Probable reaction scheme for

Figure 2. A probable reaction scheme for the M -mediated dechlorination of PCP. Reproduced with permission from reference 10. Copyright 2002, Royal...

Of probably greater importance is the effect of local concentration gradients. For example, analysis for a given constituent in the entire meat mass does not reflect the real concentration at a given point. For example, DNA is localized in the nuclei and lipid is localized predominantly in the adipose cells. Another factor of potential influence in reaction schemes for nitrite is the fact that polar-nonpolar interfaces are present as a result of structural compartmentalization. In an adipose cell, the lipid is contained as the body of the cell, but it is surrounded by a thin layer of sarcoplasmic protein. Therefore, large surface areas are involved. 

A second example has been reported by Barton and Yeh. They found that silane 5 gave rise to the formation of dimethylnortricyclane 8 when heated to about 600°C in the gas phase.4 A probable reaction route for this process is depicted in Scheme 1. 

Very little work has been carried out on the kinetics of the PdCl2 only system. This is probably because of the slowness of the reaction. A stereochemical study using 3,3,6,6-cyclohexene-d4, however, provided evidence for frarw-acetoxypalladation in this system (5). For example the reaction scheme for formation of 3-cyclohexen-l-yl acetate is shown in Equation 22 (from Ref. 5). This scheme involves frans-acetoxypallada-tion and ci -Pd(II)-H (D) eliminations and additions. This agrees with the exchange kinetics which also suggest frarw-acetoxypalladation. 

To date, mechanistic studies into the carbonylations of secondary alcohols with the same type of rhodium/RI catalyst system have used 2-propanol as a model substrate. At least part of the reason for this has been to minimize the expected complexities of the product analyses. The carbonylation of 2-propanol gives mixtures of n- and isobutyric acids. Two studies have been (24b, 32) reported with this system. The first of these (32) concluded that the reactivity could be described in terms of the same nucleophilic mechanism as has been described above, despite the fact that the reaction rates at 200°C were approximately 140 times faster than predicted by this type of chemistry (24b). Other data also indicated that this SN2-type reactivity was probably not the sole contributor to the reaction scheme. For example, the authors were not able to adequately explain either the effect of reaction conditions on product distribution or the activation parameters. They also did not consider the possible contribution of a hydrocarboxylation pathway, which is known to be extremely efficient in analogous systems (55). For these reasons, a second study into the carbonylation of 2-propanol was initiated (24b, 57). 

It is obvious from the above discussion that the existing number of observations is insufficient to allow the formulation of a general catalytic reaction scheme for these enz5mies. In spite of the differences in the presteady state behavior of fungal and Rhus laccases, it seems unlikely that they utihze different catalytic mechanisms, and the two enzymes probably behave identically during steady state catalysis. Elucidation of the mechanism will thus require knowledge of their steady state structure and behavior. 

Scheme 14.4 Probable reaction pathways for propene epoxidation over Au/Ti02 catalyst [12].

Base catalyzed reaction As for the base catalyzed reaction, two out of the four major products were isolated as shown in Table 2. Figure 11 shows the probable reaction scheme. Unlike photochemical or thermal reactions, base catalyzed reaction seems to lake place upon the nucleophilic attack of hydroxy or phenoxy anions to the diazo group. It should be reminded that BC-F3 also implies a crosslinking structure. On the real resist surface NQD would crosslink with novolak resin as shown in Figure 12. This reaction is regarded as one of the mechanisms of the dissolution inhibition effect. 

Scheme 6 depicts a typical penicillin sulfoxide rearrangement (69JA1401). The mechanism probably involves an initial thermal formation of a sulfenic acid which is trapped by the acetic anhydride as the mixed sulfenic-acetic anhydride. Nucleophilic attack by the double bond on the sulfur leads to an episulfonium ion which, depending on the site of acetate attack, can afford either the penam (19) or the cepham (20). Product ratios are dependent on reaction conditions. For example, in another related study acetic anhydride gave predominantly the penam product, while chloroacetic anhydride gave the cepham product (7lJCS(O3540). The rearrangement can also be effected by acid in this case the principal products are the cepham (21) and the cephem (22 Scheme 7). Since these early studies a wide variety of reagents have been found to catalyze the conversion of a penicillin sulfoxide to the cepham/cephem ring system (e.g. 77JOC2887).

This is an interesting exercise, but we should not become excessively concerned with formal schemes for the identification of the rds. We want to know the rds because it is a piece of information about the reaction mechanism. If we have already acquired so much information about the system that we can construct a reaction coordinate diagram displaying ail intermediates and transition states, we probably have no need to specify the rds. As an example of the experimental detection of the rds we will describe Jencks study of the reaction of hydroxyiamine with acetone. The overall reaction is... 

In principle, reaction schemes similar to that given in the preceding paragraph may be developed for other comparable rate processes, for example spinel formation. However, Stone [27] has pointed out that, where the barrier phase is not an efficient ionic conductor, the overall reaction may be controlled by the movement of a single cation and anion. In addition, there is the probability that lattice imperfections (internal surfaces, cracks, leakage paths [1172], etc.) may provide the most efficient route to product formation.]... 

The structurally simplest silicon reagent that has been used to reduce sulphoxides is the carbene analog, dimethylsilylene (Me2Si )29. This molecule was used as a mechanistic probe and did not appear to be useful synthetically. Other silanes that have been used to reduce sulphoxides include iodotrimethylsilane, which is selective but unstable, and chlorotrimethylsilane in the presence of sodium iodide, which is easy to use, but is unselective since it cleaves esters, lactones and ethers it also converts alcohols into iodides. To circumvent these complications, Olah30 has developed the use of methyltrichlorosilane, again in the presence of sodium iodide, in dry acetonitrile (equation 8). A standard range of sulphoxides was reduced under mild conditions, with yields between 80 and 95% and with a simple workup process. The mechanism for the reaction is probably very similar to that given in equation (6), if the tricoordinate boron atoms in this reaction scheme are replaced... 

Mg powder can probably be activated for any subsequent Grignard reaction by treating the metal with MesSiCl 14 in either ether or THF, or entirely without solvent, followed by removal of unreacted MesSiCl 14 and HMDSO 7 and any ether or THF in vacuo before adding the halogen compound dissolved in ether or THF (Scheme 13.14). 

Figure 4.10 Graph showing domains of AE and reaction yield for first reaction in Scheme 4.7. Shaded area versus rectangle ABCD indicates probability that this reaction has an RME of at least 0.618 (p = 0.77 (77%)).

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