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

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.

Photoreduction of triplet thioxanthone derivative by azolium tetraphenylborate: a way to photogenerate N-heterocyclic carbenes

“…Ph 2 BOH was found to react with the photogenerated IMes to give an IMes-Ph 2 BOH ate-complex. While it is difficult to avoid the formation of NHC-boron adduct, our recent investigation showed that such complex did not prevent the activity of its NHC photogenerator in ringopening methathesis polymerization or for synthesis of polyurethane, 22 but could have a detrimental effect on other reactions. Knowledge about the precise photochemical mechanism offers now the possibility for improving the photoinitiating system in a rational way, and create a new generation of NHC photogenerator.…”

“…-(E ox = 0.94 eV). 22 It also highlights a more complex mechanism where photogenerated NHC can also interact with triplet ITX and compete with BPh 4 for charge transfer.…”

“…The utility of this two-component NHC photogenerator NHCH + BPh 4 -/ITX was also demonstrated in photopolymerization reactions to form polyurethane, polyester and polynorbornene. 22 Indeed, although NHCs have brought profound changes in catalytic organic synthesis, [23][24][25] they generally require inert atmosphere and harsh conditions for their generation. As a result, an air-stable system based on mild components, able to generate NHC on demand and on simple UV exposure has the potential to significantly simplify implementation of the broad spectrum of NHC-catalyzed reactions.…”

Combining a ligand photogenerator and a Ru precatalyst: a photoinduced approach to cross-linked ROMP polymer films

“…Quantitative conversion of IMes into IMes-carbodiimide (CDI) adduct was described elsewhere. 29 The irradiation sources include a medium pressure Xe-Hg lamp (75 mW cm À2 , Hamamatsu, LC-9588/01A), a LED@365 nm (65 mW cm À2 , Hammamatsu LC-L1V3) and a LED@405 nm (100 mW cm À2 , Thorlabs M405L2). The irradiances were determined by using an optical power meter PM100D from Thorlabs.…”

“…22 Recently, we replaced the PAG with a NHC photogenerator and used a stable, commercially available dimeric Ru(II) complex as metathesis inactive catalyst (Scheme 1). 28,29 Our tandem system made up of 1,3-bis(mesityl)imidazolium tetraphenylborate IMesH + BPh 4 À (1) as NHC progenitor and [RuCl 2 (p-cymene)] 2 precatalyst (Ru-1) proved to be very efficient at ROMP of NB. In comparison with the examples discussed above, a safer UVA radiation (320-400 nm) was employed, and no ligand displacement occurred.…”