Allylic substitution metal-mediated reactions

The concept of C2-symmetric ligands has widely been recognized as an ideal design of asymmetric ligands, which include DIOP, chiraphos, and BINAP. These ligands have been applied to a variety of transition metal-catalyzed asymmetric reactions and have been fairly successful. However, this situation is not always applied to rr-allylpalla-dium-mediated asymmetric allylic substitutions. In the reaction, which has been the most frequently examined asymmetric reaction catalyzed by 7r-allylpalladium complexes, two factors need to be controlled for the sake of high stereoselectivity. One is... 

Metal-mediated carbonyl allylation, allenylation, and propargylation of optically pure azetidine-2,3-diones were investigated in aqueous environments.208 Different metal promoters showed varied regioselec-tivities on the product formation during allenylation/propargylation reactions of the kcto-fi-lactams. The stereochemistry of the new C3-substituted C3-hydroxy quaternary center was controlled by placing a chiral substituent at C4. The process led to a convenient entry to densely functionalized hydroxy-ji-lactams (Eq. 8.82). 

The transition metal-catalyzed allylation of carbon nucleophiles was a widely used method until Grieco and Pearson discovered LPDE-mediated allylic substitutions in 1992. Grieco investigated substitution reactions of cyclic allyl alcohols with silyl ketene acetals such as Si-1 by use of LPDE solution [95]. The concentration of LPDE seems to be important. For example, the use of 2.0 M LPDE resulted in formation of silyl ether 88 with 86 and 87 in the ratio 2 6.4 1. In contrast, 3.0 m LPDE afforded an excellent yield (90 %) of 86 and 87 (5.8 1), and the less hindered side of the allylic unit is alkylated regioselectively. It is of interest to note that this chemistry is also applicable to cyclopropyl carbinol 89 (Sch. 44). 

Qualitatively, most metal-mediated allylation reactions in aqueous medium display similar regio- and stereoselectivities. Such studies have been carried out most extensively on the indium-mediated allylations. For regioselectivities of the allylic moiety, both electronic and steric effects are important. Usually, the carbon-carbon bond formation occurs at the more substituted carbon of the allyl halide, irrespective of the position of halogen in the starting material (Eq. 4.28). However, the carbon-carbon bond forms at the less substituted carbon when the y-substituents of allyl halides are large enough (e.g. trimethylsilyl or tert-butyl) as shown by Chan and Isaac. 

Microporous crystalline solids in which transition metals are tetrahedrally substituted via template-mediated hydrothermal synthesis have remarkable properties in selective oxidation reactions [66]. Unfortunately, the microporous structure and the rigidity of the crystalline frameworks limit the substitution degree, variety of substituted metal, and their general applicability. For this reason, the amorphous microporous mixed oxides (AMMs) with uniform microporosity and wide compositional variability are devoid of Bronsted acidity, but are not associated with the redox-active elements. However, metals such as Ti, V, and Mo incorporated in amorphous silica were good catalysts in allylic oxidation or epoxidation of olefins [67]. 

Schobert and co-workers have performed a study on 77 -alkene and 77 -allylcarbene complexes of transition metals. This particularly includes studies on the chemistry of metallacyclic alkenyl complexes of iron, which are reviewed in this chapter. An example of the rf if transformation is shown in Scheme 15. Iron-mediated allylic substitution reactions involve rf rf interconversion and can show chirality transfer.A working hypothesis involves the reaction shown in Scheme 16. 

Four reviews of allylic substitution reactions have been published. The first covers the metal-mediated allylic substitution reactions in water, the second discusses the mechanisms and scope of iridium-catalysed asymmetric allylic substitution reactions, the third ° reviews the development and use of iridium salt-phosphoramidite ligand catalysts for enantioselective allylic substitution reactions, and the fourth covers the transition metal-catalysed reactions of allylic alcohols. Special attention is focussed on the ar-allyl metal intermediates and their influence on the regio-, stereo-, and enantio-selectivities of these reactions. 

Abstract Chiral ferrocenyl phosphine ligands are certainly one of the most developed and successful classes of chiral ligands used in asymmetric catalysis. The literature describing their synthetic and coordination chemistry, as well as their metal-mediated applications in the field of catalysis, is extremely rich and varied. Moreover, they represent a rare example in which enantioselective chemical catalysts were used in industrial processes. The present chapter provides an account of the planar-chiral ferrocene ligands developed in the Authors laboratory, including their coordination chemistry with various metals as well as their use in different asymmetric catalytic reactions (allylic substitution, Suzuki coupling, methoxycarbonylation of alkenes, hydrogenation of ketones). 

By far the most generally useful synthetic application of allyltributyltin is in the complementary set of transition metal- and radical-mediated substitution reactions. When the halide substrates are benzylic, allylic, aromatic or acyl, transition metal catalysis is usually the method of choice for allyl transfer from tin to carbon. When the halide (or halide equivalent) substrate is aliphatic or alicyclic, radical chain conditions are appropriate, as g-hydrogen elimination is generally not a problem in these cases. 

In addition, silver-catalyzed asymmetric aza-Diels-Alder reactions provide a useful route to optically active nitrogen-heterocyclic compounds such as piperidines or pyrid-azines. Substituted dihydrobenzofurans can also be enantioselectively prepared through silver-promoted allylation of aldehydes. Other types of silver-mediated cyclizations can also be used in the synthesis of tetrahydrofnrans, tetrahydropyrans, 1,2-dioxetanes, 1,2-dioxolanes, medium-sized lactones, dihydroisoqninolines, and so on. Silver salts can also be used as cocatalysts with other transition metals. Unique activity was observed for these silver-based systems in several cases. Conseqnently, the use of silver can enrich several available heterocyclization methods, and fnrther developments in the application of chiral silver complexes will hopefnlly appear in the near future. 

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