INDEX nucleophilic addition

CONTENTS Nucleophilic Addition to the a-Carbon of H c-lams, P. R. Guzzo and M. J. MHler. Chemical Synthesis of Porphobilinogen and Studies of Its Biosynthesis. R Neier. Synthesis and Cycloaddiiion Reactiorts of Iso-Cortdensed Heteroaromatic F rroles. C. K. Sha. Azacyclopentadienyl Metal Compounds Historical Background and Recent Advances. C. Janiak and N. Kuhn. Recent Developments in the Synthesis of Marine Pyridoacridine Alkaloids. A. M. Echavar-ren. Alkaloid Synthesis Using 1 -Acylpyridinium Salts as Intermediates, 0. L. ConyinsandS. P. Joseph. Index. 

W. Langenaeker, K. Demel and P. Geerlings. Quantum chemical study of the Fukui function as a reactivity index. Part 3. Nucleophilic addition at activated CC double bonds. J. Mol. Struct. (THEOCHEM) 259, 1992, 317. 

In addition to the above prescriptions, many other quantities such as solution phase ionization potentials (IPs) [15], nuclear magnetic resonance (NMR) chemical shifts and IR absorption frequencies [16-18], charge decompositions [19], lowest unoccupied molecular orbital (LUMO) energies [20-23], IPs [24], redox potentials [25], high-performance liquid chromatography (HPLC) [26], solid-state syntheses [27], Ke values [28], isoelectrophilic windows [29], and the harmonic oscillator models of the aromaticity (HOMA) index [30], have been proposed in the literature to understand the electrophilic and nucleophilic characteristics of chemical systems. 

In addition to allowing comparison among experiments carried out in different solvents, the m—Y system serves as an important tool for study of mechanism. Sensitivity to ionizing power, measured by m, is an index of the degree of charge separation at the transition state. The ratio of rates in two solvents of equal Y but different nucleophilicity provides evidence about nucleophilic assistance by a solvent molecule during the ionization. 

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

From the relation between Fukui function and local softness, electrophilic and nucleophilic local softnesses can be computed. Donor and acceptor sites can also be identified by large values of both types of local softnesses in addition, it can be used to compare sites of different molecules and to identity which one is softer or harder. The elec-trophilicity index can also be extended to a local context,21 and a comparison of the electrophilicity of sites in different species can be made. 

Using their CNDO results Helland and Skancke calculated indices of reactivity (frontier electron density, FED, for electrophilic substitution frontier orbital density, FOD, for nucleophilic substitution frontier radical density, FRD, for radical substitution) for the thienopyridines. It was indicated that the FED index has its highest value for C-3 in the [2,3-]- and 3,2- -fused systems and for C-2 in the [3,4-1-fused isomers. As far as the former group is concerned, the predictions are in agreement with experimental observations (see Section IV,A.). Little experimental evidence is available for the [3,4-]-fused systems, but it seems highly probable that they would have a considerable tendency to undergo addition reactions at the 1,3-positions, since the product would contain a normal rather than a quinoid pyridine ring [Eq. (23)]. 

Eisch et al. (24) performed a mechanistic study of the desulfurization of dibenzothiophene by a nickel(0)-bipyridyl complex and reported that a radical anion of the thiophene nucleus was formed and underwent C-S bond cleavage into S and an aromatic radical. In addition, they suggested that the oxidative reaction of the nickel(0)-bipyridyl complex toward dibenzothiophene had the characteristics of stepwise electron transfer rather than nucleophilic attack. However, no correlations occurred between the desulfurization rate and the reaction indexes of Fr(E), Fr(N), and Fr(R), as shown in Table II. The results suggested no evidence for either electron transfer or nucleophilic attack in this study. Moreover, the radical reaction was not... 

It is worth emphasizing at this point that the general reaction mechanism depicted in Scheme 7.1 is not unequivocal, but several other reaction routes leading to the same reaction product in Scheme 7.1 can be operative—for example, it may be a concerted nucleophilic attack followed by deprotonation of the nucleophile or proton transfer assisted by the solvent and others [77]. However, for the purpose of illustrating the reliability of the reactivity indexes approach to describe reaction mechanisms, the analysis will be performed on the basis of Scheme 7.1. Additionally, Scheme 7.1 is valid only for neutral nucleophiles. It has been proposed, from computational studies, that the use of charged nucleophiles (anions) may lead to concerted routes [78], but for practical purposes, the discussion in this chapter will consider only neutral nucleophiles. 

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