Reactivity and selectivity

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

Nitration in sulphuric acid is a reaction for which the nature and concentrations of the electrophile, the nitronium ion, are well established. In these solutions compounds reacting one or two orders of magnitude faster than benzene do so at the rate of encounter of the aromatic molecules and the nitronium ion ( 2.5). If there were a connection between selectivity and reactivity in electrophilic aromatic substitutions, then electrophiles such as those operating in mercuration and Friedel-Crafts alkylation should be subject to control by encounter at a lower threshold of substrate reactivity than in nitration this does not appear to occur. 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

Selectivity and reactivity of mixed anhydrides toward primary and secondary aliphatic and aromatic amines has been studied in details [42, 43] (equation 22) Fluorocarboxyhc acids and then denvahves are used as building blocks m condensa non reactions widi dmucleophihc species for the synthesis of fluoroalkyl-subshtuted lieteroaromahc systems [S 9, 40, 44, 45, 46, 47 4S] (equations 20 21, and 23)... 

For secondary derivatives, the selectivity and reactivity vary somewhat. To what extent this depends on the highly functionalized glucose derivative has not been determined. The table that follows gives the cleavage selectivity for the reaction... 

A related study used the air- and moisture-stable ionic liquids [RMIM][PFg] (R = butyl-decyl) as solvents for the oligomerization of ethylene to higher a-olefins [49]. The reaction used the cationic nickel complex 2 (Figure 7.4-1) under biphasic conditions to give oligomers of up to nine repeat units, with better selectivity and reactivity than obtained in conventional solvents. Recycling of the catalyst/ionic liquid solution was possible with little change in selectivity, and only a small drop in activity was observed. 

Attempts to realize enzymatic reactions have been reported over the past four decades in the context of host-guest chemistry, presently a well-established research field. In the field of CPOs, much attention has been paid to identical research objectives. The host-guest chemistry based on CPOs holds a special position, because specific selectivity and reactivity will be achieved using the coordination-bond-forming reactions between the substrate and the incorporated metals in the porphyrins, as well as the redox reaction associated with the porphyrin s rr-electron system. 

Figure 10.8. Temperature-programmed reaction of NO and CO on two surfaces of rhodium. The initially molecularly adsorbed NO dissociates entirely at relatively low temperatures, but NO does not desorb. Note the difference in selectivity and reactivity between the surfaces on Rh(lOO) most of the CO oxidizes to CO2 and the reaction already...

Use of an ethylene atmosphere can be beneficial to increase catalyst selectivity and reactivity [176] but it can also lead to the unwanted production of the corresponding 1,3-butadiene due to competition of ethylene with the desired cross-partner [177]. On the other hand ECM with ethylene as the cross-partner can be used as a very convenient method for the production of 2,3-disubstituted butadiene systems. The successful application of this methodology has for example been achieved by Mori et al. in the total synthesis of anolignan A (Scheme 3.20) [178]. 

McCracken, P. G. Bolton, J. L. Thatcher, G. R. J. Covalent modification of proteins and peptides by the quinone methide from 2-ZerZ-butyl-4,6-dimethylphenol selectivity and reactivity with respect to competitive hydration, j. Org. Chem. 1997, 62, 1820-1825. 

K. Mizutani, T. Electronic and structural requirements for metabolic activation of butylated hydroxytoluene analogs to their quinone methides, intermediates responsible for lung toxicity in mice. Biol. Pharm. Bull. 1997, 20, 571-573. (c) McCracken, P. G. Bolton, J. L. Thatcher, G. R. J. Covalent modification of proteins and peptides by the quinone methide from 2-rm-butyl-4,6-dimethylphenol selectivity and reactivity with respect to competitive hydration. J. Org. Chem. 1997, 62, 1820-1825. (d) Reed, M. Thompson, D. C. Immunochemical visualization and identification of rat liver proteins adducted by 2,6-di- m-butyl-4-methylphenol (BHT). Chem. Res. Toxicol. 1997, 10, 1109-1117. (e) Lewis, M. A. Yoerg, D. G. Bolton, J. L. Thompson, J. Alkylation of 2 -deoxynucleosides and DNA by quinone methides derived from 2,6-di- m-butyl-4-methylphenol. Chem. Res. Toxicol. 1996, 9, 1368-1374. 

With ferrocenes, an alternative approach has been to attach the phosphorus moieties only to side chains. The WalPhos family (36) forms an eight-mem-bered chelate with the metal. Members of this family provide good selectivity and reactivity for the reductions of dehydroamino and itaconic acid derivatives as well as a,/ -unsaturated carboxylic acids [145, 156],... 

We will present mechanistic aspects of the Diels-Alder reaction, its selectivity and reactivity in order to explain solvent effects on the one hand, and the effects of Lewis acids on the other. Other catalytic systems like micelles will also be addressed. Some of the explanations may seem trivial or are well-known but, as we will use these in later sections, a clear terminology is desirable. 

The selectivity and reactivity of the dihalocarbenes is independent of the catalysts used in their generation [165],... 

The 7-chloro derivative (Cl-PIQ) 46 was found to provide even better selectivity and reactivity than CF -PIP 45 for aryl alkyl iec-alcohols and, moreover, was effective for certain cinnamyl-based aUyUc xec-alcohol substrates s = 17-117, Scheme 15) [153, 154],... 

Determine the selectivity and reactivity of. CH, from the yields of products from methylene insertion in pentane ... 

Two main mechanistic hypotheses were considered for this reaction type38, a zinca ene 39 reaction and a metallo-Claisen rearrangement (equations 31 and 32). 70 and 71 are drawn as monomers for the sake of simplicity. The former probably exists in oligomeric form (see Section . . ), whereas the transition state of the metallo-Claisen rearrangement may involve two molecules of 7138. Since the simplified structures are perfectly suitable to rationalize the selectivity and reactivity of these reactions, they are used throughout this chapter. 

The selectivity and reactivity in indirect electrochemical syntheses can be enhanced by coordination of the substrate or an intermediate to the redox catalyst, for example through metal centers. In direct electrolyses, however, the selectivity and reactivity is mainly controlled by the difference between the electrode potential and the redox potentials of the different functions within the substrate. 

A selective and reactive oxidizing agent. Will epoxidize a,3-unsaturated carbonyl compounds. In epoxidation reactions, there is a strong steric influence directing the facial selectivity ... 

Site selectivity and reactivity of acceptor OH groups (e.g., axial, equatorial, primary D-gluco, D-galacto, C-3, C-4, or other)... 

Selectivity and reactivity in 1,3-dipolar cycloadditions of the nonclassical A,B-diheteropentalenes... 

---