Mono with metathesis catalysts

Although metathesis catalysts polymerize mono- and disubstituted acetylenes, they do not polymerize monomers having polar and/or protic groups because of their lack of tolerance of these groups. For the same reason, the polymerization with metathesis catalysts is carried out in relatively nonpolar solvents. 

Recently. Shirakawa et al. also synthesized mono-substituted acetylenes with liquid crystallinemoieties by Ziegler—Natta and metathesis catalysts (see Chart... 

Large-ring products can be accessed readily by ring-closing metathesis. If more than one alkene is present in the substrate then the less-hindered, typically mono-substituted, alkene reacts preferentially. For example, the anticancer epothilone compounds can be prepared by using metathesis as the key ring-forming step. Treatment of the substrate 95 with the catalyst 84 resulted in the formation of both the desired Z-alkene 96 and the E-alkene 97 (2.115). Control of alkene stereochemistry in macrocycle formation is often difficult unless a conformational constraint... 

The experiments that are most relevant to the present discussion involved the cross metathesis of mixtures of 1-pentene and l-pentene-catalyst system, Calderon and coworkers monitored isotopic scrambling to form the d2 and mass spectrometry). After only short reaction times, the mixture reached 70% of the theoretical scrambling limit, with almost no formation of any octene products. To explain pentene s proclivity toward generating itself rather than octene in the presence of metathesis catalysts, Calderon suggested that either the mono-P-substituted metallacycle was favored over the mono-a-substituted isomer, or the a,a -disubstituted metallacycle was preferred relative to the a,p-substituted one (Scheme 10.4). In other words, degenerate metathesis can be performed by either M=CH2 or M=CHR species. These pathways were indistinguishable, but the results proved that one of the two inequalities must be true [5]. 

On the other hand, metathesis catalysts based on group V and VI metals effectively polymerize mono- and disubstituted alkynes to the corresponding substituted PAs. These catalysts are typically the metal chlorides, used with or without main-group organometallic cocatafysts, or metal carbonyls activated with light (Fig. 7) [110]. The latter e of catalyst is known for Mo and W only. Water can even be used as a cocatalyst with these catalysts for some monomers. For example, WQ5 I/2H2O polymerizes phenylacetylene to a soluble, powdery poly(phenylacetylene) with M = 15,000 g/mol and PDI = 2.06 [113]. 

The pioneering work on enantioselective ruthenium olefin metathesis was carried out by Grubbs and co-workers in 2001 [69] (Fig. 3.23). Catalysts 55a-b and 56a-b were designed and prepared from C -symmetric NHC hgands with a combination of chiral backbone and mono-ortfto-substituted aryl side chains, a motif that was expected to form a chiral environment around the metal centre. 

Since group 4 derived species are of particular interest as catalysts for olefin polymerization and epoxidation reactions, the thermal stability of surface metal-alkyl species, as weU as their reactivity towards water, alcohols and water, deserve some attention. On the other hand, mono(siloxy) metaUiydrocarbyl species can be converted into bis- or tris(siloxy)metal hydrides by reaction with hydrogen [16, 41, 46-48]. Such species are less susceptible to leaching and can be used as pre-catalysts for the hydrogenolysis of C-C bonds, alkane metathesis and, eventually, for epoxidation and other reactions. 

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... 

Perez-Castells and coworkers devised a tandem enyne methathesis Diels-Alder reaction strategy for the assembly of polycyclic indole structures [109]. The enyne metathesis reaction using Grubbs s catalyst (311) with the 2-alkynylaniline 310 in the absence of a dienophile proceeded to form the mono- and bis-indole derivatives 313 and 314 (Scheme 66). Testing the hypothesis that a Diels-Alder cycloaddition with an activated diene might be faster than the undesired cross-metathesis reaction which led to the formation of 314, a one-pot reaction with maleic anhydride (312) as the dienophile was conducted. Disappointingly, the above reaction resulted in a... 

Metathesis of mono- and diolefins can be performed with both homogeneous and heterogeneous catalysis. The most important processes involving metathesis steps, the SHOP process and the Phillips triolefin process, are based on heterogeneous catalysts. Homogeneous catalysts are used in the ring opening metathesis of norbor-nene (Norsorex, CDF-Chemie) and cyclooctene (Vestenamer, Hills) [7]. 

Our recent study on the activity for metathesis revealed a high reactivity of 1,9-decadiene and cyclooctene in their reactions with triethoxy- and trisiloxy-substituted vinylsilanes [20]. When the mixture containing vinylsilane and 1,9-decadiene was heated in the presence of Grubbs catalyst, the formation of mono- and bis(silyl)dienes, accompanied by a polymeric product, was detected. The replacement of 1,9-diene with cyclooctene in the reaction mixture has resulted in the same products. The reactions can be illustrated by Eqs. 6 and 7. l,10-bis(silyl)-substituted dienes were isolated with 50-70 % yields and characterized spectroscopically. In the case of disubstituted products separated by distillation, no double bond migration was observed. The only process observed in that case was E/Z isomerization, so only Z,Z-disubstituted products could be isolated. 

PLA-functionalized polyoxanorbomenes with one or two exo-FLA chains as well as two endo-, exo-chains were prepared using Sn(Oct)2 as a catalyst in the presence of mono- or dialcohol derivatives of oxanorbomenes [73]. These macromonomers are then subjected to ring-opening metathesis polymerization (ROMP) to yield graft copolymers (Figure 4.8). 

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