Catalysis tandem

Kumar, R., Sithambaram, S. and Suib,S.L. (2009) Cyclohexane oxidation catalyzed by manganese oxide octahedral molecular sieves - effect of acidity of the catalyst. Journal of Catalysis, 262,304—313. Sithambaram, S., Kumar, R., Son, Y. and Suib, S.L. (2008) Tandem catalysis direct catalytic synthesis of imines from alcohols using manganese octahedral molecular sieves. Journal of Catalysis, 253, 269-277. 

The use of two or more different catalysts in the same reactor, sometimes known as in situ reactor blending or tandem catalysis, has been widely employed industrially as means of controlling the properties of a polyolefin (e.g. molecular weight and the molecular weight distribution). Recent years have seen a variety of reports emerge on the use of bis(imino)pyridine iron/cobalt systems as one component of the process [169, 170, 171, 172, 173, 174, 175, 176, 177,178, 179],... 

Heterogeneous tandem catalysis involving at least one of the components being supported has also been reported [178, 179]. For example, calcosilicate has recently been used as an effective carrier for simultaneous immobilisation of a dual-functional system based on a bis(imino)pyridine iron compound and a zirconocene to form a heterogeneous catalyst precursor. On activation with triethylaluminium, ethylene was converted to LLDPE the layered structure of the calcosilicate was used to account for the improved thermal stability and higher molecular weights of the LLDPE formed [179],... 

Scheme 24.8 Unnatural amino-acid derivatives accessed via tandem catalysis.

O-stannylene acetals,84 glycals and 1,2-anhydrosugars,85 and to selective de-O-benzylation of position-2 with TIBAL, DIBAL-H57 or Lewis acid catalysts,86 or to the one-pot access to 3-O-benzyl-4,6-0-benzylidene glucosides by tandem catalysis recently reported.87 The 1,2-lactones recently reported by Linker and co-workers35 are also synthons which provide... 

Pahnans et al. prepared 5 by the reaction of [RuCl2(p-cymene)]2 and 2-phenyl-2-aminopropionamide in the presence of potassium carbonate. They used 5 in an iterative tandem catalysis for the synthesis of chiral oligoesters. The enzymatic ring opening of 6-methyl-e-caprolactone was combined with ruthenium-catalyzed alcohol racemization to produce optically active oligomers of 6-methyl-e-capro-lactone [23] (Scheme 1.17). 

Scheme 12 Chiral polymers from racemic co-methylated lactones by iterative tandem catalysis [105, 106]...

Scheme 48 Proposed tandem catalysis cycle for 13/Cp2ZrCl2. Reagents (i) MAO activation.

Fig. 12.3 (a) Tandem catalysis cycle coordinating the actions of three well... 

These observations are all consistent with the proposed tandem-catalysis mechanism shown in Fig. 8. Central to this proposal is that the reaction proceeds via several parallel reaction steps as might be expected for formal eight-electron coupling of two methane molecules to acetic acid. The observation that the reaction is effectively stopped by high concentrations of added CO but proceeds at low levels of CO can be explained if the rate-limiting step is oxidation of Pd(0) by sulfuric acid. From independent experiments we have found that under high CO pressure... 

Figure 8. Proposed tandem-catalysis mechanism for oxidative condensation of methane directly to acetic acid.

Immobilized CPO and glucose oxidase (GOx) on SBA-15 were mixed for tandem catalysis By the use of the tandem system, a conversion of 92% of indole was reached at pH = 5.5. Also, the mixture of immobilized CPO and GOx was recycled several times without significant loss of activity [9]... 

However, the latter type of metal-catalyzed cascade reactions turns out to be even more challenging since issues of selectivity and efficiency are crucially dependent on the particular catalyst structure. This type can either be performed in a parallel or sequential fashion [16,21], Whereas parallel catalysis is significantly more difficult to develop, sequential catalysis offers the possibility of altering reaction conditions and additives from step to step in the sense of bi- or multicatalytic one-pot processes, assisted tandem catalysis, or auto tandem catalysis [1]. Therefore, a demanding goal is the development of one-catalyst multireaction sequences that set the stage for new reactions in diversity-oriented syntheses of complex molecular structures (for reviews on diversity-oriented syntheses see [27-33]). 

The combination of cobalt with other metals like Pd increases the synthetic utility of this methodology. Thus, a concurrent tandem catalysis reaction of 102 and 103 using first an homogeneous chiral palladium complex, followed by the action of heterogeneous cobalt/C led to high enantioselective synthesis of PK products 104 (Scheme 30) [129]. 

It is noteworthy that an illustrative review on tandem catalysis, including some ruthenium-catalyzed transformations, has recently appeared [3], which also proposed a useful definition rationalization effort. 

Wasilke JC, Obrey SJ, Baker RT, Bazan GC (2005) Concurrent tandem catalysis. Chem Rev 105 1001-1020... 

Metal enolates have played a Umited role in the metal-catalyzed isomerization of al-kenes . As illustrated in a comprehensive review by Bouwman and coworkers, ruthenium complex Ru(acac)3 (51) has been used to isomerize a wide range of substituted double bonds, including aUylic alcohols (131), to the corresponding ketones (132) (equation 38) . The isomerization of aUylic alcohols affords products that have useful applications in natural product synthesis and in bulk chemical processes. An elegant review by Fogg and dos Santos shows how these complexes can be used in tandem catalysis, where an alkene is subjected to an initial isomerization followed by a hydroformylation reaction ... 

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