Covalent toolbox

Lewis Structures Lewis structures are one of the most useful and versatile tools in the chemist s toolbox. G. N. Lewis reported this model for chemical bonding in 1902. Lewis structures are nonmathematical models that allow us to qualitatively describe the chemical bonding in a molecule and then gain insights about the physical and chemical properties we can expect of that molecule. Don t discount the power of Lewis structures just because the underlying mathematics isn t evident. In a Lewis structure, the atoms are represented by their chemical symbol. Lines between atoms represents shared pairs of electrons in covalent bonds. Valence electrons that are not used for covalent bonds are lone pairs, and they are represented as pairs of dots on the atom. 

Inspired by the seminal report of Nagel [34], which described a very active and selective Rh-pyrphos catalyst attached covalently to sihca gel, Pugin and colleagues have developed the modular toolbox which his depicted schematically in Figure 12.4. The main elements of their system are functionalized chiral diphosphines, where three different hnkers are based on isocyanate chemistry and various carriers [37, 45, 58]. This approach allows for a systematic and rapid access to a variety of immobilized chiral catalysts, with the possibility of adapting their catalytic and technical properties to specific needs. 

The chapter Supramolecular Information/Programming from a Boolean Perspective, Concepts delves deeper into the reversible covalent bond toolbox. The equilibration of these strong covalent bonds, often referred to as dynamic covalent chemistry, is kineticaUy slower in comparison to weaker noncovalent interactions and often requires a catalyst. An extreme example might also include the formation of carbon nanotubes and fullerenes. While not spontaneous at room temperature, carbon vapor at high temperature does assemble to form these intricate and beautiful stractures. 

For many covalent reactions, conditions that meet some or aU of the criteria listed above have been found (Table 1). However, in many cases, studies have been limited to a proof-of-principle demonstration of reversibility. The vast majority of actual applications of dynamic covalent chemistry appear to predominantly rely on disulhde exchange or imine-type exchange reactions. A detailed mechanistic description of some important reversible covalent bonds is provided in the chapter titled Reversible Covalent Bond Toolbox, Concepts. 

Ongoing research is aimed at increasing the toolbox of available reversible covalent bonds. The amide bond is probably the one that is attracting most interest, especially from the perspective of DCC (see below). Reversible formation of the robust amide bond would permit a scrambling of peptides, which would give easy access to combinatorial libraries of vast structural diversity. Consider that the 20 naturally occurring amino acids can form 1.6 X 10 (20 ) different tetrapeptides. Methods to shift the... 

The successful synthesis of large molecular structures brings an intrinsic joy to chemists. The ability to rationally create well-defined architectures has risen to great heights, and in many cases the obtained structnres can be considered miniature pieces of art. " " It is also the area that most abundantly employs the complete toolbox of chemical bonds available, covering the entire range of chemical stabilities. Here, only a narrow selection of structures that are entirely organic in nature (i.e., without the use of metal ions) and that exclusively rely on dynamic covalent bonds for their formation is discussed. The focus is on the dynamic synthesis of these structures, rather than on their applications which are extensively discussed elsewhere in this series. 

The examples discussed in this chapter illustrate that the dynamic covalent bond is now forming an integral part of the toolbox available to organic chemists. It also illustrates that the classical distinction between covalent and noncovalent bonds in terms of kinetic and thermodynamic stabilities is losing relevance. This, in turn, points to the important role supramolecular chemistry is playing in all areas of chemistry. 

We have examined five types of chemical interactions that occur between monolayers and molecules electrostatic binding, covalent linking, complexation interactions, proton transfer, and hydrogen bonding (9-14). These interactions are five of the six tools presently in our "toolbox" the sixth is a physical recognition... 

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