Stepwise ionic type

When a stepwise ionic addition reaction involves nucleophilic attack at carbon as a first step, it is described as a nucleophilic addition. Reactions of this type often are catalyzed by bases, which generate the required nucleophile. For example, consider the addition of some weakly acidic reagent HX to an alkene. In the presence of a strong base ( OH), HX could give up its proton to form the conjugate base Xe, which is expected to be a much better nucleophile than HX ... 

Diels-Alder reactions of heterodienophiles have been known for decades, but only recently has this methodology become widely accepted by the synthetic community. There is enormous diversity in the structural types of compounds which can act as heterodienophiles, and a wide array of heterocyclic adducts can be prepared via these [4 + 2] cycloadditions. It seems clear that hetero Diels-Alder reactions span a range of mechanism from concerted to stepwise ionic processes. In many instances, mechanistic information is totally lacking. The discussion below therefore classifies heterodienophiles by structural rather than mechanistic class. Only the major types of synthetically useful heterodienophiles have been included. Moreover, the significant regio- and stereo-chemical features of the reactions have been exemplified as much as possible using recently reported cases. Other more comprehensive and more specialized reviews should be consulted for older material and more obscure hetero Diels-Alder cycloadditions. 

Acyclic C-acyl imines have recently been studied as dienophiles.32-34 p j example, Prato and coworkers examined the reaction of imines (41) (equation 13) with several cyclic and acyclic 1,3-dienes. Under neutral conditions, (41) is unreactive as a dienophile. However, under Lewis acid catalysis these imines react to afford mixtures of adducts. With 1,3-cyclohexadiene, bicyclic adducts (42) and (43) are produced along with (44) in which the imine has acted as an azadiene. The ratios of these sorts of products are dependent upon the particular imine and diene used. The formation of adducts of type (43) proved to be both regio- and stereo-selective. Product formation in these cases can be rationalized lx>th by concerted and by stepwise ionic mechanisms. ... 

Two studies on the mechanism of this type of [4 + 2] cycloaddition which have led to very di erent interpretations have appeared. Mock and Nugent suggested that the Diels-Alder reactions of N-sulfi-nyl-p-toluenesulfonamide are stepwise, ionic processes. On the other hand, Hanson and Stockbum prefer a concerted, pericyclic mechanism in accord with frontier molecular orbital theory. Both proposals satisfactorily rationalize the observed regioselectivity of these reactions. 

Case 14. The examples of this type may only be formal ones, since most of the known reactions187 could be stepwise ionic reactions.188... 

With the preceding reviews of the enzymology of monoterpene cyclization and of model studies relevant to the cyclization process, it is possible to formulate a unified stereochemical scheme for the enzymatic cyclization of geranyl pyrophosphate (Figure 4). The proposal which follows is consistent with the implications of parallel advances in related fields, most notably the contributions of Cane (8,16,24,25,52), Arigoni (67) and Coates (68,69) on the stereochemistry of sesquiterpene and diterpene cyclizations, and of Poulter and Rilling (29,70) on the stepwise, ionic mechanism of prenyl transferase, a reaction type of which several monoterpene, sesquiterpene and diterpene cyclizations are, in a sense, the intramolecular equivalents. 

Membrane absorbers are continuous chromatographic supports, which circumvent some of the above-mentioned problems of particulate stationary phases. They were originally derived from membrane (filtration) technology. The immobilization of interactive (ionic, hydrophobic, or biospecific) groups on the surface of microfiltration membranes was found to increase the selectivity of certain separation procedure. Ideally such activated membranes, or membrane adsorbers, allow the selective adsorption of certain substances and substance classes, which may subsequently be eluted by means of a stepwise change of the mobile phase (elution buffer). More complete information on the various types of modern membrane technology can be found in some recent reviews [e.g., 31-33]. 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

There is far less information in the scientific literature about template copolymerization than about template homopolymerization. As in the case of template homopolymerization, template copolymerization can be realized according to different types of reaction stepwise (template polycondensation), copolyaddition, radical or ionic polymerization, ring-opening copolymerization, etc. 

Liquid chromatography can be operated under mild conditions in terms of pH, ionic strength, polarity of liquid, and temperature. The apparatus used is simple in construction and easily scaled up. Moreover, many types of interaction between the adsorbent (the stationary phase) and solutes to be separated can be utilized, as shown in Table 11.1. Liquid chromatography can be operated isocratically, stepwise, and with gradient changes in the mobile phase composition. Since the performance of chromatography columns was discussed, with use of several models and on the basis of retention time and the width of elution curves, in Chapter 11, we will at this point discuss some of the factors that affect the performance of chromatography columns. 

When Z is a simple aquacation, two types of complex are formed depending upon the ionic radius of Z. For alkali, alkaline earth and most transition metal cations the product contains Z"+ in quasi-octahedral coordination. Equilibrium constants for reaction (6) have been determined for Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Fe2+, Co2+, Ni, Cu2+ and Zn2+.93 For the transition metals, log K lies between 3 and 9, and is sensitive both to Z and to the lacunary polyanion involved. Larger cations, Sr24, Ba2+, and tri- and tetra-valent lanthanides and actinides are also able to bind two lacunary ligands in a manner similar to that illustrated in Figure 18. Although the stepwise formation of 1 1 and 2 1 complexes of the... 

Many reactions involve a cyclic transition state. Of these, some involve radical or ionic intermediates and proceed by stepwise mechanisms. Pericyclic reactions are concerted, and in the transition state the redistribution of electrons occurs in a single continuous process. In this chapter, we will consider several different types of pericyclic reactions, including electrocyclic transformations, cycloadditions, sigmatropic rearrangements, and the ene reaction. 

For complexes of the type discussed here one often finds a regular decrease in the stepwise equilibrium constants. In this respect the data (based on the least-squares method) look satisfactory. However, the data in 8 M and 13 M HNO3 have not been used in the selection of equilibrium constants because of the high ionic strength. 

Stepwise Elution of Antithrombin III or Heparin Both types of material have affinities for several other plasma proteins (6,10). Antithrombin III, for example, has an affinity constant of about 10 1 M for either PAOM or PSSO solid surface (8,J ) Consequently, retention on these resins was observed in low-pressure affinity chromatography, and only at a low ionic strength (10). It should be noted that the adsorption of thrombin was far greater (100-fold) than that of its inhibitor under the same experimental conditions. 

Polymers are macromolecules which are composed of smaller molecules linked by covalent bonds. In terms of the reaction kinetics, polymerizations are traditionally classified into several categories stepwise polymerization, free-radical polymerization, ionic polymerization, ring-open polymerization, and coordination polymerization or polyinsertion. Each polymerization method has a combination of requirements for reaction conditions, and they exhibit certain types of product and process features (Caneba, 1992a, 1992b Odian, 1991). Even though in principle, the FRRPP process can be implemented with a wide variety of polymerization mechanisms, its discovery and immediate implementation has occurred in conjunction with free-radical kinetics. 

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