Catalytic variation

A catalytic variation of the cobalt method is the vitamin BI2a-mediated reaction to provide C-glycosides [24], The intermediate in this chain mechanism is the sugar cobalamine 45 formed by reaction of the precursor 43 with vitamin B 2a 44 (the brackets in Scheme 10 represent the corrin system) [25]. 

Some enzymes—notably CYP2C9 and CYP2C19—are encoded by genes which contain polymorphisms that alter structure and catalytic function of the enzyme rather than altering the level of expression for 2C9 and 2C19 the range of catalytic variation in vivo is determined much more by the polymorphisms that alter enzyme structure than by the genetic and environmental factors that alter expression levels of the enzyme. 

The power of the Sharpless epoxidation method is augmented by the versatility of the resultant 2,3-epoxy alcohols4 and the development of the catalytic variation.5... 

Summing up, it is conceivable that both the state of surface topography as well as the presence of other elements on the Pd surface are decisive for its activity in alkane reactions. However, two things seem obvious and unquestionable. First, all these catalytic variations result from the fact that hydrogenolysis, rather than isomerization, is sensitive to changes in... 

Structural and Catalytic Variations Within the Three Families of Molybdenum Enzymes... 

Neumann and co-workers reported a catalytic variation of this chemistry. Labile Keggin heteropolymolybdates such as K5PV2M010O40 after activation by H2 catalyze the deoxygenation of aldehydes and ketones. For example, PhCOMe is deoxygenated to PhEt in 100% yield in the presence of K5PV2Moi0O40 supported on A1203.324... 

High E selectivity (80-95%) is observed in mercury(II)-catalyzed rearrangements of tertiary 1.5-hexadien-3-ols 6 and 8 (Table 33)78V 785. In contrast to the catalytic variation (see Table 33 and experimental procedure below), use of stoichiometric amounts of the mercury salt results in a decrease in selectivity. 

The initial work on the reaction of osmium tetroxide with olefins was carried out by Hofmann early in the last century [3,4]. In the 1930s, Criegee [5] showed that the addition of a base, such as pyridine, accelerates the reaction rate. Later on, catalytic variations of the reaction were developed employing relatively inexpensive reagents for the reoxidation of the osmate species [6]. More recently, several advances were achieved to immobilize the catalyst and to improve the catalytic oxidation chain. In particular, recent work couples the oxidation of the osmium(VI) compound to a green terminal oxidant such as air or hydrogen peroxide [7-9]. 

Kagan and Pitchen ° and Modena and coworkers independently reported the oxidation of sulfides to sulfoxides using modified Sharpless epoxidation catalyst (titanium/diethyl tartrate). By 1987, Kagan had already reported a catalytic variation of the reaction and an improved catalytic system allows for the use of lower (10 mol%) loading of catalyst. For example, sulfide (5.143) undergoes sulfoxidation with good enantioselectivity. An alternative catalyst based on Ti(0 Pr)4 and BINOL is also effective for sulfoxidation, providing up to 96% ee. ... 

Over the years with the advent of milder and more controlled reaction conditions, the aldehyde component has increased in its diversity with a concomitant increase in functional group inter-compatibility. These changes also have enabled the extension of the nature of the active hydrogen (nucleophilic) species beyond enols. More recently, the utility of the Mannich reaction has been enhanced with enantioselective and catalytic variations. 

Apphcations of the reaction have been seen in the rearrangement of tetraethylbenzenesulfonic acids [85], wherein the reaction of meta- and para-di-, 1,3,5-tri- and 1,2,3,5- and 1,2,4,5-tetra-ethylbenzenes with concentrated aqneous sulfuric acid leads to clean sulfodeprotonation. Recent examples by Solari et al. witnessed a catalytic variation of the Jacobsen rearrangement using ZrCl [86]. The rearrangement, previously only observed in concentrated sulfuric acid, appeared to be extremely mUd under ZrCl -mediated conditions on permethylated arenes at room temperature in halogenated solvents. 

Metal-activated alkene additions can be classified as stoichiometric or catalytic processes. Stoichiometric processes for THP synthesis typically involve the use of mercury(II) salts and to a lesser extent iodo and seleno reagents. The progress of intramolecular oxymercuration is determined by the stabiUty of the cationic intermediates. Product stereochemistry is under substrate control and usually leads to the thermodynamically more stable THP product. Catalytic variations generally involve palladium complexes [44], but other transition metals are becoming more common (e.g., Pt [45], Ag [46], Sn [47], Ce [48]). The oxidation state of Pd determines the catalyst reactivity. Palladium(O) complexes are nucleophilic and participate in tetrahydropyran synthesis through jt-allyl cation intermediates, whereas Pd(II) complexes possess electrophilic character and progress through a reversible t-complex. 

---