Tandem catalyst

With the tandem catalyst Pd/Sn, the hydroesterification of limonene delivered a 90% yield of the linear product, while in the absence of tin, more branched esters were formed. The hydroesterification of camphene delivers the linear product with a selectivity of 90%, whereas the two diastereomers were built in an equal ratio. The additive SnCl2 had a high impact on catalytic activity without this additive, the palladium/diphosphinobutane catalyst had lower activity. 

We look at the epoxidation of an alkene using a green chemistry approach as an example of an attempt to design and implement a tandem catalyst to carry out a range of sequential functions, i.e. the in situ generation of a selective oxidant and the use of this oxidant in a selective oxidation reaction. 

In this area, Yang and colleagues have designed a bilayer stracture by combining platinum and cerium oxide nanocube into sub-lOnm layers on silica substrate [7]. Two distinct reactions can be utilized on a bilayer structure with two metal-metal oxide interfaces (CeO -Pt and Pt-SiO ). The CeO -Pt interface is used to catalyze the decomposition of methanol into CO and H, while ethylene hydroformylation happens on the Pt-SiO interface. This is an intriguing concept on making nanocrystal bilayer structures designed for the application of multifunctional catalysts, the so-called tandem catalyst. 

With the observed alkane distribution of both systems (multifunctional catalysts or dual-tandem catalysts), a more selective alkane metathesis reaction would be desirable. Going toward this direction, a homogenous, dual-tandem catalyst system using Ta/Ir has recently been developed, in which the metathesis process has been modified with a dimerization system [146,147]. In this, -heptane was converted into alkene isomers in the presence of tbe in 18% yield. Unfortunately, the final step, the hydrogenation of the resulting alkenes to alkanes, was not accomplished using these reaction conditions. 

Figure 3.3 Ethylene hydroformylation with MeOH over a tandem catalyst. (Derived from Ref. [36].)...

Peng H-g XuL, Wu H, Zhang K, Wu P (2013) One-pot synthesis of benzamide over a robust tandem catalyst based on center radially fibrous silica encapsulated TS-1. Chem Common 49 (26) 2709-2711... 

Zanardi A, Corberan R, Mata JA, Peris E. Homo- and heterodinuclear complexes with triazolyl-diyhdene. An easy approach to tandem catalysts. Organometallics. 2008 27 3570-3576. 

Tandem cyclization/3-substitution can be achieved starting with o-(trifluoro-acetamido)phenylacetylenes. Cyclization and coupling with cycloalkenyl trif-lates can be done with Pd(PPh3)4 as the catalyst[9]. The Pd presumably cycles between the (0) and (II) oxidation levels by oxidative addition with the triflate and the reductive elimination which completes the 3-alkenylation. The N-protecting group is removed by solvolysis under the reaction conditions, 3-Aryl groups can also be introduced using aryl iodides[9]. 

The rhodium-catalyzed tandem carbonyl ylide formation/l,3-dipolar cycloaddition is an exciting new area that has evolved during the past 3 years and high se-lectivities of >90% ee was obtained for both intra- and intermolecular reactions with low loadings of the chiral catalyst. 

Shibasakihas also extended the use of LLB catalyst to tandem nltro-aldolreacdons providing bicyclic adducts with 65% ee fEq. 3.7S. ... 

The Rh2(DOSP)4 catalysts (6b) of Davies have proven to be remarkably effective for highly enantioselective cydopropanation reactions of aryl- and vinyl-diazoacetates [2]. The discovery that enantiocontrol could be enhanced when reactions were performed in pentane [35] added advantages that could be attributed to the solvent-directed orientation of chiral attachments of the ligand carboxylates [59]. In addition to the synthesis of (+)-sertraline (1) [6], the uses of this methodology have been extended to the construction of cyclopropane amino acids (Eq. 3) [35], the synthesis of tricyclic systems such as 22 (Eq. 4) [60], and, as an example of tandem cyclopropanation-Cope rearrangement, an efficient asymmetric synthesis of epi-tremulane 23 (Eq. 5) [61]. 

Carbonyl ylides continue to be targets of opportunity because of their suitability for trapping by dipolar addition. High enantiocontrol has been achieved in the process described by Eq. 16 [109], but such high enantioselectivity is not general [110] and is dependent on those factors suggested by Scheme 11. Using achiral dirhodium(II) catalysts, Padwa and coworkers have developed a broad selection of tandem reactions of which that in Eq. 17 is illustrative [111] these... 

RCM of a dienyne was also a key step in Mori s recent total synthesis of the alkaloid erythrocarine (447) [183]. The tetracyclic framework of447 was elaborated in the penultimate step, by exposing the hydrochloride of metathesis precursor 445 (1 1 diastereomeric mixture at the carbinol center) to first-generation catalyst A. The tandem process occurred smoothly within 18 h at room temperature leading to tetracycles 446 (1 1 mixture) in quantitative yield. Deprotection of the a-acetoxy isomer 446a led to 447 (Scheme 88). 

A domino Diels-Alder reaction (the term was chosen from the well-known game) is a one-pot process involving two or more Diels-Alder reactions carried out under the same reaction conditions without adding additional reagents or catalyst such that the second, third, etc., cycloaddition is the consequence of the functionality generated in the previous reaction. A historical example is illustrated in Equation 1.28 [60]. This type of transformation is sometimes named tandem or cascade, but these terms seem less appropriate for describing a time-resolved transformation. 

Scheme 5.1 Tandem cyclopropanation/ring closing metathesis of dienyne using Gmbbs catalyst...

Tandem syn addition of alkyl and trimethylsilyl groups can be accomplished with dialkylzinc and trimethylsilyl iodide in the presence of a Pd(0) catalyst.79... 

Entries 10 to 12 are examples of oxidative generation of radicals, followed by tandem cyclization. The reaction in Entry 10 includes a lanthanide catalyst. Entry 11... 

Chapter 10 considers the role of reactive intermediates—carbocations, carbenes, and radicals—in synthesis. The carbocation reactions covered include the carbonyl-ene reaction, polyolefin cyclization, and carbocation rearrangements. In the carbene section, addition (cyclopropanation) and insertion reactions are emphasized. Recent development of catalysts that provide both selectivity and enantioselectivity are discussed, and both intermolecular and intramolecular (cyclization) addition reactions of radicals are dealt with. The use of atom transfer steps and tandem sequences in synthesis is also illustrated. 

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