Affiliation: | 1. Institut de Science des Matériaux de Mulhouse, IS2M UMR 7361 CNRS, Université de Haute-Alsace, France Université de, Strasbourg, France;2. Institut Charles Gerhardt Montpellier, ICGM UMR 5253, Université de Montpellier, CNRS, ENSCM, Montpellier, France;3. CNRS, Bordeaux INP, LCPO, UMR 5629, Université de Bordeaux, 33600 France;4. Institut de Science des Matériaux de Mulhouse, IS2M UMR 7361 CNRS, Université de Haute-Alsace, France |
Abstract: | In the search of smarter routes to control the conditions of N-heterocyclic carbene (NHCs) formation, a two-component air-stable NHC photogenerating system is reported. It relies on the irradiation at 365 nm of a mixture of 2-isopropylthioxanthone (ITX) with 1,3-bis(mesityl)imidazoli(ni)um tetraphenylborate. The photoinduced liberation of NHC is evidenced by reaction with a mesitoyl radical to form an NHC-radical adduct detectable by electron spin resonance spectroscopy. The NHC yield can be determined by 1H NMR spectroscopy through the formation of a soluble and stable NHC–carbodiimide adduct. To deprotonate the azolium salt and liberate the NHC, a mechanism is proposed in which the role of base is played by ITX radical anion formed in situ by a primary photoinduced electron-transfer reaction between electronically excited ITX (oxidant) and BPh4− (reductant). An NHC yield as high as 70 % is achieved upon starting with a stoichiometric ratio of ITX and azolium salt. Three different photoNHC-mediated polymerizations are described: synthesis of polyurethane and polyester by organocatalyzed step-growth polymerization and ring-opening copolymerization, respectively, and generation of polynorbornene by ring-opening metathesis polymerization using an NHC-coordinated Ru catalyst formed in situ. |