Transition metal catalysis, initiators

One of the success stories of transition metal catalysis is the rhodium-complex-catalyzed hydrogenation reaction. Asymmetric hydrogenation with a rhodium catalyst has been commercialized for the production of L-Dopa, and in 2001 the inventor, Knowles, together with Noyori and Sharpless, was awarded the Nobel Prize in chemistry. After the initial invention, (enantioselective) hydrogenation has been subject to intensive investigations (27). In general, hydrogenation reactions proceed... 

Ring-opening polymerization is one of the most important applications of SCBs in organic chemistry. Polymerization of SCBs, which gives rise to carbosilane polymers, has been carried out thermally, by transition metal catalysis, or, most commonly, by anionic initiation. Thermal polymerization is rare, however, and is not covered in this chapter. For leading references into thermal polymerization of SCBs, refer to <1996CF1EC-II(1B)867> and <1995COMC-II(2)50>. 

The reaction is catalysed by many transition-metal complexes, and a mechanism for the hydrosilylation of an alkene under transition-metal catalysis is depicted in Figure Si5.7. Initial coordination of the alkene to the metal is followed by cis addition of the silicon-hydrogen bond. A hydride migratory insertion and elimination of the product silane complete the cycle. 

A first study on the combination of transition metal catalysis with radical chemistry was published in 2002 by Ryu [158], Under CO pressure (40 atm), and in the presence of a palladium catalyst, cyclopentanones were formed from 4-pentenyl iodide in a photochemically initiated reaction. 

Fluoride activation of Si-C bonds toward electrophiles has recently been exploited to synthesise alternating thiophene-perfluoroarene copolymers without using transition metal catalysis. This has the advantage of leading to products that are devoid of even traces of metal residues <2006JA2536>. Here the electrophiles are perfluoroarenes (ttF) the potential nucleophilic sites are the 2- and 5-positions of 3,4-dibutoxy-2,5-bis(trimethylsilyl)thiophene. The reaction is initiated with catalytic fluoride ion, which is regenerated with each C-C bond formed (Equation 106). 

Other fused thiiranes are more thermally stable. For example, the compound 45 desulfurizes at 160 °C (Scheme 11) <1987JA3801, 1990JA3029>. The reaction provided alkenes with retention of stereochemistry. In the presence of a desulfurizing agent such as phosphines or phosphites, the desulfurization temperature may be decreased to room temperature (Section 1.06.6.6) <1984CHEC(7)131>. The desulfurization of thiiranes is also subjected to transition metal catalysis. Since the initial step involves coordination of the metal to thiirane sulfur atom, the topic is discussed further in Section 1.06.6.4. 

Only some of the syntheses with acetylene which are practiced industrially are carried out with homogeneous metal catalysis initially it was predominantly the late transition metals Fe, Co, Ni, Cu, Zn, Cd, Hg, and Pd which were employed. In recent times the use of the noble metals Rh, Ru, Pd, and Pt has opened up a multiplicity of new reaction possibilities for alkynes. These methods have quickly found their way into the toolkit of the synthetic organic chemist. 

The metallocarbene intermediates are most often formed from thermal, photolytic, or metal-catalyzed deconposition of diazocarbonyl compounds, with concomitant loss of dinitrogen. Under transition metal catalysis, the initially formed species is a metallocarbene rather than a free carbene, and this is usually desirable due to the moderated reactivity (and, hence, fewer undesired side reactions) of the metal-complexed carbene. The two most common methods for introduction of the diazo group are acylation of diazoalkanes with suitably activated carboxylic acid derivatives and diazo transfer reactions in the case of more acidic active methylene substrates fScheme 16.12T... 

Electrochemical functionalization methods 3 Electrochemical reaction 6 Photochemical functionalization methods 4 Photochemical reaction 6 Silicon-carbon bond formation 1 Thermal functionalization methods 3 Thermal-Grignard and organolithium reagents 5 Thermal-heat reaction 3 Thermal-hydride abstraction 5 Thermal-Lewis acid catalysis 4 Thermal-radical initiators 3 Thermal-transition metal catalysis 4... 

However, VDF-IDT-CRPs always require a free radical source (e.g., butyl peroxide (TBPO)), as direct metal catalyzed initiation from perfluoroiodides or any other halides is not available. This is a serious drawback with respect to the precise synthesis of block or graft copolymers based on FMs, as such systems would inevitably lead to mixtures of homo- and copolymers with the current technology. Therefore, the availability of an initiation method directly from halides, most likely mediated by transition metal catalysis, would be highly valuable. 

Substitutions of MeSiCls can also be performed under radical conditions or with transition metal catalysis.The radical reactions can be initiated either by higher temperature or photolysis. However, the selectivity of the reaction is generally poor. Better results were obtained with transition metal-catalyzed reactions. It was reported that MeSiCl3 reacted with terminal alkynes in the presence of a catalytic amount of copper(I) chloride to form alkynyldichlorosilanes (eq 5). ... 

In this rapidly growing field of asymmetric catalysis [3,4], the use of chiral Lewis acid catalysts has been well appreciated by us during the past three decades [5-7]. Although we treat transition-metal catalysis separately from Lewis acid catalysis, it should be noted that, as long as electron pair donors/acceptors are involved, the interactions between transition metals and corresponding substrates are always Lewis acid/Lewis base interactions and thus, any electron pair acceptor catalyst initiated asymmetric reaction could be regarded as chiral Lewis acid catalyzed reaction in its broadest sense. 

Mechanistically, the Duan group suggests that diphenylphosphine oxide is first reacted with diphenylacetylene under the catalysis of transition metals. The initial assumption is to utilize the coordination ability of the P-atom toward transition metals for the formation and functionalization of possible metallacycle intermediates via ortho-C-H bond activation. An unusual aryl migration on the P-atom derived from a C-P bond cleavage and a new C-P bond formation was also observed and demonstrated to proceed via the radical process (Scheme 9.39). 

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