Late transition metal-catalyzed

Late Transition Metal-catalyzed Polymerization of Ethylene... 

So far, progress on the late transition metal-catalyzed reactions utilizing S-H bond activation has been surveyed. Finally, the recent advancement of chiral Lewis... 

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... 

Recent Progress in Late Transition Metal-Catalyzed Polymerization... 

Late transition metal-catalyzed processes also proved to be very useful tools for formation of the C-O bond of the 1,3-oxazine ring from the corresponding alkynes. In the presence of 1-5 mol% of a cationic gold(l) complex, A -BOC-protected alkynylamines 450 were converted to 6-alkylidene-l,3-oxazin-2-ones 451 under very mild conditions (Equation 49) <2006JOC5023>. 

MMA can function as a chain-transfer agent for late transition metal-catalyzed ethylene polymerization. Gibson and Tomov revealed that Ni complexes with bulky phosphino-enolate ligands, F13-17, were capable of polymerizing ethylene in the presence of MMA to afford MMA-end-capped PEs through immediate /3-H transfer after MMA 337... 

By far the most common way for organic molecules to enter late transition metal catalyzed reactions is oxidative addition. In this process a low valent palladium(O)3 or nickel(O) atom inserts into a carbon-heteroatom bond, usually of an aryl halide or sulfonate (Figure 1-2). The formation of the carbon-metal bond is accompanied by an increase in the oxidation number of the metal by 2. There are a series of factors determining the speed of the process. 

In a series of late transition metal catalyzed processes the first step in the catalytic cycle is the coordination of the reagent to the metal atom, which is in a positive oxidation state, followed by its covalent attachment through the concomitant breaking of an unsaturated carbon-carbon bond or a carbon-hydrogen bond. These processes usually require a highly electrophilic metal centre and are frequently carried out in an intramolecular fashion. The carbometalation processes that follow a similar course, but take place only at a later stage in the catalytic cycle, will be discussed later. 

Probably the most common attachment reaction in a late transition metal catalyzed reaction is transmetalation. This reaction, depicted in Figure 1-6, is the reversible exchange of covalently bonded ligands between two metal centres. The placement of the equilibrium is usually determined by the difference between the thermodynamic stability of the sacrificed and the formed bonds. From the practical point of view the placement of the equilibrium is less interesting as long as it is able to provide enough of the transmetalated complex for the follow up reaction. 

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. 

Probably the most common detachment step in late transition metal catalyzed processes is reductive elimination. In this transformation two groups, that are both attached to the same metal centre, will be released and form a covalent bond, with the concomitant formation of a metal whose formal oxidation state, coordination number and electron count are decreased by two. Figure 1-9 presents a general order of the ease of reductive elimination for the most common complexes. 

The use of this classification might help to identify the ways a selected compound might participate in late transition metal catalyzed transformations, and might also help to establish potential reaction partners. Although Table 2 suggests that there is an abundance of potential reactions for a given substrate, we have to emphasize that certain classes are well represented in the synthetic literature (e.g. 3, 8, 11, 17), while for other classes there are only a very limited number of examples. 

Triggered by the developments in late transition metal catalyzed polymerization, new catalyst systems were described very recently for the oligomerization of ethene. Nickel and palladium complexes based on a-diimine ligands 13 and imi-nophosphines 14 were reported to be very active and selective oligomerization catalysts [57, 58], Activation of the Ni(II) diimine halides with a large excess of MAO (210 equiv.) leads to oligomerization catalysts with activities of between... 

This extensive section deals with the late transition metal-catalyzed homopoiymerization of cyclic olefins to afford cycloaliphatic polymers, or saturated polymers in which the cyclic structure of the monomer remains intact in the polymer backbone. 

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. 

The group of Busch also presented evidence that the Mn(n) complex of a cross-bridged cyclam ligand, 4,ll-dimethyl-l,4,8,ll-tetraazabicyclo[6.6.2]hexadecanc, denoted as Mn"(Me2EBC)CL, forms a Mn(IV) adduct with lodosylbenzene which is a new active intermediate in epoxidation reactions [600,609,610]. These examples with Mn, and in the previous section with Fe, are instances of late transition metals catalyzing epoxidation reactions by both the redox and the Lewis acid mechanisms (see Chapter 3). 

Earher mechanistic studies by Milstein on a achiral Ir catalyst system indicated that the iridium catalyzed norbornene hydroamination involves amine activation as a key step in the catalytic cycle [27] rather than alkene activation, which is observed for most other late transition metal catalyzed hydroamination reactions [28]. Thus, the iridium catalyzed hydroamination of norbornene with aniline is initiated by an oxidative addition of aniline to the metal center, followed by insertion of the strained olefin into the iridium amido bond (Scheme 11.4). Subsequent reductive elimina tion completes the catalytic cycle and gives the hydroamination product 11. Unfor tunately, this catalyst system seems to be limited to highly strained olefins. 

Eq. 1 Late transition metal-catalyzed polymerization reaction of MeH2SiSiH2Me. 

More often in organometallic chemistry, the catalytic reaction occurs by a mechanism that is completely different from the mechanism of the uncatalyzed process. In this case, the reaction typically occurs by more steps, but the activation energy of each of the individual steps is lower than the activation energy of the imcatalyzed process. The overall barrier is then lower than that of the uncatalyzed reaction. A comparison of the uncatalyzed and catalyzed hydroboration of alkenes with a dialkoxyborane (ROl BH, such as cat-echolborane (see Chapter 16), illustrates this scenario. Qualitative reaction coordinates for tihe uncatalyzed and rhodium-catalyzed process are shown in Figure 14.4. In the absence of a catalyst, the B-H bond adds across the alkene through a concerted four-center transition state, albeit at elevated temperatures in neat alkene. hi contrast, late transition metal-catalyzed hydroborations first cleave the B-H bond by oxidative addition. Coordination... 

Late transition-metal-catalyzed asymmetric Claisen rearrangement takes place in a different mode from that of Lewis-acid-catalyzed Claisen rearrangement Late transition metal catalysis is based on affinity for the Claisen diene system. Among late transition metals, palladium complexes are the most useful and effective for the Claisen rearrangement. 

In the recent years, all new mechanistic implications on the late transition metal-catalyzed hydrosilylation of olefins reported involve ruthenium complexes as model transition metal centers of molecular catalysis (86-88). 

Scheme 15.14 General mechanism of late-transition-metal-catalyzed hydroamination with nucleophilic attack on neutral rt-complexes.

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