Alkylidene metathesis

The first well-defined Ru alkylidene metathesis initiator, (231), was reported by Grubbs et al. in 1992.632 This complex initiates the ROMP of norbornene and other highly strained monomers such as bicyclo[3.2.0]hept-6-ene.633 Examination of alternative ligands634-636 led to the development of more active initiators, in particular (232)637 and (233).638-640... 

In 1970, Chauvin and Herisson presented a study of the co-metathesis of cycloalkene/alkene mixtures using a WOCLj/SnBuj pro-catalyst mixture [12]. Whilst the fully quantitative analysis of the product mixtures was made complicated by the range of techniques that were required for the low, medium and high molecular weight products (mono alkenes, telomers and polymers), it became clear that product ratios were not consistent with what would be predicted by either mechanism in Scheme 12.14. The analysis and associated mechanistic interpretation were seminal and worthy of consideration in some detail here. The key point is that both mechanisms in Scheme 12.14 are pairwise, i.e. each turnover of the catalyst cycle involves two alkenes that undergo concerted alkylidene exchange. When a single alkene, e.g. pent-2-ene (C5), is considered, the products of alkylidene metathesis... 

Figure 4.23 Second-generation W-and Re-alkylidene metathesis catalysts anchored on silica, with their corresponding TOF and TON values at25°C [131],...

Figure 5 Alkylidene metathesis catalysts. (a-e R = Ph, Cy, Cyp R — Ph, CH—CMej Ar = mesityl.)...

These 1987 resnlts concluded that classical metathesis catalyst systems were not sufficient and that Lewis acid cocatalyst-free systems were necessary if successM ADMET condensation polymerization were to become a reality. The key to snccessM ADMET polymerization was demonstrated " nsing the Lewis acid-free tungsten alkylidene metathesis catalyst (5a), the structure of which had been reported by Schrock et just one year earlier. When this... 

Figure 8.5 Examples of Grubbs ruthenium alkylidene metathesis...

Synthesis of well-defined Mo imido alkylidene metathesis eatalysts [65]... 

As mentioned above in Section 1.25.5.2, rhodium and iridium pincer complexes have been used to catalytically dehydrogenate alkanes, giving terminal olefins as the kinetic products. In a recent report by Goldman and Brookhart, the iridium Pincer complexes were combined with Schrock s alkylidene metathesis catalyst... 

The emulsion polymerization of norbornene and cyclooctadiene in water was catalyzed by well-defined, water-soluble Ru-alkylidene metathesis catalysts (such as [RuCl2(TPPTS)2(=CHC02Et)] in the presence of surfactants. Well-controlled polynorbornene latexes were obtained with particle sizes in the range of 50-100 nm (194). The easily available water-soluble allenylidene complex [RuCl2(TPPMS)2(=C=CH—CeHs)] was found catalytically active in the crossmetathesis of methylacrylate with cyclopentene in biphasic mixtures of acidic water and diethyl ether to give C12H19CO2CH3 in 42% yield (195). 

Scheme 8.5 Ruthenium phosphonium alkylidene metathesis catalysts (22a-c) reported by...

Figure 3.12 The preparation of bis-hydroxy telechelic PBD using a ruthenium alkylidene metathesis catalyst in combination with an allylic functionalized CTA [34].

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... 

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