Reactivity selectivity studies

Reactivity/selectivity studies M. Marx, T. T. Tidwell, J. Org. Chem. 49, 788 (1984). Reviews A. J. Mancuso, D. Swem, Synthesis 1981, 165-185 passim T. T. Tidwell, Org. React. 39, 297-572 passim (1990). Cf. Corev-Kim Oxidation Pfitzner-Moffatt Oxidation. 

The (Schiffbase)vanadium(V) complex 53 with tridentate imine auxiliaries acts as a catalyst for the oxidation of Br with tert-butyl hydroperoxide (TBHP) in nonaqueous solvents. The principal objective of this study performed by Hartung and coworkers [72] was associated with the search for an adequate combination of bromide source and primary oxidant that would reduce the inherent propensity of (SchifFbase) vanadium]V) complexes in the presence of TBHP to convert alkenols (54) directly into hydroxy-functionalized tetrahydrofurans (56) [73]. Reactivity-selectivity studies on the vanadium(V)-catalyzed oxidation of bromide were investigated through ESI (—)-MS" in order to interpret crucial steps in the process such as peroxide activation... 

Absolute rate data for Friedel-Crafts reactions are difficult to obtain. The reaction is complicated by sensitivity to moisture and heterogeneity. For this reason, most of the structure-reactivity trends have been developed using competitive methods, rather than by direct measurements. Relative rates are established by allowing the electrophile to compete for an excess of the two reagents. The product ratio establishes the relative reactivity. These studies reveal low substrate and position selectivity. 

It becomes intriguing to inquire what leads to the observed contrasteric reactivity. Intensive studies to disclose the origin of Tt-facial selectivity examined various dienes having unsymmetrical 7t-plane, since their reactions potentially generate five or more consecutive stereocenters with one operation. In this chapter, we review the theories to disclose the origin of 7t-facial selectivity in Diels-Alder reactions of the substrates having unsymmetrical 7t-planes. Recent works are discussed. 

In conclusion, bromination is a particularly attractive reaction for studying the origin of reactivity-selectivity effects in detail, since it is now well established that substituent and solvent effects arise not only from changes in the stability of the cationic intermediate but also from transition-state shifts, in agreement with the Bema Hapothle, i.e. RSP, Hammond postulate and Marcus effects. 

The structure of SSZ-35 (IZA structure code STF) as viewed in the [001] direction is shown in Fig. 17. The dimensions of the 10-MR structures are 5.5 x 6.1 A and the diameter of the 18-MR structures is 12.5 x 9 A. This pore structure is in contrast to the structure of SSZ-44 (IZA structure code SFF) shown in Fig. 18, where the 10-MR structures are nearly spherical (5.8 A) and the 18-MR structures are slightly larger (12.9 x 9 A). These small differences in pore size apparently translate into startling differences in reactivity. A study of m-xylene conversion shows a high degree of isomerization versus disproportionation, which is consistent with a 10-MR pore system (47). The interesting data is the para to ortho selectivity in the isomerization products, where SSZ-44 exhibited a higher para/ortho... 

Chatteijee, A., Iwasaki, T., and Ebina, T. 2002. 2 1 Dioctahedral smectites a selective sorbent for dioxins and furans—a reactivity index study. J. Phys. Chem. A. 106 641-648. 

In this instance, not only do the calculations bring into question the validity (or at least the generality) of the reactivity-selectivity principle but, more importantly, they open up a route to actually improving reaction selectivity. Note, in particular, the close parallels between the way this investigation was carried out and the way in which an experimental study would be performed. 

Dynamic ETEM experiments on CS defects have shown mat mey consume anion vacancies and grow (figure 3.7). These correlation studies indicate mat CS planes are secondary or detrimental to catalytic reactivity. They eliminate anion vacancies by accommodating the supersaturation of the vacancies in the reacting oxide catalyst and me catalyst reactivity (selectivity) begins to decrease with the onset of CS formation, i.e. CS planes are the consequence of catalyst reduction reactions rather than the origins of catalytic reactivity (Gai 1981,1992, 1993, Gai etfl/ 1982). 

The roles of carbocations in commercially important hydrocarbon transformations are still not perfectly understood. The same can be said for carbocations in biological systems. Significant questions concerning reactivity still need to be explained. Why do so many reactions of carbocations show constant selectivity, in violation of the reactivity-selectivity principle Is it possible to develop a unified scale of elec-trophilicity-nucleophilicity, in particular one that incorporates these parameters into the general framework of Lewis acidity and basicity. Finally, quite sophisticated synthetic transformations are being developed that employ carbocations, based upon insights revealed by the mechanistic studies. 

The selectivity studies show that concave pyridines 3 (Table 1) not only catalyze the addition of alcohols to ketenes, but they may also differentiate between different OH groups in inter- and intramolecular competitions. They are not only reactive but also selective. First experiments with a chiral concave 1,10-phenanthroline show that enantioselectivity is also possible [20]. Structure 9 shows the concave 1,10-phenanthroline 21 g which catalyzes the addition of R-1-phenylethanol (R-68) to diphenylketene (59a) 20% faster than the addition of the S-enantiomer S-68. 

Mechanisms, of proton transfer between oxygen and nitrogen acids and bases in aqueous solutions, 22, 113 Mechanisms, organic reaction, isotopes and, 2, 1 Mechanisms of reaction, in solution, entropies of activation and, 1, 1 Mechanisms of reaction, of /3-lactam antibiotics, 23, 165 Mechanisms of solvolytic reactions, medium effects on the rates and, 14, 10 Mechanistic analysis, perspectives in modern voltammeter basic concepts and, 32, 1 Mechanistic applications of the reactivity-selectivity principle, 14, 69 Mechanistic studies, heat capacities of activation and their use, 5, 121 Mechanistic studies on enzyme-catalyzed phosphoryl transfer, 40, 49 Medium effects on the rates and mechanisms of solvolytic reactions, 14, 1 Meisenheimer complexes, 7, 211... 

Reid DL, Armstrong DA, Rauk A, Nese C, Schuchmann MN, Westhoff U, von Sonntag C (2003) H-atom abstraction by C-centered radicals from cyclic and acyclic dipeptides. A theoretical and experimental study of reaction rates. Phys Chem Chem Phys 5 3278-3288 Roberts BP (1996) Understanding the rates of hydrogen abstraction reactions empirical, semi-em-pirical and ab initio approaches. J Chem Soc Perkin Trans 2 2719-2725 Russell GA (1973) Reactivity, selectivity, and polar effects in hydrogen atom transfer reactions. In Kochi JK (ed) Free radicals. Wiley, New York, pp 275-331 Russo-Caia C, Steenken S (2002) Photo- and radiation-chemical production of radical cations of methylbenzenes and benzyl alcohols and their reactivity in aqueous solution. Phys Chem Chem Phys 4 1478-1485... 

Kinetic studies at 100 °C revealed that ethyl /V-ethylthionocarbamate, EtOC(S)NHEt, was hydrolysed in acid by an Al mechanism and in base by a BAc2 mechanism.53 The concerted mechanism proposed for the aminolysis at 30 °C in MeCN of aryl /V-ethyl thionocarbamates, (XC6H40)C(S)NHEt, by benzylamine was supported by a negative cross-interaction constant, pxz = —0.87 and failure of the reactivity-selectivity principle.54 Similar conclusions, for the same reasons, were made for the aminolysis of the corresponding thiolocarbamates, (XC6H4S)C(0)NIIFL, by benzylamine in MeCN at 10 °C.55... 

Chandrakumar, K.R.S., and Pal, S., DFT and local reactivity descriptor studies on the nitrogen sorption selectivity from air by sodium and calcium exchanged zeolite-A, Colloids Surf. A - Physicochemical Eng. Aspects, 205, 127-138, 2002a. 

Table II. A Brief Summary of Literature Relating Generally to Reactivity—Selectivity-Poisoning Studies of Bimetallic Catalysts...

This review, therefore, attempts to survey the current status of reactivity-selectivity relationships by exploring the basis and variety of such relationships. The part they have played in a number of selected areas of mechanistic interest is also discussed. No attempt is made to cover every facet of mechanistic study. Rather, the aim has been to point out different reactivity-selectivity relationships that have been noted in such studies and to focus attention on the use of such relationships as a mechanistic tool. In addition, an attempt has been made to outline the limitations of the reactivity-selectivity principle. This serves not only to increase the utility of the principle but also strengthens the basis and justification for its use by rationalizing apparent failures that have been reported. 

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