Latent ruthenium catalysts

Fig. 16 Mechanoactivation of latent ruthenium catalyst in the solid state, initiating in situ polymerisation of norbomene monomer in response to stress. Reprinted with permission from [87] (Copyright American Chemical Society 2013)...

Latent Ruthenium Catalysts for Ring Opening Metathesis Polymerization (ROMP)... 

In current reports of mechanochemically latent ruthenium alkylidenes, metathesis activity is known only in solution, in which access to mechanical force is achieved through solvodynamic shear stresses during ultrasonication [115, 116]. These catalysts are touted as being apphcable to self-healing polymers, but several refinements toward self-healing applications are necessary. First, mechanochemically induced ligand dissociation must be demonstrated through macroscopic stresses on a bulk polymer. Second, once catalyst initiation... 

The amount of effort invested in the advancement of latent ruthenium olefin metathesis catalysts for ROMP bodes well for the near future. Certainly, more of these developments will afford ever-more useful macromolecules. The most... 

Concomitantly, Lemcoff et al. prepared a closely related series of sulfur-chelated latent ruthenium alkene metathesis catalysts 35 that possessed an uncommon ris-dichloro arrangement and were mostly inactive at room temperature, but could be activated either thermally or photochemically. Modification of the size of the remote substituents on the sulfur atom significantly affected the catalytic activity at different temperatures. Bulkier substituents raised the activity at lower temperatures. Catalysts 35 were stable in solution and retained their catalytic activity in RCM reactions even after being exposed to air for two weeks. 

The first reported example using macromolecule-supported catalysts in latent biphasic systems was work by Chan s group that employed a dendrimer-bound BINAP 127 that was used to form a chiral ruthenium hydrogenation catalyst [164]. The dendritic Ru-BINAP complex formed from the reaction of [RuCl2(benzene)2]2 and 127 was successfully used in four cycles in the hydrogenation of 2-phenylacrylic acid (Eq. 65) in a 1 1 (vol/vol) ethanol/hexane mixture. Addition of 2.5 vol% water to this mixture produced a biphasic mixture where >99% of the dendritic catalyst was in the hexane phase. Addition of a fresh ethanolic substrate solution to this hexane phase produced another miscible solution of catalyst and substrate. The second and subsequent cycles of hydrogenation carried out in this manner led to consistent conversions of substrate with synthetic yields of >91% with e.e. values of 90%. 

The corresponding ruthenium complexes [Ru(/7-cymene)Cl2(PR3)] C3 show a much higher reactivity in NBE polymerization but none of these complexes are thermally latent towards NBE. If R in C3 is n-butyl, the complex is almost inactive in the thermal polymerization of NBE. However, if this catalyst is irradiated for 5 min at room temperature high yields of poly(NBE) are obtained. Irradiation will cleave off the arene ligand, thus enabling the reactive carbene formation. C3 (R=n-butyl) can also be used to photopolymerize endo configurated monomers like dicyclopentadiene (DCPD) even at R.T. Figure 2 shows a typical photo DSC of the photo polymerization of crude (=unpurified) DCPD. 

A widely used strategy for latent catalyst activation is the addition of a chemical entity or a co-catalyst to promote the formation of the active species. Co-catalysts are usually a carbene source (such as diazo compounds) or an acidic compound (Bronsted or Lewis) that stimulates the dissociation of hgands from the ruthenium complex to generate an active species. 

In 2014, two independent reports from Poland disclosed the preparation of ruthenium-alkylidene complexes chelated via a phenoxide anion [29)7 After activation with hydrogen chloride or other suitable acidic additives, these stable catalyst precursors became efficient promoters for various CM, RCM, and enyne metathesis reactions, including butenolysis. It is noteworthy that they were soluble in neat dicyclopentadiene, thereby enabling their use as latent catalysts for the ROMP of this highly reactive monomer. 

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