Synlett 2017; 28(17): 2262-2266
DOI: 10.1055/s-0036-1590820
letter
© Georg Thieme Verlag Stuttgart · New York

Synthesis of Unsymmetrical Aza[7]helicenes by One-Pot Wittig–Heck and Photodehydrogenation Sequence

Blessy Rajan, Ashutosh V. Bedekar*
  • Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390 002, India   Email: avbedekar@yahoo.co.in
Grant received from Science and Engineering Research Board (SERB), New Delhi No. SR/S1/OC-74/2012.
Further Information

Publication History

Received: 29 April 2017

Accepted after revision: 06 June 2017

Publication Date:
14 July 2017 (eFirst)

We dedicate this paper to Professor Pradeep Kumar of National Chemical Laboratory (Pune, India) on the occasion of his 60th birthday

Abstract

An efficient protocol for the synthesis of unsymmetrical aza[7]helicenes has been developed. The bis-styryl-type substrates for regiospecific photodehydrogenative oxidative double cyclization to give helicenes were prepared by a one-pot Wittig–Heck sequence as well as a stepwise procedure. The derivatives were characterized by single-crystal X-ray diffraction analysis and fluorescence spectroscopy.

Supporting Information

 
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  • 18 Experimental Details Synthesis of 9-Butyl-3-(4-methylstyryl)-6-(styryl)-9H-carbazole (4a) by One-Pot Wittig–Heck Reaction A catalyst solution was prepared under nitrogen atmosphere using Pd(OAc)2 (0.010 g, 0.045 mmol, 1 mol%) and dppp (0.037 g, 0.091 mmol, 2 mol %) in DMA (5 mL) at r.t. with constant stirring until a homogenous solution was obtained. A two-necked round-bottom flask was charged with 6-bromo-9-butyl-9H-carbazole-3-carbaldehyde (1, 1.500 g, 4.54 mmol), 4-methyl styrene (3x, 0.640 g, 5.450 mmol), benzyltriphenylphosphonium chloride (2x, 2.120 g, 5.450 mmol), dry K2CO3 (2.510 g, 18.200 mmol), TBAB (0.290 g, 0.908 mmol, 20 mol%), and DMA (10 mL), and the mixture was heated up to 100 °C. At 100 °C, the catalyst solution was added dropwise, and the mixture was heated to 140 °C for 48 h. After the completion of the reaction, the mixture was poured into water and extracted with EtOAc (3 × 100 mL). The combined organic phase was washed with water, brine, and dried over anhydrous Na2SO4. The solvent was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel using PE–EtOAc (95:5) as eluent to afford cis-trans isomers of the compound 4a (0.92 g; 46%); Rf = 0.5 (5:95 EtOAc–PE). Physical state: white crystalline solid; mp 178–180 °C. IR (KBr): 3423, 3016, 2947, 1863, 1596, 1489, 1388, 1349, 1306, 1250, 1179, 1116, 1038, 970, 861, 823, 750 cm–1. 1H NMR (400 MHz, CDCl3): δ = 8.28 (d, J = 3.2 Hz, 2 H), 7.70–7.67 (m, 2 H), 7.60–7.58 (d, J = 7.6 Hz, 2 H), 7.50–7.48 (d, J = 8.0 Hz, 2 H), 7.16 (d, J = 16.4 Hz), 7.12–7.08 (d, J = 16.4 Hz, 2 H), 6.93–6.91 (br d, 4 H), 4.32 (t, J = 7.2 Hz, 2 H), 2.4 (s, 3 H), 1.93–1.86 (m, 2 H), 1.47–1.38 (m, 2 H), 0.97 (t, J = 7.2 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 140.73, 140.53, 136.95, 136.48, 135.12, 132.47, 130.19, 129.43, 129.02, 128.82, 128.50, 128.32, 127.37, 126.19, 126.16, 124.74, 124.66, 124.63, 123.28, 123.19, 118.79, 118.51, 109.12, 43.08, 31.20, 21.30, 20.57, 13.93. General Procedure for Photocyclodehydrogenation Synthesis of 2-Methyl-9-butyl-9H-aza[7]helicene (6a) A solution of 3,6-distyryl-N-butylcarbazole (4a, 0.200 g, 0.45 mmol), iodine (0.253 g, 0.99 mmol), dry THF (3.26 g, 3.67 mL, 45.3 mmol), and toluene (610 mL) was irradiated using a 250 W HPMV lamp (10 h monitored by TLC). After the completion of the reaction, the excess of iodine was removed by washing the solution with aq Na2S2O3 and water. The organic layer was concentrated under reduced pressure to obtain the crude product. The crude product was purified by column chromatography over silica gel using PE–EtOAc (98:2) as eluent to obtain a pale yellow solid (0.06 g; 30%). Rf = 0.4 (EtOAc–PE = 5:95). Physical state: yellow solid; mp >200 °C. IR (KBr): 3427, 3042, 2952, 2927, 2867, 1725, 1660, 1588, 1522, 1495, 1450, 1339, 1283, 1218, 795, 746, 645 cm–1. 1H NMR (400 MHz, CDCl3): δ = 8.14 (d, J = 8.8 Hz, 2 H), 8.06 (d, J = 8.4 Hz, 2 H), 7.99–7.96 (m, 3 H), 7.90–7.88 (d, J = 8.4 Hz, 1 H), 7.83–7.79 (m, 2 H), 7.75 (d, J = 8.0 Hz, 1 H), 7.40 (d, J = 8.4 Hz, 1 H), 7.22–7.18 (m, 2 H), 7.05–7.03 (dd, J = 8.0, 1.2 Hz, 1 H), 6.28–6.24 (m, 1 H), 4.79 (t, J = 7.2 Hz, 2 H), 1.44 (s, 3 H), 1.04 (t, J = 7.2 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 139.23, 132.47, 131.49, 130.11, 129.91, 129.43, 128.34, 127.74, 127.43, 127.20, 126.82, 126.73, 126.61, 126.57, 126.53, 126.40, 126.14, 126.10, 125.91, 125.78, 124.03, 123.96, 122.34, 120.18, 116.75, 116.70, 109.57, 109.45, 43.44, 31.96, 20.77, 20.71, 13.98 ppm. HRMS (ESI-TOF): m/z calcd for C33H27N: 437.2143; found: 437.2140. The spectral and analytical data of other compounds are presented in the Supporting Information.