Transition metal catalysis oxidative coupling

Biaryls have also been prepared by coupling support-bound aryl halides with aryl-zinc compounds (Figure 5.20) or with aryl(fluoro)silanes [203]. As with Suzuki or Stille couplings, these reactions also require transition metal catalysis. An additional strategy for coupling arenes on solid phase is the oxidative dimerization of phenols (Figure 5.20). 

Vinyl stannanes also undergo oxidative homo-coupling under transition metal catalysis to result in dienes (equation An intramolecular version of this method... 

From C-H to C-C Bonds Cross-Dehydrogenative-Coupling 27 Renewable Resources for Biorefineries 28 Transition Metal Catalysis in Aerobic Alcohol Oxidation 29 Green Materials from Plant Oils... 

The possibilities for the formation of carbon-carbon bonds involving arenes have been dramatically increased in recent years by the use of transition metal catalysis. Copper-mediated reactions to couple aryl halides in Ulknann-type reactions [12, 13] have been known for many years, and copper still remains an important catalyst [14, 15]. However, the use of metals such as palladium [16,17] to effect substitution has led to such an explosion of research that in 2011 transition metal-catalyzed processes comprised more than half of the reactions classified as aromatic substitutions in Organic Reaction Mechanisms [18]. The reactions often involve a sequence outlined in Scheme 6.6 where Ln represents ligand(s) for the palladium. Oxidative addition of the aryl halide to the paiiadium catalyst is followed by transmetalation with an aryl or alkyl derivative and by reductive elimination to give the coupled product and legeuCTate the catalyst. Part 6 of this book elaborates these and related processes. 

To date, most research on NHCP transition-metal catalysis has been devoted to cross-couplings [13] or related reactions such as hydroarylation of alkenes [18], direct arylation of alkynes [17], or oxidative homocoupling of terminal alkynes [19]. All NHCP systems used in these studies feature one or two... 

There are other examples of oxidative coupling of metal-olefin complexes with early (Ti, Ta) - as well as late (Fe, Ni) - transition-metals. The metallocycles play an important role in organometallic chemistry and particnlarly in catalysis because of their ease of formation and rich reactivity (see Chap. 15). 

The elimination of a hydrogen atom positioned on a carbon to the central metal constimtes an important reaction in transition metal catalysis. In the classical example, an alkylmetal intermediate is reversibly converted to an alkene and a metaUiydride (scheme 1.12). Despite the fact, that the resulting hydridometal complex can be exploited in various catalytic processes including polymerization reactions, [57] cycloisomerizations, [58] annulations, [59] etc., the ]S-hydride elimination is often considered undesired in transition metal catalyzed cross couplings. Thus, efforts have often been concentrated towards the prohibition of this fundamental reaction [60]. Nevertheless, the ]S-hydride elimination is a vital transformation in a number of catalytic processes including the ene-yne coupling reported by Trost [61] and Skrydstrup, [62] oxidation of alcohols, [63] the Heck reaction etc [64]. 

In this chapter, we mainly discussed, selective industrial scale organic synthetic applications on C-C and C-0 bond forming ractions involving transition metal catalysis and asymmetric catalysis. It focused on recent advancements of catalytic reactions developed by Nobel laureates, viz., cross-coupling reactions (Heck, Suzuki and Negishi), asymmetric oxidation (Sharpless), metathesis (Grubbs ) and two other noteworthy reactions, Jacobsen and Shi ep-oxidations. 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

BINOL and its derivatives have been utilized as versatile chiral sources for asymmetric catalysis, and efficient catalysts for their syntheses are, ultimately, required in many chemical fields [39-42]. The oxidative coupling of 2-naphthols is a direct synthesis of BINOL derivatives [43, 44], and some transition metals such as copper [45, 46], iron [46, 47] and manganese [48] are known as active metals for the reaction. However, few studies on homogeneous metal complexes have been reported for the asymmetric coupling of 2-naphthols [49-56]. The chiral self-dimerized V dimers on Si02 is the first heterogeneous catalyst for the asymmetric oxidative coupling of 2-naphthol. 

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