In Situ Generated Ruthenium–Arene Catalyst for Photoactivated Ring‐Opening Metathesis Polymerization through Photolatent N‐Heterocyclic Carbene Ligand

Abstract

1,3-Bis(mesityl)imidazolium tetraphenylborate (IMesH BPh ) can be synthesized in one step by anion metathesis between the corresponding imidazolium chloride and sodium tetraphenylborate. In the presence of 2-isopropylthioxanthone (sensitizer), an IMes N-heterocyclic carbene (NHC) ligand can be photogenerated under irradiation at 365 nm through coupled electron/proton transfer reactions. By combining this tandem NHC photogenerator system with metathesis inactive [RuCl (p-cymene)] precatalyst, the highly active RuCl (p-cymene)(IMes) complex can be formed in situ, enabling a complete ring-opening metathesis polymerization (ROMP) of norbornene in the matter of minutes at room temperature. To the best of our knowledge, this is the first example of a photogenerated NHC. Its exploitation in photoROMP has resulted in a simplified process compared to current photocatalysts, because only stable commercial or easily synthesized reagents are required.

Mixture of Azolium Tetraphenylborate with Isopropylthioxanthone: A New Class of N‐Heterocyclic Carbene (NHC) Photogenerator for Polyurethane, Polyester, and ROMP Polymers Synthesis

“…1 H NMR spectroscopy provides a deprotonation degree (from NC H N proton), but the results can be distorted when the reaction is not complete because of N −CH−N/ N −C:−N hydrogen bonds forming in the 1H + ‐ 1 complex (see evidences in Figure S4). Alternatively, NHC concentration can be determined indirectly through a quantitative reaction between 1 and CS 2 . However, the zwitterionic 1 ‐CS 2 adduct is only partially soluble in solvents such as THF and acetonitrile, making yield calculation complex because the adduct is partitioned between solid and liquid phases.…”

“…NaBPh 4 induces the lowest rate of photobleaching (1.58×10 −7 m s −1 ), compared to 1H + BPh 4 − (2.40×10 −7 m s −1 ) and 2H + BPh 4 − (4.04×10 −7 m s −1 ) even the Δ G et,NaBPh4 value is slightly more negative (Δ G et,NaBPh4 =−0.18 eV). One possible explanation for the differences of ITX decomposition rate could be the slower electron‐transfer rate of NaBPh 4 compared to other borate salts …”

“…Synthesis of azolium tetraphenylborate salts : 1H + BPh 4 − and 2H + BPh 4 − were prepared following a procedure published elsewhere …”

“…The proposed solution is to produce a suitable radical anion by means of a photoinduced electron‐transfer (PET) reaction, which can subsequently deprotonate the azolium cation (Scheme ). To this end, we begin with a mixture of isopropylthioxanthone (ITX) with an air‐stable azolium tetraphenylborate salt, such as 1,3‐bis(mesityl)imidazolium tetraphenylborate 1H + BPh 4 − or 1,3‐bis(mesityl)imidazolinium tetraphenylborate 2H + BPh 4 − . First, ITX is an inexpensive, commercially available aryl ketone, known for its ability to mediate single electron‐transfer after photochemical excitation in the near UV/Vis region (350–420 nm); in particular, excited ITX is used as an electron acceptor sensitizer in many photoreactions .…”

Organische Photosäuren‐ und Photobasenbildner für Polymerisationen: Jüngste Fortschritte und Herausforderungen

“…[49] Diese Verbindungen ermçglichten erstmals die Freisetzung von tertiären Aminen, was die Verwendung von PBGs auf eine grçßere Vielfalt von Basen erweiterte (Abbildung 5b). [ [52] was die Anzahl der Basen, die unter Einwirkung von Licht aktiviert werden kçnnen, deutlich erweiterte. [ [52] was die Anzahl der Basen, die unter Einwirkung von Licht aktiviert werden kçnnen, deutlich erweiterte.…”