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Palladium catalyzed synthesis of highly substituted naphthalenes via direct ring construction from amides with alkynesw Junliang Wu, Xiuling Cui,* Xia Mi, Ying Li and Yangjie Wu*

Downloaded by University of California at Berkeley on 01 September 2010 Published on 23 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CC01448F

Received 17th May 2010, Accepted 14th July 2010 DOI: 10.1039/c0cc01448f The direct ring construction of amides with alkynes catalyzed by palladium acetate with cheap oxidant under an air atmosphere has been realized. A variety of novel highly substituted naphthalenes 3a–3l have been prepared chemo- and regioselectively in 55–97% yields under mild conditions. Product 3j emits intense blue luminescence peaked at 435 nm with a good blue purity. The synthesis of substituted polycyclic aromatic compounds has attracted considerable attention because of their increasing applications as p-conjugated functional materials, such as organic semiconductors and luminescent materials.1 Of particular interest is the synthesis of polyarylated aromatic cores owing to their enhanced ability to transport charge and fluorescent properties brought by the aryl groups. The synthetic methodologies for such structures have been developed recently, among which, transition metal-catalyzed ring construction of an aromatic substrate with two alkyne molecules has emerged particularly as an effective tool.2 However, the di- or mono- functionalized aromatic substrates, such as aryl halide and aryl boronates, were required. Polyarylated heteroaromatic compounds, for instance, indoles, isoquinolines, benzothiazoles and pyridines, could be obtained by catalytic activation of C–H bonds with subsequent C–C bond formation,3 which is one of most valuable endeavours from atom- and step-economic points of view.4 Miura et al. has obtained naphthalene derivatives via the dual C–H bond activation of 1-phenylpyrazole catalyzed by a rhodium complex in the presence of excessive Cu(OAc)2 as the oxidant.5a However, the requirement of Cu(OAc)2 as the oxidant, an inert gas atmosphere and ortho-azole as the directing group, which requires several steps for installation and detachment, may limit its structural modification and applications. Thus, cycloaromatization of alkynes with arenes through activation of C–H bonds still remains challenge.5 Based on the ability of N-acetyl anilines to undergo ortho-metalation,6 the ease to be de-protected and modified, and stability of Pd(OAc)2 under an air atmosphere, here we studied the synthesis of 5,6,7,8-tetraaryl-N-acetyl-1-aminonaphthalenes from N-acetyl anilines with two alkyne molecules through dual C–H bond activation catalyzed by Pd(OAc)2 using a cheap oxidant under mild conditions. The products with fluorescent properties were obtained. Henan Key Laboratory of Chemical Biology and Organic Chemistry, Key Laboratory of Applied Chemistry of Henan Universities, Department of Chemistry, Zhengzhou University, 75 Da Xue Road, Zhengzhou 450052, China. E-mail: [email protected], [email protected]; Fax: (+86)-371-67767753 w Electronic supplementary information (ESI) available: Preparation and characterization. CCDC 778161. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cc01448f

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The Royal Society of Chemistry 2010

Initially, the condensation of acetanilide 1a with diphenylacetylene 2a was chosen as a model reaction in the presence of palladium acetate (5 mol%) to examine the impact of various parameters on the reaction (Table 1). The results revealed that 5,6,7,8-tetraphenyl-N-acetyl-1-aminonaphthalene 3a was obtained as a main product in 83% yield in toluene at 80 1C when potassium persulfate and TsOH (p-toluenesulfonic acid) (0.5 equiv.) were used as oxidant and additive, respectively (Table 1, entry 11). Polar solvents, such as HOAc, DMSO and a mixture of HOAc and toluene (1 : 2 v/v), did not favour this reaction (Table 1, entries 1–6). Cu(OAc)2 (Table 1, entry 9) and Ag2O (Table 1, entry 10) afforded only 13% and trace amount of the desired product, respectively. The use of potassium persulfate as oxidant showed a similar performance as benzoquinone (BQ) (Table 1, entry 11 vs. 7), and was cheaper and easier to be isolated from the product. TsOH was superior to trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TfOH) as additives (Table 1, entry 11 vs. entries 12 and 13). Sulfuric acid was not suitable as additive since acetanilide was partly decomposed although the yield was improved slightly (see ESI).w Table 1 Screening of various reaction conditions for ring construction of 1a with 2a via dual C–H bond cleavagea

Entry Solvent

Additives

Oxidants

T/1C Yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

TsOH TsOH TsOH TsOH TsOH TsOH TsOH — TsOH TsOH TsOH CF3COOH CF3SO3H TsOH TsOH TsOH

BQ BQ BQ BQ BQ BQ BQ BQ Cu(OAc)2H2O Ag2O K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8

80 80 80 80 80 80 80 80 80 80 80 80 80 60 70 100

HOAc DMSO HOAc–toluene Acetonitrile Ethanol DMF Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene

58 3 66 15 58 36 82(78) 7 13 Trace 83(80) 10 71 65 79 83

a

Reaction conditions: 1a (0.3 mmol), 2a (0.63 mmol), Pd(OAc)2 (5 mol%), additives (0.15 mmol), oxidant (0.6 mmol), solvent (1.5 mL), 16 h. b Yields determined by HPLC analysis and based on 1a. Isolated yield in parentheses.

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Table 2 Entry

Palladium catalyzed direct ring construction of amides 1 with alkynesa 1

2

Product yield (%)b


1

Entry

1

2

7

1c

2b

2

2a

8

3

2a

9

1a

4

2a

10

1b

2c

5

2a

11

1c

2c

6

1a

Product yield (%)b

2b

12

2a

a Reaction conditions: 1 (0.3 mmol), 2 (0.63 mmol), Pd(OAc)2 (5 mol%), TsOH(0.15 mmol), K2S2O8 (0.6 mmol), toluene (1.5 mL), 16 h. yield in parentheses.

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b

Isolated



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Under the optimized reaction conditions {5% Pd(OAc)2, 1.0 equiv. anilide, 2.1 equiv. diarylacetylene, 0.5 equiv. TsOH, 2.0 equiv. K2S2O8, at 80 1C in toluene}, the scope of the substrates was examined (Table 2). Diphenylethylene 2a reacted smoothly with acetanilide and its derivatives 1a–1e to give 5,6,7,8-tetraphenyl-N-acetyl-1-aminonaphthalenes 3a–3e in good to excellent yields (entries 1–5). 5,6,7,8-Tetra(4-methyl)phenyl-N-acetyl-1-aminonaphthalenes 3f–3h were obtained as main products from the reaction of di-(4-methyl)phenylacetylene 2b with acetanilides 1a, 1c and 1f (entries 6–8). Di-(4-trifluoromethyl)phenylacetylene was also a suitable substrate, providing 5,6,7,8-tetra-(4-trifluoromethyl)phenyl-N-acetyl-1-aminonaphthalenes 3i–3k in 81–97% yields (entries 9–11). N-Propionylaniline can also be applied, affording the desired product 3l in moderate yield (entries 12). It is worth noting that the reactions of 2,3-dimethylacetanilide 1d and 2-methylacetanilide 1f with diarylacetylene provided the corresponding desired products in 77% (entry 4) and 61% (entry 8) yields, respectively, although it was reported that the direct ortho-alkenylation of ortho-substituted acetanilides catalyzed by Pd(OAc)2 via C–H bond activation failed.7 No products were afforded when mono aryl-, alkyl- substituted and dialkyl- substituted alkyens were used as substrates. The results of Table 2 revealed that this procedure exhibited electronic dependence. Electron-deficient diarylacetylenes exhibited higher reactivity than those of electron-rich diarylacetylenes. The substituents on the moiety of the acetanilides also influenced the efficiency of the cycloaromatization significantly. Acetanilides substituted with electron-donating group, such as methyl, converted into the desired products smoothly, whereas, no desired products were observed for acetanilides with electron-withdrawing groups, e.g. trifluoromethyl and nitro, indicating that the lower electron density on ortho-carbon of the N-acetyl group has the cyclometallation obstructed. These results suggested a reaction pathway via electrophilic attack of cationic [PdOAc]+ species on the p-system of the arenas.8 Product 3 showed solid state fluorescence within a range of 350–480 nm (Fig. 1). Interestingly, red-shifts of fluorescence spectra were observed when electron-withdrawing groups, e.g. trifluoromethyl, and electron-donating groups, e.g. methyl, were introduced into the diarylacetylene and acetanilide, respectively. As a result, compound 3j emitted intense blue luminescence peaked at 435 nm with a full width at halfmaximum of 75 nm, presenting a good blue purity, which implied that 3j may be employed as a promising blue-emitting material to fabricate OLED device. In summary, we have developed an efficient method for the direct ring construction of amides with alkynes catalyzed by Pd(OAc)2 under mild reaction conditions. A series of novel, highly substituted naphthalenes have been prepared in good to excellent yields. The products obtained can be applied as a promising blue-emitting material. Further investigations on the scope of the reaction and employing the products as blueemitting materials to fabricate OLED device are in progress. This work was supported by NSF of China (20772114, 20972139), NSF of Henan (082300423201) and Outstanding Doctoral Dissertation Fund of Zhengzhou University. We

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Fig. 1

Fluorescence spectra of products 3 in the solid state.

also thank Prof. Chenxia Du from Zhengzhou University for fluorescence spectra and valuable discussion.

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