Ruthenium alkylidene initiators

Olefin metathesis has demonstrated vast potential in polymer chemistry. Thanks to both the good tolerance of ruthenium-alkylidene initiators to many different functionalities and their capacity to promote ATRP of vinyl monomers, combination of ROMP and ATRP should have a bright future in the field of advanced material synthesis. 

Acyclic diene metathesis (ADMET) is a step-growth polycondensation reaction for the polymerization of o -dienes 729 The process is catalyzed by the same metal alkylidene initiators used for ROMP, and is driven by the removal of ethylene from the system (Scheme 13). Both molybdenum and ruthenium-based initiators have been used to prepare a variety of materials including functionalized polyethy-... 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

Norbornene polymerization was initiated selectively on the surface of SWCNTs via a specifically adsorbed pyrene-linked ring-opening metathesis polymerization initiator (Fig. 1.20). The adsorption of the organic precursor was followed by cross-metathesis with a ruthenium alkylidene, resulting in a homogeneous noncovalent poly (norbornene) (PNBE) coating [249]. 

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]. 

HeterobimetalUc ruthenium alkylidenes, which may be prepared by reaction of ClaRuIPCyalafCHR) with [Ru(pactive initiators are the bimetallic complexes (p-cymene)RuCl(p-Cl)2RuCl(CHPh)(NHC) and (Cp )RhCl(p-Cl)2RuCl(CHPh)(NHC) (NHC=N-heterocyclic carbene, Cp =pentamefhylcyclopentadienyl) [236, 237]. 

In the 1980s, well-defined metal alkylidenes were introduced as catalyst precursors for olefin metathesis [99, 109-111]. Especially for aqueous ROMP, ruthenium alkylidenes represent readily activated, well-defined, easy to handle catalyst precursors respectively initiators (for a living ROMP without chain-transfer, the term initiator appears more appropriate). Whereas in initial work vinyl-substituted carbenes (cf. 16a) were employed [112], more straightforward routes to aryl-substituted carbenes (16b) were soon developed [113]. Today, vinyl-substituted carbenes are also accessible in one-pot procedures [114], and 16a and 16b are both commercially available. 

Recycled solutions of 2 initiated ROMP as quickly as the recycled Ru(III) solutions, closer examination of which revealed NMR resonances identical to those of the alkene protons in 2 [25]. It was therefore suggested that a key step in the initiation process using Ru(III) was the in situ formation of a Ru(II)-alkene complex [27]. Current evidence supports the disproportionation of the Ru(III) species to form Ru(II) and Ru(IV) species, followed by formation of a Ru(II)-alkene complex [25]. The equilibrium constant for disproportionation is small, accounting for the poor initiation efficiency of the Ru(III) systems [30]. An alternative, the disproportionation of an equilibrium amount of Ru(III)-alkene complex to a Ru(II)-alkene complex and a Ru(IV) species, is unlikely since Ru(III)-alkene complexes are generally unstable. Formation of a ruthenium alkylidene, the requi-... 

The insight derived from the investigation of ill-defmed ruthenium ROMP initiators was successfully applied to the development of Ru(II) alkylidenes 8 and 9 [15-18], In contrast to the classical complexes, these well-defined alkylidenes initiated ROMP quickley and quantitatively, reacted readily with acyclic alkenes, and could be used to initiate living polymerizations in organic solvents. 

The lessons learned from these complexes were eventually applied to the synthesis of well-defined ruthenium alkylidenes 8 and 9. Although they were insoluble in water, these alkylidenes could be used to initiate the living ROMP of functionalized norbornenes and 7-oxanorbomenes in aqueous emulsions. Substitution of the phosphine ligands in 9 for bulky, electron-rich, water-soluble phosphines produced water-soluble alkylidenes 10 and 11, which served as excellent initiators for the ROMP of water-soluble monomers in aqueous solution. These new ruthenium alkylidene complexes are powerful tools in the synthesis of highly functionalized polymers and organic molecules in both organic and aqueous environments. 

The cycle outlined in Fig. 4.32 indicates that the overall metathesis activity of the catalysts is determined by the relative magnitudes of several rate constants (i) the rate constant for phosphine dissociahon ( i) dictates the rate at which the 16-electron pre-catalyst complex enters the catalytic cycle, (ii) the ratio of k- to k2 controls the rate of catalyst deachvation (by re-coordination of phosphine) versus catalyhc turnover (by coordination of olehnic substrate and subsequent steps), and (iii) the rate constant for metallacycle formahon k ) determines the rate of the carbon-carbon bond formation. High olefin metathesis activity is expected when a ruthenium alkylidene catalyst exhibits fast initiation (a large value of k ), high selectivity for binding olefins relative to phosphines (a small value of k-x/k ), and fast metallacyclobutane formation (a large value of k ). 

In a later study, bis-NHC ruthenium-alkylidene complex was activated under compressive strain [87] (Fig. 16). In order to initiate Ru-mediated polymerisation of norbomene in solid state, polymer catalyst (34 kg mol ) and a norbomene monomer were incorporated in a high molecular weight poly(tetrahydrofuran) (pTHF) matrix (Mn=170 kDa, PDI=1.3) which provided the physical cross-linking through the crystalline domains and allowed macroscopic forces to be transferred to the metal-ligand bonds. Consecutive compressions showed that up to 25% of norbomene monomer was polymerised after five loading cycles. 

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... 

Equally important as recycling, problems involved with the use of certain reagents can often be completely avoided if they are attached to a resin. For instance, the ruthenium alkylidene catalysts typically employed to initiate metathesis reactions often decompose into darkly colored, metal-containing by-products, unwanted materials that sometimes require special protocols to remove and which are... 

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