaction conditions, the desired dibenzo[c,g]phenanthrene (2k) was obtained in 32% yield even with prolonged reaction time (48 h) and increased catalyst amount (30 mol%). The reaction was found to be applicable to the synthesis of benzo[k]tetra­phene (2l) via twofold dehydrative cycloaromatization of substrate 1l. The dehydrative cycloaromatization of 1m possessing a heteroatom also afforded benzonaphthothiophene 2m in 78% yield.

To gain insight into the cationic chromium porphyrin catalyzed dehydrative cycloaromatization, we performed the reaction with deuterium-labeled substrate (Scheme [3]). The reaction of 1b (99% D) under the standard conditions afforded phenanthrene 2b in 82% yield with 63% deuterium labeling at the 9-position of phenanthrene. The incomplete deuteration of product 2b could be explained by the formation of aldehyde intermediate 3 via 1,2-hydride shift,[8] which reacts with arene through intramolecular electrophilic aromatic substitution and subsequent dehydration. To test whether the reaction of 1b to give 2b involves the formation of aldehyde 3, deuterium-labeled aldehyde 3 (99% D) was subjected to the standard reaction conditions (Scheme [4]). And it was found that the reaction of aldehyde 3 (99% D) in place of oxirane 1b also furnished phenanthrene 2b in good yield with 63% deuterium labeling at the 9-position of phenanthrene. The observed data clearly indicated that the conversion of 1b into 2b proceeded via the formation of aldehyde 3.

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Scheme 3 The dehydrative cycloaromatization with deuterium-labeled substrate 1b to afford phenanthrene 2b
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Scheme 4 The dehydrative cycloaromatization with deuterium-­labeled aldehyde 3 to afford phenanthrene 2b

Taking into account the aforementioned data, we proposed the following reaction pathway for the dehydrative cycloaromatization catalyzed with cationic chromium porphyrin complex (Scheme [5]). The reaction would be initiated by coordination of the cationic chromium to the oxirane moiety of 1 to promote the formation of benzylic carbocation intermediate 4 via ring opening, which undergoes 1,2-hydride shift to afford intermediate 5. Intramolecular electrophilic aromatic substitution type reaction of 5 provides arenium intermediate 6. Subsequent aromatization of 7 via dehydration yields phenanthrene 2 and regenerates the catalyst.

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Scheme 5 A plausible reaction mechanism

In conclusion, we developed the dehydrative cycloaromatization of biphenylyloxiranes to afford phenanthrenes. The readily available cationic chromium porphyrin complex effectively catalyzed the reaction. The reaction is also applicable for the synthesis of polycyclic aromatic compounds and polycyclic heteroaromatic compounds. Detailed studies to elucidate the unique reactivity of the cationic metalloporphyrin catalyst and efforts to expand the scope of the reaction are under way.


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Acknowledgment

This work was supported by JST, ACT-C, and Grants-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. T.K. also acknowledges the Asahi Glass Foundation, The Uehara Memorial Foundation, Tokuyama Science Foundation, and Kurata Memorial Hitachi Science and Technology Foundation.

Supporting Information

    for this article is available online at http://www.thieme-connect.com.accesdistant.sorbonne-universite.fr/ejournals/toc/synlett.
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Zoom Image
Scheme 1 Strategy for the synthesis of phenanthrenes with dehydrative cycloaromatization
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Scheme 2 Construction of polycyclic aromatic compounds with dehydrative cycloaromatization with cationic chromium(III) porphyrin catalyst
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Scheme 3 The dehydrative cycloaromatization with deuterium-labeled substrate 1b to afford phenanthrene 2b
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Scheme 4 The dehydrative cycloaromatization with deuterium-­labeled aldehyde 3 to afford phenanthrene 2b
Zoom Image
Scheme 5 A plausible reaction mechanism