Heteroatom ruthenium carbenes

Mechanistic implications of a general cross-metathesis of vinylsilicon with allyl-substituted heteroorganic compounds have been studied in detail for the reaction with allyl alkyl ethers [13]. The detailed NMR study of the stoichiometric reaction of Grubbs catalyst with allyl-n-butyl ether has provided information on individual steps of the catalytic cycle. A general mechanism of the cross-metathesis of vinyltri(alkoxy, siloxy)silanes (as well as octavinylsilsesquioxane) with 3-heteroatom-containing 1-alkenes in the presence of ruthenium carbene is shown in Scheme 5. 

DEGRADATION OF ROMP-POLYMERS HETEROATOM SUBSTITUTED RUTHENIUM CARBENES... 

Heteroatom substituted ruthenium carbenes like 7 and 8 (Cy=cyclohexyl, i-Pr=iso-propyl) were synthesized similar to 6, just adding the appropriate vinylethers and vinylthioethers instead of styrene in the last step (Scheme 4) [20]. Phenyl vinyl sulfide was prepared according to Scheme 5a. 

Table 6 compares the ROMP activity of heteroatom substituted ruthenium carbenes in the polymerization of DCPD (low and high purity) and Table 7 shows some results with other norbornene type monomers (see below). 

TABLE 6. Polymerization of DCPD of different purity with heteroatom substituted ruthenium carbenes. Curing conditions 1 h 120°C, 2 h 150°C. Tg Glass transition temperature. 

Table 6 clearly shows that heteroatom substituted ruthenium carbenes like 8 allow to lower the necessary catalyst concentration by a factor of 2-3, compared with phenyl or vinyl substituted ruthenium carbenes (3 and 4) and by a factor of 4-8 compared with the older catalyst generation (1). Using DCPD (98%) as monomer and 8 as catalyst in an amount corresponding to 60 ppm ruthenium, a Tg of 127°C is obtained. Similar results were obtained with intramolecular coordinated carbenes (see parts 2.4). 

The ROMP of DCPD by reaction injection molding (RIM) with ruthenium catalysts is possible with such heteroatom substituted ruthenium carbenes (7, 8) A molar ratio of DCPD/7 of 4700/1 was used and curing profiles measured as a function of the mold temperature, Figure 1. The polymerization and crosslinking of DCPD was very fast, completed within ca. 3 min. Conversions >95% and glass transition temperatures >140°C were obtained by RIM of DCPD with the new generation of ruthenium carbenes. 

From a methodological point of view, it should be pointed out the formation of 51, which is a result of the addition of acetone to an allenylidene ligand. Heteroatom-containing cyclic metal-carbene complexes [24] have been conveniently prepared via metal co-haloacyl, carbamoyl, alkoxycarbonyl, or imido intermediates [25], opening of epoxides by deprotonated Fischer-type carbene complexes [26], and activation of homopropargylic alcohols with low-valent d complexes [27], including ruthenium(II) derivatives [28]. In general, the preparation of unsaturated cyclic carbene complexes requires the previous preparation of functional carbenes to react with P-dicarbonyl derivatives, acrylates, and enol ethers [29]. 

The catalytic production of olefins, diethyl maleate and fumarate, from ethyl diazoacetate has been reported with osmium [ 149] and ruthenium [ 128] porphyrins. Despite the periodic relationship of ruthenium to iron and osmium and the syntheses of different carbene complexes of ruthenium porphyrins, developed by Collman et al. [125-127], it is only very recently that cyclopropanation [135,171] and ethyl diazoacetate insertion into heteroatom bond reactions [172] were observed using ruthenium porphyrins as catalysts. The details of the catalytic reaction of diazo esters with simple olefins catalyzed with ruthenium porphyrins have been reported [173]. Product yields. 

The past decade has witnessed extensive modifications of Af-heterocyclic carbene ligands for ruthenium olefin metathesis catalysts. This includes symmetrical and unsymmetrical NHCs, 1,3- and 4,5-substitutions, introduction of heteroatoms into the backbone, NHC ring size variation, and introduction of chirality. Most of these changes were initially targeted to improve stability and activity of the catalyst, while recent approaches are mainly focused on affording well-defined stereoselectivity. However, the activity and stability of the ruthenium-based metathesis catalysts are not solely ruled by the type of neutral NHC ligand the anionic ligands, chelation mode, substrates used, and the reaction conditions naturally also influence catalytic properties. One of the main lessons learned from ruthenium olefin metathesis development is that there is no one catalyst fits all and every type of application must be studied in detail in order to discover the most efficient catalytic complex. 

In order to explore the effect of the heteroatom in Fischer-carbene type ligands on the reactivity and thermal stability of ruthenium complexes, Grubbs et al. prepared and characterized a series of well-defined bis-phosphine 4-8, NHC imidazole (IMes) 9-12, and NHC imidazolidine (HglMes) 13 complexes (Figure 12.3) [17]. The exceptionally high stability of 8 at 55 C (20 days before half the complex was decomposed) could be explained by the chelation ofthe amide carbonyl to the ruthenium center. When tested in ROMP reactions, all Fischer carbene complexes demonstrated rapid and quantitative polymerization of norbornene (NBE) derivatives at room temperature, although the polymerization... 

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