Oxidation reactions, transition-metal resolution

Further resolution of the details of oxidative dehydrogenation requires the measurement of a catalyst s degree of reduction carried out during steady state reaction. We note that UV-visible spectroscopy offers a way to perform this measurement since many of the transition metal oxides which are active as oxidation catalysts exhibit striking color changes between their oxidized and reduced states. 

Synthesis of chiral atropisomeric diphosphine type ligands is a current challenge in chemical research because their late transition metal complexes usually provide high enantioselectivity in homogenous catalytic reactions. [28] In practical point of view, preparation and optical resolution of racemic diphosphine oxides followed by the reduction of the separated enantiomers are usually more advantageous than an expensive enantioselective synthesis of one diphosphine enantiomer. [29]... 

Preparative Methods racemic l,l -bi-2,2 -naphthol (BINOL) is most conveniently prepared by the oxidative coupling reaction of 2-naphthol in the presence of transition metal complexes (eq 1). The resolution of racemic BINOL with cinchonine may be performed via the cyclic phosphate (eq 2). An alternative procedure to provide directly optically active BINOL is the oxidative coupling of 2-naphthol catalyzed by Cu salt in the presence of chiral amines (eq 3). The best procedure uses (+)-amphetamine as the chiral ligand and provides BINOL in 98% yield and 96% ee. Above 25 °C the Cu /(+)-amphetamine/(5)-BINOL complex precipitates, while the more soluble Cu /(+)-amphetamine/(I )-BINOL complex is slowly transformed into the former complex. 9,9 -Biphenanthrene-10,10 -diol has also been prepared in 86% yield and with 98% ee by a similar asymmetric oxidative coupling of 9-phenanthrol in the presence of (I )- 1,2-diphenylethylamine. ... 

Since the number of atoms on the surface of a bulk metal or metal oxide is extremely small compared to the number of atoms in the interior, bulk materials are often too costly to use in a catalytic process. One way to increase the effective surface area of a valuable catalytic material like a transition metal is to disperse it on a support. Figure 5.1.5 illustrates how Rh metal appears when it is supported as nanometer size crystallites on a silica carrier. High-resolution transmission electron microscopy reveals that metal crystallites, even as small as 10 nm, often expose the common low-index faces commonly associated with single crystals. However, the surface to volume ratio of the supported particles is many orders of magnitude higher than an equivalent amount of bulk metal. In fact, it is not uncommon to use catalysts with 1 nm sized metal particles where nearly every atom can be exposed to the reaction environment. 

P-chiral phosphines, which are potential ligands for transition metal-catalyzed reactions, were synthesized through hpase-catalyzed optical resolution of the corresponding racemic phosphine oxide compounds (Fig. 10.29). For example, lipase from C. rugosa (CRL) was used for the enantioselective hydrolysis of acetoxynaphthyl phosphine oxide (Fig. 10.29(a)). The P-enantiomer was hydrolyzed selectively, leaving the (S)-acetoxy compound, which was further subjected to chemical hydrolysis. Both enantiomeric phosphine oxides were obtained in >95% after recrystallization. Methylation followed by reduction with triethyl amine/trichlorosilane, with inversion of configuration, yielded the desired chiral phosphine. 

Other examples include OKR of racemic secondary alcohols (Scheme 25A), oxidative desymmetrizations of meso-diols, etc. The kinetic resolution is generally defined as a process where two enantiomers of a racemic mixture are transformed to products at different rates. Thus, one of the enantiomers of the racemate is selectively transformed to product, whereas the other is left behind. This method allows to reach a maximum of 50% yield of the enantiopure remaining sec-alcohol. To overcome this fim-itation, a modification of the method, namely dynamic kinetic resolution (DKR), was introduced. In this case, the kinetic resolution method is combined with a racemization process, where enantiomers are interconverted while one of them is consumed (e.g., by esterification. Scheme 25B). Therefore, a 100% theoretical yield of one enantiomer can be reached due to the constant equifibrium shift. In most of the proposed DKR processes, several catalytic systems, e.g., enzymes and transition-metal catalysts, work together. Both reactions (transfer hydrogenation of ketones and the reverse oxidation of secondary alcohols using ketone as a hydrogen acceptor) can be promoted by a catalyst. The process can involve a temporary oxidation of a substrate with hydrogen transfer to a transition-metal complex. 

The direct epoxidation of terminal aromatic alkenes using classic chemocatalysts, such as Jacobsen s salen transition metal catalysts, suffer from poor stereoselectivity ( -36%ee) [95] and/or imprachcal reaction temperatures of -78 °C (86% ee) [96]. To achieve opfically pure sfyrene oxide, a more universal approach is to use hydrolytic kinetic resolution following unselective epoxidation reactions, which leads to a product with high optical purity but only 50% yield [11]. 

---