Catalytic double activation

Conditions under which the catalytic activity is measured should ensure that the rate of the reaction is proportional to the concentration of the enzyme that is, if the enzyme concentration is doubled, the catalytic activity must double. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

The propagation reaction consists in repetitive interaction of the cycloolefin double bond with a metal alkylidene species formed via the initiation reaction, and differs from this reaction only in the rate constant value. The rate of the propagation reaction should be first order with respect to the monomer concentration. However, the kinetics of polymerisation of cycloolefins is complicated by the fact that any active catalytic species in the polymerisation system will be able not only to coordinate the monomer double bond to lead to a propagation step but also to coordinate a C=C bond from the polymer chain. If the coordination of the latter C=C bond proceeds intramolecularly, then a new... 

Several thermodynamic and kinetic behaviors of enzyme-catalyzed reactions performed in ILs, with respect to enzymatic reactions carried out in conventional solvents, could lead to an improvement in the process performance [34—37]. ILs showed an over-stabilization effect on biocatalysts [38] on the basis of the double role played by these neoteric solvents ILs could provide an adequate microenvironment for the catalytic action of the enzyme (mass transfer phenomena and active catalytic conformation) and if they act as a solvent, ILs may be regarded as liquid immobilization supports, since multipoint enzyme-1L interactions (hydrogen. Van der Waals, ionic, etc.) may occur, resulting in a flexible supramolecular not able to maintain the active protein conformation [39]. Their polar and non-coordinating properties hold considerable potential for enantioselective reactions since profound effects on reactivities and selectivities are expected [40]. In recent years attention has been focused on the appUcation of ILs as reaction media for enantioselective processes [41—43]. 

Methylene ( CH2) generated photochemically or thermally from diazomethane is highly reactive and is prone to incur side reactions to a substantial extent. In order to avoid these undesirable complexities, the cyclopropanation of multiple bonds with diazomethane has usually been carried out under catalytic conditions The catalysts most frequently employed are copper salts and copper complexes as well as palladium acetate. The intermediate produced in the copper salt-catalyzed reactions behaves as a weak electrophile and exhibits a preference to attack an electron-rich double bond. It is also reactive enough to attack aromatic nuclei. In contrast, the palladium acetate-catalyzed decomposition of diazomethane cyclopropanates a,a- or a,jS-disubstituted a,jS-unsaturated carbonyl compounds in high yields (equation 47). The trisubstituted derivatives, however, do not react. The palladium acetate-catalyzed reaction has been applied also for the cyclopropanations of some strained cyclic alkenesstyrene derivatives and terminal double bondsHowever, the cyclopropanation of non-activated, internal double bonds occurs only with difficulty. The difference, thereby. 

Several early attempts at ADMET polymerization were made with classical olefin metathesis catalysts [57-59]. The first successful attempt was the ADMET polymerizations of 1,9-decadiene and 1,5-hexadiene with the WClg/EtAlf l,. catalyst mixture [60]. As mentioned in the introduction, the active catalytic entities in these reactions are ill-defined and not spectroscopically identifiable. Ethylene was trapped from the reaction mixture and identified. In addition to the expected ADMET polymers, intractable materials were observed, which were presumed to be the result of vinyl polymerization of the diene to produce crosslinked polymer. Addition to double bonds is a common side reaction promoted by classical olefin metathesis catalysts. Indeed, reaction of styrene with this catalyst mixture and even wifh WCl, alone led to polystyrene. Years later, classical catalysts were revisited in fhe context of producing tin-containing ADMET polymers wifh tungsten phenoxide catalysts [61], Alkyl tin reagents have long been known to act as co-catalysts in classical metathesis catalyst mixtures, and in this case the tin-containing monomer acted as monomer and cocatalyst [62]. Monomers with less than three methylene spacers between the olefin and tin atoms did not polymerize (Scheme 6.14). 

Similarly, hydrogenation involves addition of hydrogen to an unsaturated moiety such as an olefin. This reaction can be catalyzed with most of the d-block elements, but Group 8 is especially important because it produces the most active catalytic systems. Hydrasilation of alkenes is similar to hydrogenation except that hydrogen and SiRj from a silane (RgSiH) is added across the reactive double bond ... 

Based on the catalytic double activation machinery concept (tertiary phosphines/ thiourea catalyst 56), Shi and co-workers [124] were able to achieve the allylic amination of MBH acetates with phthalimides in good yields and selectivities (Scheme 11.43). Higher reactivity and enantioselectivity were observed when 25 mol% of the catalyst, 1,2-dichlorobenzene as a solvent to increase the solubility... 

Subsequently, the Feng group developed an enantioselective cyanosilylation of ketones by a catalytic double-activation catalyst system composed of chiral (J ,J )-salen 16-triethylaluminium complex and N-oxide 17 (Scheme 19.10). High catalytic turnovers (200 for aromatic ketones, 1000 for aliphatic ones) with high enantioselectivity (up to 94% enantiomeric excess for aromatic ketones, up to 90% enantiomeric excess for aliphatic ones) were achieved under mild reaction conditions. Based on the control experiments, a double-activation model was suggested (Scheme 19.10). The chiral aluminium complex performed as a Lewis acid to activate the ketone, whereas the N-oxide acted as a Lewis base to activate trimethylsilyl cyanide and form an isocyanide species. The activated nucleophile and ketone attracted and approached each other, and so the transition state was formed. The intramolecular transfer of cyanide to the carbonyl group gives the product cyanohydrin O-TMS ether. 

Caille and coworkers finished the synthesis of AMG 221 in 2009, by the use of an enantioselective cyanosilylation of 3-methylbutan-2-one as the key step (Scheme 19.11). Using the double-activation catalytic system mentioned above, the key intermediate cyanohydrin derivative was isolated in 88% yield (47.2 g) with 85% enantiomeric excess. Six additional steps allowed to the synthesis of AMG 221, which is an inhibitor of llp-hydro g steroid dehydrogenase type 1. 

On the basis of the multinuclei NMR study and the fact that the reactions of Pt(IV) and Pt(II) chlorides with (CH2=CHSiMe2)20 yield polysiloxanes, vinyl chloride, 1,3-butadiene and ethene, Lappert and coworkers have proposed a plausible mechanism illustrated in Scheme 4, which includes a rather unique vinyl-chlorine exchange (26 -> 27) and reductive elimination of vinyl chloride (28 29). The homolytic fission of Pt—CH=CH2 bond is also suggested. If a divinyl-Pt complex is formed by double vinyl-chlorine exchange, the observed formation of 1,3-butadiene can be explained as well. This study concludes that 16-electron species such as 24 and 25 are considered to be highly active catalytic species due to the availability of a vacant site for oxidative addition by a hydrosilane. ... 

Catalytic activity of double molybdenum and nickel carbides and nickel-promoter molybdenum carbides... 

We performed three series of experiments to study the catalytic activity of double molybdenum and nickel carbides and nickel-promoter molybdenum carbides (Table 4.11.1 series A, B, C). 

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