Theoretical calculations electrophilic reactions

Jug and co-workers investigated the mechanism of cycloaddition reactions of indolizines to give substituted cycl[3,2,2]azines <1998JPO201>. Intermediates in this reaction are not isolated, giving evidence for a concerted [8+2] cycloaddition, which was consistent with results of previous theoretical calculations <1984CHEC(4)443>. Calculations were performed for a number of substituted ethenes <1998JPO201>. For methyl acrylate, acrylonitrile, and ethene, the concerted [8+2] mechanism seems favored. However, from both ab initio and semi-empirical calculations of transition states they concluded that reaction with nitroethene proceeded via a two-step intermolecular electrophilic addition/cyclization route, and dimethylaminoethene via an unprecedented two-step nucleophilic addition/cyclization mechanism (Equation 1). 

It is accepted that the acmal nucleophile in the reactions of oximes with OPs is the oximate anion, Pyr+-CH=N-0 , and the availability of the unshared electrons on the a-N neighboring atom enhances reactions that involve nucleophilic displacements at tetravalent OP compounds (known also as the a-effect). In view of the fact that the concentration of the oximate ion depends on the oxime s pATa and on the reaction pH, and since the pKs also reflects the affinity of the oximate ion for the electrophile, such as tetra valent OP, the theoretical relationship between the pATa and the nucleophilicity parameter was analyzed by Wilson and Froede . They proposed that for each type of OP, at a given pH, there is an optimum pK value of an oxime nucleophile that will provide a maximal reaction rate. The dissociation constants of potent reactivators, such as 38-43 (with pA a values of 7.0-8.5), are close to this optimum pK, and can be calculated, at pH = 7.4, from pKg = — log[l//3 — 1] -h 7.4, where is the OP electrophile susceptibility factor, known as the Brpnsted coefficient. If the above relationship holds also for the reactivation kinetics of the tetravalent OP-AChE conjugate (see equation 20), it would be important to estimate the magnitude of the effect of changes in oxime pX a on the rate of reactivation, and to address two questions (a) How do changes in the dissociation constants of oximes affect the rate of reactivation (b) What is the impact of the /3 value, that ranges from 0.1 to 0.9 for the various OPs, on the relationship between the pKg, and the rate of reactivation To this end, Table 3 summarizes some theoretical calculations for the pK. ... 

The reductive NO chemistry will cover some new developments on the electrophilic reactions of bound nitrosyl with different nucleophiles, particularly the nitrogen hydrides (hydrazine, hydroxylamine, ammonia, azide) and trioxodinitrate, along with new density functional theoretical (DFT) calculations which have allowed to better understand the detailed mechanistic features of these long-studied addition reactions, including the one with OH-. The redox chemistry of other molecules relevant to biochemistry, such as O2, H2O2 and the thiolates (SR-) will also be presented. 

An L-shaped arrangement of bromine atoms has been identified by spectroscopy and theoretical studies on the 2 1 71-complex (8) generated by electrophilic bromination of tetraneopentylethylene (6). The reaction stops at the stage (8) which, for the first time, allowed its detection and determination of its thermodynamic parameters by UV spectroscopy (Scheme 1). Theoretical calculations predict an alkene-Br2 Br2 rather than the Br2-alkene-Br2 structure.22... 

This chapter represents an update to the previous two editions, published in 19771 and 19892, and covers the literature of the period 1989-1994 with some references to 1995 papers. It deals mainly with electrophilic additions across the C=C, C=Si and Si=Si bonds and includes both theoretical (ab initio calculations, orbital approach, molecular modelling etc.) and experimental aspects. Particular attention is paid to mechanistic studies, facial selectivity and neighbouring group participation. Synthetic utilization of electrophilic addition is discussed only if including substantial mechanistic insight purely synthetic work is not covered. Aside from the classical reactions, such as hydration, bromination etc., newly included material comprises aziridination (Section VI), attack at C=C bond by an electron-deficient carbon (Section VII) and those electrophilic reactions which utilize a transition or non-transition metal as the electrophile (Section VIII). 

Our book is about the emerging field of Superelectrophiles and Their Reactions. It deals first with the differentiation of usual electrophiles from superelectrophiles, which show substantially increased reactivity. Ways to increase electrophilic strength, the classification into gitionic, vicinal, and distonic superelectrophiles, as well as the differentiation of superelec-trophilic solvation from involvement of de facto dicationic doubly electron deficient intermediates are discussed. Methods of study including substituent and solvent effects as well as the role of electrophilic solvation in chemical reactions as studied by kinetic investigations, spectroscopic and gas-phase studies, and theoretical calculations are subsequently reviewed. Subsequently, studied superelectrophilic systems and their reactions are discussed with specific emphasis on involved gitionic, vicinal, and distonic superelectrophiles. A brief consideration of the significance of superelectrophilic chemistry and its future outlook concludes this book. 

SCF-Xa calculations have indicated that the ix-rj1 rf peroxide is less negative than other structural peroxides (such as trans-jx-1,2 or cis-fi-1,2) because the it donor interacts with the Cu d orbitals more strongly than with the others (254). In addition, the peroxide theoretical predictions, Solomon and co-workers (254, 278) proposed the electrophilic reaction of the /u-172 rj1 peroxide in Tyr with phenol (Fig. 19). Here, the key feature is that the copper ion... 

By theoretical calculations (B3LYP/6-31G ) four reaction pathways were investigated formation of endo or egzo product with initial bond formation to C2 or C3 in indole. For each mechanism theoretical 13C KIE were analysed and the best agreement of theoretical and experimental KIEs was found for the reaction involving the intermediacy of the radical cation 11, resulting from electrophilic aromatic substitution of indole at C3 by cyclohexadiene in the rate-limiting step ... 

As predicted by theoretical calculations <89AP(322)885>, the site of electrophilic attack on pyrazolo[3,4-c]pyridines is C-3. Reactions such as chlorination, bromination, and nitration of the parent compound (158) all give the 3-substituted derivatives (159) <73JCS(Pl)290l, 89AP(322)885>. 

In that way, two independent paths for the reaction of carbonyl compounds with nitriles under alternative electrophilic activation of either components have been found experimentally. Both are catalyzed by protic and Lewis acids. In this connection, the problem arises which of the two reaction mechanisms takes place. In order to solve this problem theoretical calculations on the reaction pathways have been performed. 

Thus, one may conclude that theoretical calculations as well as experimental results (see Sections V.A and V.C) suggest that both nucleophilic and electrophilic reactions occur predominantly from the back-side of the bicyclobutane molecule. These may result at least in part from the fact that both the HOMO and the LUMO reside heavily on this side of the molecule. In addition, due to the inverted geometry of the bridgehead carbons, the equatorial side is sterically more exposed than the axial one. 

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