Chiral metal complexes oxidation

Chiral sulfoxides have emerged as versatile building blocks and chiral auxiliaries in the asymmetric synthesis of pharmaceutical products. The asymmetric oxidation of prochiral sulfides with chiral metal complexes has become one of the most effective routes to obtain these chiral sulfoxides.We have recently developed a new heterogeneous catalytic system (WO3-30% H2O2) which efficiently catalyzes both the asymmetric oxidation of a variety of thioethers (1) and the kinetic resolution of racemic sulfoxides (3), when used in the presence of cinchona alkaloids such as hydroquinidine 2,5-diphenyl-4,6-pyrimidinediyl diether [(DHQD)2-PYR], Optically active sulfoxides (2) are produced in high yields and with good enantioselectivities (Figure 9.3). ... 

In 1995, Morgan et al. synthesized a layered aluminophosphate compound by using a chiral cobaltammine complex as the template for the first time.[61] Recently, the Jilin group has synthesized a number of 2-D layered and 3-D open-framework metal phosphates by using a racemic mixture or an optically pure chiral metal complex as the template, and has systematically studied the chirality transfer from the guest chiral complex templates to the host inorganic open frameworks.1901 Table 7.15 lists some metal phosphates and oxides with open-framework structures templated by optically pure or racemic cobalt ammine complexes. 

The BV transformations with chiral metal complexes are based on either transition metals such as Cu [37b, 40, 41], Pt [42, 43], Co [44], Pd [45], Zr [46], Hf [47] or nontransition metals such as Mg [48] and Al [49]. For example, Kocovsky and coworkers have developed a series of new terpene-derived pyridinephosphine ligands, whose complexes with Pd(II) have been proven to catalyze the BV oxidation. Prochiral cyclobutanones (1) were oxidized at low temperature (—40°C), with 5 mol % catalyst loading and the urea-H202 complex as the stoichiometric oxidant to give lactones 2 in good yields and up to 81% ee (Eq. (10.5)) [50],... 

In tenns of understanding the mechanistic aspects involved in such additimis on vinylic substrates via organometallic catalysts, analogies have been drawn to the hydroamination reactimis [28-30], Chiral metal complex-promoted asymmetric hydroaminations have been proposed to follow two different pathways. The first involves a sequence that commences with the oxidative addition of the N-H bond onto the metal ion followed by the insertion of the olefin and subsequent reductive elimination of the chiral substrate. An alternative pathway has also been proposed which involves the nucleophilic attack by the free amine on a coordinated olefin and a final protonolysis sequence, which leads to the release of the final product. Similar studies on metal irm-induced hydrophosphinations have been reported, and the mechanisms suspected to be in play include those proposed by Glueck and coworkers which basically involves the oxidative addition of a secondary phosphine followed by an olefin insertion [31], Togni and coworkers have also observed in certain scenarios the coordination of the olefin to the catalyst metal center followed by the addition of a secondary phosphine across the C-C double bond [32]. 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

The most well-developed recent examples of catalysis concern catalysts for oxidation reactions these are essentially achiral or chiral metal-salen complexes. Taking into account a number of results suggesting the importance of a degree of mobility of the bound complex, Sherrington et al. synthesized a series of polymer-supported complexes in which [Mn(salen)Cl] units are immobilized in a pendant fashion by only one of the aromatic rings, to polystyrene or poly(methacrylate) resin beads of various morphology (Figure 6).78,79... 

In 1965, Denney et al. (98) reported the reaction of a number of alkenes with ferf-butyl hydroperoxide (TBHP) and cupric salts of chiral acids. The use of ethyl camphorate copper complex 144 in the allylic oxidation of cyclopentene provides, upon reduction of the camphorate ester, the allylic alcohol in low yield and low selectivity, Eq. 82. The initial publication only provided the observed rotation of cyclopentenol, but comparison to subsequent literature values (99) reveals that this reaction proceeds in 12% ee and 43% yield (based on the metal complex). 

However, many of heterogenized chiral catalysts suffer from the leaching of the active metal or the chiral auxiliary into the solvent and from the decrease of e.s. Another problem associated with catalysts made from metal complexes that are attached to either a modified polymer or metal oxide surface is that the techniques used for their preparation are rather specific and are driven by the nature... 

Rogers MM, Stahl SS (2006) N-Heterocyclic Carbenes as Ligands for High-Oxidation-State Metal Complexes and Oxidation Catalysis. 21 21-46 Roland S, Mangeney P (2005) Chiral Diaminocarbene Complexes, Synthesis and Application in Asymmetric Catalysis. 15 191-229 Roll R, see Behr A (2008) 23 19-52... 

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