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A novel regiospecific synthesis of 1-chloro-2-arylcyclohexenes.

1. Introduction

Cross-coupling reactions of unsaturated carbon centres bearing halogens/triflates with aryl boronic acids catalyzed by palladium metal complexes have become a powerful tool in organic synthesis for the formation of carbon-carbon bonds. One of the important cross-coupling reactions in this genre is the Suzuki reaction [1-4]. These organopalladium complex catalyzed couplings of unsaturated halides with boronic acids/esters have been employed in key steps to form a wide variety of pharmaceuticals and natural products [510].

1,2-Dihalocycloalkenes are precursors to cycloalkynes [11, 12]. However, there are very few reports for the conversion of 1,2-dihalocycloalkenes to substituted 1-halo-2arylcycloalkenes. One report shows the chemoselective cross coupling of 2-bromo-3-chloronorbornadiene to 2-chloro-3aryl-norbornadiene [13]. In this regard, the Suzuki coupling reaction of cyclohexenyl bromides with arylboronic acids is still almost unexplored. A recent report shows its importance in the synthesis of polysubstituted phenol derivatives [14].

Our laboratory is involved in the synthesis and reactions of some novel cyclic vinyl silanes. We employ the Wurtz-Fittig coupling reaction of corresponding cyclic vinyl halide with sodium and chlorotrimethylsilane in suitable solvent to prepare the cyclic vinylsilanes. Using the Wurtz-Fittig reaction, we have been able to synthesize a large number of simple and substituted cyclic vinylsilanes [15-19]. The organosilicon compounds are anionic synthons serving as the starting materials in the total synthesis of natural products [20-23].

Cyclic vinyl halides and 1,2-dihalocycloalkenes serve as precursors to the cyclic vinylsilanes. There are several protocols for the preparation of cyclic vinyl halides and 1,2-dihalocycloalkenes [24]. The protocols for 1,2dihalocycloalkenes normally afford the two halogen atoms symmetrically attached to the carbon-carbon double bond [25, 26]. In comparison to the preparation of symmetrical 1,2-dihalocycloalkenes, there exist very few protocols for the synthesis of unsymmetrical 1,2-dihalocycloakenes especially 1-bromo-2-chlorocycloalkenes [27-31]. We had earlier adopted the addition reaction of bromine chloride to 1chlorocyclopentene followed by dehydrochlorination for the synthesis of 1-bromo-2-chlorocyclopentene [25].

In this paper, we report for the first instance the synthesis of 1-bromo-2-chlorocyclohexene (2) and the chemoselective Suzuki cross-coupling reaction with eleven different aryl boronic acids (3a-k) to form eleven novel 1-chloro-2arylcyclohexenes (4a-k) in high yields.

2. Materials and Methods

All reactions were performed in oven dried apparatus. Reactions were monitored on Merck F-254 precoated TLC plastic sheets using hexane (60-80[degrees]C) fraction as mobile phase. GC was run on SE-30 SS2 m x 1/8" column on Mayura 9800 gas chromatograph. IR spectra were recorded on Shimadzu FTIR 8400S using NaCl flats as neat thin liquid film samples. The values are reported in wave numbers ([cm.sup.-1]). [sup.1]H NMR and [sup.13]C NMR were obtained on a Bruker AMX400 spectrometer using CD[Cl.sub.3] with tetramethylsilane as internal standard.

Chemical shifts are reported in [delta] (ppm downfield from tetramethylsilane). Coupling constants are reported in Hz with multiplicities denoted as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), and br (broad). GC-MS spectra were obtained using a Shimadzu GC-MS QP 5050A instrument equipped with a 30 mm x 0.32 mm BP-5 capillary column. Elemental analyses were obtained using Elementar Vario Microcube-15106062 instrument. All yields refer to the isolated yields of the products.

2.1. Synthesis of 1-Bromo-2-chlorocyclohexene (2). To a stirred suspension of P[Cl.sub.5] (1.4 g, 6.723 mmol) in 10 mL anhydrous ether was added a 10 mL ethereal solution of 1 (1.0 g, 5.525 mmol), dropwise over 30 min. The mixture was stirred for further 8 hours, when all the starting material disappeared as followed by GC. The reaction mixture was poured into crushed ice (50 g) and extracted with ether (3 x 30 mL). The combined organic extracts were washed with brine (20 mL), dried over anhydrous [Na.sub.2]S[O.sub.4], and concentrated in vacuo to isolate 1.05 g of concentrate.

To 0.63 g (5.614 mmol) of potassium tert-butoxide in 10 mL tert-butyl alcohol was added the 1.05 g of crude in 10 mL tert-butyl alcohol drop wise over 15 min and refluxed for 2 hours, when the aliquot indicated a single peak in gas chromatogram. The mixture was diluted with water (150 mL), extracted with ether (4 x 30 mL), washed with brine (20 mL), dried (an. [Na.sub.2]S[O.sub.4]), and concentrated. Distillation under reduced pressure gave pure 2 (0.62 g, 56%).

2.2. General Procedure for the Synthesis of 1-Chloro2-arylcyclohexenes (4a-k). A mixture of 1-bromo-2chlorocyclohexene 2 (196 mg, 1.0 mmol), aryl boronic acid (1.0 mmol), [K.sub.2]C[O.sub.3] (3.0 mmol), and Pd(dppf)[Cl.sub.2] x C[H.sub.2][Cl.sub.2] (5 mol%) in 10 mL 1,4-dioxane was heated in a sealed tube placed in an oil bath at 70-80[degrees]C for 2-3 hr. The reaction mixture was cooled to ambient temperature and filtered, and the solvent was removed in vacuo. The crude product was then subjected to column chromatography using silica gel(60-100 mesh) and 1:10 ethyl acetate/hexane (60-80[degrees]C fraction) to isolate 4a-k in greater than 90% yield (Table 2).

2.3. Spectral Data for the Products

2.3.1. 1-Bromo-2-chlorocyclohexene (2). IR: 2941, 2862, 2839, 1641,1434,1325,1006 [cm.sup.-1]; :H NMR(CD[Cl.sub.3]) [delta]: 1.70-1.80 (m, 4H), 2.40-2.45 (m, 2H) 2.53-2.58 (m, 2H); [sup.13]C NMR(CD[Cl.sub.3]) [delta]: 23.30, 23.89, 34.60, 36.47, 119.50, 130.83; GC-MS m/e (rel. int.): 198 (0.29), 196 (1.22), 194 (0.80), 168 (0.18), 166 (0.17), 160 (0.42), 159 (0.33), 158 (0.27), 129 (0.24), 117 (2.63), 116 (1.06), 115 (7.30), 114 (0.72), 113 (0.57), 100 (0.94), 99 (1.14), 89 (1.16), 87 (3.19), 81 (2.18), 80 (8.05), 79 (100.00), 78 (7.21), 77 (29.9), 75 (3.88), 74 (3.40), 73 (3.66), 63 (7.03), 53 (5.90), 52 (12.56), 51 (34.76), 50 (20.39) 49 (4.31); Anal. Found: C, 36.95; H, 4.19. Calcd: C, 36.86; H, 4.12.

2.3.2.1-Chloro-2-phenylcyclohexene (4a). IR: 698,1004,1440, 2858, 2929 [cm.sup.-1]; [sup.1]H NMR (CD[Cl.sub.3]) [delta]: 1.70-2.84 (m, 4H), 2.36-2.47 (m, 2H), 2.48-2.57 (m, 2H), 7.24-7.60 (m, 5H); [sup.13]C NMR (CD[Cl.sub.3]) [delta]: 22.68, 23.90, 32.95, 34.09, 126.09, 127.15, 127.23, 127.80, 128.03, 128.73, 134.40, 141.56; Anal. Found: C, 74.76; H, 6.75. Calcd: C, 74.80; H, 6.80.

2.3.3. 1-Chloro-2-phenyl-3'-methyl-cyclohexene (4b). IR: 1440, 1693, 2880, 2931 [cm.sup.-1]; [sup.1]H NMR (CD[Cl.sub.3]) [delta]: 1.69-1.84 (m, 4H), 2.35 (s, 3H), 2.36-2.58 (m, 4H), 7.04-7.59 (m, 4H); [sup.13]C NMR (CD[Cl.sub.3]) [delta]: 22.70, 23.34, 23.93, 33.02, 34.64, 119.56, 124.26, 125.12, 127.67, 127.94, 128.67, 137.59, 141.54; Anal. Found: C, 75.45; H, 7.27. Calcd: C, 75.53; H, 731.

2.3.4.1-Chloro-2-phenyl-4'-formyl-cyclohexene (4c). IR: 1209, 1602,1701, 2731, 2837, 2860, 2933 [cm.sup.-1]; [sup.1]H NMR (CD[Cl.sub.3]) [delta]: 1.26-1.87 (m, 4H), 2.38-2.41 (m, 2H), 2.48-2.52 (m, 2H), 7417.44 (d, 2H, J = 12 Hz) 7.85-7.88 (d, 2H, J = 12 Hz), 10.00 (s, 1H); [sup.13]C NMR (CD[Cl.sub.3]) [delta]: 22.48,23.74,32.52,34.06,128.36,129.23, 129.60,130.05,130.26,133.52,135.02,147.93; GC-MS m/e (rel. int.): 221.93 (32), 219.82 (100), 184.99 (28), 154.88 (32). Anal. Found: C, 70.77; H, 5.99. Calcd: C, 70.75; H, 5.94.

2.3.5. 1-Chloro-2-phenyl-2'-methoxy-cyclohexene (4d). IR: 1249,1490,1596, 2837, 2856, 2931 [cm.sup.-1]; [sup.1]H NMR (CD[Cl.sub.3]) [delta]: 1.75-1.85 (m, 4H), 2.51 (br, 4H), 3.83 (s, 3H), 6.92-6.99 (m, 2H), 7.10-7.12 (m, 1H), 7.26-7.30 (m, 1H); [sup.13]C NMR (CD[Cl.sub.3]) [delta]: 22.50,23.97, 31.65, 33.71, 55.62,111.15,120.41,128.28,128.36, 130.00,130.57,132.60,156.26; Anal. Found: C, 70.15; H, 6.72; Calcd: C, 70.11; H, 6.79.

2.3.6. 1-Chloro-2-phenyl-2'-methoxy-5-isopropyl-cyclohexene (4e). [sup.1]H NMR (CD[Cl.sub.3]) [delta]: 1.23-1.25 (d, 6H, J = 6.8 Hz), 1.751.90 (br m, 4H), 2.10-2.35 (br, 4H), 2.40-2.60 (br, 2 H), 2.822.92 (sep, 1H), 3.79 (s, 3H), 6.83-6.85 (d, 1H, J = 8.4 Hz), 6.96-6.97 (d, 1H, J = 2.4 Hz), 7.10-7.12 (m, 1H); [sup.13]C NMR (CD[Cl.sub.3]) [delta]: 22.99,24.45,24.59, 30.13, 32.17,33.67, 34.23, 56.24, 111.48, 126.32, 128.56, 128.71, 130.73, 133.40, 141.18, 154.80; Anal. Found: C, 72.59; H, 797 Calcd: C, 72.57; H, 799.

2.3.7.1-Chloro-2-phenyl-4'-ethoxy-cyclohexene (4f). IR: 1286, 1685, 2862, 2935 [cm.sup.-1]; [sup.1]H NMR (CD[Cl.sub.3]) [delta]: 1.40-1.43 (t, 3H, J = 7.2 Hz), 1.74-1.82 (m, 4H), 2.35-2.49 (m, 4H), 4.01-4.06 (q, 2H, J = 6.8 Hz), 6.86-6.88 (d, 2H, J = 8.4 Hz), 7.18-7.20 (d, 2H, J = 8.8 Hz); GC-MS m/e (rel. int.): 23780 (38), 235.81 (100), 207.70 (28), 200.82 (36), 172.76 (56). Anal. Found: C, 71.09; H, 730. Calcd: C, 71.03; H, 724.

2.3.8. 1-Chloro-2-phenyl-3',4-dimethoxy-cyclohexene (4g). IR: 1159,1207,1504,1606, 2837, 2933 [cm.sup.-1]; [sup.1]H NMR (CD[Cl.sub.3]) S: 1.17-1.18 (broad m, 4H), 2.1-2.4 (broad s, 2H), 2.4-2.6 (broad s, 2H), 3.7 (s, 3H), 3.8 (s, 3H), 6.5 (s, 2H), 6.98-7.01 (d, 1H, J = 12 Hz), [sup.13]C NMR (CD[Cl.sub.3]) [delta]: 22.59, 24.01, 31.89, 33.82, 55.28, 55.63, 99.00, 104.19, 106.19, 128.48, 130.39, 132.29, 15735,160.01; Anal. Found: C, 66.50; H, 6.72; Calcd: C, 66.53; H, 6.78.

2.3.9.1-Chloro-2-phenyl-3'-acetyl-cyclohexene (4h). IR: 1286, 1685, 2862, 2935 [cm.sup.-1]; [sup.1]H NMR (CD[Cl.sub.3]) [delta]: 1.73-1.86 (m, 4H), 2.38-2.42 (m, 2H), 2.48-2.51 (m, 2H), 2.61 (s, 3H), 7.46 (s, 1H), 7.42-7.48 (m, 1H) 7.85-7.86 (q, 2H); [sup.13]C NMR (CD[Cl.sub.3]) [delta]: 22.57,23.81,26.64, 32.77,34.04,126.91,128.11,128.34,128.77, 132.98,133.56,137.05,141.94,198.04; Anal. Found: C, 71.60; H, 6.40. Calcd: C, 71.64; H, 6.44.

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2.3.10. 1-Chloro-2-phenyl-3'-chloro-cyclohexene (4i). IR: 1438, 1703, 2860, 2935 [cm.sup.-1]; [sup.1]H NMR (CD[Cl.sub.3]) [delta]: 1.691.84 (m, 4H), 2.33-2.58 (m, 4H), 7.12-7.58 (m, 3H); [sup.13]C NMR(CD[Cl.sub.3]) [delta]: 23.34, 23.94, 34.64, 36.51, 119.56, 126.38, 127.04, 127.88, 128.30, 129.34, 130.12, 130.89; Anal. Found: C, 63.49; H, 5.38. Calcd. C, 63.46; H, 5.33.

2.3.11. 1,3', 4-Trichloro-2-phenyl-cyclohexene (4j). [sup.1]H NMR (CD[Cl.sub.3]) [delta]: 1.72-1.84 (m, 4H), 2.32-2.35 (m, 2H), 2.45-2.49 (m, 2H), 7.08-7.11 (q, 1H, J = 4 Hz) 7.35-7.63 (m, 2H); [sup.13]C NMR (CD[Cl.sub.3]) [delta]: 22.49, 23.71, 32.61, 34.04, 126.15, 127.72, 128.80, 129.33, 130.07, 130.21, 130.96, 141.34; Anal. Found: C, 55.15; H, 4.29; Calcd. C, 55.10; H, 4.24.

2.3.12. 1-Chloro-2-(2'-benzo[b]furano)-cyclohexene (4k). 1H NMR (CD[Cl.sub.3]) [delta]:1.82-1.84 (m, 4H), 2.61-2.73 (m, 4H), 7237.32 (m, 3H), 7.47-7.49 (d, 1H, J = 7.6 Hz), 7.60-7.62 (d, 1H, J = 7.6 Hz); [sup.13]C NMR (CD[Cl.sub.3]) [delta]: 22.39, 23.99, 28.703, 35.82,106.85,111.35,121.50,123.14,124.62,124.67,129.18,131.34, 153.97,154.38; Anal. Found: C, 72.30; H, 5.68. Calcd. C, 72.26; H, 5.63.

3. Results and Discussion

Reaction of 2-bromocyclohexanone (1) 32-35] with P[Cl.sub.5] in anhydrous ether gave a mixture of chloro-compounds [26], which without further purification was subjected to dehydrohalogenation using potassium tert-butoxide/tert-butyl alcohol to isolate pure 2 in 50% overall yield from 1.

The nonsymmetrical 1-bromo-2-chlorocyclohexene 2 was subjected to the Suzuki cross-coupling reaction with phenyl boronic acid (3a) in the presence of palladium acetate catalyst [36]. The reaction however yielded biphenyl as the major product, instead ofthe expected cross-coupled product 1-chloro-2-phenylcyclohexene (4a).

Literature survey indicated several reports which show the use of differing palladium catalysts for the cross-coupling reactions. Some of the catalysts reported are Pd[(OAc).sub.2] [36], Pd[Cl.sub.2] [37], ([PPh.sub.3])2Pd[Cl.sub.2] [38], [([PPh.sub.3]).sub.4]Pd [39], Pd(dppf)[Cl.sub.2] [40], and Pd(dppf)[Cl.sub.2] x C[H.sub.2][Cl.sub.2] [41]. We employed all the above catalysts for the Suzuki cross-coupling reaction in an effort to prepare 1-chloro-2-arylcyclohexenes 4a-k (Table 1).

Our investigations have shown that the best catalyst for the cross-coupling reaction of 2 to be Pd(dppf)[Cl.sub.2] x C[H.sub.2][Cl.sub.2] under inert nitrogen atmosphere and sealed tube conditions (Scheme 1).

In a typical reaction 2 (1mmol), phenylboronic acid 3a (1 mmol), potassium carbonate (3 mmol), and Pd(dppf)[Cl.sub.2] x C[H.sub.2][Cl.sub.2] (5mol%) in 8-10 mL 1,4-dioxane solvent were heated on an oil bath at 80[degrees]C in a sealed tube for 3 hours. Monitoring using TLC and GC indicated the completion of reaction. The reaction mixture was filtered and subjected to column chromatography using silica gel (60-100 mesh) and 1: 10 ethyl acetate/hexane (60-80[degrees]C fraction) to isolate 4a in 95% isolated yield.

The reaction of 2 was extended to a wide variety of aryl boronic acids 3b-k to isolate the products 4b-k. Each coupling reaction was repeated for a minimum of three trials and the optimum yields of the products are indicated in Table 2. All products 4a-k were characterised completely by IR, [sup.1]H-NMR, [sup.13]C-NMR, GC-MS, and elemental analysis.

The mechanism for the formation of the 1-chloro-2arylcyclohexenes may be explained based on the bond dissociation energies of the carbon-bromine and carbonchlorine which are 289 and 335kJ [moL.sup.-1], respectively. In the first step, due to the lower bond energy between carbon and bromine, oxidative addition of the Pd(dppf)[Cl.sub.2] x C[H.sub.2][Cl.sub.2] catalyst to 2 occurs across the carbon-bromine bond. In presence of potassium carbonate/arylboronic acid 3a-k, the species eliminates KBr, C[O.sub.2], and KOB[(OH).sub.2] followed by reductive elimination of the 1-chloro-2-arylcyclohexene 4ak. The restored palladium catalyst further continues the cycle.

4. Conclusion

A general methodology for the regiospecific replacement of bromine in 1-bromo-2-chlorocycloalkenes and formation of some novel 1-chloro-2-arylcyclohexenes using the Suzuki cross-coupling reaction are reported. The method is simple and cost efficient and yields the 1-chloro-2-aryl-cyclohexenes in high yields in short duration of time.

http://dx.doi.org/10.1155/2014/871595

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors thank the University Grants Commission Grant no. F. 36-42/2008 (SR) and Government of India, New Delhi, for financial assistance. Grateful thanks are placed on record to the Indian Institute of Science, Bangalore, for providing NMR facility.

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Lokesh Krishnappa and HariPrasad Suresh

Department of Studies in Chemistry, Central College Campus, Bangalore University, Bangalore, Karnataka 560 001, India

Correspondence should be addressed to HariPrasad Suresh; hariprasad@bub.ernet.in

Received 23 February 2014; Accepted 2 April 2014; Published 10 July 2014

Academic Editor: William Setzer
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Title Annotation:Research Article
Author:Krishnappa, Lokesh; Suresh, HariPrasad
Publication:Organic Chemistry International
Article Type:Report
Date:Jan 1, 2014
Words:4104
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