Catalyzed selective oxidations control

The first example of Pd-catalyzed enantioselective allylation to be reported was the reaction of l-(l -acetoxyethyl)cyclopentene and the sodium salt of methyl benzenesulfonylacetate in the presence of 10 mol % of a DIOP-Pd complex, which led to the condensation product in 46% ee (Scheme 85) (200). This reaction used a racemic starting material, but the enantioselection was not a result of kinetic resolution of the starting material, because the chemical yield was above 80%. However, in certain cases, the selectivity is controlled at the stage of the initial oxidative addition to a Pd(0) species. In a related reaction, a BINAP-Pd(0) complex exhibits excellent enantioselectivity the chiral efficiency is affected by the nature of the leaving group of the allylic derivatives (Scheme 85) (201). It has been suggested that this asymmetric induction is the result of the chiral Pd catalyst choosing between two reactive conformations of the allylic substrate. 

In industry many selective oxidations are carried out in a homogeneously catalyzed process. Heterogeneous catalysts are also applied in a number of processes, e.g. total combustion for emission control, oxidative coupling of methane, the synthesis of maleic acid from butanes, the epoxidation of ethylene. Here we focus upon heterogeneous catalysis and of the many examples we have selected one. We will illustrate the characteristics of catalytic oxidation on the basis of the epoxidation of ethylene. It has been chosen because it illustrates well the underlying chemistry in many selective oxidation processes. 

Oxidative transformations driven by oxygen and catalyzed by transition metal complexes play a very important role in the control of selective oxidations. The role of the catalyst in oxidations is to interrupt the pathway leading to the most favorable thermodynamic products, water and carbon dioxide, and to provide a low temperature pathway for the controll formation of the desired product. Monsanto has commercialized several 02-driven oxidations which illustrate extremely well the principle of providing a selective pathway to desired products. 

The success of this triple catalytic system relies on a highly selective kinetic control. From a thermodynamic point of view, there are 10 possible redox reactions that could occur in this system. However, the energy barrier for six of these (O2 + diene, O2 + Pd(0), etc.) are too high, and only the kinetically favored redox reactions shown in Scheme 11.14 occur. A likely explanation for this kinetic control is that the barrier is significantly lowered by coordination. Thus, the diene coordinates to Pd(II), BQ coordinates to Pd(0), HQ coordinates to (ML,), and Oj coordinates to ML ,. In a related system for aerobic oxidation, a heteropolyacid was employed in place of the metal macrocyclic complex (ML ,) as oxygen activator and electron transfer mediator [72]. Recent immobilization of the macrocyclic complex in ZeoHte-Y, led to eflBcient reoxidation of the HQ in the palladium-catalyzed 1,4-diacetoxylation [73]. 

Iodine-catalyzed and solvent-controlled selective method for the synthesis of iodopyrano[4,3-6]quinolines and o-alkynyl esters from o-alkynyl aldehydes in mild reaction conditions is developed for the first time by Verma et al. This novel oxidative esterification provides a powerful tool for the preparation of a wide range of functionalized pyranoquinolinones as well as isocoumarins, and it is applicable for a variety of functional groups including primary alcohol, carboxyl, and methoxy groups. 

By combining heteropolyanions (polyoxometalates POMs for convenience) and selected cations in acetonitrile, more than 150 combinations were assayed for their catalytic activity towards selective CEES oxidation to CEESO by dioxygen under ambient (room temperature and atmospheric pressure) conditions. The main criteria in choosing POMs were their ability to undergo reversible redox transformations and to catalyze homogeneous oxidations either by peroxides or other terminal oxidants. The cations chosen included redox-active transition metal ions or cations conventionally used as the counterions in POMs. In control experiments the chloride, nitrate or perchlorate salts of the same transition metal ions were also examined. The list of these catalytic systems and some selected results were recently published. ... 

For the reactions of other 1,3-dipoles, the catalyst-induced control of the enantio-selectivity is achieved by other principles. Both for the metal-catalyzed reactions of azomethine ylides, carbonyl ylides and nitrile oxides the catalyst is crucial for the in situ formation of the 1,3-dipole from a precursor. After formation the 1,3-di-pole is coordinated to the catalyst because of a favored chelation and/or stabiliza-... 

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

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