Electrophilicity relative

Relative electrophilic localization energies vs. logarithms of partial rate factors for nitration (a) Hiickel, (6) PPP with fixed /3. (From Dewar Thompson. ) (iv) Plot of log K vs. AB c. (From Dewar. )... 

The reactivity of these metal hydride-metal carbonyl reactions can be correlated with the nature of the reactants in a manner consistent with the proposed mechanism nucleophilic attack by hydride on coordinated CO. Thus reactions involving the highly nucleophilic group IV hydride, Cp gZrHg, are much faster than those of group V metal hydrides. On the other hand, the relatively electrophilic neutral binary metal carbonyls all react with Cp2NbH under mild conditions (20-50° C), whereas more electron-rich complexes such as cyclopentadienylmetal carbonyls (Cp2NbH(C0), CpV(CO) ) or anionic carbonyls (V(CO)g ) show no reaction under these conditions. 

Evaluation of the only appropriate Fukui function is required for investigating an intramolecular reaction, as local softness is merely scaling of Fukui function (as shown in Equation 12.7), and does not alter the intramolecular reactivity trend. For this type, one needs to evaluate the proper Fukui functions (/+ or / ) for the different potential sites of the substrate. For example, the Fukui function values for the C and O atoms of H2CO, shown above, predicts that O atom should be the preferred site for an electrophilic attack, whereas C atom will be open to a nucleophilic attack. Atomic Fukui function for electrophilic attack (fc ) for the ring carbon atoms has been used to study the directing ability of substituents in electrophilic substitution reaction of monosubstituted benzene [23]. In some cases, it was shown that relative electrophilicity (f+/f ) or nucleophilicity (/ /f+) indices provide better intramolecular reactivity trend [23]. For example, basicity of substituted anilines could be explained successfully using relative nucleophilicity index ( / /f 1) [23]. Note however that these parameters are not able to differentiate the preferred site of protonation in benzene derivatives, determined from the absolute proton affinities [24],... 

These indices have been used to study the reactivity for a series of chlorobenzenes and a good correlation is observed, for example, between W and toxicity of chlorobenzene [41]. For a detail discussion of this concept and its applications, we refer the readers to a recent review [41,42]. For studying intramolecular reactivity, these philicity indices and local softness contain the same information as obtained from the Fukui functions, because they simply scale the Fukui functions. In some cases the relative electrophilicity and relative nucleophilicity may be used although they provide similar trends as s(r) and co(r) in most cases [43]. In the same vein, the spin-donicity and spin-philicity, which refer to the philicity of open-shell systems [44], could also be utilized to unravel the reactivity of high-spin species, such as the carbenes, nitrenes, and phosphinidenes [45]. 

For any of the nucleophilic aromatic reactions covered in this chapter, regioselectivity, when more than one activated position is available, depends in most cases on the selectivity of attack by the nucleophile. However, when the conversion of the cr-adduct to product is the rate-limiting step, as in the VNS reaction, the final product distribution may differ from that expected, based on the relative electrophilic reactivity of the possible reaction sites.15 Important roles are played by deactivating steric effects and by stabilizing specific interactions such as those, for example, between an ion-paired nucleophile and a nitro activating group, which favor attack at the ortho position. 

Recently, a quantitative study of the regiochemistry of addition of a number of different fluoroalkyl radicals to CHF = CF2, summarized in Table 6, indicated that the observed selectivity could be correlated with the postulated relative electrophilicity of the radicals, with the conclusion being reached that the secondary H-C5Fn(CF3)CF radical was the most electrophilic [94]. 

Table 8 provides the absolute rates of addition of (CF3)3C, (CF3)2CF-, CF3CF2-, and CF3 to a group of alkenes of variable reactivity. It can be seen from the Table that both the perfluoro-iso-propyl and perfluoro-ferf-butyl radicals give evidence of much greater electrophilicity in their alkene addition reactions. For example, the latter radical reacts significantly (6.8 times) faster than CF3- with the nucleophilic a-methylstyrene (IP = 8.9 eV), while reacting somewhat (1.6 times) slower than CF3- with the more electrophilic pentafluorostyrene (IP = 9.2 eV). Comparative plots of all of the available rate data for alkene additions of CF3- and (CF3)3C- vs alkene IPs, as seen in Fig. 4, leave no doubt as to the relative electrophilicity of the two species. Moreover, the rates of addition of the very electrophilic, but non-o, planar perfluoro-ferf-butyl radical to the more nucleophilic... 

Fig. 7.1. Nitration of polynuclear hydrocarbons by nitric acid in acetic anhydride, (i) Plot of logio (KJKo) against N. , 9-Anthryl positions A,a-napthyl positions V, 4-phenanthryi positions O, other positions. (From Dewar et a/.286) (ii) and (iii) Relative electrophilic localization energies vs. logarithms of partial rate factors for nitration (a) Hiickel, (6) PPP with fixed p. (From Dewar Thompson.83 ) (iv) Plot of log K vs. AEiac- (From Dewar.23 )...

An additional type of element effect, which will be called the vicinal element effect is possible when two leaving groups are attached at both ends of the same double bond. Whether X or Y leaves earlier, or which one leaves alone when only monosubstitution occurs, depends on the preferred position of the attack of the nucleophile, and on the reversibility of this step. For irreversible attack, the (19)/(20) ratio would be determined by the relative electrophilicity of the a- and the /l-carbon... 

An ab initio MO study of the approach of hydride to 1-hydronicotin-amide has been used to examine the relative electrophilicity of the 4-, 2- and 6-positions of the cation (Umeyama, 1980). 

It occurred to us that the interhalogens might serve as a more potent source of electrophilic iodine than I2 itself. The relative electrophilic character o sue... 

It is also possible to observe [6 + 4] cycloadditions when the very electron-rich 6-dimethylaminofulvenes undergo reaction with electron-deficient dienes, thiophene dioxides86 and coumalic esters87, as well as the relatively electrophilic 1,3-dipole, benzonitrile oxide88. The frontier orbital picture here is somewhat more complex, in that the dimethylamino substituent causes the HOMO and SHOMO to be nearly degenerate78 (Fig. 29). In such a case, [6 + 4] cycloaddition again occurs, since now... 

In contrast to the dihydroxylations, the hydroboration of allylic alcohols takes place without an inside alkoxy effect in the sense 5.132 with the oxygen atom treated as the medium-sized group. It is probably significant that the inside alkoxy effect is most noticeable with reagents which are relatively electrophilic in nature, and not with boranes, which are only mildly electrophilic. 

Diborane, B2H6, also reduces many carbonyl groups. In contrast to the metal hydride reagents, diborane is a relatively electrophilic reagent, as witnessed by its ability to add to carbon-carbon double bonds. 

The peroxy radicals are strongly resonance stabilized and are relatively electrophilic species abstracting tertiary bonded hydrogen in preference to secondary or primary bonded. 

As a general trend, the homogeneous hydrogenation of thiophenes to thioethers is catalyzed by complexes that are not sterically demanding with relatively electrophilic metal centers (e. g., d metal ions such as Rh , Ir , Ru , or Os ). This is because moderate electron density and low steric hindrance at the metal favor the rf--C,C coordination mode of the thiophene (precursor to hydride migration) over rj -S binding (precursor to C-S bond cleavage) [7-10]. 

Cascade reactions can be designed that depend on the relative electrophilicities of the intermediate organic radicals, and carbon monoxide can provide one of the components (equation 9-28).60... 

Two other reactivity descriptors called relative electrophilicity RE and relative nucleophilicity... 

The relative electrophilic strength (or electrophilicity) of a cation depends on the stability of the positive charge. Inductive (+1), mesomeric (+M) and/or steric effects (see Section 4.4) can all lower the reactivity of the cation. 

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