Catalysts single-component

A Ta carbene complex (2 in Table 15.2) is the first example of a single-component metathesis catalyst that induces the living polymerization of substituted acetylene, where the monomer used is 2-butyne [37]. The initiation efficiency is quantitative. 

Ring-substituted PhAs have also been polymerized by well-defined Mo carbenes ligated by less bulky alkoxy groups (4, Table 15.2) [38]. Like metal halide-induced living polymerizations, bulky ring substituents at the ortho position are required for controlled polymerization. The advantage of the Mo carbene-catalyzed polymerization system is that the initiation and propagation steps can be readily monitored by an NMR technique. 

In general, Ru carbenes do not show high catalytic activity for the polymerization of substituted acetylenes, and therefore have not been studied in detail. Nuyken and coworkers [63,64] developed a well-defined Grubbs-type Ru carbene for living polymerization of diethyl dipropargylmalonate (5, Table 15.2). 

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Formation of single-walled carbon nanotubes (SWNTs) was found to be catalyzed by metal nanoparticles [207]. Wang et al. [114] investigated bimetallic catalysts such as FeRu and FePt in the size range of 0.5-3 nm for the efficient growth of SWNTs on flat surfaces. When compared with single-component catalysts such as Fe, Ru, and Pt of similar size, bimetallic catalysts Fe/Ru and Fe/Pt produced at least 200% more SWNTs [114]. 

It has since been shown that if less coordinating anions are used, then cationic zirconocene alkyls may serve as highly active single-component catalysts. Hence, treatment of Cp2ZrMe2 with... 

The resulting complexes can be effectively employed as single component catalysts to homopolymerize ethylene or copolymerize ethylene with acrylates [50, 51] and a variety of other polar monomers including vinyl ethers, [51,52] vinyl fluoride [53], iV-vinyl-2-pyrrolidinone, and AMsopropylacrylamide [54], In fact, the resulting catalysts are so robust that they can be used as single component catalysts in aqueous emulsion homo-polymerization of ethylene and copolymerization of ethylene with norbomenes and acylates [55]. 

Following pioneering work by Sen [50] and Risse [51] in the 1980 s, B.F. Goodrich launched a new family of amorphous norbomene-based polymers aimed at a number of microelectronic applications. The polymers are high-priced specialties (up to 6,000 per kg). These new polymers were made possible by a breakthrough in the area of single component catalysts based on Group 10 (Ni and Pd) transition metals [52], These catalysts are characterised by their ability to ... 

Homogeneous, single-component catalysts such as, e.g., W(=CCMe3)(OCMe3)3 or W(=CMe)(OCMe2CF3)3, cannot only be used for exchange metathesis of alkynes but also for ROMP of cycloalkynes, ADMET of a,to-diynes, and RCM of a,co-diynes [751]. 

The known catalyst systems for olefin metathesis can be grouped into heterogeneous catalysts, homogeneous multi-component catalysts, and homogeneous single-component catalysts. 

Small-scale preparations and the chemical modification of fine chemicals and elaborate intermediates are usually conducted in solution. For this purpose soluble metathesis catalysts of predictable and reproducible activity are generally preferred. The catalytic systems presently known can be grouped into multi-component and single-component catalysts (Tables 3.14-3.16). 

Most known single-component catalysts can be easily modified by simple ligand exchange this enables precise fine-tuning of the activity and the stereochemical output of a given catalyst. 

Table 3.16. Alkylidene and carbyne complexes as single-component catalysts for homogeneous-phase alkene metathesis.

Because of the enormous synthetic potential of molybdenum- [22] and ruthenium-based [57,806] single-component catalysts, a closer look at the scope and limitations of the most promising compounds known to date is appropriate. The systematic exploration of the synthetic possibilities offered by these new catalysts has just begun, and many new developments are to be expected in the near future [744,746,747,807]. As quick reference for the organic chemist, the most relevant chemical properties of two types of frequently used catalyst (Figure 3.46) are listed below. These carbene complexes are quite robust and well-suited to the metathesis of elaborate organic intermediates. 

To a certain extent the expression multicomponent catalysts is an arbitrary one. There is no doubt that the pure chemical elements and pure chemical compounds have to be called single component catalysts. It is, however, questionable whether a material such as steel should be classified as a single component system or as a multicomponent system. Some of the multicomponent catalysts, for instance, the iron-alumina catalyst consist of two separate solid phases but it would be misleading to accept the presence of more than one phase as the decisive criterion for multicomponent catalysts. The more than additive catalytic action of Cu-ions and Fe-ions in an homogeneous aqueous medium represents obviously a case of multicomponent catalysis, although it occurs in a single-phase system. As to solid multicomponent catalysts, they usually consist of more than one single phase, but there are exceptions to this rule, such as in cases in which mixed crystals or solid solutions are formed from the components. 

Further developments have been made possible by the introduction of third-generation catalysts. These are the commercially available, highly active Schrock-type Mo-based catalyst 19 and W catalyst 20, coordinated by a bulky neopentylidene group and alkoxides [2], They are well-defined, single-component catalysts. The W complex 20 is very active, but the Mo complex 19 is more versatile. These catalysts have some... 

Figure 3.11 Single-component catalyst, dimeric homochirotopic rac.-(S, S)-dimethyl-silylenebis[l -(2-trimethylsilyl-4-r-butylcyclopentadienyl)]yttrium hydride [rac.-Me2Si (Me3Si,t-BuCp)2YH]2, for obtaining highly isotactic poly(a-olefins)s. Side view. Reproduced by permission from Ref. 31. Copyright 1992 American Chemical Society...

Also, neutral group 3 and lanthanide series metallocenes form with olefins n complexes having formally a d° 16-electron configuration, which is favourable for the insertion reaction during olefin polymerisation (or oligomerisation) in the presence of these single-component catalysts [187]. 

Further, since the activity of zinc oxide has been found to be higher than that of other oxides such as alumina or chromia, no single-component catalyst, oxide, or metal is known at the present time that would effectively catalyze methanol synthesis at low temperatures and pressures, i.e., below 250°C and 100 atm. It is the synergic promotion in multicomponent catalysts that brings about greatly enhanced activity at low temperatures. 

The ansa-metallocene complex (CpCMe2Flu)Nd(C3H5)(THF) is an effective single-component catalyst for the production of syndiotactic styrene-rich co-polymer materials modified by isoprene and/or ethylene. The properties of the final polymer could be tuned by altering the monomer ratio.55... 

Csihony S, Culkin DA, Sentman AC, Dove AP, Waymouth RM, Hedrick JL (2005) Single-component catalyst/initiators for the organocatalytic ringopening polymerization of lactide. J Am Chem Soc 127 9079-9084... 

DP 3 Consider qualities such as Lewis-acidic and weakly coordinating with regard to the B- and Al-containing activators mentioned in Section 7.4.1. Consider also single-component catalysts. 

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