Cooperative activation effect

In this chapter, we summarize the recent advances in the development of nanaoreactors based on porous solid materials for chemical reactions, including the general methods for the fabrication of typical porous materials, (mesoporous silicas (MSs), carbon nanotubes (CNTs), and the MOFs), the assembly of the molecular catalysts in the cavities and pores of the porous materials, the chemical reactions in the porous-material-based nanoreactors, and some important issues concerning the porous-material-based nanoreactor, such as the pore confinement effect, the isolation effect, and the cooperative activation effect We close this chapter with an outlook of the future development of the nanoreactors. 

Through covalent and noncovalent bonding methods, different kinds of molecular catalysts could be incorporated into MSs and MOFs. These porous materials with the incorporated molecular catalyst could catalyze various kinds of chemical reactions. A review of all the related works is impossible and not necessary in this chapter. We only review some representative examples for demonstrating the unique properties of the nanoreactor for catalytic reactions, including the pore confinement effect, the enhanced cooperative activation effect, and the isolation effect, as well as the microenvironment and the porous structure engineering of the nanoreactor and the catalytic nanoreactor engineering. 

Mechanistic study shows that the high activity of the catalyst is mainly derived from the enhanced cooperative activation effect and the enrichment of reactants in the nanoreactor (Scheme 10.20b). This result strongly confirms the cooperative activation effect in the asymmetric reaction and is corroborated by a DFT calculation that the activation energy could be greatly reduced when the reaction goes through a bimolecular activation pathway (Scheme 10.22) [104]. 

In addition to the enhanced cooperative activation effect of the nanoreactor, the isolation effect could also be expected in the confined nanospace if the diameter of nanopore is similar to the size of the molecular catalysts, because the limited nanospace could restrict the free movement of the molecular catalysts. Two issues relevant to the isolation effect of the nanoreactor, namely selectivity control in organic reactions and inhibition dimerization of the molecular catalysts, will be discussed. 

DMT, TMT, DBT, TBT and DPhT chlorides exhibited in vitro spindle disturbance in V79 Chinese hamster cells of brain tubulin. The V79 cells lose stainable spindles at higher concentration. The cell mitosis activity effect at low concentration increased with the lipophilicity of the OTC, but all compounds showed a concentration dependence on microtubules. The OTC seem to act through two different cooperative mechanisms inhibition of microtubule assembly and interaction with hydrophobic sites. The latter mechanism might involve Cl/OH ion exchange28. 

Ca2+/calmodulin. The effect of calmodulin binding is to increase the affinity of the substrate Ca2+ site by 20- to 30-fold. This highly cooperative activation mechanism makes the PMCAs very sensitive to small changes in [Ca2+]j. A group of at least five PMCAs forms a multigene family. Three isoforms, PMCA1-3, occur in brain and each has a distinct distribution [20]. 

Since silica is such a common support for immobilization, the effect of the weakly acidic silanol groups has been studied. A cooperative silanol effect is thought to improve the catalytic activity of mesoporous silica-supported amines in base-catalyzed reactions such as the nitroaldol (Henry) condensation [6, 7], Knoevenagel condensation [6, 8,9], and Michael addition [6]. Thus immobilizing amines onto supports with stronger acid groups could be expected to further increase the catalytic activity. 

In a similar [4+2] reaction of a, -unsaturated esters, the aluminum catalyst complexed with the ligand S-VAPOL resulted in autoinduction , because of cooperative interaction of the product with the catalyst to generate a more selective catalytic species (Scheme 6.48) [68]. The ee% gradually increased as the reaction time lengthened. In the proposed intermediate, penta-coordinated aluminum complex 77, the cycloadduct is recognized as a complementary ligand, leading to substantial asymmetric induction. The acrylate is activated effectively within this hybridized complex which adopts pentacoordination [87]. 

The activity and regioselectivity of 1 and 5 therefore contrast to those of monometal rhodium or ruthenium complexes, and the regioselectivity of complex 1 is particularly noteworthy. These results indicate that the active species uses some sort of bimetal cooperativity to effect high regioselectivities. 

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