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Pd(OAc)2/DABCO as an efficient and phosphinefree catalytic system for the synthesis of single and double Weinreb amides by the aminocarbonylation of aryl iodides† Sandip T. Gadge and Bhalchandra M. Bhanage* This work reports a mild, stable and efficient Pd(OAc)2/DABCO catalysed protocol for the synthesis of single and double Weinreb amides. Double Weinreb amides, having 1,4-phenylene- and biphenylenelinkers – important backbones for the synthesis of biologically active symmetrical resorcylate oligomer

Received 7th April 2014, Accepted 19th May 2014

units – were synthesized by the double carbonylation of aryl diiodides. Notably, the reaction does not require any expensive or air/moisture sensitive phosphine ligands. DABCO was found to be an inexpensive

DOI: 10.1039/c4ob00729h

and stable ligand for the Pd(OAc)2 catalysed carbonylation of aryl iodides under an atmospheric pressure

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of carbon monoxide, and offered excellent yields of the single and double Weinreb amides.

Introduction N-Methoxy-N-methyl amides (Weinreb amides)1 are an important functional moiety for the synthesis of various functional groups such as aldehydes,2 ketones,3 trifluoromethylketones,4 and α-chloro ketones.5 Recent studies have shown that N-methoxy-N-methyl amides act as directing groups for ortho olefination6 and ortho oxygenation reactions7 of aryl rings. Furthermore, Barrett and co-workers showed that double Weinreb amides with 1,4-phenylene 1 and biphenylene 2 linkers may be used for the synthesis of biologically active symmetrical resorcylate oligomer units8 (Fig. 1).

Fig. 1 Double Weinreb amides as building blocks for the synthesis of biologically active symmetrical resorcylate oligomer units.8

Department of Chemistry, Institute of Chemical Technology, N. Parekh Marg, Matunga, Mumbai-400019, India. E-mail: [email protected], [email protected]; Tel: +91 22-33612601 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4ob00729h

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Various methodologies utilising starting materials such as acids,9 acid chlorides,10 aldehydes,11 amides,12 and esters,13 have been employed for the synthesis of Weinreb amides. However, the above methodologies suffer from one or more drawbacks such as the use of an activator or coupling reagent, thermally unstable acid chlorides, or stoichiometric reagents. The aminocarbonylation of aryl halides is an alternative methodology for the synthesis of Weinreb amides (Scheme 1). Buchwald and co-workers synthesized a Weinreb amide by the aminocarbonylation of an aryl halide using a Pd(OAc)2/xanthphos catalytic system.14 Kollár and co-workers used Pd(OAc)2 along with PPh3 as a ligand for the synthesis of Weinreb amides by the aminocarbonylation of aryl iodides.15 In 2011, Odell and co-workers demonstrated the microwave-assisted synthesis of Weinreb amides by the aminocarbonylation of aryl halides.16 They used expensive and stoichiometric amounts of metal carbonyls such as W(CO)6, along with a Pd(OAc)2 catalyst, an expensive xanthphos ligand and N,N-dimethylaminopyridine to facilitate the aminocarbonylation. Barrett and co-workers used a two step method for the synthesis of double Weinreb amides8 (N1,N4-dimethoxy-N1,N4dimethylterephthalamide and N4,N4′-dimethoxy-N4,N4′dimethyl-[1,1′-biphenyl]-4,4′-dicarboxamide) using symmetrical diacids as starting materials (Scheme 1). Carbonylation methods use phosphine ligands which are generally used to activate and stabilize the palladium species. However, most of the phosphine ligands used are air-sensitive and expensive, require tedious work-up procedures to separate them from the products and involve high workup costs which place significant limits on their synthetic applications. Therefore, the development of phosphine-free palladium catalysts is

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mono and diiodo derivatives under phosphine-free conditions, and provided the desired product in excellent yield as compared with previous carbonylation methods.

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Results and discussion

Scheme 1 Previous methods for the synthesis of single and double Weinreb amides.

gaining increased attention.17 In this respect, palladium catalysts, along with various N-coordinating ligands, have been well studied for various cross-coupling and carbonylation reactions.18 These amine ligands are stable, inexpensive, stabilize the reactive palladium intermediates and provide the products in excellent yields.18,19 In continuation of our research into phosphine-free carbonylation reactions,17a,20 herein we report a phosphinefree Pd-catalyst with 1,4-diazabicyclo[2.2.2]octane (DABCO) as an inexpensive, stable ligand for the synthesis of single and double Weinreb amides by the aminocarbonylation of aryl iodides (Scheme 2). The DABCO ligand has the advantage that it is air/moisture stable and hence is easy to handle as compared to conventional PPh3 based catalysts. The double Weinreb amides were efficiently synthesized by the aminocarbonylation of aryl diiodides. This catalytic system provided the aminocarbonylation products in complete selectivity for a wide range of aryl iodides and N-methoxy-N-methyl amine nucleophiles under an atmospheric pressure of carbon monoxide. The scope of the reaction was explored for both

Initial attempts focused on exploring the feasibility of the proposed process. The reaction between iodobenzene 3 and N,Odimethyl hydroxylamine hydrochloride 4 was chosen as a model reaction. We first tested various Pd-precursors and N-coordinating ligands such as DABCO, DBU, DMAP and TMEDA in the carbonylation reaction, and the results are summarized in Table 1. Pd(OAc)2, PdCl2, and Pd(acac)2 were found to be the most effective catalysts with DABCO as a ligand (Table 1, entries 1–3). Using 10% Pd/C as a catalyst the yields were low (Table 1, entry 4). Without any ligands, only a 30% yield of the corresponding cross-coupled product 5 was isolated in the presence of 5 mol% of Pd(OAc)2 and 2.5 equiv. of Na2CO3 (Table 1, entry 5), whereas the yield of 5 increased sharply to 96% when 10 mol% of DABCO was added (Table 1, entry 2). This implied that the addition of ligand was necessary. This demonstrated that Pd(OAc)2/DABCO is the real catalytic system in this process, and that DABCO acts as a ligand and stabilizes the reactive palladium intermediate. In order to optimize the reaction conditions, other commonly used ligands such as DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), DMAP, TMEDA and 1,10-phenanthroline were also compared with DABCO under the same conditions (Table 1, entries 6–9). DBU and DMAP resulted in good yields, but by using TMEDA and 1,10-phenanthroline only moderate yields of the desired product were observed. The catalytic activity strongly

Table 1

Optimization of the reaction parametersa

Entry Catalyst

Ligand

Base

Solvent

% Yieldb

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

DABCO DABCO DABCO DABCO — DBU DMAP TMEDA 1,10-Phenanthroline DABCO DABCO DABCO DABCO DABCO DABCO

Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3 K3PO4 Et3N Na2CO3 Na2CO3 Na2CO3 Na2CO3

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN Dioxane DMF CH3CN CH3CN

91 96 92 20 30 81 75 34 31 78 60 72 22 77 65

PdCl2 Pd(OAc)2 Pd(acac)2 10% Pd/C Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2

a

Scheme 2 Synthesis of single and double Weinreb amides by the aminocarbonylation of aryl iodides.

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Reaction conditions: 3 (1 mmol), 4 (1.5 mmol), Pd(OAc)2 (5 mol%), DABCO (10 mol%), Na2CO3 (2.5 mmol), CO (1 atm), temperature 80 °C, time 8 h. b G.C. yields. c Pd(OAc)2 (2 mol%). d Pd(OAc)2 (1 mol%).

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depended on the choice of ligand, and dramatic changes in yields were observed by using DABCO as a ligand (Table 1, entry 2). The effect of base was also examined, and our study showed that the replacement of Na2CO3 with other bases such as K3PO4 and Et3N drastically reduced the yield of the amide product (Table 1, entries 10 and 11). Screening of solvents was also carried out, and it was found that acetonitrile was the best solvent among several others screened (Table 1, entries 2, 12 and 13). A catalyst loading of 5 mol% was found to be optimal as decreasing the catalyst loading to 2 mol% or 1 mol% reduced the yield to 77% and 65%, respectively (Table 1, entries 14 and 15). Optimization of the reaction times showed that the reaction was completed within 8 h at 80 °C under an atmospheric pressure of carbon monoxide. To enlarge the scope of the reaction, aryl iodides containing various functional groups were used as substrates with 4 as a nucleophile using the Pd(OAc)2/DABCO catalytic system. The results are summarized in Table 2. The model reaction of iodobenzene with 4 under the optimized reaction conditions provided a 95% yield of the corresponding amide (Table 2, entry 1). Aryl iodides possessing electron-withdrawing groups (–CF3, –Cl, –NO2 and –CN) were found to be good substrates for the aminocarbonylation reaction (Table 2, entries 2–7). Aryl iodides such as 1-iodo-2-methylbenzene, 1-iodo-4-methoxybenzene and 2-iodopyridine provided the carbonylated products in good to excellent yield (Table 2, entries 8–10). Furthermore, we could synthesize the N-methoxy-N-methyl-2naphthamide in excellent yield starting from 2-iodonaphthalene (Table 2, entry 11). As the incorporation of double Weinreb amide functionality on aryl rings is of importance, and only one method is available for the synthesis of such functionality,8 we have targeted and developed a new efficient method for the synthesis of double Weinreb amide derivatives using the aminocarbonylation of aryl diiodides (Scheme 3). The reaction provided the product in excellent yield using Pd(OAc)2/DABCO as the catalytic system. 1,4-diiodobenzene 6a and 4,4′-diiodo-1,1′-biphenyl 6b provided excellent yields of N1,N4-dimethoxy-N1,N4dimethylterephthalamide 7a and N4,N4′-dimethoxy-N4,N4′dimethyl-[1,1′-biphenyl]-4,4′-dicarboxamide 7b respectively. Notably the double carbonylation proceeds under phosphine free conditions and provided the double Weinreb amides in excellent yield.

Paper Table 2 Synthesis of Weinreb amides by the aminocarbonylation of aryl iodidesa

Entry

Aryl iodide

Product

% Yieldb

1

95

2

90

3

92

4

86

5

88

6

83

7

93

8

94

9

89

10

85

11

91

a Reagents and conditions: aryl iodide (1 mmol), 4 (1.5 mmol), Pd(OAc)2 (5 mol%), DABCO (10 mol%), Na2CO3 (2.5 mmol), CO (1 atm), temperature 80 °C, time 8 h. b Isolated yields.

A plausible reaction mechanism for the synthesis of Weinreb amides by the aminocarbonylation of aryl iodides using a Pd(OAc)2/DABCO catalytic system is shown in Scheme 4. The catalytically active species during the catalytic cycle is believed to be a Pd(0) complex [A], which can be formed

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during the reaction in the presence of CO and the DABCO ligand. Oxidative addition of the aryl iodide on the palladium (0) complex [A], formed in situ from palladium(II) acetate and DABCO takes place. The resulting palladium(II) intermediate [B] yields the Pd-acyl intermediate [C] by the migratory insertion of CO. Species [C] undergoes nucleophilic attack by the

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Experimental

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General

Scheme 3 Synthesis of double Weinreb amides by the aminocarbonylation of aryl diiodides. Reagents and conditions: 6a or 6b (1 mmol), 4 (2.5 mmol), Pd(OAc)2 (10 mol%), DABCO (20 mol%), Na2CO3 (4.5 mmol), CO (1 atm), temp. 80 °C, time 10 h.

All chemicals were purchased from Sigma Aldrich, S.D. Fine Chemicals, Lancaster (Alfa-Aesar) and commercial suppliers. The progress of the reaction was monitored by gas chromatography, using a Perkin Elmer Clarus 400 GC instrument equipped with a flame ionization detector (FID) and capillary column (30 m × 0.25 mm × 0.25 μm). Thin layer chromatography was performed using Merck silica gel 60 F254 plates. The product was visualized with a 254 nm UV lamp. Products were purified by column chromatography on a silica gel (100–200) mesh. All compounds were confirmed by GCMS, FT-IR, 1H and 13C NMR spectroscopic techniques. Mass spectra were obtained on a GCMS-QP 2010 instrument (Rtx-17, 30 m × 25 mm ID, film thickness 0.25 µm df, column flow 2 mL min−1, 80 °C to 240 °C at 10° min−1 rise). IR spectra were recorded using a FT-IR instrument (Perkin Elmer). GC analysis was carried out on a Perkin Elmer (Clarus-400) instrument equipped with a flame ionization detector with a capillary column (Elite-1, 30 m × 0.32 mm × 0.25 µm). Chemical shifts are reported in parts per million (δ) relative to tetramethylsilane as an internal standard. J (coupling constant) values were reported in Hz, and splitting patterns of protons are described as s (singlet), d (doublet), t (triplet), q (quartet), or m (multiplet). General experimental procedure

Scheme 4 A plausible reaction mechanism for the Pd(OAc)2/DABCO catalysed synthesis of a Weinreb amide by the aminocarbonylation of an aryl iodide.

N-methoxy-N-methyl amine moiety, resulting in the formation of the corresponding Weinreb amide. Complex [D] then reductively eliminates HI and regenerates complex [A], marking the completion of the catalytic cycle.

Conclusions In summary we have developed an efficient, phosphine free catalytic system for the synthesis of single and double Weinreb amides via the aminocarbonylation of aryl iodides and diiodides respectively. Pd(OAc)2/DABCO is the real catalytic system in this process, where DABCO was found to be a stable, inexpensive ligand, which stabilized the reactive palladium intermediate and provided the Weinreb amide in excellent yield under phosphine free conditions. The double Weinreb amide was successfully synthesized by the aminocarbonylation of aryl diiodides. The protocol will be of practical use as an economical synthetic method and offers an alternative synthetic strategy for the practical construction of Weinreb amide derivatives.

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Aryl iodide (1 mmol), 4 (1.5 mmol), Pd(OAc)2 (5 mol%), DABCO (10 mol%), Na2CO3 (2.5 mmol) and CH3CN (15 mL) were charged in a 100 mL-autoclave. The autoclave was flushed three times with nitrogen, pressurised with CO (1 bar) and the reaction was performed at 80 °C for 8 hours. The mixture was cooled to room temperature and vented to discharge the excess CO. The reaction mixture was washed with brine solution (3 × 5 mL) and extracted with ethyl acetate (3 × 5 mL), dried over Na2SO4 and then evaporated in vacuo. The residue was purified by column chromatography on silica gel with hexanes–ethyl acetate as the eluent to afford the corresponding Weinreb amide. Characterisation data of products N-Methoxy-N-methylbenzamide (Table 2, entry 1). Colourless oil; isolated yield: 95%; IR: 2922, 1638, 1236, 1090 cm−1; 1 H NMR (CDCl3, 400 MHz): δ 7.67 (d, 6.8 Hz, 2H), 7.47–7.38 (m, 3H), 3.56 (s, 3H), 3.36 (s, 3H); 13C NMR (CDCl3, 100 MHz) δ 170.0, 134.2, 130.5, 128.1, 128.0, 61.0, 33.8; GCMS (m/z): 165 (M+ 3%), 105 (100), 91 (0.5), 77 (59), 51 (20), 40 (3). 2-Chloro-N-methoxy-N-methylbenzamide (Table 2, entry 2). Colourless oil; isolated yield: 90%; IR: 2921, 1655, 1236, 1089 cm−1; 1H NMR (CDCl3, 400 MHz): δ 7.34 (m, 4H), 3.44 (s, 3H), 3.33 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 168.4, 135.2, 130.6, 130.3, 129.5, 127.6, 127.5, 61.3, 32.2; GCMS (m/z): 199 (M+ 2%), 168 (1), 139 (100), 111 (28), 85 (2), 75 (20), 50 (9). 3-Chloro-N-methoxy-N-methylbenzamide (Table 2, entry 3). Colourless oil; isolated yield: 92%; IR: 2923, 1644, 1236,

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1090 cm−1; 1H NMR (CDCl3, 400 MHz): δ 7.67 (s, 1H), 7.57 (d, J = 7.6 Hz, 1H), 7.43 (d, J = 8.4 Hz, 1H), 7.36–7.32 (m, 1H), 3.56 (s, 3H), 3.38 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 168.3, 135.7, 134.0, 130.6, 129.3, 128.3, 126.3, 61.1, 33.5; GCMS (m/z): 199 (M+ 5%), 168 (1), 139 (100), 111 (43), 85 (3), 75 (23), 50 (9). N-Methoxy-N-methyl-4-nitrobenzamide (Table 2, entry 4). Yellowish solid; isolated yield: 86%; IR: 3116, 2925, 2852, 1524, 975 cm−1; 1H NMR (CDCl3, 400 MHz): δ 8.28 (d, J = 8.8 Hz, 2H), 7.86 (d, J = 8.8 Hz, 2H), 3.55 (s, 3H), 3.41 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 167.6, 148.7, 140.0, 129.1, 123.1, 61.3, 33.1; GCMS (m/z): 210 (M+ 4%), 179 (2), 150 (100), 120 (17), 104 (40), 92 (23), 76 (31), 50 (16). N-Methoxy-N-methyl-3-(trifluoromethoxy)benzamide (Table 2, entry 6). Colourless oil; isolated yield: 83%; IR: 2922, 1644, 1236, 1090 cm−1; 1H NMR (CDCl3, 400 MHz): δ 7.65 (d, J = 7.5 Hz, 1H), 7.58 (s, 1H), 7.47–7.43 (m, 1H), 7.31 (d, J = 8.4 Hz, 1H), 3.59 (s, 3H), 3.41 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 168.1, 148.7, 135.8, 130.0, 129.6, 127.0, 123.0, 121.2, 61.2, 31.9; GCMS (m/z): 249 (M+ 4%), 218 (1), 189 (100), 161 (27), 95 (38), 75 (7), 64 (8), 40 (3). 4-Cyano-N-methoxy-N-methylbenzamide (Table 2, entry 7). Colourless oil; isolated yield: 93%; IR: 3094, 3068, 2977, 2940, 2820, 2232, 1935, 1650, 1610, 1559, 1510, 1461, 1422, 1383, 1180, 1149, 1114, 1063, 1022 cm−1; 1H NMR (CDCl3, 400 MHz): δ 7.79 (dd, J = 2, 6.8 Hz, 2H), 7.72 (dd, J = 2, 6.8 Hz, 2H), 3.54 (s, 3H), 3.39 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 167.8, 138.2, 131.8, 128.8, 118.1, 114.1, 61.3, 33.1; GCMS (m/z): 190 (M+ 3%), 159 (2), 130 (100), 102 (47), 75 (11), 51 (8). N,4-Dimethoxy-N-methylbenzamide (Table 2, entry 9). Colourless oil; isolated yield: 89%; IR: 3285, 3075, 2940, 2842, 1637, 1510, 1460, 1422, 1373, 1255, 1174, 1027, 980, 844, 757 cm−1; 1H NMR (CDCl3, 400 MHz): δ 7.70 (s, 4H), 3.54 (s, 6H), 3.38 (s, 6H); GCMS (m/z): 195 (M+ 1%), 194 (2), 152 (10), 135 (100), 107 (15), 92 (14), 77 (22), 64 (10). 7a: N1,N4-dimethoxy-N1,N4-dimethylterephthalamide. White solid; isolated yield 92%; IR: 1626, 1508, 1460, 1447, 1422, 1397, 1383, 1193, 968 cm−1; 1H NMR (CDCl3, 400 MHz): δ 7.69 (s, 4H), 3.53 (s, 6H), 3.36 (s, 6H); 13C NMR (CDCl3, 100 MHz): δ 169.1, 136.0, 127.8, 61.1, 33.5; GCMS (m/z): 252 (M+ 2%), 192 (100), 161 (7), 133 (28), 104 (37), 76 (21), 50 (7). 7b: N4,N4′-dimethoxy-N4,N4′-dimethyl-[1,1′-biphenyl]-4,4′dicarboxamide. Yellow solid; isolated yield 87%; IR: 1621, 1607, 1569, 1414, 1380, 1190, 981 cm−1; 1H NMR (CDCl3, 400 MHz): δ 7.59 (d, J = 8 Hz, 4H), 7.35 (d, J = 8.4 Hz, 4H), 3.59 (s, 6H), 3.39 (s, 6H); 13C NMR (CDCl3, 100 MHz): δ 169.5, 142.1, 138.0, 128.9, 126.4, 61.1, 33.7; GCMS (m/z): 328 (M+ 1%), 268 (100), 209 (28), 180 (30), 152 (17), 119 (2), 104 (22), 90 (10), 76 (16), 63 (3).

Acknowledgements STG is thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing the Senior Research Fellowship and DST-JSPS to conduct collaborative work at the University of Tokyo, Kashiwa campus, Japan.

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DABCO as an efficient and phosphine-free catalytic system for the synthesis of single and double Weinreb amides by the aminocarbonylation of aryl iodides.

This work reports a mild, stable and efficient Pd(OAc)2/DABCO catalysed protocol for the synthesis of single and double Weinreb amides. Double Weinreb...
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