Preorganized Systems

It was Swain [15] who pointed out more than half a century ago that when a reaction between a nucleophile N and a substrate S is catalyzed by an electrophile E (case a), enhanced catalysis is expected if two of the components, either N and S (case b), S and E (case c), or N and E (case d) are bound together in the same molecule or in the same complex by covalent bonding or noncovalent interactions, respectively. An additional possibility is given by case e, where the two reactants and catalyst are held together in a ternary complex. 

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To achieve synergy and cooperativity, it is desirable to employ preorganized systems that possess multiple complementary H-bond arrays defining... 

Despite the vast number of outstanding examples of enzymatic catalysis that rely on the collaboration of two or more vicinal metal centers hitherto disclosed, the design and development of efficient binuclear organometallic complexes able to enhance the performance of mononuclear catalyst by means of an intermetallic cooperative process remains widely unexplored [22-24]. In fact, the formation of bi- or polynuclear complexes has been often described as a catalyst deactivation pathway [25-30]. However, the availability of more electron density at the active site, extra coordination positions, and the possibility to develop more preorganized systems that allow for (enantio)selective reactions shows great promise for an improved catalytic performance [9]. 

A possible disadvantage of an induced-fit receptor is the intrinsically lower binding constants compared to those of the preorganized systems. Typically, an unfavorable entropic contribution is expected when binding an anion as the system becomes more ordered. In macrocyclic systems, with less conformational freedom, this entropy cost... 

The use of chiral ruthenium catalysts can hydrogenate ketones asymmetrically in water. The introduction of surfactants into a water-soluble Ru(II)-catalyzed asymmetric transfer hydrogenation of ketones led to an increase of the catalytic activity and reusability compared to the catalytic systems without surfactants.8 Water-soluble chiral ruthenium complexes with a (i-cyclodextrin unit can catalyze the reduction of aliphatic ketones with high enantiomeric excess and in good-to-excellent yields in the presence of sodium formate (Eq. 8.3).9 The high level of enantioselectivity observed was attributed to the preorganization of the substrates in the hydrophobic cavity of (t-cyclodextrin. 

Enforced by the preorganized bimetallic array, a more symmetrical cyanato bridge has been observed in (106). Triple /r-l,l,l-NCO linkages have been reported for heterobimetallic nickel/ lanthanide systems.423... 

Type (820) dinickel complexes offer the opportunity of substrate binding within the bimetallic pocket, and highly preorganized complexes of this type have also been employed as model systems for the urease metalloenzyme (see Section 6.3.4.12.7). The Ni—Ni separation in type (820) complexes can be... 

The mentioned properties make polymer brushes ideal systems for the preparation of preorganized inteUigenf materials to serve as functional devices on significantly larger length scales [189, 190]. 

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