Comparison of Pyrylium and Thiopyrylium Photooxidants in Metal-Free Ring-Opening Metathesis Polymerization

Abstract

Systematically varied pyrylium and thiopyrylium photo-oxidants have been evaluated in the metal-free ring-opening-metathesis polymerization (MF-ROMP) of norbornene. Across the series, we observed higher conversion into polynorbornene from thiopyrylium species in comparison with pyrylium salts that were otherwise similarly functionalized. Additionally, more electron-rich photo-oxidants (i.e., weaker oxidants) correlated with higher conversions.

Key words

metathesis polymerization - metal-free - photoredox - radical cation

• References and Notes

• 1 New address: K. A. Ogawa, 104 Chemistry Research Building, The Pennsylvania State University, University Park, PA 16802, USA.
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• 11 A 2-dram vial was equipped with a magnetic stir bar and then a (thio)pyrylium salt (1.0 equiv) was added. The vial was then transferred into a nitrogen-filled glovebox. To this vial was then added norbornene (424 mg, 4.50 mmol, 2000 equiv) and CH₂Cl₂ (2 mL). Then, ethyl propenyl ether (5 μL, 0.045 mmol, 20 equiv) was added via syringe. The vial was then sealed with a PTFE-lined screw cap, transferred out of the glovebox to a fume hood, and irradiated with 2 W blue LEDs for 75 min. The LEDs were placed ca. 1 cm from the vial. After 75 min, the vial was opened, and a small amount (ca. 5 mg) of hydroquinone was added. A small aliquot was then taken for ¹H NMR analysis to determine conversion into polymer. The remaining contents of the vial were diluted with CH₂Cl₂ and passed through a thin plug of neutral alumina using CH₂Cl₂ as eluent to remove any (thio)pyrylium salt. The filtrate was then concentrated to a volume of ca. 5 mL and then added dropwise into an excess of MeOH (60 mL), which resulted in precipitation of the PNB. The solids were collected by vacuum filtration, washed with cold MeOH, and then dried under reduced pressure.