Double catalytic

Figure 2.27 Double catalytic cycle that gives rise to the current in the presence of p-D-glucose. COx(FAD) oxidizes the enzyme substrate p-D-glucose to yield the reduced form of the enzyme GOx(FADH2) that is then reoxidized by the osmium polymer mediator PAH-Os(ll I). This process yields PAH-Os(ll). Electrons should diffuse to the electrode surface by electron...

Scheme 2.4. Double catalytic cycle for the eiBctrochemica conversion of alk-I -cnes...

In this double catalytic cycle the olefin is first transformed into aldehyde and then into alcohol. Owing to the increased sieric crowding of the bulky tertiary phosphine Hgand, all the stcric factors discussed in the previous section, which favour the production of normal aldehydes, are much favoured here. 

The mechanism has been extensively studied by Forster and coworkers at Monsanto and is shown in Scheme 19. It is actually a double catalytic cycle involving the Hl-catalyzed conversion of MeOH to Mel and H2O at the beginning of the Rh-catalyzed carbonylation reaction, and regeneration of HI at the end of the Rh cycle by hydrolysis of the acyl iodide. 

Kanemasa S, Ito K (2004) Double catalytic activation with chiral lewis acid and amine catalysts. Eur J Org Chem 2004 4741... 

Wang et al. (1994) analyzed by MD the roles of the "double catalytic triad" in papain catalysis, based on the structure of the enzyme, which is not completely known from crystallography (Kamphuis et al., 1984) due to the oxidation state of Cys-25 (present as cysteic acid in the crystal). Stochastic boundary MD (Brooks and Karplus, 1983) was carried out on the whole enzyme + 350 water molecules. Three "layers" were treated according to their distance from the sulfur atom of Cys-25 - atoms within 12A, atoms between 12-16A and the more distant atoms were kept fixed. CHARMM forcefield was employed. The active site geometry was examined as a function of pH, for various mutual states of S-/SH and Im/ImH+. In addition, the mutations of Asp-158 (Menard et al., 1991) were studied. 

Double catalytic activation with chiral Lewis acid and amine catalysts in the syntheses of heterocycles, in particular, enol lactones 04EJO4741. 

Simultaneous action of a bio- and a chemo-catalysl is an attractive possibility for certain catalytic processes in solution. For example, a starting compound could be enzymatically equilibrated with a product that could be irreversibly transformed by the chemo-catalyst into the final product. Problems associated with such double catalytic systems arc the limited range of conditions for enzyme activity, the preferential conversion of the intermediate by the chemo-catalyst, and the possible mutual poisoning of the two catalysts. 

Enantioselective addition of 3-crotonyl-2-oxazolidinone 423 to 1 under double catalytic conditions using 2,2,6,6-tetramethylpiperidine (TMP) and R,R-complex of nikel(II) perchlorate hexahydrate in THF... 

Double Catalytic Activation Using a Bifunctional Catalyst with Both Acid and Base on Solid Surfaces... 

The size of V-Al particles in double catalytic systems is about 23% less than the size of the Ti-Al system particles (Table 3.6), all other things being equal. Ti-Al particle size in catalytic systems or catalysts modified using piperylene additives is 25%... 

This can be regarded as a double catalytic system, reminiscent of the ones described by Backvall et ai. [33]. 

For instance in the double catalytic cycle shown below, and for which the reaction rate depends both on the mono- and dinuclear systems (r = k + (H)(C0)5] [Rh(COR)(CO)4]), the rhenium hydride [Re(H)(CO)5] is ca 1,000 times more effective than molecular hydrogen toward attack on the acyl rhodium species [Rh(COR)(CO)4]. 

As shown in Figure 1, 5-LOX, via a double catalytic step, transforms arachidonic acid into the first of the leukotrienes, LTA [5(6)-epoxy-7,9-rrans-... 

In 2002, Itoh and Kanemasa found that the combined use of both amine and chiral Lewis acid (R,R)-DBFOX-Ph complex of Ni(II) can be an active catalyst for enantioselective Michael addition of nitromethane or malononitrile to unsaturated carbonyl compounds [37a,b]. Recently, they have reported a new enol ketone synthesis through the reactions between cyclic 1,3-dicarbonyl donors and a,p-unsaturated carbonyl acceptors under the double catalytic activation conditions (10mol% each) of Ni(11)-perchlorate hexahydrate and (2,2,6,6-tetramethylpiperidine (TMP) (114))(Scheme 16.33) [38a,b]. Thus, 1,3-cyclohexanedione (112) is allowed to react with 4-bromo-l-crotonoyl-3,5-dimethylpyrazole (113), in THF at room temperature in the presence of both catalytic amounts to give 4,7,7-trimethyl-3,4,5,6,7,8-hexahy-drobenzopyran-2(H),5-diones (115) in good yields along with high enantioselectivity up to 99% ee. 

This double catalytic cycle proceeds in Ihe following way (Monsanto process) ... 

SCHEME 7.29. Synthesis of cyanthiwigin F using double catalytic enantioselective aUylation. 

Enquist JA, Stoltz BM. The total synthesis of (—)-cyanthiwi-gin F by means of double catalytic enantioselective alkylation. Nature 2008 453 1228-1231. 

This reaction used a catalytic amount of chiral catalyst thanks to the use of a double catalytic cycle... 

SCHEME 31.17. Double catalytic cycle involved in the first enantioselective organocatalytic protonation of silicon enolate. 

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