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Front-side attack

Fluorination with retention of configuration (such as in 327) has often been attributed to steric crowding on the opposite side of the hydroxyl group, hindering the back side of the fluoride ion for Sn2 attack. However, a possible Sn/ mechanism by loss of an OSp2NEt2 fragment to give a car-bonium cation, followed by front-side attack of fluoride ion, should be... [Pg.147]

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. [Pg.297]

For the favorable reaction mechanism, the direct front-side attack, three findings are worth noting, (i) The activation barriers for analogous species decrease along the series Cr > Mo > W. (ii) The barriers for the di(peroxo) species are lower than those of the corresponding mono(peroxo) complexes, (iii) Coordination of a second base ligand significantly increases the activation barrier. [Pg.297]

Figure 9. Stabilization energies of base adducts CHjReOfOi L relative to the energy of the base-free di(peroxo) complex 5a and activation barriers of the corresponding epoxidation TSs for front-side attack of ethene as model olefin. Figure 9. Stabilization energies of base adducts CHjReOfOi L relative to the energy of the base-free di(peroxo) complex 5a and activation barriers of the corresponding epoxidation TSs for front-side attack of ethene as model olefin.
The lowest TS pc>2l5b lies at almost the same energy as the TS F2b of front-side attack of 2b because of the higher stability of the preceding intermediate 15b while the activation barrier is much higher (27.5 kcal/mol) than that of 2b. The relative stability of 2b and 15b may be sensitive to environmental effects that were not taken into account in the models. Note also that TS al5a of a attack lies only 4.4 kcal/mol higher than TS F3b. [Pg.317]

According to this mechanism, the first formed ion pair is 19a. Owing to dispersal of charge in the allylic system, the bond between halogen and C(2) is weakened so that an open carbenium ion (19c) readily forms, allowing for the possibility of front-side attack by the anion with the resulting formation of syn 1,2-adducts. This intermediate explains the formation of the cis-],2-adducts by chlorine addition to cyclic systems. However, syn 1,2-dichlorides can also result from linear dienes by rotation around the C(l)—C(2) bond in 19c to produce 19d, followed by back-side attack by the anion with respect to its position in 19d. Syn 1,4-adducts should instead arise by attack of the anion on C(4) in either 19a, 19c or 19d. Formation of anti dichlorides (1,2- or 1,4-) can only occur when there is appreciable translocation in the ion pair 19a to give 19b. Attack by the anion at C(2) in 19b yields anti 1,2-dichloride and attack at C(4) yields anti 1,4-dichloride. [Pg.565]

When the chiral a,jS-enone enoate 98 was treated with magnesiocuprates in the presence of 1.5-2 equivalents of diethylaluminium chloride, the anti addition product 99 was obtained in moderate yield and with good diastereoselectivity (Scheme 6.21) [43, 44]. A reasonable explanation might assume a chelating coordination of the aluminium reagent [45]. Thus, if the enone 98 were to adopt an s-trans conformation, as indicated for complex 100, subsequent front side attack of the nucleophile would furnish the major diastereomer anti-99. [Pg.200]

Notice that this back-side attack corresponds to an attack on the small lobe of 0c- It follows that front-side attack may become competitive if it is possible (a) to... [Pg.93]

Glycogen phosphorylases belong to the group of vitamin B6 enzymes bearing a catalytic mechanism that involves the participation of the phosphate group of pyridoxal-5 -phosphate (FTP). The proposed mechanism is a concerted one with a front-side attack, as can be seen in Fig. 5 [109]. In the forward direction, e.g.. [Pg.31]

Front-side attack, corresponding to an attack on the big lobe of silicon, leads to retention. When unfavorable, out-of-phase overlap between the nucleophile and the orbitals of the leaving group predominates, nucleophilic attack occurs at the rear of the molecule, opposite X, leading to... [Pg.287]

The two former factors favor a front-side attack leading to the retention. Moreover, numerical calculations indicate that in this example, the Si—X bond shortening does not compensate for the two former effects. Therefore, retention is favored. [Pg.291]

In the case of the p -methoxyphenoxide anion, Taft et al. (77) have shown that the oxygen atom has a high degree of sp3 character. This nucleophile is quite similar to hard alkyl anions from an electronic point of view, i.e., it is a hard nucleophile with contracted valence orbitals around oxygen, unfavorable out-of-phase overlap with the leaving group is minimized (Scheme 9), and a front-side attack leading to retention is therefore possible. The stereochemical data are summarized in Table XI. [Pg.298]

When Si and OR are both in a five-membered ring, the crg, 0R MO has more p character, and front-side attack is less favorable (Scheme 9). However the experimental data indicate a shift of the stereochemistry to inversion only in borderline cases since the angular tension is not very high (Table XIX). [Pg.306]

Figure 8-2 Stereochemistry of displacement of 2-chlorobutane with hydroxide by (1) front-side attack (not observed) and (2) back-side attack... Figure 8-2 Stereochemistry of displacement of 2-chlorobutane with hydroxide by (1) front-side attack (not observed) and (2) back-side attack...
The S 2 transition states and the encounter complexes for back- and front-side attack in the gas-phase reactions between X - and CF3X have been calculated at the... [Pg.250]

The. S n reactions between HF and protonated methyl, ethyl, isopropyl, and /-butyl fluorides in the gas phase have been examined at the MP2/6-31+- -G(d,p) level of theory.112 113 The reaction of CH3FH+ clearly occurs via back-side attack as the transition state for this process is of lower energy than the transition state for frontside attack. The EtFH+ can react via a more stable back-side. S N2 reaction or an. S n 1 reaction via front-side attack since the. S N 1 pathway is 4.4 kJmol-1 lower in energy. No. S n2 path could be found for i-PrFH+ and the front- and back-side pathways had equal activation energies for /-BuFH+, which effectively reacts by an. S N1 mechanism. The conclusion is that the preference for back-side attack is reduced as the size of... [Pg.265]

The reported data indicate that the substitution occurs with retention of configuration at the attached carbon, and there seems to be little doubt that the reaction involves front-side attack. Possible mechanisms for this reaction are SE1, SE2, and SEi. It has also been suggested that the reaction involves a four-center transition state arising from molecular or ion-pair attack (72). [Pg.258]

A one-stage displacement giving retention by a front-side attack via (89) is no longer analogous to the SN2 reaction, and requires distortion of the two lobes of the p-orbital involved in the substitution to ca. 110° angle. Such a structure for the transition state was dismissed for... [Pg.33]

Therefore, it follows that in a backside trajectory, we obtain both the lowest crossing point as well as the largest TS resonance energy. Computationally, the backside barrier is smaller by 10—20 kcal/mol compared with a front side attack (42). Equation 6.18 defines an orbital selection rule for an Sn2 reaction. Working out this rather trivial prediction is nevertheless necessary since it constitutes a prototypical example for deriving orbital selection rules in other reactions, using FO—VB configurations. Thus, a simple rule may be stated as follows ... [Pg.139]

The calculations predict that the degenerate homolytic substitution by silyl radical at the silicon atom of disilane proceeds by mechanisms that involve either a back-side or a front-side attack, having similar activation barriers of 12.6 and 13.9 kcalmol-1, respectively. Similar conclusions were obtained for the degenerate homolytic substitution reactions involving GeH3 and SnH3, with barriers of 15.6 kcalmol-1 ( back-side ) and... [Pg.143]


See other pages where Front-side attack is mentioned: [Pg.516]    [Pg.516]    [Pg.200]    [Pg.247]    [Pg.244]    [Pg.301]    [Pg.318]    [Pg.94]    [Pg.95]    [Pg.95]    [Pg.669]    [Pg.288]    [Pg.291]    [Pg.298]    [Pg.305]    [Pg.1690]    [Pg.265]    [Pg.147]    [Pg.148]    [Pg.148]    [Pg.194]    [Pg.43]    [Pg.647]    [Pg.144]    [Pg.865]    [Pg.868]    [Pg.516]   
See also in sourсe #XX -- [ Pg.265 ]

See also in sourсe #XX -- [ Pg.462 ]




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Nucleophilic ‘front-side’ attack

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