Activation of Transition Metal Catalysts

Copper(I) triflate was used as a co-catalyst in a palladium-catalyzed carbonylation reaction (Sch. 27). The copper Lewis acid was required for the transformation of homoallylic alcohol 118 to lactone 119. It was suggested that the CuOTf removes chloride from the organopalladium intermediate to effect olefin complexation and subsequent migratory insertion [60]. Copper(I) and copper(II) chlorides activate ruthenium alkylidene complexes for olefin metathesis by facilitating decomplexation of phosphines from the transition metal [61]. 

The rearrangement of propargyl chlorides to chloroallenes occurs stereoselectively in the presence of BU4NCUCI2 (Sch. 28) [62]. Copper(I) dichloride performs as a reagent to deliver chloride, and CuCl might be acting as a Lewis acid to activate the pro-pargylic chloride for elimination. 

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The activity of transition metal catalysts depends on both the metal and the ligands. In addition, solvent effects, etc. can play a role. Table 3.10 shows examples of transition- metal catalysts with the reactions for which they are active (Farkas, 1986). 

The enhanced activity of transition metal catalysts supported on reducible oxides in the selective hydrogenation of the C=0 bond is generally attributed to... 

MAO is commonly used as an activator of transition metal catalysts in the syndiotactic polymerization of styrene. MAO acts as a reductant and as an alkylating reagent of the transition metal. Many different aluminum alkyls were examined for the syndiospecific polymerization of styrene (see Table 3.1) [2], A rapid alkylation of the metallocene by MAO takes place and the active species arises from a ligand transfer reaction between the metallocene alkyls and MAO. 

We have summarized below recent results concerning spectroscopic / flow reactor investigations of hydrocarbons partial and total oxidation on different transition metal oxide catalysts. The aim of this study is to have more information on the mechanisms of the catalytic activity of transition metal oxides, to better establish selective and total oxidation ways at the catalyst surface, and to search for partial oxidation products from light alkane conversion. 

Transition-metal catalysts are, in general, more active than the MPVO catalysts in the reduction of ketones via hydrogen transfer. Especially, upon the introduction of a small amount of base into the reaction mixture, TOFs of transition-metal catalysts are typically five- to 10-fold higher than those of MPVO catalysts (see Table 20.7, MPVO catalysts entries 1-20, transition-metal catalysts entries 21-53). The transition-metal catalysts are less sensitive to moisture than MPVO catalysts. Transition metal-catalyzed reactions are frequently carried out in 2-propanol/water mixtures. Successful transition-metal catalysts for transfer hydrogenations are based not only on iridium, rhodium or ruthenium ions but also on nickel [93], rhenium [94] and osmium [95]. It has been reported that... 

In the search to develop new materials for immobilization of homogeneous transition metal catalyst to facilitate catalyst-product separation and catalyst recychng, the study of dendrimers and hyperbranched polymers for application in catalysis has become a subject of intense research in the last five years [68], because they have excellent solubility and a high number of easily accessible active sites. Moreover, the pseudo-spherical structure with nanometer dimensions opens the possibility of separation and recycling by nanofiltration methods. Although dendrimers allow for controlled incorporation of transition metal catalysts in the core [69] as well as at the surface [70], a serious drawback of this approach is the tedious preparation of functionalized dendrimers by multi-step synthesis. 

Cycloisomerization of 1,6-diene 25 is effected by a number of transition metal catalysts. For example, both rhodium trichloride [22, 23] and Wilkinson s catalyst [24, 25] promote this reaction efficiently to give methylenecyclopentane 26 (Scheme 7.12). In the latter case, the active catalyst species is beheved to be [Rh(PPh3)2HCl2]. A mechanism proposed for this cycloisomerization is shown in Scheme 7.13. Coordination of a diene to [Rh(PPh3)2HCl2] and insertion of one of the olefin moieties of the diene into the [Rh]-H bond gives complex II.3a. Carbocychzation affords alkyl-[Rh] intermediate II.3i,. Subsequent reductive ehmination of the methylenecyclopentane regenerates the active catalyst species. 

This chapter does not intend to provide a complete collection of newly synthesized organometallic or coordination complexes for alkene polymerization, but rather aims to review a cross-section of transition metal catalysts from the viewpoint of polymers and polymerization reactions. We focus particularly on polymers that are difficult or virtually impossible to prepare using conventional catalysts. In this light, we narrow our attention to well-defined molecular catalysts, including a study of progress in the understanding of active species, reactive intermediates, and reaction mechanisms that are indispensable for the synthesis of such polymers. 

A very active field of research is the in situ generation of transition metal catalysts. After the fundamental work by Lehmkuhl et al. there have been very interesting new results... 

As discussed in Section 6.9 1, 3-dienes and dienophiles in which multiple bonds are not activated by electron-withdrawing or electron-releasing substituents fail to undergo cycloaddition except under the most severe conditions. Particular difficulty is encountered in the cycloaddition of two unactivated species since homodimerization can be a competitive and dominant reaction pathway. The use of transition-metal catalysts, however, has proved to be a valuable solution. Complexation of unactivated substrates to such catalysts promotes both inter- and intramolecular cycloadditions. Consequently, the cycloaddition of such unactivated compounds, that is, simple unsubstituted dienes and alkenes, catalyzed by transition metals is a major, important area of study.655 In addition, theoretical problems of the transformation have frequently been addressed in the more recent literature. 

The catalytic activities of transition metal sulphides were classified in the DHN of THT, the HDS of thiophene and the HN of BP. The results clearly evidence strong similarities between the activities of the most active catalysts in the three reactions, which implies that the catalytic sites involved in each of these reactions are comparable. 

Since the epoxidation step involves no formal change in the oxidation state of the metal catalyst, there is no reason why catalytic activity should be restricted to transition metal complexes. Compounds of nontransition elements which are Lewis acids should also be capable of catalyzing epoxidations. In fact, Se02, which is roughly as acidic as Mo03, catalyzes these reactions.433 It is, however, significantly less active than molybdenum, tungsten, and titanium catalysts. Similarly, boron compounds catalyze these reactions but they are much less effective than molybdenum catalysts 437,438 The low activity of other metal catalysts, such as Th(IV) and Zr(IV) (which are weak oxidants) is attributable to their weak Lewis acidity. 

In considering the effect of the electronic structure of catalysts on activity, Dowden (33) suggested that carbides, and similarly nitrides and carbonitrides, should be less active for synthesis than the corresponding metal since the interstitial atoms may contribute electrons to the unfilled d-shells of the metal, which are believed to be essential for the catalytic activity of transition metals in hydrogenation reactions. This hypothesis is supported by the low activity of cobalt carbide compared with that of reduced cobalt (28,29). For iron catalysts the hypothesis... 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

While commercial HDS catalysts are based on Mo, other transition metals are also active, some more active than Mo. In Figme 1 is a plot of the HDS activities of transition metal... 

Terminal alkenes react with CO and trialkylsilane in the presence of transition metal catalysts to give the corresponding silyl enol ethers. Ru3(CO)i2 and Co2(CO)g each show high catalytic activity [114]. Ru3H(CO)n [115] shows moderate catalytic activity with high selectivity for linear isomers (Eq. 11.62). 

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