The Product-Determining Steps

It may seem, at first sight, paradoxical that a competition reaction carried out under conditions in which the measured rate is independent of the concentration of the aromatic can tell us about the relative reactivities of two aromatics. Obviously, the measured rate has nothing to do with the rate of the product-determining step, and what is important in determining relative reactivities is the ratio of the values of ( 3.2.4) for two compounds. The criteria to be met for a correct application of the competitive method are well understood. ... 

In the Sakurai reaction, the product-determining step should be the nucleophilic addition of the allylsilane to the Lewis acid coordinated enone28. 

The existence of an ion pair stabilized by a solvent molecule in the product-determining step of the reaction has been established by calculations and also supported by the product composition (equation 89). While the formation of the diiodo derivative is characteristic of all the cited solvents, in tetrahydrofuran this iodination takes place with the predominant formation of l-iodomethyl-3-(4-iodobutoxy)adamantane (equation 89). 

Simple 1,2-additions to this compound have been observed123131132 also in other sulfenylation reactions, and in other electrophilic additions involving strongly bridged intermediates. Although these results have been interpreted as evidence that additions of sulfenyl halides to symmetrical alkenes do not involve open carbenium ions before the product-determining step, the different behavior observed in the case of 49 suggests123 that close proximity is necessary to have transannular participation of 7r-bonds, at least in additions of sulfenyl derivatives and of some other electrophiles carried out in the presence of efficient nucleophiles. 

Addition to alkene double bonds where the product-determining step usually occurs on the face opposite to the angular methyl groups ( a attack ). 

In the product-determining step of Scheme 6, the nucleophilic oxygen of water attacks one of the carbons this carbon then withdraws its orbital from the three-center bond and an ordinary a bond is formed between the remaining carbon and the hydrogen. 

The above definition of S is employed independently of the actual kinetic order of the product-determining step. Solvolytic reactions may be of higher kinetic order, or it could be argued that their kinetic order is ill-defined. There is no need to derive Equation 2.9 - it can stand alone as the definition of S. To illustrate how the equation works, suppose that the solvent is an equimolar mixture of alcohol and water, so the solvent mole ratio is 1.0. If the product mole ratio is also 1.0, then S = 1.0, and the product-forming reactions are unselective (so log S = 0 for an unselective reaction). If twice as much ether is formed as alcohol in an equimolar solvent mixture, then S = 2. In practice, many solvent compositions are employed, and the equation corrects automatically for variations in solvent composition. 

Substituents present in the starting material affected product composition according to a series of perceptions extracted from the outcome of the reaction of selected 2-phenyl-l,2,3-triazole 1-oxides with AcCl as illustrated in Scheme 120 (1981JCS(P1)503,1997BSB717). The reasoning is based upon initial formation of an O-acetylated species shown in the left column, which then reacts with acetate ion in the product-determining step. The O-acylated species may react in a similar fashion with other nucleophiles like the chloride ion. 

The product-determining step involves the partitioning of this intermediate between two paths one is the reaction with water and the other is loss of a proton ... 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

Different isotope effects from substrate and product analysis originate not only by the side-reactions, but also can be observed when the partially labelled molecule passed the rate-determining step and can react with different distribution of isotopes in the product-determining step. Changes of the substrate isotopic constitution provide the intermolecular isotope effect on the rate-determined step, whereas the analysis of product can inform about the intramolecular isotope effect. 

In contrast to what is observed in both neutral and acidic solutions, in basic medium there is no change in product distribution when the hydrogen availability is varied. This, and the fact that a,/3-unsaturated ketones are hydrogenated in preference to olefins in base but not in neutral media (18) indicate that a strongly adsorbed species, such as an enolate anion, is present in basic solutions of unsaturated ketones. If it is assumed that the enolate anion is involved in the base-promoted hydrogenation of unsaturated ketones and also that the initial adsorption of this species is the product-determining step in the reaction, a reasonable mechanistic hypothesis, based on that initially proposed by Wilds et al. (19) (see Scheme 3), can be put forth for this reaction. Such a process involves a hydride ion transfer from the catalyst to an adsorbed enolate... 

Cristol ef a/. have demonstrated that the sensitized conversion of the dibenzobarrelene (52a) into the semibuUvalenes (53) and (54) does not show a sensitizer energy effect. A similar lack of dependence was shown for the conversion of the isomer (52b) into (55) and (53). The authors suggest that the product-determining step occurs after energy transfer and that the selectivity is not due to the formation of an exciplex. Direct irradiation of the... 

Normal, inverse addition (Table 18) and the dilution experiments (Tables 18 and 19) lead to almost the same amounts of net retention for 49. This essentially excludes an involvement of a bimolecular SET in the product-determining step. In the case of a bimolecular reaction increasing concentrations of MN should lead to an increase of net retention in 49. 

In agreement with the first-order kinetics in the product-determining step in the reactions of 5(1-52 and 118-Hal with MN, and with all other experimental facts outlined in detail above, the following intramolecular SET reactions are proposed to account for the net retention observed with these cyclopropyl halides (Scheme 15). 

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