Use of late transition metals

Abstract Significant advances have been made in the study of catalytic reductive coupling of alkenes and alkynes over the past 10 years. This work will discuss the progress made in early transition metal and lanthanide series catalytic processes using alkyl metals or silanes as the stoichiometric reductants and the progress made in the use of late transition metals for the same reactions using silanes, stannanes and borohydrides as the reductant. The mechanisms for the reactions are discussed along with stereoselective variants of the reactions. 

The use of late transition metals as olefin polymerization catalyst requires the suppression of chain transfer while at the same time a high chain growth rate should be maintained. These new catalysts have an electron-deficient, in most cases, 14-electron and cationic metal center with a vacant coordination site. The most... 

Whereas Trost and Takano established the usefulness of late transition metals and their complexes for the construction of the dendrobine skeleton, Mori et al. demonstrated the advantage of the more stable early transition-metal complexes in the synthesis of dendrobine (82) (167-169). They presented their formal EPC-synthesis centered on Negishi s (Taber s) zirconium-assisted cyclization in several publications (189,190). Mori et al. tried to test their key step with a model synthesis (191). 

The addition copolymerization of norbornene-type monomers with a-olefins [21] forms the basis of EPDM (ethylene propylene diene monomer) technology. Incorporation of smaU amounts of DCPD or ethylidene norbornene (ENB) in olefinic vinyl addition polymers provides latent crosslink sites in EPDM elastomers. It is weU known in the hterature that incorporation of higher amounts of rigid, bulky multicychc olefins results in materials with higher TgS [22]. In fact, more recent work has concentrated on increasing the Tg of norbornene-type monomer/a-olefin copolymers [23]. The use of late transition metal catalysts to prepare such copolymers is reviewed in Section 4.3. 

Functionalized polyethylene would be of great industrial importance, and if synthetic methods to control the microstructure of functionalized polymers using transition-metal-based catalysis are developed, it would significantly broaden the utility and range of properties of this class of polymers. Recent progress in the field of late transition metal chemistry, such as Brookliart s use of nickel-based diimine catalysts, has enabled the copolymerization of ethylene with functional a-olefins.29 However, these systems incorporate functionalized olefins randomly and with limited quantity (mol percent) into the polymer backbone. 

Such calculations have also been performed for isolated impurities of late transition metals alloyed into the surface of other transition metals, and the trends are the same. The accuracy of the numbers in Fig. 6.33 is limited since many approximations had to be made to calculate them. Nevertheless, they reflect trends very well and give useful insight into reactivity trends that have actually been measured for a number of pseudomorfic overlayers [J.A. Rodriquez and D.W. Goodman, Science 257 (1992) 897]. 

The cationic imidazolium rhodium complex (56) has been found to catalyze the intramolecular hydroamination of alkynes in refluxing THF. In the case of 2-ethynylaniline, indole is formed in 100% yield over 9h at 55 °C (Scheme 38).173 One of the earliest examples of late transition metal-catalyzed hydroamination involved the use of the iridium(I) complex [Ir(PEt3)2(C2H4)Cl] as... 

This chapter will provide an overview of the development and use of early transition-metal complexes in hydrogenation, and in consequence has been divided into several sections. Section 6.2 will focus on the mechanistic differences in the hydrogenation reaction between early and late transition metals. The following three sections will describe the various systems based on Group IV (Sec-... 

Ferraris et al.108 demonstrated an asymmetric Mannich-type reaction using chiral late-transition metal phosphine complexes as the catalyst. As shown in Scheme 3-59, the enantioselective addition of enol silyl ether to a-imino esters proceeds at —80°C, providing the product with moderate yield but very high enantioselectivity (over 99%). 

Carbonylation of unsaturated substrates has been known for decades but the reaction selectivity has been progressively improved by tuning the coordination sphere of late transition metal-based catalysts. Palladium assumes a privileged place in this chemistry and its versatility allows the use of mild conditions for the selective incorporation of CO into acyclic and cyclic compounds. Further improvements open a path to more sophisticated reactions, particularly cascade reactions. Similarly, asymmetric versions of most of these carbonylations can be envisioned. Atom economy and the green character of the process will probably be the key criteria for evaluating any new catalytic system. 

In contrast to the free-radical polymerizations, there have been relatively few studies on transition metal catalysed polymerization reactions in water. This is largely due to the fact that the early transition metal catalysts used commercially for the polymerization of olefins tend to be very water-sensitive. However, with the development of late transition metal catalysts for olefin polymerizations, water is beginning to be exploited as a medium for this type of polymerization reaction. For example, cationic Pd(II)-bisphosphine complexes have been found to be active catalysts for olefin-CO copolymerization [21]. Solubility of the catalyst in water is achieved by using a sulfonated phosphine ligand (Figure 10.5) as described in Chapter 5. 

Using established principles of late-transition metal catalysis, several research groups have engineered multi-component coupling reactions from the basis set of known Group 9 metal vinylidene-mediated reactions. In 2004, Jun and coworkers described a new method for the synthesis of enones via rhodium vinylidene-mediated hydrative dimerization of alkynes (Table 9.12) [24]. 

Because of their frequent use, some late transition metal catalyzed carbon-carbon bond forming reactions evolved into name reactions. The most prominent examples are cross-coupling reactions, where distinction is usually made on the basis of the transmetalating agent used. The common mechanism of cross-coupling reactions and its name variants are discussed in Chapter 2.1. 

Tilley and coworkers reported a more direct procedure for the synthesis of late transition metal silylenoid complexes by reaction with silanes [equation (7.1)].36,37 Exposing iridium complex 14 to dimesitylsilane afforded iridium silylenoid 15.36 In addition to dimesitylsilane, other silanes could be used, including diphenyl-, diethyl-, or dimethylsilane. Primary silanes such as mesitylsilane and 2,4,6-triisopropylphe-nylsilane were also tolerated as substrates.37... 

It is possible to use ancillary ligands in addition to phosphonic acids in building up nanosized cluster compounds of late transition metal ions. Thus, the reaction of CuCl2 with tert-butylphosphonic acid in the presence of 3,5-dimethylpyrazole affords a dodecanuclear copper phosphonate with an interesting cage structure,3 Similarly, large vanadium phosphonate clusters with up to 18 vanadium atoms have been assembled from phosphonic acids.35... 

In particular, Schrock-type catalysts suffered from extreme moisture and air sensitivity because of the high oxidation state of the metal center, molybdenum. Due to the oxophilicity of the central atom, polar or protic functional groups coordinate to the metal center, poisoning the catalyst and rendering it inactive for metathesis. Since late transition metal complexes are typically more stable in the presence of a wide range of functionalities, research was focused on the creation of late transition metal carbene complexes for use as metathesis catalysts. 

One exception to this exclusion of late transition metals may be copper. CuH—MgXa complexes, prepared in situ from MgH2/CuX or from NaH/CuX/MgX2, react with terminal alkynes to give alke-nylcopper species. So far, these have only been used as sources for dialkenyl-coupling products, as in equation (54), but there is no obvious reason why other copper-based procedures should not be accessible, as was found for alkenylcopper obtained by transmetallation from Zr (Section 3.9.3.4.2). Al-kylcopper cannot be made by hydrometallation (nor by transmetallation), as CuH does not add to alkenes, except for special ones such as enones. ... 

Decamethylferrocene has also been used as a donor for the formation of CT complexes with inorganic acceptors. Such acceptors are mainly composed of late transition metal complexes containing planar ligands. Some of these acceptors are illustrated in Scheme 8-3. As for their organic counterparts, the inorganic partners have at least two reversibly accessible oxidation states. The reduced form present in the CT complex is usually a radical anion. 

Brookhart and co-workers [79-81] introduced catalysts based largely on chelating, nitrogen-based ligands that are active for the homopolymerization of ethylene and the copolymerization of ethylene with 1-olefins and polar comonomers (31). Ni, Co, Fe or Pd are used as late transition metals. The diimine ligands have big substituents to prevent 6-hydride elimination. Ni(II) or Pd(II) complexes form cations by combination with MAO and polymerize ethylene to highly branched polymers with molecular weights up to one million. The activities reach TON... 

Surprisingly there have been no reports of late transition metal-catalyzed addition polymerization of cyclobutenes. Since the homopoiymerization of cyclobutene using metallocene catalysts is exemplified in the literature [16] it is only a matter of time before a report of a cationic palladium or nickel catalyst for the polymerization appears in the literature. 

Most commonly used chiral Lewis acids have been derived from main group and early transition series elements. An initial attempt at utilizing optically active catalysts of late transition metal complexes for the enantioselective addition of allyltributylstannane to aldehydes was made by Nuss and Rennels [30]. Employment of Rh(COD)[(-)-DIOP]BF4 (11) as a catalyst, however, resulted in only a small degree of asymmetric induction (17% ee). 

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