Direct Covalent Bond Formation Method

Laboratory procedures are presented for two divergent approaches to covalent structure controlled dendrimer clusters or more specifically - core-shell tecto(dendrimers). The first method, namely (1) the self assembly/covalent bond formation method produces structure controlled saturated shell products (see Scheme 1). The second route, referred to as (2) direct covalent bond formation method , yields partial filled shell structures, as illustrated in Scheme 2. In each case, relatively monodispersed products are obtained. The first method yields precise shell saturated structures [31, 32] whereas the second method gives semi-controlled partially shell filled products [30, 33],... 

The second method referred to as the direct covalent bond formation method , produces semi-controlled, partial shell filled structures. It involves the reaction of a limited amount of nucleophilic dendrimer core reagent with an excess of... 

Inspired by these derived values for shell filling around a centfal dendrimer core, we devised several synthetic approaches to test this hypothesis. The first method involved the direct covalent reaction of a dendrimer core with an excess of dendrimer shell reagent referred to as the (a) Direct Covalent Method. The second method involved self-assembly by electrostatic neutralization of the dendrimer core with excess shell reagent to give the (b) Self-Assembly with Sequential Covalent Bond Formation Method. These strategies are described in Section IV. In each case, relatively monodispersed products are obtained. We call these new dendritic architectures core-shell (tecto)dendrimers. 

The first catenanes and rotaxanes were constructed by either statistical [2] or directed [3] synthetic approaches. The statistical method relies [2] on the formation of small quantities of a species in which a cyclic molecule is threaded by an acyclic molecule. After experiencing appropriate covalent bond formation, these threaded species are con-... 

Adsorption control in catalyst preparation can be achieved from both liquid and gas phase once the necessary conditions for the strong interaction between precursor and support have been created. This review has focused on the atomic layer epitaxy (ALE) method where the gas-solid reactions of precursors are directed to the strong interaction of covalent bond formation. In ALE, surface saturation is systematically utilized, providing the means for precise control of metal density and rendering the method truly adsorption controlled. 

The methods for the determination of the basicity of aromatic compounds discussed hitherto have as their starting point the formation of a proton addition complex in an acid solution. In addition to this interaction, numerous intermolecular interactions are known which are also directly connected with the basicity of unsatimated compounds but which do not lead to the formation of a true covalent bond. This interaction was already mentioned in connection with the vapour pressure measurements of the system of aromatic substance-HCl, and leads to a 77--complex (Dewar, 1946). 

As discussed earlier, the concepts of chiral chromatography can be divided into two groups, the indirect and the direct mode. The indirect technique is based on the formation of covalently bonded diastereomers using an optically pure chiral derivatizing agent (CDA) and reacting it with the pair of enantiomers of the chiral analyte. The method of direct enantioseparation relies on the formation of reversible quasi diastereomeric transient molecule associates between the chiral selector, e.g., i /t)-SO, and the enantiomers of the chiral selectands, [R,S)-SAs [(Ry SA + (S)-SA] (Scheme 1). 

The problem of directed valence is treated from a group theory point of view. A method is developed by which the possibility of formation of covalent bonds in any spatial arrangement from a given electron configuration can be tested. The same method also determines the possibilities of double and triple bond formation. Previous results in the field of directed valence are extended to cover all possible configurations from two to eight s, p, or d electrons, and the possibilities of double bond formation in each case. A number of examples are discussed. 

We can also specialize these methods according to physical requirements and chemical requirements. This provides an abstraction barrier between what is true (i.e., the physical laws) and what is believed to be true (i.e., current theory). For example, suppose that the methods favor-able-interatomic-distance-p and potentially-stable-bond-formation-p are physical requirements whereas the methods available-bonding-electron-p and proper-orbital-symmetry-p are chemical requirements. We represent this to covalent-bond by associating the top-level methods (i.e., methods associated with K j ), chemical requirements, and physical requirements, to covalent-bond directly (i.e., we override the method inherited by the mother model class). This is accomplished using the semantic relationship is-method-of, as was demonstrated earlier ... 

The molecular assemblies described above have inspired us, in recent years, to develop finite assemblies in the solid state that exhibit chemical reactivity. Specifically, we,69 and others,70 have been utilizing principles of molecular recognition and self-assembly to develop a method to direct the formation of covalent bonds in organic solids. The method builds on the work of Schmidt on the reactivity of cinnamic acids in the organic solid state.45 Specifically, Schmidt has described topochemical postulates that dictate geometry criteria for a [2 + 2] photodimerization to occur in a solid. The postulates state that two carbon-carbon double (C=C) bonds should be aligned in parallel and separated by a distance <4.2 A to react. 

Oxygen forms binary compounds with nearly all elements. Most may be obtained by direct reaction, although other methods (such as the thermal decomposition of carbonates or hydroxides) are sometimes more convenient (see Topic B6). Oxides may be broadly classified as molecular, polymeric or ionic (see Topics B1 and B2). Covalent oxides are formed with nonmetals, and may contain terminal (E=0) or bridging (E-O-E) oxygen. Especially strong double bonds are formed with C, N and S. Bridging is more common with heavier elements and leads to the formation of many polymeric structures such as Si02 (see Topics FT and F4). 

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