Ruthenium metathesis initiators

Oxoalkylidene complexes of the type (W=CH-CH=CPh2)(0)[0CMe(CF3)2]2 L (16) were obtained by Grubbs and utilized for ROMP of norbomene [159]. His synthetic strategy employed 3,3-diphenylcyclopropene for the synthesis of the alkylidene moiety [159,160]. This method was later adapted for the synthesis of the first well-defined ruthenium metathesis initiators [161]. Furthermore, (0Ar)2W(CH-/-Bu)(0)(PMe3) (17) was synthesized by Schrock et al. and reported to polymerize 2,3-dicarbomethoxynor-bornadiene in a living manner [162]. 

Despite the numerous reports concerning NHC-Ru olefin metathesis initiators, a complex incorporating a carbene that has only one exocyclic substituent adjacent to the carbenic centre was not reported until 2008. Studies by Grubbs and co-workers led to the development of ruthenium-based catalysts bearing such carbene ligands, in this case incorporating thiazole-2-ylidenes [63] (Fig. 3.19). 

By contrast, much of the work performed using ruthenium-based catalysts has employed well-defined complexes. These have mostly been studied in the ATRP of MMA, and include complexes (158)-(165).400-405 Recent studies with (158) have shown the importance of amine additives which afford faster, more controlled polymerization.406 A fast polymerization has also been reported with a dimethylaminoindenyl analog of (161).407 The Grubbs-type metathesis initiator (165) polymerizes MMA without the need for an organic initiator, and may therefore be used to prepare block copolymers of MMA and 1,5-cyclooctadiene.405 Hydrogenation of this product yields PE-b-PMMA. N-heterocyclic carbene analogs of (164) have also been used to catalyze the free radical polymerization of both MMA and styrene.408... 

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-... 

Ruthenium Carbene-Based Olefin Metathesis Initiators Catalyst Decomposition and Longevity M. Ulman, R.H. Grubbs,/. Org.Chem. 1999, 64, 7202— 7207. 

In 1998 it was revealed that allenylidene-ruthenium complexes, arising simply from propargylic alcohols, were efficient precursors for alkene metathesis [12], This discovery first initiated a renaissance in allenylidene metal complexes as possible alkene metathesis precursors, then it was observed and demonstrated that allenylidene-ruthenium complexes rearranged into indenylidene-ruthenium intermediates that are actually the real catalyst precursors. The synthesis of indenylidene-metal complexes and their efficient use in alkene metathesis are now under development. The interest in finding a convenient source of easy to make alkene metathesis initiators is currently leading to investigation of other routes to initiators from propargylic derivatives. 

Propargylic Ethers as Alkene Metathesis Initiator Precursors Generation of Alkenyl Alkylidene-Ruthenium Catalysts... 

Ulman, M., Grubbs, R. H. Relative Reaction Rates of Olefin Substrates with Ruthenium(ll) Carbene Metathesis Initiators. Organometallics 1998, 17, 2484-2489. 

Matron and Grubbs formed block copolymers by combining ring opening metathesis polymerization with ATRP [437]. Use was made of fast initiating ruthenium metathesis catalyst to form three different monotelechelic poly(oxa)norbomenes. The ends were functionized and ATRP polymerizations of styrene and ferf-butyl acrylate followed. 

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. 

The living nature of ROMP catalyzed by well-defined metathesis initiators allowed the synthesis of homo and block copolymers with controlled lengths and compositions. Moreover, the introduction of well-defined and functional-group-tolerant Grubbs ruthenium initiators allowed the synthesis of polymers with biologically active molecules. 

Table 3 Examples of ruthenium-based metathesis initiators.

Ulman, M., Grubbs, R.H., Ruthenium carbene-based olefin metathesis initiators catalyst decomposition and longevity, J. Org. Chem. 1999, 64 7202-7207. 

De Clerq, B., Verpoort, F., Assessing the seope of the introduction of Schiff bases as eo-ligands for monometallic and homobimetallic ruthenium ringopening metathesis polymerisation and ring-elosing metathesis initiators, Adv. Synth. Catal. 2002, 344 639-648. 

The first report of NHC-containing osmium compounds acting as catalysts came from Esteruelas and co-workers in 2005. Thus, cationic benzyli-dene complexes 44 were prepared by reaction of the corresponding 16-electron precursors [(NHC)OsCl(p-cymene)][OTf] (NHC = IMes or IPr) with phe-nyldiazomethane, and their potential as initiators for olefin metathesis was probed in the RCM of diethyl diallylmalonate, the ROMP of cyclooctene, and a variety of self- and cross-metathesis reactions (Equation (7.10)). Although they were not as efficient as standard ruthenium-benzylidene metathesis initiators, compounds 44 displayed, nevertheless, a fairly decent activity. More importantly, in addition to being the first NHC-Os catalytic application, this study constituted a rare example of osmium catalysed C-C bond formation. 

Frenzel, U. and Nuyken, Oskar (2002) Ruthenium-based metathesis initiators development and use in ring-opening metathesis polymerization. Journal of Polymer Science Part A-Polymer Chemistry, 40,2895-2916. 

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. ... 

One of the most successful embodiments of this concept combined a metathetical step with the selective isomerisation of a terminal C=C double bond, and found applications in natural product synthesis. As discussed in Sections 7.3.2.2 and 7.3.2.3, it is indeed possible to convert the metathetically active benzylidene initiator 20 into hydrido complex 43, simply by adding trimethyl(vinyloxy)silane or methanol to the reaction mixture, thereby triggering a consecutive catalytic isomerisation process. Other methods for the decomposition of ruthenium metathesis catalysts for use in tandem with olefin isomerisation reactions include treatment with hydrogen, formic acid, sodium borohydride, or sodium hydroxide in isopropanol. ... 

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