Rhodium-catalyzed cyclocarbonylation of azobenzene

Inorganica Chimica Acta 357 (2004) 3057–3063 www.elsevier.com/locate/ica

Rhodium-catalyzed cyclocarbonylation of azobenzene q Da-Yang Zhou, Tetsuharu Koike, Satoshi Suetsugu, Kiyotaka Onitsuka, Shigetoshi Takahashi * The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Received 2 March 2004; accepted 6 March 2004 Available online 21 April 2004

Abstract Carbonylation of azobenzene derivatives catalyzed by rhodium carbonyls in the presence of nitrobenzene as a hydrogen acceptor gave a four-ring heterocyclic product, indazolo[2,1-a]indazole-6,12-dione, in a good yield, which is derived from a novel cyclocarbonylation with C–H bond activation and CO insertion at each benzene nucleus of azobenzene. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Cyclocarbonylation; Azobenzene; Rhodium catalyst; Indazolone; C–H bond activation

1. Introduction Carbon–carbon unsaturated compounds such as alkenes and alkynes are smoothly carbonylated by the catalysis of transition metal complexes to give carbonyl compounds such as aldehydes and carboxylic acid derivatives (see, for example [1]). In the carbonylation of alkenes and alkynes bearing hydroxy and amino functional groups, cyclocarbonylation often occurs to yield heterocyclic compounds such as lactones and lactams, in which the functional groups are incorporated in the carbonylation (see, for example [2]). Lactones and lactams are a fundamental skeleton frequently found in bioactive substances. Many efforts for the effective synthesis of such heterocyclic compounds have been made so far in synthetic organic chemistry, and the cyclocarbonylation is realized to provide an effective method for the direct and practical synthesis of heterocyclic compounds from simple starting substrates (see, for example [3]). One of more direct and convenient methods for the synthesis may be the carbonylation of unsaturated compounds consisting of heteroatoms such as azo and azomethine compounds. However, a few req Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2004.03.009. * Corresponding author. Tel.: +81668798455; fax: +81668798459. E-mail address: [email protected] (S. Takahashi).

0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.03.009

ports on the carbonylation of such unsaturated heteroatom compounds have appeared in the literature [4]. In 1955, Murahashi and Horiie [4b] reported a cobalt-catalyzed carbonylation of azobenzene (Eq. (1)) which proceeds via two steps to afford a final product, quinazoline 3a. The first step of carbonylation proceeds at 190 °C to give indazolone 2a and then the second step at 230 °C transforms 2a to the final product with further uptake of CO. Co2(CO)8 N N

+ CO

190 ˚C

H N N O

2a

1a Co2(CO)8 CO 230 ˚C

H N

O N

O

3a

ð1Þ It should be noted that the first-step reaction is not only the first example of the carbonylation of unsaturated heteroatom compounds but also the first example of catalytic carbonylation involving C–H bond activation. Here, we wish to report the cyclocarbonylation of azobenzene catalyzed by rhodium carbonyls which proceeds in a different fashion from that of cobalt and

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gives a four-ring heterocyle, indazolo[2,1-a]indazole6,12-dione, in a good yield.

2. Experimental IR spectra were taken with a Perkin–Elmer 2000 infrared spectrophotometer. NMR spectra were recorded on a JEOL JNM-LA400 FT NMR system in CDCl3 with tetramethylsilane as an internal standard. Mass spectra were obtained using Shimadzu GCMS-QP2010 and JEOL JMS-600H spectrometers. Analytical GC was carried out by Shimadzu GC-14A with a CPB1 column. Elemental and HRMS analyses were performed by Materials Analysis Center, ISIR, Osaka University. 2.1. Materials Solvents and reagents were purified prior to use according to standard procedures. Rh6 (CO)16 [5a] and [Rh(CO)2 Cl]2 [5b] were prepared by methods described in the literature. Azobenzene derivatives are commercially available, but some were prepared by the literature methods [6]. 2.2. General procedure for carbonylation of azobenzene A mixture of azobenzene 1 (5 mmol), [Rh(CO)2 Cl]2 (0.075 mmol) and nitrobenzene (10 ml) was placed in a 100 ml stainless-steel autoclave and stirred under 70 atm of carbon monoxide at 190 °C for 18 h. The reaction mixture was dissolved in dichloromethane and the products were separated by column chromatography on silica gel with AcOEt–hexane as an eluent to give product 5, which was purified by recrystallization from dichloromethane–hexane. Indazolo[2,1-a]indazole-6,12-dione (5a) (R ¼ H) [7]: m.p. 288 °C; IR (KBr, cm
1 ): 1688 (mco ); 1 H NMR (CDCl3 ): d 7.98 (d, 2H, J ¼ 8.3 Hz, Arom), 7.91 (d, 2H, J ¼ 8.3 Hz, Arom), 7.73 (t, 2H, 7.3 Hz, Arom), 7.37 (t, 2H, J ¼ 7.3 Hz, Arom); 13 C NMR (CDCl3 ): d 156.0 (C@O), 136.2 (C), 138.0 (C), 135.0 (CH), 125.5 (CH), 125.2 (CH), 121.8 (C), 113.4 (CH); MS (EI) m/z 236 (Mþ ). Anal. Calc. for C14 H8 N2 O2 : C, 71.18; H, 3.41; N, 11.86. Found: C, 70.92; H, 3.69; N, 11.86%. 2,8-Dimethylindazolo[2,1-a]indazole-6,12-dione (5b) (R ¼ p-CH3 ) [8]: m.p. 254 °C; IR (KBr, cm
1 ): 1703 (mco ); 1 H NMR (CDCl3 ): d 7.83 (d, 2H, J ¼ 8.3 Hz, Arom), 7. 67 (s, 2H, Arom), 7.51 (d, 2H, J ¼ 8.3 Hz, Arom), 2.46 (s, 6H, CH3 ); 13 C NMR (CDCl3 ): d 156.2 (C@O), 136.2 (C), 136.0 (CH), 135.7 (C), 125.0 (CH), 122.2 (C), 112.8 (CH), 21.2 (CH3 ); MS (EI) m/z 264 (Mþ ). Anal. Calc. for C16 H12 N2 O2 : C, 72.72; H, 4.58; N, 10.60. Found: C, 72.42; H, 4.82; N, 10.66%. 2,8-Dimethoxyindazolo[2,1-a]indazole-6,12-dione (5c) (R ¼ p-OCH3 ): m.p. 318 °C; IR (KBr, cm
1 ): 1681 (mco );

1

H NMR (CDCl3 ): d 7.85 (d, 2H, J ¼ 8.3 Hz, Arom), 7.32–7.26 (m, 4H, Arom), 3.88 (s, 6H, CH3 ); 13 C NMR (CDCl3 ): d 157.9 (C@O), 156.1 (C), 132.6 (C), 123.6 (CH), 123.0 (C), 114.2 (CH), 106.9 (CH), 56.0 (CH3 ); MS (EI) m/z 296 (Mþ ). Anal. Calc. for C16 H12 N2 O4 : C, 64.86; H, 4.08; N, 9.46. Found: C, 64.64; H, 4.27; N, 9.30%. 6,12-Dioxoindazolo[2,1-a]indazole-2,8-dicarboxylic acid dimethyl ester (3d) (R ¼ p-CO2 CH3 ): m.p. 276 °C; IR (KBr, cm
1 ): 1714 (mco ); 1 H NMR (CDCl3 ): d 8.62 (s, 2H, Arom), 8.43 (d, 2H, J ¼ 8.5 Hz, Arom), 8.02 (d, 2H, J ¼ 8.5 Hz, Arom), 3.98 (s, 6H, CH3 ); 13 C NMR (CDCl3 ): d 165.3 (C@O), 155.3 (C@O), 140.3 (C), 136.7 (CH), 128.0 (C), 127.7 (CH), 121.6 (C), 113.1 (CH), 52.7 (CH3 ); MS (EI) m/z 352 (Mþ ). Anal. Calc. for C18 H12 N2 O6 : C, 61.37; H, 3.43; N, 7.95. Found: C, 61.08; H, 3.67; N, 8.08%. 6,12-Dioxoindazolo[2,1-a]indazole-2,8-dicarboxylic acid diethyl ester (3e) (R ¼ p-CO2 CH2 CH3 ): m.p. 248 °C; IR (KBr, cm
1 ): 1718 (mco ); 1 H NMR (CDCl3 ): d 8.64 (s, 2H, Arom), 8.44 (d, 2H, J ¼ 8.3 Hz, Arom), 8.04 (d, 2H, J ¼ 8.3 Hz, Arom), 4.43 (q, 4H, J ¼ 7.1 Hz, CH2 ), 1.46 (t, 6H, J ¼ 7.1 Hz, CH3 ); 13 C NMR (CDCl3 ): d 165.2 (C@O), 155.7 (C@O), 140.9 (C), 137.1 (CH), 128.7 (C), 127.9 (CH), 121.9 (C), 113.4 (CH), 16.1 (CH2 ), 14.6 (CH3 ); MS (EI) m/z 380 (Mþ ). Anal. Calc. for C20 H16 N2 O6 : C, 63.16; H, 4.24; N, 7.37. Found: C, 62.91; H, 4.28; N, 7.32%. 2,8-Difluoroindazolo[2,1-a]indazole-6,12-dione (5f) (R ¼ p-F): m.p. 317 °C; IR (KBr, cm
1 ): 1694 (mco ), 1226 (mCF ); 1 H NMR (CD2 Cl2 ): d 7.91 (dd, 2H, J ¼ 8.8, 3.9 Hz, Arom), 7.56 (dd, 2H, J ¼ 7.3, 2.4 Hz, Arom), 7.48 (td, 2H, J ¼ 8.8, 2.4 Hz, Arom); 13 C NMR (CD2 Cl2 ): d 159.0 (C@O), 122.9 (C), 122.7(CH), 114.6(C), 114.5 (CH), 111.5 (C), 111.3 (CH); MS (EI) m/z 272 (Mþ ); Anal. Calc. for C14 H6 N2 O2 F2 : C, 61.31; H, 2.40; N, 10.01. Found: C, 61.77; H, 2.22; N, 10.29%. 3,9-Dimethylindazolo[2,1-a]indazole-6,12-dione (91 g) (R ¼ m-CH3 ): m.p. 276 °C; IR (KBr, cm
1 ): 1694 (mco ); 1 H NMR (CDCl3 ): d 7.77–7.75 (m, 4H, Arom), 7.15 (d, 2H, J ¼ 8.5 Hz, Arom), 2.53 (s, 6H, CH3 ); 13 C NMR (CDCl3 ): d 156.5 (C@O), 146.8 (C), 138.7 (C), 126.7 (CH), 124.9 (CH), 119.3 (C), 113.2 (CH), 22.3 (CH3 ); HRMS Calc. for C16 H12 N2 O2 : 264.0899. Found: 264.0880. 1,9-Dimethylindazolo[2,1-a]indazole-6,12-dione (92 g) (R ¼ m-CH3 ): m.p. 215 °C; IR (KBr, cm
1 ): 1697 (mco ); 1 H NMR (CDCl3 ): d 7.74–7.72 (m, 3H, Arom), 7.53 (t, 1H, J ¼ 7.7 Hz, Arom), 7.13 (d, 1H, J ¼ 7.7 Hz, Arom), 7.05 (d, 1H, J ¼ 7.0 Hz, Arom), 2.72 (s, 3H, CH3 ), 2.51 (s, 3H, CH3 ); 13 C NMR (CDCl3 ): d 157.1 (C@O), 156.3 (C@O), 146.7 (C), 140.0 (C), 138.54 (C), 138.50 (C), 134.6 (CH), 126.9 (CH), 126.7 (CH), 124.9 (CH), 119.5 (C), 119.3 (C), 113.1 (CH), 110.4 (CH), 22.4 (CH3 ), 17.3 (CH3 ); HRMS Calc. for C16 H12 N2 O2 : 264.0899. Found: 264.0920.

D.-Y. Zhou et al. / Inorganica Chimica Acta 357 (2004) 3057–3063

3,9-Difluoroindazolo[2,1-a]indazole-6,12-dione (91 h) (R ¼ F): m.p. 143 °C; IR (KBr, cm
1 ): 1712 (mco ), 1229 (mC–F ); 1 H NMR (CDCl3 ): d 7.91–7.88 (q, 1H, J ¼ 5.1 Hz, Arom), 7.78–7.65 (m, 3H, Arom), 7.11–6.98 (m, 2H, Arom); 13 C NMR (CDCl3 ): d 153.9 (C@O), 137.6 (C), 130.9 (C), 130.3 (CH), 129.5 (CH), 124.9 (C), 121.3 (CH); HRMS Calc. for C14 H6 N2 O2 F2 : 272.0397. Found: 272.0423. 1,9-Difluoroindazolo[2,1-a]indazole-6,12-dione (92 h) (R ¼ m-F): m.p. 255 °C; IR (KBr, cm
1 ): 1712 (mco ), 1229 (mCF ); 1 H NMR (CDCl3 ): d 7.89 (dd, 2H, J ¼ 8.8, 5.1 Hz, Arom), 7.63 (dd, 2H, J ¼ 8.1, 2.2 Hz, Arom), 7.07 (td, 2H, J ¼ 8.8, 2.2 Hz, Arom); 13 C NMR (CDCl3 ): d 169.0 (C@O), 166.4 (C@O), 156.0 (C), 140.1 (C), 139.9 (C), 128.1 (CH), 128.0 (CH), 117.95 (C), 117.93 (C), 114.5 (CH), 114.3 (CH), 101.5 (CH), 101.2 (CH); HRMS Calc. for C14 H6 N2 O2 F2 : 272.0397. Found: 272.0417. 3,9-Diisopropyl-indazolo[2,1-a]indazole-6,12-dione (91 i) (R ¼ m-i Pr): m.p. 181 °C; IR (KBr, cm
1 ): 1712 (mco ); 1 H NMR (CDCl3 ): d 7.80–7.78 (m, 4H, Arom), 7.21 (d, 2H, J ¼ 8.3 Hz, Arom), 3.07 (sept, 2H, J ¼ 6.8 Hz, CH), 1.33 (d, 12H, J ¼ 6.8 Hz, CH3 ); 13 C NMR (CDCl3 ): d 159.2 (C@O), 158.0 (C), 140.3 (C), 126.6 (CH), 125.9 (CH), 121.2 (C), 112.3 (CH), 36.6 (CH), 25.2 (CH3 ); HRMS Calc. for C20 H20 N2 O2 : 320.1525. Found: 320.1553.

3. Results and discussion Previously, we have reported the cobalt-catalyzed cyclocarbonylation of diphenylacetylene affording indanones [9], which also involves a C–H bond activation and uptake of one molecule of CO. We have also shown a new type of cyclocarbonylation of diphenylacetylene catalyzed by rhodium carbonyls affording furanones [10], which are formally produced from the reaction of the acetylene with two molecules of CO and one molecule of H2 . Now we have extended our catalytic system using rhodium carbonyl cluster to the carbonylation of azobenzene, since a rhodium complex often exhibits a different catalytic activity from a cobalt one, and found that the carbonylation of azobenzene in the presence of rhodium carbonyl cluster Rh6 (CO)16 at 190 °C produced indazolone (2a), imidazolone (4a) and indazolo[2,1-a]indazole-6,12-dione (5a) (Eq. (2)). The reaction at 230 °C gave essentially the same result as that at 190 °C although the Co-catalyzed reaction at elevated temperature causes a second-step carbonylation (Eq. (1)). In addition, the present reaction gave products 4a and 5a which were not found in the products from the Co-catalyzed carbonylation. The carbonylation of azobenzene using a rhodium(III) hydroxide catalyst was reported 35 years ago from a group in ISIR of Osaka

3059

University, however the products were not identified correctly [11]. Rh 6(CO) 16 + CO

N N

190 ˚C

1a H N

H N N

O

+

N

O

ð2Þ 2a

4a NH2

O N N

+

+

O 5a

6

Products 2a, 4a and 5a in Eq. (2) were identified by IR, MS and, 1 H and 13 C NMR spectra as well as by comparison with the spectral data reported in the literature [7]. The molecular structure of 5a was finally confirmed to be indazolo[2,1-a]-indazole-6,12-dione [12] 1; 2 by an X-ray crystallographic analysis (Fig. 1). Since the Rh carbonyls catalyzed a different type of carbonylation from the Co one, we checked the catalytic activity of various metal carbonyls of groups 8 and 9 metals towards the carbonylation of azobenzene. The results are summarized in Table 1. The Co, Ir and Fe carbonyl complexes are not active for the carbonylation of azobenzene under the reaction conditions in Table 1, although Co carbonyl showed a high activity under CO pressure of 150 atm [4b]. Rh complexes are effective and [Rh(CO)2 Cl]2 showed the highest selectivity for 5a (Entry 8). Table 2 shows the solvent effect for the carbonylation when [Rh(CO)2 Cl]2 was employed as a catalyst. The reactions in dioxane, benzene and hexane gave 5a in moderate yields. Product 5a has a unique four-ring structure and seems to be formed from the occurrence of C–H bond activation at an ortho-position of each phenyl group of azobenzene 1

Crystallographic data (excluding structure factors) for the structure of 5a have been deposited with the Cambridge Crystallographic Data Centre as Supplementary Publication No. CCDC 231704. 2 Crystal data for Indazolo[2,1-a]indazole-6,12-dione 5a: C14 H8 N2 O2 , M ¼ 236.23, crystal dimensions 0.10  0.20  0.30 mm, monoclinic, space group P 21 =c, a ¼ 5:803ð4Þ, b ¼ 6:409ð3Þ,  V ¼ 522:6ð4Þ A 3 , Z ¼ 2, Dc ¼ 1:501 g/cm3 , graphite c ¼ 14:252ð3Þ A,  l ¼ 1:03 cm
1 , monochromated Mo Ka radiation with k ¼ 0:71069 A, 1332 reflections were collected at )75 °C on a Rigaku AFC7R four circle diffractometer in the x–2h scan mode to 2hmax ¼ 55:1°. The structure was solved by direct methods and expanded using Fourier techniques, and refined to give R ¼ 0:045, Rw ¼ 0:062 for 692 observed reflections [I > 3:00rðIÞ].

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 and Fig. 1. An ORTEP drawing of 5a. Selective bond distances (A) angles (°): N(1)–N(1) , 1.387(4); N(1)–C(1), 1.392(3); N(1) –C(3), 1.404(4); C(1)–O(1), 1.224(3); C(2)–C(3), 1.401(4); C(3)–C(4), 1.377(4); C(4)–C(5), 1.388(5); C(5)–C(6), 1.396(4); C(6)–C(7), 1.383(5); N(1) – N(1)–C(1), 112.4(3); N(1)–C(1)–C(2), 102.9; O(1)–C(1)–C(2), 132.5(3); C(1)–C(2)–C(3), 109.5(3); C(2)–C(3)–N(1) , 107.2(2).

followed by CO insertion. The formation of 5a was always accompanied by side-product aniline 6 (Entries 1– 4) which may derive from the reductive cleavage of the N@N bond in azobenzene by hydrogen liberated from

phenyl rings via C–H bond activation (vide infra). The C–H bond activation by the Rh catalyst probably results in the formation of a hydridorhodium species Rh–H, which may cause the reductive cleavage of N@N bond to give aniline. High yield synthesis of 5a essentially requires depressing the consumption of the azobenzene substrate to aniline. To improve the selectivity for 5a, we searched out a suitable hydrogen acceptor from the Rh–H species. Thus, large amounts of olefins such as cyclopentene and norbornene were added in the reaction system, but appreciable improvement in the selectivity was not observed. On the other hand, it is well known that nitrobenzene is reduced to aniline in the presence of Rh carbonyl catalysts under water–gas shift reaction conditions, and a Rh–H species is postulated as an active intermediate in the reaction. [13] Thus, we used nitrobenzene as a solvent in the present carbonylation and found remarkable improvement in the selectivity for 5a (Entry 8). This experimental result shows that nitrobenzene can effectively accept the hydrogen coming from the phenyl groups. The yield of aniline formed in

Table 1 Catalytic activity of groups 8 and 9 metal carbonylsa Entry

1 2 3 4 5 6b 7b 8b

Catalyst

Conv. (%)

c

Co2 (CO)8 Ir4 (CO)12 d Fe3 (CO)12 e Ru3 (CO)12 e Rh6 (CO)16 f RhCl3 g [Rh(cod)2 Cl2 ]2 c [Rh(CO)2 Cl]2 c

Trace Trace Trace 40 100 93 72 88

Yield (%) 2a

4a

5a

6

Trace 0 Trace Trace 26 0 10 8

0 0 0 7 12 0 1 1

Trace 0 Trace 7 33 24 29 43

Trace 0 Trace 7 19 0 19 24

a

Reaction conditions: azobenzene 5 mmol; benzene 10 ml; CO 70 atm; 190 °C; 4 h. Dioxane 10 ml. c 0.075 mmol. d 0.038 mmol. e 0.05 mmol. f 0.025 mmol. g 0.15 mmol. b

Table 2 Effect of solventa Entry

Catalyst

Conv. (%)

Yield (%) 2a

4a

5a

6

1 2 3 4 5 6 7 8

1,4-Dioxane THF Benzene Hexane Cyclohexane CH2 Cl2 Nitromethane Nitrobenzene

88 72 54 100 100 100 12 81

8 8 1 0 2 0 0 Trace

1 Trace 1 3 1 1 0 2

43 11 31 33 2 2 2 62

24 29 16 9 68 0 Trace 15

a

Reaction conditions: azobenzene 5 mmol; [Rh(CO)2 Cl]2 1.2 mol%; solvent 10 ml; CO 70 atm; 190 °C; 4 h.

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Table 3 Carbonylation of p-substituted azobenzenesa Entry

Substrate

9 10 11 12 13 14

R

Conv. (%)

H Me MeO CO2 Me CO2 Et F

1a 1b 1c 1d 1e 1f

Product

100 100 100 100 100 100

5a 5b 5c 5d 5e 5f

Yield (%)b

Yield (mmol)c

5

6

7

79 85 40 64 58 60

0.39 0.35 0.26 0.22 0.12 0.15

0.37 0.27 0.36 0.59 0.37 0.63

a

Reaction conditions: substrate 1 mmol; [Rh(CO)2 Cl]2 1.2 mol%; nitrobenzene 2 ml; CO 70 atm; 190 °C; 18 h. Based on the substrate used. c The amount formed from the reaction. b

Table 4 Cyclocarbonylation of m-substituted azobenzenesa Entry

Products

R

15 16 17

Me F i Pr

Molar ratio

91 g 91 h 91 i

92 g 92 h 92 i

5:1:0 4:1:trace 1:0:0

93 g 93 h 93 i

Yield (%)b

Yield (mmol)c

9

6

7

68 35 32

0.19 0.42 0.59

0.43 0.78 0.61

a

Reaction conditions: substrate 1 mmol; [Rh(CO)2 Cl]2 1.2 mol%; nitrobenzene 2 ml; CO 70 atm; 190 °C; 18 h. Based on the substrate used. c The amount formed from the reaction. b

Entry 8 seemed to be rather lower than an expected one based on the yield of 5a, suggesting that aniline underwent a further carbonylation forming diphenylurea (vide infra). Then, we have applied this new type of cyclocarbonylation to several azobenzene derivatives (Eqs. (3) and (4)). The results obtained from the carbonylation of p-substituted azobenzenes (Eq. (3)) are summarized in Table 3. Azobenzene derivatives such as 4,40 -dimethyland 4,40 -dimethoxycarbonylazobenzene were smoothly cylocarbonylated to give indazoloindazoledione derivatives in good to moderate yields. Nitrobenzene behaves as an effective hydrogen acceptor, and the formation of aniline derivatives such as p-methyl- and p-methoxycarbonyl-aniline from the substrates was not observed. Instead diphenylurea 7, which is the carbonylation product of aniline, was formed.

The cyclocarbonylation of m-substituted azobenzene derivatives was thought to afford three structural isomers of indazoloindazoledione, 91 , 92 and 93 as shown in Eq. (4). Thus, 3,30 -dimethylazobenzene 8a was carbonylated to give products 91 g and 92 g with a molar ratio of 5:1, but not 93 g (Entry 15). 3,30 -Difluoroazobenzene 8h also gave 91 h as a major product. The carbonylation of 3,30 -diisopropylazobenzene 8i afforded exclusively product 91 i although the yield was not high. The product distribution may be understandable based on the director and steric factor of the substituents (vide infra).

NO2

R N N

190 ˚C

R +

N N

R

8

NO 2 R

O

R

CO +

[Rh(CO)2Cl]2

+ CO +

O

R

N N

1

O a: R=H b: R=CH3 c: R=CH3O d: R=CO2CH3 e: R=CO2C2 H5 f: R=F

[Rh(CO)2Cl]2

N N

R

190 ˚C

H N +

6

92 O

N N

+ NH2

O

91

O 5

+

R

O

R

H N

R

C O

R

N N

R + 6 + 7

O

93 g: R=CH3; h: R=F; i: R=iPr

7

ð3Þ

ð4Þ

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D.-Y. Zhou et al. / Inorganica Chimica Acta 357 (2004) 3057–3063 NH2 NO2

[Rh]

N N

(A) N

[Rh] H (E)

CO

N Rh H (B)

O N N

H N

N C [Rh] O (F)

[Rh] [Rh]

O 5a O N

H N

H [Rh] CO N

N

H [Rh]

N [Rh]

[Rh] O

N O 2a

H (C)

H (D)

Scheme 1. Proposed carbonylation mechanism.

On the basis of the above experimental results as well as azobenzene–metal complexes reported in the literature, we propose a tentative cyclocarbonylation mechanism as shown in Scheme 1. It is already known that azobenzene reacts with metal complexes of palladium [14], nickel [15] and other metals [16] to produce stable ortho-metallated complexes via C–H bond activation. Subsequent treatment of the palladium complex with CO gives indazolone 2a (Eq. (5)), while the dinuclear ortho-metallated complex of nickel gives indazoloindazoledione (5a) (Eq. (6)) [17].

PdCl 2(PhCN)2

N N Pd

Cl Pd Cl

H N

CO

N N

N O 2a

(5)

N N

Cp2Ni

O

Ni N

N Ni

CO

N N O 5a

(6)

Although these are stoichiometric reactions of metallated azobenzene complexes with CO, they suggest the catalytic transformation route of azobenzene to 2a and 5a by the Rh species in Scheme 1. Key intermediate C may be postulated on the basis of the reaction of azobenzene with nickelocene followed by treatment with CO yielding 5a (Eq. (6)) [17]. A similar transformation of azobenzene yielding an imino analogue of 5 was observed for the reaction with isonitrile, which has an isoelectronic structure with CO [17]. The carbonylation of azobenzene at 230 °C with the Rh catalyst in benzene

or in nitrobenzene did not give quinazoline 3a, but gave indazoloindazoledione 5 as mentioned above. In addition, the treatment of 2a with CO in the presence of the Rh catalyst at 230 °C gave neither 3a nor 5a, and 2a was recovered intact, indicating clearly that 2a is not an intermediate for the transformation of azobenzene to 5a. Mono-carbonylated product 2a may be formed via a route similar to that proposed for an azobenzene–Pd analogue [14a]. C–H Bond activation by a metal species is often observed in the reactions of metal complexes even with hydrocarbons [18a]. In particular, a C–H bond at an ortho-position of phenyl group in aromatic amines and phosphines, which can coordinate to the metal, frequently undergoes facile activation, so-called orthometallation, and forms a phenyl–metal r-bond [18b]. As seen in Eqs. (5) and (6), azobenzene also undergoes ortho-metallation in the reaction with several metal complexes. Since the ortho-metallation is essentially governed by initial interaction of the azo group to the metal and subsequent C–H bond activation at the orthoposition, the reactivity of azobenzene derivatives towards the present carbonylation would be affected by electronic and steric factors of substituents on azobenzene. Thus, we examined the reactivity of azobenzene derivatives bearing electron-donating and -withdrawing groups at ortho, meta- or para-position relative to the azo group. A phenylazo group has an electron-withdrawing character and meta-director similar to those of an acetyl group [19]. The results obtained from the carbonylation of the para- and meta-substituted azobenzenes are summarized in Tables 3 and 4, respectively, showing that the substituents give a little electronic influence on the reactivity of azobenzene towards the

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carbonylation. Introduction of electron-withdrawing substituents on the phenyl group resulted in a somewhat lower yield of product 5. Azobenzene bearing a methoxy group gave a low yield of 5c and an oligomeric product was formed. Although the electronic influence of the substituents on the interaction between the azo group and Rh cannot be evaluated at present, decrease in the electron density on the phenyl ring seems to depress the C–H bond activation at ortho-positions. However, steric factor of the substituents actually gave a strong effect on the carbonylation. In the reaction of meta-substituted azobenzenes 8, the exclusive formation of 91 i and the low total-yield of 9i should be due to a steric reason of a bulky isopropyl group. In addition, ortho-substituted azobenzenes such as o,o0 -dimethylazobenzene did afford neither indazolone 2 nor indazoloindazoledione 5 and most of the starting substrate was recovered, implying that a substituent at one of two ortho-positions in each phenyl group prevents the interaction between the azo group and the central metal atom of catalyst.

4. Conclusions Both cobalt and rhodium carbonyls are good catalysts for the carbonylation of alkenes, but they often exhibit a different catalytic activity from each other. Azobenzene is carbonylated by a cobalt carbonyl catalyst to indazolone and quinazoline, whereas rhodium carbonyl complexes catalyze the novel cyclocarbonylation of azobenzene in nitrobenzene to give indazolo[2, 1-a]indazole-6,12-dione. The formation of indazoloindazoledione is interpreted by a mechanism involving the C–H bond activation of benzene nucleus followed by insertion of a CO molecule to the Rh–C bond and then by reductive elimination affording the product along with a Rh–H species. Nitrobenzene behaves as an effective hydrogen acceptor and takes hydrogen from the Rh–H species, resulting in a high yield of indazoloindazoledione in the present carbonylation.

Acknowledgements This work is partially supported by a grant-in-aid for Scientific Research on Priority Areas (A) ‘‘Exploitation of Multi-Element Cyclic Molecules’’ from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. We are grateful to the Materials Analysis Center, ISIR, Osaka University, for their support with the spectral measurements, X-ray diffraction studies and elemental analyses.

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