Ruthenium alkylidene catalyst acids

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

This finding is a significant improvement over aqueous ROMP systems using aqueous ROMP catalysts. The propagating species in these reactions is stable. The synthesis of water-soluble block copolymers can be achieved via sequential monomer addition. The polymerization is not of living type in the absence of acid. In addition to eliminating hydroxide ions, which would cause catalyst decomposition, the catalyst activity is also enhanced by the protonation of the phosphine ligands. Remarkably, the acids do not react with the ruthenium alkylidene bond. 

Copper(I) triflate was used as a co-catalyst in a palladium-catalyzed carbonylation reaction (Sch. 27). The copper Lewis acid was required for the transformation of homoallylic alcohol 118 to lactone 119. It was suggested that the CuOTf removes chloride from the organopalladium intermediate to effect olefin complexation and subsequent migratory insertion [60]. Copper(I) and copper(II) chlorides activate ruthenium alkylidene complexes for olefin metathesis by facilitating decomplexation of phosphines from the transition metal [61]. 

By 1992, Grubbs and co-workers had discovered an alternative catalyst that overcame many of these shortcomings. Indeed, although ruthenium alkylidene 12 (Scheme 5) displays a lower metathesis activity than Schrock s molybdenum systems, it importantly demonstrated air stability and the ability to initiate metathesis in the presence of alcohols, water, and carboxylic acids. Thus, 12 represents the first true catalyst for general bench top olefin metathesis reactions, and over time has been optimized to 13 (Scheme 6), which has proven far easier to prepare than the parent structure 12 and constitutes the current gold standard with which all new catalyst systems are compared. Without question, this... 

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. 

Addition of small amounts of acid (up to 1 equiv. DCl) is advantageous for catalyst performance. Remarkably the metal-alkylidene moiety is not attacked by the acid, but a monophosphine complex and the phosphonium salt of the hgand are formed instead. Monitoring of ruthenium-alkyhdene species during the polymerization reaction by NMR confirms their high stability towards water. 

Cavallo and coworkers [56] have explored the decomposition of the second-generation ruthenium methylidene and benzylidene catalysts induced by the coordination of % acids. Carbon monoxide (CO) was used as a model it-acid ligand in these computations, although it is not normaUy added during metathesis. The DFT calculations indicated that the coordination of CO trans to the Ru-alkylidene bond was highly exothermic and promoted a cascade of reactions with very low energy barriers (Scheme 7.11) [57]. The coordination of the % acid reduced the electron density on the alkylidene and thus promoted the... 

---