Formation of unusual dithiaphlorins from condensation of 2,5-bis(arylhydroxymethyl)thiophene and pyrrole

Abstract

The first example of an unusual 21,23-dithiaphlorin containing pyrrole and aryl groups at the sp³ meso carbon that is present between the pyrrole and thiophene rings was isolated from the condensation of 2,5-bis(arylhydroxymethyl) and pyrrole under mild acid catalyzed conditions. The crystal structure revealed that the macrocycle is significantly distorted because of the presence of the sp³ meso carbon.

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Introduction

Ulman and Manassen reported¹ in 1974 that condensation of 2,5-bis(arylhydroxymethyl)thiophene with pyrrole under acid catalyzed conditions yielded 5,10,15,20-meso-tetraaryl-21,23-dithiaporphyrin I. Latos-Grazynski and co-workers found² that the condensation of bis(arylhydroxymethyl)thiophene with a slight excess of pyrrole (2 : 3 molar ratio) in the presence of BF₃·OEt₂ (3 equivalents) resulted in the formation of minor amounts of expanded porphyrin, 5,10,15,20-tetraaryl-26,28-dithiasapphyrin II (1.2%) in addition to major amounts of I (Scheme 1). Chandrashekar and co-workers isolated³ three macrocycles such as I, II and, 10,19,24-tetraaryl-30,33-dithiarubyrin III from condensation of 2,5-bis(arylhydroxymethyl)thiophene with pyrrole (1 : 2 molar ratio) in the presence of one equivalent of protic acid such as TFA, p-TsOH and HBr (Scheme 1). In 2001, Latos-Grazynski and co-workers isolated⁴ 5,10,15,20-tetraaryl-2-aza-21-carba-22,24-dithiaporphyrin IV along with I when 2,5-bis(arylhydroxymethyl)thiophene and pyrrole (1 : 1) were condensed under BF₃·OEt₂ catalysed conditions. However, the same research group also isolated⁴ 5,10,15,20-tetraaryl-25,27-dithiasapphyrin V in addition to I and IV when condensation of the same two substrates was carried out under CH₃SO₃H catalyzed conditions. Thus, the condensation of thiophene diol with pyrrole under slight variation in acid catalyzed reaction conditions resulted in the formation of five different types of macrocycles till date. Herein, we report the formation of unusual dithiaphlorins by condensing 2,5-bis(arylhydroxymethyl)thiophenes with pyrrole (1 : 1.2 molar ratio) in the presence of dilute concentration of BF₃·OEt₂. The dithiaphlorins which were isolated in 4–5% yields contains one sp³ meso-carbon having one pyrrole and one-tolyl group and three sp² meso-carbons containing aryl groups. This kind of dithiaphlorins cannot be obtained easily by adopting any rational approaches.^5,6 The reaction was tested by varying three different types 2,5-bis(arylhydroxymethyl)thiophenes and condensed with pyrrole under acid catalyzed conditions to afford different meso-substituted dithiaphlorins (Scheme 2). The dithiaphlorins were characterized by HRMS, 1D, 2D NMR spectroscopy, absorption, and electrochemical techniques and obtained the crystal structure for one of the compound. Our preliminary studies indicated that dithiaphlorins sense various ions in their protonated form without specificity towards any particular anion.

Scheme 1 Condensation of 2,5-bis(p-arylhydroxymethyl)thiophene and pyrrole in different conditions.

Scheme 2 Synthesis of dithiaphlorins 1a–3a.

Result and discussion

We carried out the reaction by condensing one equivalent of 2,5-bis(p-tolylhydroxymethyl)thiophene 1 with 1.5 equivalents of pyrrole in CH₂Cl₂ in the presence of BF₃·OEt₂ (3.3 mM) in 150 mL CH₂Cl₂ for 2.5 h followed by oxidation with 1.5 equivalents of p-chloranil for 3 h at room temperature (Scheme 2). The TLC analysis showed the one major reddish brown spot which was followed closely by the green spot and other polar minor spots. The crude compound was subjected to silica gel column chromatographic purification and the major reddish brown spot corresponding to meso-tetra(p-tolyl)phenyl-21,23-dithiaporphyrin 1b was collected and afforded in 14% yield. The green spot which followed the reddish brown spot of 21,23-dithiaporphyrin 1b was subsequently collected and subjected further to two basic alumina column chromatographic purifications to afford pure green colored compound 1a. The mass spectrum of green coloured compound showed a molecular ion peak at 772.28 which was matching with meso-tetraaryl dithiasapphyrin II. However, the ¹H NMR spectrum of compound 1a showed signals only in 5.5 to 8 ppm region indicating that it was not the reported aromatic meso-tetraaryl dithiasapphyrin II but appears to be a macrocycle with disrupted π-electron delocalization. Fortunately, we obtained the single crystal structure of the green compound (vide infra) which confirmed that the green coloured compound was indeed 21,23-dithiaphlorin 1a having one sp³ and three sp² meso-carbon bridges. To confirm the formation of thiaphlorin 1a under dilute acid catalyzed conditions, we condensed different 2,5-bis(arylhydroxymethyl)thiophenes 2 and 3 with pyrrole under identical reaction conditions and afforded pure green coloured 21,23-dithiaphlorins 2a and 3a in 4–5% yields along with 21,23-dithiaporphyrins 1b–3b in 12–15% yields. To improve the yields of dithiaphlorins 1a–3a, we carried out the reaction under different conditions. The increase of pyrrole equivalents in the condensation did not increase the yield of the desired dithiaphlorin 1a but resulted in the formation of an additional macrocycle 10,19,24-tetraaryl-30,33-dithiarubyrin III. The change of oxidant from p-chloranil to different oxidizing agents such as DDQ, KMnO₄, Br₂, Ag₂O, CAN, FeCl₃ did not yield the dithiaphlorin. The formation of compounds 1a–3a was confirmed by HRMS analysis and the crystal structure of compound 1a. We propose the plausible mechanism for the formation of dithiaphlorins 1a–3a as shown in Scheme 3. The dithiaporphyrinogen that is formed, while undergoing oxidation to form dithiaporphyrin in the presence of mild oxidizing agent like p-chloranil, is possibly attacked by pyrrole at one of the electrophilic meso-carbon followed by the usual sequential oxidation steps to form dithiaphlorin.

Scheme 3 Plausible mechanism for synthesis of dithiaphlorin.

X-ray crystallography

We attempted to grow single crystals for compound 1a, and fortunately we could get crystals suitable for X-ray diffraction via slow diffusion of petroleum ether into CH₂Cl₂ over a period of one week. The X-ray crystal structure of compound 1a (CCDC no. 1421852) is shown in Fig. 1 and the relevant crystallographic parameters are presented in Table S1 (ESI†). The compound 1a was crystallized in orthorhombic space group Pbcn. The crystal structure showed that the compound 1a contains one sp³ meso-carbon bearing one pyrrole and one-tolyl groups as substituents along with three meso sp² hybridized carbons having tolyl groups. The macrocycle is highly distorted due to presence of one sp³ hybridized carbon which disrupts the π-delocalization of the molecule. The crystal structure of compound 1a was compared with the reported crystal structure of 5,10-bis(p-chlorophenyl)-10,20-bisphenyl-21,23-dithiaporphyrin (4a) that contain four sp² meso carbons.⁷ The macrocycle 1a was distorted compared to 4a which was found to be almost planar. A close inspection of the crystal structure of compounds 1a and 4a indicated that in compound 4a, the pyrrole and thiophene rings are almost in plane (∼6.70° and 1.15° respectively) from the 24 atoms mean plane of the macrocycle. In compound 1a, the pyrrole ring connected to sp³ hybridized carbon center was significantly deviated by 44.14° from the mean plane of the macrocycle whereas, the thiophene ring that is connected to sp³ hybridized carbon center was deviated by only 12.83°. However, the pyrrole and thiophene rings attached to sp² hybridized carbon center were not significantly deviated (9.83° and 8.56° respectively) from the mean plane of the macrocycle. Furthermore, the flexible sp³ meso carbon was significantly deviated upward by 0.869 Å from the mean plane which is comparable to our recently reported 21-thiaisoporphyrin (of 0.996 Å)⁸ and significantly higher than compound 4a (in 0.013–0.139 Å). However, the deviation of three sp² meso carbon centres are almost in the same range (0.214, 0.141, 0.246 Å) from the mean plane. The S1 and S2 thiophene sulphurs of compound 4a are essentially (0.034 Å) in the mean plane, whereas the N1 and N2 pyrrole nitrogens are slightly deviated (0.149 Å) from the mean plane of the macrocycle. In case of 1a, the N1 of pyrrole and S1 of thiophene ring connected to sp³ hybridised meso carbon centre are significantly deviated upward (0.462 and 0.295 Å), whereas the N2 of pyrrole and S2 of thiophene are almost in the plane of the macrocycle (0.027 and 0.029 Å respectively). The tolyl ring at sp³ hybridized carbon center is almost perpendicular to the plane of the macrocycle. Thus, the crystal structure analysis reveal that compound 1a is highly distorted as compared to compound 4a, due to presence of sp³ carbon center which induces flexibility in compound 1a.

Fig. 1 Single crystal X-ray structure of compound 1a (a) perspective ORTEP view (b) front view (meso-aryl groups are omitted for clarity) and (c) side view showing the non-planarity distortation in molecular plane due to sp³ meso carbon centre of the macrocycle (all hydrogens are omitted for clarity). Thermal ellipsoids are drawn in 50% probability.

NMR studies

The dithiaphlorins 1a–3a were freely soluble in common organic solvents and characterized in detail by 1D and 2D NMR studies. The dithiaphlorins 1a–3a were characterized in detail by 1D and 2D NMR studies. The resonances were identified based on integration, coupling constant and proton–proton correlations observed in 1D and 2D NMR spectra. The ¹H, ¹H–¹H COSY and NOESY NMR spectra of dithiaphlorin 1a are shown in Fig. 2. The close inspection of ¹H NMR spectrum showed one broad signal at 7.93 ppm (type r) corresponding to inner NH proton and one relatively sharp signal at 8.05 ppm (type q) corresponding to NH proton of pyrrole present at the sp³ meso-carbon. In ¹H–¹H COSY, the type q NH signal at 8.05 ppm showed cross-peak correlation with a signal present at the 6.73 ppm which we identified as type a proton of pyrrole present at the sp³ meso-carbon. The type a proton showed a cross peak connectivity with a signal at 6.14 ppm which we assigned as type b proton which in turn showed correlation with a signal at 5.75 ppm corresponding to type c pyrrole proton present at the sp³ meso-carbon. Furthermore, the eight protons corresponding to four thiophene and four pyrrole protons appeared as eight sets of signals in 6.00–7.40 ppm region. In NOESY spectrum, the q-type NH proton at 8.05 ppm showed a cross-peak correlation with the doublet at 6.75 ppm which we identified as type d pyrrole proton. The type d pyrrole proton showed cross-peak correlation in COSY spectrum with a signal at 7.35 ppm which was assigned as type e pyrrole proton. To identify the type f and g protons of thiophene, we followed the NOE correlations of methyl group of p-tolyl present at the sp³ meso-carbon. The ¹H NMR spectrum showed four singlets at 2.30, 2.44, 2.45 and 2.48 ppm corresponding to three protons each. The signal at 2.30 ppm was identified as type I methyl signal of tolyl group present at the sp³ meso-carbon. The signals at 2.44, 2.45 and 2.48 ppm corresponding to three protons each were identified as type II, type III and type IV methyl protons of meso-tolyl groups present at the sp² carbons. The type I methyl signal at 2.30 ppm showed NOE correlation with a signal at 7.03 ppm which we identified as type m protons of meso-aryl group present at the sp³ carbon. The signal at 6.90 ppm was identified as type l proton as it was showing cross peak correlation with type m proton. The type l proton at 6.90 ppm showed NOE correlation with a signal at 7.12 ppm which we assigned as type f proton of thiophene ring. The type f proton at 7.12 ppm in turn showed cross-peak correlation with a signal at 7.29 ppm which was identified as type g proton of thiophene. To identify the type h and i protons of pyrrole as well as type k and j protons of thiophene, we followed the NOE correlations observed between type II, III and IV methyl protons signals with meso-aryl protons signals. However, the meso-aryl protons appeared as two sets of signals at 7.48 and 7.25–7.36 ppm corresponding to four and eight protons respectively and these protons were difficult to assign to particular type of meso-aryl protons. But the close inspection of Fig. 2 clearly showed that the set of meso-aryl signals appeared at 7.48 ppm showed cross peak correlation with a signals at 6.40 and 7.12 ppm. We assigned the signal at 6.40 ppm to type i pyrrole proton and a signal at 7.12 ppm to the type j proton of thiophene. The type i pyrrole proton at 6.40 ppm showed a cross-peak correlation with a doublet at 6.01 ppm which we identified as type h pyrrole proton. Similarly, the type j thiophene proton appeared at 7.12 ppm showed cross-peak correlation with a signal at 6.73 ppm which we identified as type k thiophene proton. Thus, NMR studies helped in identifying all protons of thiophene and pyrrole rings, NH protons and some selected meso-aryl protons. Thus, NMR studies helped in identifying all protons of thiophene and pyrrole rings, NH protons and some selected meso-aryl protons. The ¹H NMR spectrum of the other thiaphlorins 2a and 3a also showed similar NMR spectra like 1a and all protons were identified using the same 1D and 2D NMR techniques.

Fig. 2 (a) Partial ¹H NMR, (b) partial ¹H–¹H COSY NMR and (c) partial ¹H–¹H NOESY NMR spectra of compound 1a recorded in CDCl₃ at room temperature.

Absorption and electrochemistry studies

The absorption and electrochemical properties of compounds 1a–3a were studied and the relevant data are included in Table 1. The comparison of absorption spectra of compound 1a and its protonated form is shown in Fig. 3. The compound 1a showed one broad featureless Q-type band at 690 nm and one strong Soret type band at 390 nm along with a shoulder band at 411 nm. These absorption features are characteristic of non-aromatic macrocycles. The other two macrocycles 2a and 3a also showed similar non-aromatic absorption features (Table 1). Upon addition of TFA to compound 1a in CH₃CN, the Q-type band experienced bathochromic shifts and appeared as two bands at 856 and 967 nm whereas the Soret type band also resolved into two bands at 412 and 463 due to protonation of inner pyrrole ring of compound 1a. The electrochemical properties of compounds 1a–3a were studied with cyclic voltammetry and differential pulse voltammetry using tetrabutylammonium perchlorate as supporting electrolyte in CH₂Cl₂. The representative reduction waves of cyclic voltammogram along with its differential pulse voltammogram of compound 1a is shown in Fig. 3 and the data are included in Table 1. The compounds 1a–3a showed three to four oxidations and two reductions which are mostly quasi-reversible and irreversible in nature indicating that the macrocycles are not very stable under redox conditions.

Table 1 Absorption and redox data of compound 1a–3a

	Absorption band [nm] (log ε)	Redox potential
		Oxidation (V)	Reduction (V)
1a	390 (0.13963), 411 (0.13502), 432 (sh), 636 (sh), 690 (0.06340)	0.63, 0.88, 1.17, 1.62	−0.86, −1.24
2a	389 (0.12726), 409 (0.12396), 433 (sh), 634 (sh), 683 (0.06340)	0.51, 0.77, 1.12, 1.69	−0.77, −1.25
3a	392 (0.12390), 409 (0.12177), 433 (sh), 632 (sh), 686 (0.06140)	0.74, 1.01, 1.31, 1.70	−0.77, −1.25

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Fig. 3 (a) Comparison of absorption spectra of compound 1a (solid line) and 1a.H⁺ (dotted line) recorded in CH₃CN. (b) Redox waves of cyclic voltammograms along with differential pulse voltammograms of compound 1a recorded in CH₂Cl₂ solvent using tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte and the saturated calomel electrode (SCE) as a reference electrode at scan rates of 50 mV s⁻¹.

As calixphyrins are known to bind various anions,^9–12 the preliminary anion biding studies were performed with 1a and showed that the compound 1a did not bind to any anion. We then carried out anion binding studies with compound 1a in its protonated state. Our studies indicated that macrocycle 1a showed potential ability to bind various anions without specificity for any particular anion. Thus, the thiacalixphyrins although binds anions in its protonated state but doesn't show selectivity.

Conclusion

In conclusion, we obtained an unusual dithiaphlorins containing pyrrole and aryl group at the sp³ meso carbon from the condensation of 2,5-bis(hydroxymethylaryl)thiophene with pyrrole under mild acid catalyzed conditions. These kinds of unusual dithiaphlorin cannot be prepared by any available rational methods. Our preliminary studies showed that the protonated dithiaphlorins have potential ability to bind various anions. The electrochemical properties of compounds 1a–3a showed three to four oxidations and two reductions which are mostly quasi-reversible and irreversible in nature indicating that the macrocycles are not very stable under redox conditions.

Materials and methods

Chemicals

The chemicals such as BF₃·Et₂O, 2,3,5,6-tetrachloro-1,4-benzoquinone (p-chloranil) were used as obtained from Aldrich. All other chemicals used for the synthesis were reagent grade and solvents were dried by routine procedures immediately before use. Column chromatography was performed on basic alumina and silica gel (60–120 mesh).

Instrumentation

All the NMR spectra were recorded with Bruker 400 MHz and 500 MHz instrument using tetramethylsilane (Si(CH₃)₄) as internal standard. Absorption spectra were obtained with Perkin-Elmer Lambda-35 instrument. Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) studies were carried out with BAS electrochemical system utilizing the three electrode configuration consisting of a glassy carbon (working electrode), platinum wire (auxiliary electrode) and saturated calomel (reference electrode) electrodes. The experiments were done in dry dichloromethane using 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The HR-MS were recorded with a Q-Tof Micromass spectrometer using the electron spray ionization technique.

X-ray crystal structure analysis

Single crystals of suitable size for X-ray diffractometry were selected under a microscope and mounted on the tip of a glass fiber, which was positioned on a copper pin. The X-ray data for macrocycles 1 and 3 were collected, and graphite-monochromated Mo Kα radiation at 200 K and a θ–2θ scan mode were used. The space group for macrocycles 1a was determined on the basis of systematic absences and intensity statistics, and the structures of macrocycles 1a was solved by direct methods using SIR92 or SIR97 and refined with SHELXL-97.14. An empirical absorption correction by multiscans was applied. All non-hydrogen atoms were refined with anisotropic displacement factors. Hydrogen atoms were placed in ideal positions and fixed with relative isotropic displacement parameters. Selected crystallographic data for the macrocycle (CCDC no. 1421852) are given in Table S1.†

Experimental section

Compound 1a

Samples of 2,5-bis(p-tolylhydroxymethyl)thiophene (0.617 mmol) and pyrrole (0.740 mmol) in dichloromethane (150 mL) was degassed with nitrogen for 10 minutes with stirring. The condensation was initiated by adding catalytic amount of BF₃·OEt₂ (150 μL of 2.5 M) and stirred under nitrogen atmosphere at room temperature for 2.5 h. The oxidant p-chloranil (0.740 mmol) was added and the reaction mixture was stirred in air for additional 2 h. TLC analysis showed the formation of dithiaphlorin 1a along with the major known macrocycle, 5,10,15,20-tetraaryl-21,23-dithiaporphyrin 1b. The crude compound was subjected twice to silica gel column chromatography and collected the known dithiaporphyrin macrocycle as first band followed by the desired compound 1a as second band using petroleum ether/CH₂Cl₂ (65 : 35). The solvent was removed on rotary evaporator and afforded 5,10,15,20-tetratolyl dithiaporphyrin¹³ 1b in 14% and dithiaphlorin 1a as green solid in 4% yields. ¹H NMR (400 MHz, CDCl₃, δ in ppm) 2.3 (s, 3H), 2.441 (s, 3H), 2.448, (s, 3H), 2.47 (s, 3H) 5.75 (s, 1H), 6.01 (d, J = 3.8 Hz, 1H), 6.14 (dd, J = 4.15, 2.55 Hz, 1H), 6.40 (d, J = 3.72 Hz, 1H), 6.75 (dd, J = 4.15, 2.55 Hz, 1H), 6.73 (d, J = 4.64 Hz, 1H), 6.89 (d, J = 4.0 Hz, 1H), 6.97 (d, J = 8.6 Hz, 2H), 7.03–7.05 (m, 3H), 7.12 (d, J = 5.6 Hz, 1H), 7.25–7.36 (m, 10H), 7.93 (brs, NH), 8.05 (s, NH). ¹³C NMR (100 MHz, CDCl₃, δ in ppm) 166.6, 152.9, 152.5, 148.4, 141.9, 141.1, 140.7, 138.5, 137.1, 136.6, 136.2, 135.7, 135.6, 135.1, 134.6, 134.1, 132.7, 131.9, 131.6, 131.3, 131.2, 129.3, 129.7, 129.1, 128.9, 188.5, 128.4, 127.2, 125.9, 123.6, 120.6, 117.6, 113.1, 110.1, 108.0, 54.6, 53.6, 37.2, 32.9, 32.1, 31.8, 30.2, 27.2, 22.8, 21.5, 21.4, 21.2, 21.5, 19.9, 14.3. HRMS m/z calcd for C₅₂H₄₁N₃S₂ (M + H)⁺ 772.2815, found 772.2821.

Compound 2a

The compound 2a was synthesized by following the procedure of 1a using 2,5-bis(p-phenylhydroxymethyl)thiophene. 5,10,15,20-Tetraphenyl dithiaporphyrin¹³ 2b: 15% and dithiaphlorin 2a: 4%. ¹H NMR (400 MHz, CDCl₃, δ in ppm) 5.7 (s, 1H), 6.05 (d, J = 3.7 Hz, 1H), 6.15 (d, J = 2.8 Hz, 1H), 6.42 (d, J = 3.72 Hz, 1H), 6.71 (d, J = 3.64 Hz, 1H), 6.77 (d, J = 3.6 Hz, 1H), 6.89, (d, J = 3.96 Hz, 1H), 7.02 (d, J = 4.6 Hz, 1H), 7.12 (d, J = 5.72 Hz, 1H), 7.08–7.11 (m, 3H), 7.23–7.26 (m, 2H), 7.32 (d, J = 5.8 Hz, 1H), 7.35 (d, J = 3.9 Hz, 1H), 7.42–7.50 (m, 12H), 7.59–7.61 (m, 4H), 7.93 (brs, NH), 8.05 (s, NH). ¹³C NMR (100 MHz, CDCl₃, δ in ppm) 152.8, 148.6, 144.7, 141.4, 140.5, 139.7, 139.5, 139.0, 135.7, 135.3, 134.4, 134.2, 132.9, 132.0, 131.5, 131.4, 129.9, 129.0, 128.6, 128.5, 128.3, 128.1, 127.9, 127.8, 127.5, 127.4, 127.4, 120.8, 117.8, 113.3, 110.2, 108.1, 54.3, 29.8, 29.5, 14.2, 1.18. HRMS m/z calcd for C₄₈H₃₃N₃S₂ (M + H)⁺ 716.2189, found 716.2198.

Compound 3a

The compound 3a was synthesized by following the procedure of 1a using 2,5-bis(p-bromophenylhydroxymethyl)thiophene precursor. 5,10,15,20-Tetrakis(4-bromo-phenyl) dithiaporphyrin¹⁴ 3b: yield 13%, dithiaphlorin 3a yield: 5%. ¹H NMR (400 MHz, CDCl₃, δ in ppm) 5.69 (s, 1H), 6.01 (d, J = 3.76 Hz, 1H), 6.14 (d, J = 5.8, 2.69 Hz, 1H), 6.43 (d, J = 3.7 Hz, 1H), 6.71 (d, J = 5.68 Hz, 1H), 6.78 (d, J = 4.01 Hz, 1H), 6.87 (d, J = 4.0 Hz, 1H), 6.95–7.01 (m, 3H), 7.10 (d, J = 5.72 Hz, 1H), 7.28–7.41 (m, 6H), 7.45 (d, J = 8.36 Hz, 4H), 7.52–7.65 (m, 6H), 7.93 (broad, NH), 8.05 (s, NH). ¹³C NMR (100 MHz, CDCl₃, δ in ppm) 137.7, 135.7, 135.4, 134.2, 133.6, 133.4, 133.0, 131.8, 131.6, 131.5, 131.2, 131.1, 130.7, 130.3, 130.0, 127.6, 123.2, 122.0, 121.7, 121.2, 118.3, 113.7, 110.3, 108.3, 54.6, 45.8, 32.1, 29.8, 29.5, 29.1, 22.8, 14.3, 14.2. HRMS m/z calcd for C₄₈H₂₉Br₄N₃S₂ (M + H)⁺ 1027.8609, found 1031.8570.

Acknowledgements

MR acknowledges the financial support from Department of Science & Technology, Govt. of India and RS Thanks to UGC for fellowship.

References

1. A. Ulman, J. Manassen, F. Frolow and D. Rabinovich, Tetrahedron Lett., 1978, 2, 167.
2. K. Rachlewicz, N. Sprutta, P. J. Chmielewski and L. Latos-Grazynski, J. Chem. Soc., Perkin Trans. 2, 1998, 969.
3. S. K. Pushpan, J. S. Narayanan, A. Srinivasan, S. Mahajan, T. K. Chandrashekar and R. Roy, Tetrahedron Lett., 1998, 39, 9249.
4. N. Sprutta and L. Latos-Grazynski, Org. Lett., 2001, 3, 1933.
5. S. Hong, J. Ka, D. Won and C. Lee, Bull. Korean Chem. Soc., 2003, 24, 661.
6. I. Gupta, R. Frohlich and M. Ravikanth, Chem. Commun., 2006, 3726.
7. Y. Zhu, Y. Z. Zhu, H. B. Song, J. Y. Zheng, Z. B. Liu and J. G. Tian, Tetrahedron Lett., 2007, 48, 5687.
8. A. Ghosh, W. Z. Lee and M. Ravikanth, Chem. Commun., 2013, 49, 8677.
9. R. S. Zimmerman, C. Bucher, V. Kral, B. Andrioletti and J. L. Sessler, Pure Appl. Chem., 2001, 73, 104.
10. C. Hung, G. Chang, A. Kumar, G. Lin, L. Luo, W. Ching and W. Diau, Chem. Commun., 2008, 978.
11. Y. Matano, M. Fujita, T. Miyajima and H. Imahori, Organometallics, 2009, 28, 6213.
12. M. O. Senge, S. Runge, M. Speck and K. Ruhlandt-Senge, Tetrahedron, 2000, 56, 8927.
13. K. L. Katherine, R. R. Christopher and C. Bruckner, Tetrahedron, 2005, 61, 2529.
14. A. Ulman, J. Manassen, F. Frolow and D. Rabinovich, Inorg. Chem., 1981, 20, 1987.

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Footnote

† Electronic supplementary information (ESI) available: All the NMR and HR mass spectra of compounds are given. CCDC 1421852. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra21947g

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