Scheme II.16. Post-synthetic modification
II.4. Experimental Section
II.4.1. General Information: Starting materials (acetylenes and carboxylic acids) were commercially available (Sigma-Aldrich or Alfa-Aesar or TCI chemicals) and used as
received. Some aryl and alkyl sodium sulfinates were commercially available (a′-d′) while other (e′-i′) were prepared following the literature procedure.23 The sodium sulfinates obtained were used as such without further purification. Organic extracts were dried over anhydrous sodium sulfate. Solvents were removed in a rotary evaporator under reduced pressure. Silica gel (60-120 mesh size) was used for the column chromatography. Reactions were monitored by TLC on silica gel 60 F254 (0.25 mm). NMR spectra were recorded in CDCl3 as the internal standard for 1H NMR (400, 500 MHz, and 600 MHz) and 13C{1H}
NMR (100, 125, and 150 MHz). MS spectra were recorded using ESI mode. IR spectra were recorded in KBr or neat.
Light Information:
Philips 10 W green LEDs (513 nm) were used as a light source for tlight-promotedoted reaction without any filter. Borosilicate glass was used as the reaction vessel. Distance from the light source to the irradiation vessel was approximately ~68 cm. Regular fan was used for proper aeration to maintain the temperature 2830 C (Figure II.3).
II.4.2. Crytallographic Description
Sample preparation: The compound (1db', 15 mg) was dissolved in 1 mL of DCM:MeOH (5:1) and kept at room temperature for slow evaporation.
Diffraction data were collected at 292 K with MoKα radiation (λ = 0.71073 Ǻ) using a Bruker Nonius SMART APEX CCD diffractometer equipped with graphite monochromator and Apex CD camera. The SMART software was used for data collection and for indexing the reflections and determining the unit cell parameters. Data reduction and cell refinement were performed using SAINT24a,b software and the space groups of these crystals were determined from systematic absences by XPREP and further justified by the refinement results. The structures were solved by direct methods and refined by full-matrix least-squares calculations using SHELXTL-9724c software. All the non-H atoms were refined in the anisotropic approximation against F2 of all reflections.
Crystallographic description of (Z)-1-phenyl-2-tosylvinyl 4-chlorobenzoate (1db’):
M.F. = C22H17ClO4S, crystal dimensions 0.22 x 0.17 x 0.15 mm, Mr = 412.87, monoclinic space, group P 21/n, a = 10.4280 (13), b =10.2098 (12), c =18.854 (2) Å, α = 90o (3), β =
97.004o (4) , γ = 90o, V = 1992.4(4) Å3, Z = 4, ρcalcd = 1.376 g/cm3, μ = 0.322 mm-1, F(000) = 856.0, refinement method = full-matrix least-squares on F2, final R indices [I > 2σ(I)]: R1 = 0.0534( 3362), wR2 = 0.1820( 4971), goodness of fit = 1.000. CCDC No = 2057146 for (Z)- 1-phenyl-2-tosylvinyl 4-chlorobenzoate (1db′) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
II.4.3. General Procedure for the Synthesis of Z-β-Carboxy Vinylsulfone(1bb′):
To an oven-dried 10 mL borosilicate vial, phenylacetylene (1) (0.5 mmol, 51.06 mg) sodium p-tolylsulfinate (b′) (0.5 mmol, 89.09 mg), and p-toluic acid (b) (0.75 mmol, 102 mg), eosin Y (3 mol %, 9.7 mg), I2 (1 equiv, 126 mg) and K2CO3 (2 equiv, 138 mg) were taken. After that 2 mL EtOH was added and the reaction mixture was stirred at room temperature for 48 h, tentatively at a distance of ~6-8 cm from two 10 W green LED bulbs.
After completion of the reaction (monitored by TLC analysis), the solvent was removed in vacuo and the mixture was admixed with 25 mL of ethyl acetate followed by washing with a saturated solution of aqueous Na2S2O3 (1 x 10 mL) and aqueous NaHCO3 (1 x 10 mL). The organic layer was dried over anhydrous Na2SO4, and the solvent was evaporated under reduced pressure. The crude residue thus obtained was purified by column chromatography over silica gel (60-120 mesh) using hexane and ethyl acetate (9:1) as eluent to afford the Z-1- phenyl-2-tosylvinyl 4-methylbenzoate (1bb′) in 70% yield. The identity and purity of the product were confirmed by spectroscopic analysis.
II.4.4. General Procedure for 10 mmol Scale Reaction of Z-1-Phenyl-2-tosylvinyl 4 methylbenzoate (1bb′):
To an oven-dried 50 mL borosilicate round bottom flask, phenylacetylene (1) (10 mmol, 1.021 g) sodium p-tolylsulfinate (b′) (10 mmol, 1.782 g), and p-toluic acid (b) (15 mmol, 2.040 g), eosin Y (3 mol %, 0.194 g), I2 (10 mmol, 2.520 g) and K2CO3 (20 mmol, 2.760 g) were taken. After that 30 mL EtOH was added and the reaction mixture was stirred at room temperature for 48 h, tentatively at a distance of ~6-8 cm from two 10 W green LED bulbs.
After completion of the reaction (monitored by TLC analysis), the solvent was removed in vacuo and the mixture was admixed with 100 mL of ethyl acetate followed by washing with a saturated solution of aqueous Na2S2O3 (1 x 30 mL) and aqueous NaHCO3 (1 x 30 ml). The organic layer was dried over anhydrous Na2SO4, and the solvent was evaporated under
reduced pressure. The crude residue thus obtained was purified by column chromatography over silica gel (60-120 mesh) using hexane and ethyl acetate (9:1) as eluent to afford the Z-1- phenyl-2-tosylvinyl 4-methylbenzoate (1bb′) in 52% (2.04 g) yield. The identity and purity of the product were confirmed by spectroscopic analysis.
II.4.5. General Procedure for the Synthesis of 1-Phenyl-2-tosylethan-1-one (X)
To an oven-dried 5 mL round bottom flask, Z-1-phenyl-2-tosylvinyl 4-methylbenzoate (1bb′) (137 mg, 0.5 mmol) and 1 mL of 5M HCl were taken. The flask was fitted with a condenser and the reaction mixture was stirred in a preheated oil bath at 120 C for 4 h. Next, the reaction mixture was cooled to room temperature and admixed with ethyl acetate (20 mL). The organic layer was washed with saturated sodium bicarbonate solution (2 x 5 mL) and dried over Na2SO4 and the solvent was removed under reduced pressure. The crude product so obtained was purified over a column of silica gel (hexane:ethyl acetate, 17:3) to afford the 1-phenyl-2-tosylethan-1-one (110 mg, yield 80%) (X). The identity and purity of the product were confirmed by spectroscopic analysis.
II.4.6. General Procedure for the Synthesis of N-Butyl-4-methylbenzamide (Y)
To an oven-dried 5 mL round bottom flask, Z-1-phenyl-2-tosylvinyl 4-methylbenzoate (1bb′) (137 mg, 0.5 mmol) and BuNH2 (1.2 equiv) were taken. Next, 2 mL of DCM was added and the reaction mixture was stirred at room temperature for 24 h. After completion of the reaction, the solvent was evaporated and the reaction mixture was admixed with ethyl acetate (20 mL). The organic layer was washed with brine solution (2 x 5 mL) and dried over Na2SO4 and the solvent was removed under reduced pressure. The crude product so obtained was purified over a column of silica gel to afford the 1-phenyl-2-tosylethan-1-one (X, 55%) and N-butyl-4-methylbenzamide (Y, 55%). The identity and purity of the product were confirmed by spectroscopic analysis.
II.4.7. General Procedure for Radical Trapping Experiment
To prove the photocatalytic radical pathway, a standard experiment between (1), (b), and (b′) was carried out in the presence of TEMPO (2 equiv) and BHT (2 equiv) under otherwise identical conditions. While in the former no formation of the desired product (1bb′, 0%) was observed but the later radical scavenger provided <10% yield of (1bb′). In the case of BHT as a scavenger, tosyl radical was trapped and adduct (M) was obtained in 65% yield thereby confirming the radical nature of the present protocol (Scheme 6 and S1). The adduct (M) was purified by column chromatography over silica gel (60-120 mesh) using hexane and ethyl acetate (49:1) as eluent to afford the 2,6-di-tert-butyl-4-(tosylmethyl)phenol (M) in 65% yield.
The identity and purity of the product was confirmed by spectroscopic analysis.
II.4.8. HRMS Study for the Detection of Reaction Intermediates:
In this study (1bb′) was taken as a representative example and a standard experiment between (1), (b), and (b′) was carried out. After 2 h of reaction, a small aliquot was withdrawn from the reaction mixture and subjected to HRMS analysis after diluting it with CH3CN:H2O (60:40). The formation of 2-iodo-2-phenylvinyl)sulfonyl)-4-methylbenzene (B) intermediate was judged from the appearance of a peak at [M+H]+ = 384.9751. To further confirm the intermediacy of (B), a standard experiment was carried out between pre- synthesized (B), p-toluic acid (b) in presence of K2CO3 (2 equiv) and ethanol (2 mL) in dark at room temperature. The desired product 1bb′ was obtained in 68% yield thereby suggesting the involvement of this intermediate in the present protocol.
II.4.9. The UV-Visible Spectroscopy and Fluorescence Quenching Experiment (Stern- Volmer Studies)
A 1 mM solution was prepared by mixing eosin Y in water by appropriate dilution of 0.01 M stock solution and taken in quartz UV cuvette of 1 cm path length. The UV-visible spectroscopy showed max of 515 nm. For the fluorescence measurement, the sample was excited at 515 nm, and the emission was observed at 534 nm. For each fluorescence quenching experiment, a 10 L (1 M) solution of sodium p-toluene sulfinate was added to eosin Y solution (1 mM) taken in a fluorescence cuvette and fluorescence emission spectra were recorded after each addition (Figure II.7). As evident from Figure II.7, a decrease in emission intensity was observed after each addition of sodium p-toluene sulfinate (5 35 mM). This confirms the electron transfer between eosin Y and quencher sodium p-toluene sulfinate (b′). A true fluorescence quenching phenomenon of eosin Y under various concentrations of sodium p-toluene sulfinate was demonstrated from Stern-Volmer graph using the equation I0/It = 1+KSV [Q] where I0 and It are integrated emission intensity in the absence and presence of quencher and Ksv is quenching constant. As evident from Figure II.7, a linear quenching was observed which confirmed the electron transfer between eosin Y and quencher sodium p-toluene sulfinate (b′).
II.4.10. CV Experiments Performed to Determine the Redox Potentials
Cyclic voltammetry (CV) was performed using three-electrode cell configuration comprised a platinum sphere, a platinum plate and Ag(s)/AgNO3 (0.01 M) as the working, auxiliary, and reference electrodes respectively. [Cyclic voltammetry experiment of RSO2I (RSO2Na+I2) taken at a scan rate 100mv/s. Experiment conditions: Init E = 2.0 V, High E = 2.0 V, Low E = -2.0 V, Init P/N = N, Scan Rate = 0.1 V/s, Sample Interval = 0.001 V, Quiet Time = 2s, Sensitivity = 2e-4 A/V]. The supporting electrolyte used was tetraethylammonium hexafluorophosphate (TEAHFP) (C2H5)4N(PF6). Samples were prepared with a substrate concentration of 0.01 M in a 0.1 M TEAHFP in an acetonitrile electrolyte solution. From the result, it was found that, Ered = -1.06 V vs SCE whereas E*Oxd (RSO2I) = -0.37 V vs SHE (experimental value -0.42V vs Ag/AgCl) (Figure II.8).which clearly indicated that the catalyst eosin Y (EY¯•) can easily donate an electron to the RSO2I species which provides strong evidence to the catalytic cycle suggested in mechanism (Scheme II.15).