Reactivity matching

The regioselectivity of nucleophilic addition is a function of the type of nucleophile employed and in many instances can be controlled to give Michael addition to the /3 carbon. 

Having defined the types of commonly used carbon nucleophiles and carbon electrophiles, it would seem that if you react any of the carbon nucleophiles (electron donors) with any of the carbon electrophiles (electron acceptors), then a carbon-carbon bond should be formed. While this is theoretically true, it is unworkable from a practical point of view. If, for example, a carbanion nucleophile was reacted with a cationic electrophile, it is unlikely that the desired carbon-carbon bond formation would be detected, even after the smoke cleared. Or if a silyl enol ether nucleophile was reacted with an a, /f-unsaturated ester, no reaction could be observed to take place in any reasonable time frame. 

For carbon-carbon formation to be successful, the reactivities of the nucleophile and electrophile must be matched so that reaction occurs at a reasonable 

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Radical allylations with allylsilanes 71 occur under mild conditions in good to excellent yields, provided that the radical precursor and the silane have the appropriate electronic pairing [85]. The two examples in Reactions (7.75) and (7.76) show the reactivity matching of the allylating agent with the radical. These reactions offer tin-free alternatives for the transformations that are currently carried out by allyl stannanes. 

The reaction of allyl silanes with aldehydes and ketones activated as electrophiles by Lewis acids is a very useful method for preparing homoallylic alcohols. Since allyl silanes are only modestly nucleophilic, strong electrophiles are needed to ensure a good reactivity match. 

The development of novel MCRs is an intellectually challenging task since one has to consider not only the reactivity match of the starting materials but also the reactivities of the intermediate molecules generated in situ, their compatibility, and their compartmentalization. With advances in both theory and mechanistic insights into various classic bimolecular reactions that allow for predictive analysis of reaction sequences, the development and control of new reactive chemical... 

This review focuses on free radical-mediated stereoselective bond construction in which the carbonyl group plays a key role. Reaction at the carbonyl group as well as on carbons alpha and beta are described. The general reaction characteristics of these reactive intermediates are as follows. The acyl radicals are nucleophilic in character and thus they react easily with electrophilic acceptors. On the other hand, radicals on carbon alpha to the carbonyl are electrophilic in nature and their reactivity matches with nucleophilic partners. The majority of reactions at carbon beta to the carbonyl are in a, -unsaturated systems and in these the beta carbon is electrophilic. 

MCR is composed of a series of elementary bimolecular reactions. Mixing three or more substrates, each bearing a reactive functional group, could provoke different reaction manifolds leading to different final products. In addition, the very domino nature of a MCR implies that any bimolecular reaction would generate a new intermediate function, which will in turn enter into the reaction process. Therefore, in devising any novel MCR, one has to carefully consider not only the reactivity match of the functional groups but also that of the incipient intermediate functionalities. An illustrative example is shown in Scheme 15.4. It is known that... 

Scheme 15.4 Radical-based three-component reaction example of reactivity match.

The PUs hard segments can be either aromatic or aliphatic. The aromatic isocyanates are more reactive than the aUphatic diisocyanates, which can only be utilized if their reactivities match the specific polymer reaction and special properties desired in the final product. Eor example, PUs made from aliphatic isocyanates are light stable [30-33], while materials made from aromatic isocyanates undergo photo degradation [34—36]. Furthermore, the reactivity of an isocyanate group may vary dramatically even within the same class of a diisocyanate. 

Amine Cross-Linking. Two commercially important, high performance elastomers which are not normally sulfur-cured are the fluoroelastomers (FKM) and the polyacrylates (ACM). Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diamine carbamate (Diak 1). Because the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM ia questioa. Sulfur and sulfur-beating materials can be used as cure retarders they also serve as age resistors (22). Fluoroelastomer cure systems typically utilize amines as the primary cross-linking agent and metal oxides as acid acceptors. 

Frontier orbital theory also provides the basic framework for analysis of the effect that the symmetiy of orbitals has upon reactivity. One of the basic tenets of MO theory is that the symmetries of two orbitals must match to permit a strong interaction between them. This symmetry requirement, when used in the context of frontier orbital theory, can be a very powerful tool for predicting reactivity. As an example, let us examine the approach of an allyl cation and an ethylene molecule and ask whether the following reaction is likely to occur. 

Sacrificial anode systems operate without external power source. The anodes are reactive metals such as magnesium and zinc or aluminum alloys. The energy for the process is derived from the anode material. Careful design is required to match the output and lifetime of the anodes with the polarization and life-expectancy requirements of the plant. Sacrificial anode CP is used for offshore platforms, sub-sea pipelines and the inside of ballast tanks on tanker ships. 

Although more studies need to be performed to study the scope and generality of this system, the use of amine hydrochloride salts as initiators for controlled NCA polymerizations shows tremendous promise. Fast, reversible deactivation of a reactive species to obtain controlled polymerization is a proven concept in polymer chemistry, and this system can be compared to the persistent radical effect employed in all controlled radical polymerization strategies [37]. Like those systems, success of this method requires a carefully controlled matching of the... 

The surface properties of these nano-objects match those of metal nano crystals prepared in ultrahigh vacuum, for example the C - O stretch of adsorbed carbon monoxide or the magnetic properties of cobalt particles embedded in PVP. This demonstrates the clean character of the surface of these particles and its availabihty for reactivity studies. 

FIGURE 34.2 One-dimensional scheme of the free-energy surfaces of the initial and final states. Medium polarization plays the role of the reactive modes. Matching of the electron energy levels corresponds to crossing of the free-energy snrfaces = 17y(P ). 

We ran a second set of experiments to examine the reactivity of glyceraldehyde withont interference from glucose. This set of studies is referred to as Experiment B. The resnlts of the experiment are shown in Table 46.3 and Figure 46.5. The results match what was observed in Experiment A. The molar balance of aronnd 80% indicates abont 20% formation of condensation heavies not observed by HPLC. 

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