Kinetics and Isotope Effects

In general, the mechanism involves two steps (Eq. 10.111), the first being addition of the electrophile to the aromatic ring to create a highly delocalized carbenium ion. The second step is loss of a proton to a base, most often the solvent. 

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Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

A positive iodinating species was postulated to account for the kinetics and isotope effect observed in the iodination of some amines by iodine in aqueous potassium iodide (in some cases in the presence of acetate, lactate, or phosphate ion). The isotope effects (kH/kD values in parenthesis) for these compounds studied were 2,4,6-trideutero-m-dimethylaminobenzenesulphonate ion, 25 °C (1.0) 2,4,6-trideutero-m-dimethyIbenzoate ion, 30 °C (1.4) 2,4,6-trideutero-dimethylaniline, 30 °C, lactate (3.0) 2,4,6-trideuteromethylaniline, 25 °C, acetate (3.2) 2,4,6-trideuteroaniline, 25 °C (3.5), phosphate (4.0) 2,4,6-trideutero-metanilate ion, 35 °C (2.0) 2,4,6-trideutero-m-aminobenzoate ion, 30 °C (4.8), phosphate (3.0) 2,6-dideutero-l-dimethylaminobenzene-4-sulphonate ion, 25 °C, phosphate (1.0) 4-deutero-l-dimethylaminobenzene-3-sulphonate ion, 25 °C, phosphate (1.0). The kinetics of these reactions was given by... 

Kinetics and isotope effects are consistent with this mechanism.92 The reagent is electrophilic in character and reaction is facilitated by ERG substituents in the alkene. A B3LYP/6-31G computation found the transition structures and Ea values shown in... 

Extensive studies of kinetics and isotope effects by Hartwig and coworkers support the mechanism shown in Scheme 5 for the lr(I)/dtbpy catalyzed borylation [81]. In particular, these studies indicate that the iridium(III) trisboryl bipyridine complex (10) is the species that activates the arene C-H bond this is in agreement with DFT calculations by Sakaki et al. predicting the key intermediacy of the trisboryl complex and the seven-coordinated Ir(V) species resulting from C-H addition [82]. C-H addition to Ir(III) was also proposed in the (Ind)Ir(COD)/ phosphine-catalyzed borylation by Smith et al. [76]. 

The mechanism of the aldol-Tishchenko reaction has been probed by determination of kinetics and isotope effects for formation of diol-monoester on reaction between the lithium enolate of p-(phenylsulfonyl)isobutyrophenone (LiSIBP) and two molecules of benzaldehyde. ". The results are consistent with the formation of an initial lithium aldolate (25) followed by reaction with a second aldehyde to form an acetal (26), and finally a rate-limiting intramolecular hydride transfer (Tishchenko... 

In 1998, Hasanayn and Streitwieser reported the kinetics and isotope effects of the Aldol-Tishchenko reaction . They studied the reaction between lithium enolates of isobu-tyrophenone and two molecule of beuzaldehyde, which results iu the formation of a 1,3-diol monoester after protonation (Figure 28). They analyzed several aspects of this mechanism experimentally. Ab initio molecular orbital calculatious ou models are used to study the equilibrium and transition state structures. The spectroscopic properties of the lithium enolate of p-(phenylsulfonyl) isobutyrophenone (LiSIBP) have allowed kinetic study of the reaction. The computed equilibrium and transition state structures for the compounds in the sequence of reactions in Figure 28 are given along with the computed reaction barriers and energy in Figure 29 and Table 6. 

The yields and rates of oxidation by DMDO under these in situ conditions depend on pH and other reaction conditions.75 Various computational models of the transition state agree that the reaction occurs by a concerted mechanism.76 Kinetics and isotope effects are consistent with this mechanism.77... 

Abu-Hasanayn, F., Streitwieser, A. Kinetics and Isotope Effects of the Aldol-Tishchenko Reaction between Lithium Enolates and Aldehydes. J. Org. Chem. 1998, 63, 2954-2960. 

M.H. O Leary, J.E. Rife, J.D. Slater (1981) Kinetic and isotope effect studies of maize phospho-enolpymvate carboxylase. Biochem. 20, 7308-7314... 

Extensive kinetic and isotope effect studies have also provided important information about mechanism. Replacement of the hydrogen at C-3 by deuterium (the site of proton abstraction during reaction) gives rise to a small hydrogen isotope effect under optimum conditions, and this effect is independent of CO2 concentration (81, 84). 

Thermodynamics, Kinetics, and Isotope Effects of the Binding and Cleavage of a Ligands versus Classical Ligands... 

The direct dimerization of a 17-electron hydride has rarely been documented. One example is provided by the high yield synthesis of [OsH(CO)4]2 Os-Os), upon hydrogen atom abstraction from OsH2(CO)4 by PhjC . Although the 17-electron OsH(CO)4 intermediate was not directly observed, stopped-flow kinetic and isotope effect evidence is consistent with the slow step being a H atom abstraction [33-35]. 

The palladium chloride-coppeifll) chloride couple (28, 29) used industrially in the Wacker process oxidizes olefins to carbonyl compounds. Experimental kinetic and isotope effect data (30) seem to indicate that a TT-olefin complex is initially formed in a series of preequilibrium steps. The rate-determining step is postulated to be a rearrangement of the TT-olefin complex to a cr-complex followed by the final breakdown of the cr-complex to products. Figure 13 depicts the widely accepted Henry mechanism (31). 

The type of mechanism (6-2) with a fast reversible first step, similar to that which Grovenstein and Henderson had in mind, was recently established by a more complete study of the kinetics and isotope effects in the bromination of 2-naphthol-6,8-disulphonic-l-d acid by Christen and Zollinger (1962a, b). This was the first halogenation reaction in which a kinetic isotope effect was found (Zollinger, 1956). 

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