Functional group-tolerant ruthenium systems

Ruthenium salts such as Riidb.-xIf.O or ruthenium(ll) tosylates have been known for long to effectively catalyze ROMP of several cycloalkenes. Despite the characterization of several olefin-ruthenium(II) complexes [151-154], fhe actual catalytic species in such systems is still ill-defined. Nevertheless, fhe fact fhat ruthenium-based systems did effectively catalyze fhe ROMP even in aqueous systems [155, 156] or in the presence of ofher protic functional groups (alcohols, carboxylic acids, etc.) [153, 154, 157-162] initiated an intense search for well-defined, functional group-tolerant ruthenium systems [163], mainly conducted by the group of R.H. Grubbs. In 1992, this group described fhe synfhesis of the first well-defined ruthenium alkylidene (Scheme 5.12) [75]. 

Both are stable metallocarbene complexes, but they have very different reactivity profiles. The molybdenum catalyst is highly reactive and is effective widi sterically demanding olefins. Its drawbacks are diat it is not highly tolerant of diverse functional groups and has high sensitivity to air, moisture, and solvent impurities. The ruthenium system, on die odier hand, is catalytically active in die presence of water or air, and it exhibits a remarkable functional group tolerance. It is not a reactive as the molybdenum catalyst, particularly toward sterically bulky substrates. However, it is readily available and is die reagent of choice for all but die most difficult substrates. 

On the other hand, late transition metal-based catalyst systems that had been identified by the early 1990s were characterized by low activity but high functional group tolerance, especially toward water and other protic solvents. These features led to reinvestigations of ruthenium systems and, ultimately, to the preparation of the first well-defined, ruthenium-carbene olefin metathesis catalyst (PPh3)2(Cl)2Ru=CHCH=Ph2 (Ru-1) in 1992 [5]. 

Fiirstner developed ruthenium indenylidene (readily accessible from commercially available diphenyl propargyl alcohol) (2c) and (3b) (Scheme 5) versions of the alkylidene (2a) and (3a) with (2c) being superior to (3b) in yield, reaction rate, and tolerance to polar functional groups. Catalyst (2c) was more active than (2a) for RCM and it was applied to RCM to form functionalized rings from 5 to 21 members. For all reported ring closures, compared cases (2c) was equal to or superior to (2a). The functional group tolerance and activity made (2c) the best catalyst for the RCM approach to the synthesis of the ADE ring system of Nakadomarin A. 

The convenience and functional group tolerance of the ruthenium catalysts has led these compounds to be used widely in synthesis, but the loading of catalyst needed for these applications is often high. Thus, several studies have been conducted to identify flie reactions that lead to catalyst decomposition with the objective of designing systems that resist these decomposition pathways. Three reactions that lead to catalyst decomposition are shown in Equations 21.4a-c. 

Ruthenium olefin metathesis systems are widely recognized as functional group tolerant catalysts, as they react selectively with olefins over ketones, aldehydes. 

Ruthenium has been known to catalyze olefin metathesis reactions for some time [10], and the development of discrete organometallic complexes has enabled the rational design of improved catalysts with enhanced catalytic activity, functional group tolerance, and selectivity [11]. In particular, catalyst development has provided systems that exhibit the living characteristics necessary to produce well-defined polymeric products. Three ruthenium complexes (1-3, Figure 5.1) are by far the most heavily utilized in ROMP [12,13]. 

Based on work with aqueous mthenium-based metathesis systems, stable, active, and well-defined ruthenium metathesis catalysts were developed. As will be demonstrated, the early promise of broadly functional group-tolerant and water-tolerant initiators based on mthenium have been realized. The first complex, 10, was not particularly reactive, but would polymerize norbomene and its derivatives. These complexes were much more stable to functional groups, water, and oxygen than prior systems. It was demonstrated that these systems would give a living polymer with norbomene and could be used to form block polymers. ... 

Living ring opening metathesis polymerization methods (ROMP) were first employed to synthesize LC-coil diblock copolymers by Komiya and Shrock [80] in 1993. The structure of their polymer system is shown in Scheme 7D. Recent work from Grubbs group also used a novel ruthenium catalyst which can tolerate more functional groups [81] to synthesize well-defined LC-coil block copolymers [82]. The ROMP polymer backbone can be hydrogenated to create saturated structure to improve its stability. 

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