Reagent and Solvent Effects

Efforts to establish a theoretical explanation of the reactivity of nucleophilic reagents have centered on correlations with intrinsic electron-donor properties which are the fundamental basis of nucleophilicity. According to Edwards and Pearson, in general, such properties include basicity, polarizability, and the presence of unshared electron pairs on the atom adjacent to the nucleophilic atom of the reagent. When only the first two of these properties are operative, Eq. (8), which was proposed by Edwards, has proved successful in 

The reactivity of a nucleophilic reagent may also depend on stereochemical conformation, degree of solvation and hydrogen-bonding, 

As a first approximation, within a given family of nucleophilic reagents, such as amines, basicity changes are mainly responsible for differences in nucleophilic power. The p values of some of the more familiar amines together with the rate constants for some of their reactions with chloroheteroaromatic compounds are shown 

The data show that in some cases basicity has a strong influence on reactivity. For example, the reaction of 2-chloropyridine derivatives with piperidine is about 3000 times as fast as that with pyridine the basicity change involved is in the order of 6 pA units. However, piperidine is only 4 times as reactive as morpholine with 2- or 4-chloropyrimidine as the substrate, although -dpAo in these cases is still fairly large, 2.5 units. Furthermore, even the qualitative correlation sometimes fails, and aniline is more reactive than pyridine in contrast to the expectations from their basicities. 

The position of aniline in the above reactivity order deserves special comment. Aniline is less basic than pyridine by a relatively small factor, 0.65 pA units, but is appreciably more polarizable it then seems likely that the inverted order of reactivity is caused by the polarizability term in accordance with Edwards equation. If this is correct, in the reactivity order piperidine aniline pyridine, inversion with respect to basicity appears to result from an abnormally high reactivity of aniline rather than from a particularly low reactivity of pyridine. This view differs from that based on relative steric requirements of the reagents, but other factors besides basicity and polarizability may well contribute to the quantitative experimental picture. 

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Studies of Equilibria, Shift Reagents, and Solvent Effects.—Several studies of halogen-exchange equilibria have been reported, e.g. the exchange between phosphoryl, phosphonyl, and phosphinyl halides, P and P halides (27)... 

A similar steric effect was observed in the reaction of benzyl carboxylate (44). When 44a-d were treated with Bu OK under solvent-free conditions at around 100 °C for 30 min, the corresponding condensation products 45a (75%), 45b (66%), 45c (64%), and 45d (84%) were obtained in the yields indicated [9] (Scheme 6). When the same reactions of 44a-d and Bu OH were carried out in toluene under reflux for 16 h, no condensation product was obtained and 44a-d were recovered unchanged. In solution reactions, exchange of the alkoxy group occurs among the substrate, reagent, and solvent. Therefore, the alkoxy groups of the ester, metal alkoxide, and alcohol used as a solvent should be identical. 

The reason for stressing the importance of working with relatively pure reagents and solvents is that the rates of many reactions are extremely sensitive to the presence of trace impurities in the reaction system. If there is reason to suspect the presence of these effects, a series of systematic experinlents may be carried out to explore the question by seeing how the reaction rate is affected by the intentional addition of impurities. In many cases, lack of reproducibility between experiments may be an indication that trace impurity effects are present. 

The line of thought elaborated in this paper continues to be useful for assessing functionally the purity of reagents and solvents and the effectiveness of purification procedures and the warning against spurious correlations remains topical. 

The obvious hazards in the syntheses reported in this volume are delineated, where appropriate, in the experimental procedure. It is impossible, however, to foresee every eventuality, such as a new biological effect of a common laboratory reagent. As a consequence, all chemicals used and all reactions described in this volume should be viewed as potentially hazardous. Care should be taken to avoid inhalation or other physical contact with all reagents and solvents used in this volume. In addition, particular attention should be paid to avoiding sparks, open flames, or other potential sources that could set fire to combustible vapors or gases. 

Occasionally, it may be required to study the fundamental radical reactions with organotins in benzene. However, the use of radical reactions with such toxic reagents and solvents cannot be considered in the chemical and pharmaceutical industries, even if the results in terms of organic synthesis are excellent and effective. Even in a fundamental study, it might not be wise to use 1 g of Bu3SnH and 5 ml of benzene. Hence, radical chemists should develop new and less toxic radical reagents and reaction media in order to reduce the damage to nature. 

Lindsey andco-workers [27,69,70], Weglarz and Atkin [32], and Metivier and co-workers [31,81] have all developed and applied Zymark robotic workstations to optimize chemistry. Lindsey and co-workers [69] completed a factorial design study (16 experiments) to examine the role of catalyst and reactant concentrations on porphyrin yield in less than 1 day of workstation time. Weglarz and Atkin at Dow Chemical Company [32] studied the effect of reaction parameters on (i) the alkoxy substitution of cellulosic ethers (ii) the base-catalyzed conversion ofphenethyl bromide to styrene and (iii) the onset of crystallization employing a fiber optic probe. Metivier and co-workers at Rhone-Poulenc [31,81] focused on the evaluation of catalysts, reagents, and solvents for process optimization work of numerous proprietary reactions. 

The Lewis acid catalyzed process is not applicable to aryl ketones, where ester products have been observed through aryl migration. Furthermore, anchimeiic and solvent effects have been noted during oxidation of a 3-carboxy steroidal ketone. a-Oxygenadon, although not inhibited, was altered. Desinte the frequently disappointing yields the reagent has been widely used, particularly in the steroid field, and... 

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