Cascade reactions cascade

Cascade Reactions. Cascade reactions can be defined as multi reaction one pot sequences in which the first reaction creates the functionality to trigger the second reaction and so on. Cascade reactions have also been termed tandem or domino processes by some authors. This section is concerned with Pd(PPh3)2Cl2- [to generate Pd(0)] or PdCl2-catalyzed processes in which two or more carbon-carbon/carbon-heteroatom bonds are formed. 

Multistep Cascade Reactions Synthesis of Bis-1,3,4-thiadiazolo-1,3,5-triazinium Halides... 

A more recent application of the Knoevenagel reaction is its use in domino reactions. The term domino reaction is used for two or more subsequent transformations, where the next reaction step is based on the functionality generated in the preceding step. Such reactions are also called tandem reactions or cascade reactions. 

The known, in many cases very special, synthetic approaches to azocines often proceed via valence isomerization of appropriate precursor molecules. In this section these routes are classified by the last step in the reaction cascade. The synthesis and chemistry of azocines has been described in several excellent reviews,9-11 so that the treatment here is systematic and rather concise. 

Figure 6.69 An enantioconvergent enzyme-triggered cascade reaction.

Nonetheless, it was a fairly short step from octopus compounds to dendrimers, and the step was taken by Vogtle in the late 1970s when he attempted to use a cascade reaction to prepare a molecule of the dendrimer type that would now be considered a dendron rather than a fully developed dendrimer. It began with the addition of acrylonitrile to an anfine, followed by reduction of the nitrile to amine. This was followed by a further reaction with acrylonitrile, and the process was repeated several times to yield highly branched macromolecules. There were initially problems with the reduction step but these were overcome, and the preparation of these poly(propylene imine) dendrimers was later commercialized. 

In order to confirm the proposed mechanism described above, in which O2 may have a positive effect on NO absorption, the comparative experiments have been carried out. The results are shown in Fig. 1, from which one can see that the presence of O2 will greatly improve the NO removal performance. In the absence of O2, NO coordination occurs according to Eq. (2), a reversible reaction limited by equilibrium, the NO removal decreases from the initial 100% to about 60% in one hour. In the presence of O2 however, contribution of Eq. (2) is little, the most coordination of NO is certainly attributed to the cascade reactions from Eq.(3) to Eq.(6), and the final reaction of Eq. (7), which will not be constrained by the reaction equilibrium, and thus the NO removal can be maintained 100% in 2-3 hours. 

Metal Catalyzed Cascade Reactions Volume Editor T. J. J. Muller Vol. 19, 2006... 

Malacria, M. (1996) Selective Preparation of Complex Polycyclic Molecules from Acyclic Precursors via Radical Mediated or Transition Metal-Catalyzed Cascade Reactions. Chemical Reviews, 96, 289-306. 

Nicolaou, K.C., Edmonds, D.J., Bulger, P.G. (2006) Cascade Reactions in Total Synthesis. Angewandte Chemie International Edition, 45, 7134-7186. 

With this understanding of cascade reactions for the purpose of process intensification, we now turn out attention to the relatively few literature examples that exhibit these features. 

Cascade Reactions with Incompatible Catalysts and Nanoencapsulation... 

In 2006, Poe et al. reported a cascade reaction employing two incompatible catalysts, one of which was microencapsulated [19]. In this case, an organic amine was encapsulated and used in conjunction with a nickel-based Lewis acid catalyst (Scheme 5.4). 

Scheme 5.12 Cascade reactions with acid-and base-catalyzed steps in the same pot [29]. The acids and bases are defined in the text.

The same concept of using mutually interfering acid and base catalysts was achieved by Frechet s group through the use of star polymer systems [30]. The synthesis of the catalysts is outlined in Scheme 5.13. The arms of the star polymers prevent the catalysts from interfering with each other and allow the cascade reaction to proceed unhindered. 

The same system was used by Frechet s group of to achieve a multicomponent one-pot cascade reaction with mutually interfering acid and proline-derived pyrrolidine catalysts [31]. The concept is illustrated in Figure 5.1. The protonation of imidazo-lidone (3) by the immobilized PSTA (5) gives the desired iminium catalyst (6), while... 

Figure 5.1 The use of multiple catalysts encapsulated in star polymers to enable a one-pot cascade reaction containing interfering catalysts. Catalyst 11 is formed via the protonation of imidazolidinone (9) by immobilized PSTA (10), and is responsible for the nucleophilicaddition ofN-methylindole (14)...

To the best of the authors knowledge, these examples comprise most of the literature that fits the criteria of cascade reactions with incompatible catalysts achieved with nanoencapsulation. However, there are many more examples where cascade reactions have been achieved with incompatible catalysts, but without the use... 

Other Cascade Reactions with Incompatible Catalysts - Polydimethylsiloxane (PDMS) Thimbles for Generic Site Isolation... 

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