Photodecarboxylative Additions of α-Thioalkyl-Substituted Carboxylates to Alkyl Phenylglyoxylates

Abstract

Irradiations of alkyl phenylglyoxylates with sulfur-containing carboxylates yielded the corresponding photodecarboxylative addition products in moderate to good yields of 26-58%. Reductive photodimerization competed with decarboxylative addition in all cases. The reaction protocol was successfully transferred to a microreactor. With potassium 2-(methylsulfanyl)propionate, photoadditions gave diastereomeric mixtures with low selectivity for the like-isomer.

photodecarboxylation - phenylglyoxolates - photochemistry - photoinduced electron transfer, microreactor

References and Notes

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8

General Procedure for Irradiation
The alkyl phenylglyoxylate (1.5 mmol) was dissolved in MeCN (50 mL). A solution of the potassium carboxylate (4.5 mmol) in H₂O (50 mL) was added, and the mixture was irradiated (Rayonet Photochemical Reactor RPR-200; λ = 350 ± 30 nm) at 15-20 ˚C in a Pyrex tube (λ ³ 300 nm) while purging with a slow stream of nitrogen. The progress of the reaction was monitored by TLC analysis or by passing the departing gas stream through a sat. Ba(OH)₂ solution until precipitation of BaCO₃ had ceased. Most of the MeCN was evaporated, and the remaining solution was extracted with EtOAc (4 × 25 mL). The combined organic layers were washed with 5% NaHCO₃ (1 × 25 mL) and brine (1 × 25 mL), dried over MgSO₄, and evaporated. The products were purified by flash column chromatography (eluent: n-hexane-EtOAc = 5:1).
Selected Physical and Spectral Data for the Product Methyl-2-(1,3-dithian-2-yl)-2-hydroxy-2-phenylacetate (4e)
Yellowish solid, mp 104-106 ˚C. R _f = 0.39 (SiO₂, n-hexane-EtOAc = 5:1). ^¹H NMR (400 MHz, acetone-d ₆): δ = 1.84 (m, 2 H, CH₂), 2.40-2.46 (m, 1 H, SCH₂), 2.56-2.62 (m, 1 H, SCH₂), 3.02-3.09 (m, 1 H, SCH₂), 3.19-3.26 (m, 1 H, SCH₂), 3.61 (s, 3 H, OCH₃), 4.48 (s, 1 H, CH), 5.14 (s, 1 H, OH), 7.14-7.24 (br m, 3 H, H_arom), 7.56 (dd, ^³ J = 8.4 Hz, ⁴ J = 1.6 Hz, 2 H, H_arom) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 25.1 (t, 1 C, CH₂), 28.0 (t, 1 C, SCH₂), 28.3 (t, 1 C, SCH₂), 50.4 (d, 1 C, CH), 53.9 (q, 1 C, OCH₃), 85.1 (s, 1 C, COH), 126.0 (d, 2 C, CH_arom), 128.3 (d, 1 C, CH_arom), 128.4 (d, 2 C, CH_arom), 139.1 (s, 1 C, Cq_arom), 173.8 (s, 1 C, C=O) ppm. IR (KBr): ν = 3490, 2953, 2925, 2892, 1725, 1239, 733, 692 cm^-¹. MS (EI, 70 eV): m/z (%) = 284 (<1) [M⁺], 119 (100) [M⁺ - H₂O], 105 (12), 91 (4), 77 (14), 45 (5).

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The dwell-reactor is made out of Foturan glass (λ ³ 300 nm) and has a total path length of 1.15 m (20 turns) on a 118 mm × 73 mm aperture. The reactor consisted of a(bottom) serpentine reaction channel 0.5 × 2 mm (D × W), with a second(top), heat-exchanging channel through which water is passed in order to control the reactor temperature. The degassed reaction mixture (20 mL)⁸ was pumped through the reaction channel via a syringe pump and collected in a test tube.

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Irradiation of 2 in benzene and in the presence of 5 equiv of Me₂S gave the corresponding addition product 5a in a yield of 35%. Selectivity (7 vs. 5a) was determined as 60:40. Similar results were obtained with 1,3-dithiolane.^³a

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