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Bond heterolysis

It is clear from all of the above evidence that despite earlier doubts, vinyl cations can be generated via solvolysis and bond heterolysis, especially in cases... [Pg.263]

The effect of structure of the alkyl group on the stability of monoalkyl-thallium(III) compounds can best be understood by reference to the different mechanisms by which these compounds undergo decomposition. A number of authors have attributed the instability of monoalkylthallium(III) compounds to facile C—T1 bond heterolysis and formation of carbonium ions [Eq. (25)] (52, 66, 79). This explanation is, however, somewhat suspect in cases where primary carbonium ions would be involved and either the two-step sequence shown in Eqs. (26), (27), or the fully synchronous 8 2 displacement shown in Eq. (28), is more compatible with the known facts. Examination of the oxythallation reactions that have been described reveals that Eq. (27) [or, for concerted reactions, Eq. (28)] can be elaborated, and that five major types of decomposition can be recognized for RTlXj compounds. These are outlined in Scheme 8, where Y, the nucleophile... [Pg.175]

An increase in the fraction of the four-electron reduction pathway at more reducing potentials (Fig. 18.10a, b) may be rationalized within at least two mechanisms. The first is based on the kinetic competition between the release of H2O2 from the ferric-hydroperoxo intermediate [Reaction (18.16) in Fig. 18.11] and its (reversible) reduction to a ferrous-hydroperoxo species, which undergoes rapid 0-0 bond heterolysis (18.13b). Because H2O2 and particularly HO2 are more basic ligands... [Pg.659]

Within the mechanism in Fig. 18.11, it seems implausible that simple Fe porphyrins can be effective ORR catalysts, since large overpotentials are required to access intermediates in which 0-0 bond heterolysis is facile. The only strategy discovered so far to facilitate this 0-0 bond heterolysis in the ferric-hydroperoxo intermediate is to control both the distal and the proximal environments of Fe porphyrins. In those cases, the overpotential of ORR reduction appears to be controlled by the potential of the (por)Fe / couple (see Section 18.6). [Pg.660]

The low limit on the rate constant fehetero of 0-0 bond heterolysis in the putative ferric-hydroperoxo intermediate by analyzing the turnover frequency of H2O2 reduction at potentials 0.6-0.4 V (vs. NHE at pH 7). [Pg.681]

In halogenated solvents, catalysis by a second bromine molecule, which assists the Br—Br bond heterolysis, is the main driving force. The role of the solvent is electrostatic, but the absence of an extensive Kirkwood relationship suggests that there is some other kind of contribution (Bellucci et al, 1985b). [Pg.279]

The influence of / ara-substituents on the benzamide and benzyloxyl side chains upon the pre-equilibrium protonation step is likely to be negligible considering their remoteness from the site of protonation and their electronic influence must rather impact upon the rate determining N-O bond heterolysis step. Para-substituents on the leaving group should impact upon both the protonation and bond heterolysis steps. [Pg.64]

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). [Pg.260]

Our preliminary experiments have provided the first example of Lewis acid promoted C-C bond heterolysis of epoxides and productive cycloaddition (eq 7). Under the influence of TiCl4-(THF)2 (2 equiv), epoxide 26 reacts with methyl pyruvate to provide acetal 27 (52% isolated yield), along with C-O cleavage product 28 (23 °C, 3 h). The diaste-reoselectivity for formation of 27 is 2.3 1. We have performed the analogous reaction in the absence of a Lewis acid the thermal reaction requires several days at 110 °C and gives a diastereomer ratio (dr) of ca. 1.3 1... Although not optimized from the standpoint of chemoselectivity, these results are promising because of the relatively low reaction temperature and potential for enhanced diastereocontrol. [Pg.451]

A variety of strategies for effecting dipolar cycloadditions from stable, neutral precursors, including carbon-carbon bond heterolysis induced by Lewis acidic metal complexes and chiral organic catalysts ... [Pg.489]

The formation of the parent amine 23 in these solvolysis reactions was considered to be the most dehnitive evidence for formation of a discrete nitrenium ion. Gassman and Cryberg" postulated the following, (a) Initial Cl—N bond heterolysis would occur adiabatic ally, generating the singlet nitrenium ion 21. (b) The triplet... [Pg.601]

Product analyses showed that for all five esters, the OH - and buffer-dependent components generated the corresponding hydroxamic acids. The pH-independent reaction also led to the hydroxamic acid product for 39c and 39f, but 39a and 39e generated products that appeared to be derived from N—O bond heterolysis in the pH region dominated by (pH < 8). ... [Pg.183]

The magnitude of the rate constants were such that 46e exhibited a U-shaped pH-rate profile with a broad pH-independent region from pH 3.5 to 7.0 in which /Cobs Reaction products isolated within this pH range were consistent with those previously observed for 46a-d, and CU and 1 had effects on product distribution and identity similar to those discussed above for 46a-d. It was concluded that the pH-independent reaction involved N—O bond heterolysis to yield nitrenium ion intermediates as in Scheme 24. ... [Pg.189]

TABLE 6. Free energy data (in kcal mol 1) for the C—H bond heterolysis of fluorenes85... [Pg.632]

The acid-catalysed hydrolysis of 5-methoxyacenaphthylene 1,2-oxide was shown to proceed via the carbocation (76).149 This cation reacted with water to give an approximately 3 2 ratio of cis.trans diols, despite the trans-diol being substantially more stable. Hence, transition state effects, and not thermodynamic stability, determine the products. A computational investigation examined carbocations derived from oxidized benzo aJanthraccncs.150 Examples of cations considered are (77), derived by epoxi-dation and ring opening, and (78), derived by methyl oxidation and subsequent C—O bond heterolysis. [Pg.220]


See other pages where Bond heterolysis is mentioned: [Pg.453]    [Pg.206]    [Pg.182]    [Pg.186]    [Pg.659]    [Pg.659]    [Pg.660]    [Pg.675]    [Pg.683]    [Pg.5]    [Pg.74]    [Pg.496]    [Pg.144]    [Pg.70]    [Pg.81]    [Pg.166]    [Pg.259]    [Pg.299]    [Pg.557]    [Pg.398]    [Pg.98]    [Pg.443]    [Pg.598]    [Pg.617]    [Pg.873]    [Pg.892]    [Pg.181]    [Pg.136]    [Pg.602]    [Pg.631]    [Pg.213]    [Pg.494]   
See also in sourсe #XX -- [ Pg.145 ]




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Carbon-silicon bond heterolysis

Heterolysis

Heterolysis of Bonds to Carbon Carbocations and Carbanions

N.O-bond heterolysis

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