Stereospecific reactions reactant ratio

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

On the basis of H-NMR and gas chromatographic analysis, it was shown that the product from aliphatic ketones in each case was a mixture of andZ isomers. Changing the solvent, molar ratio of reactants, reaction temperature, reaction time, or the presence of sodium bromide had little effect on the /Z ratio of reaction products. On the other hand, the structure of the substrate showed a profound effect, especially when a bulky tert-butyl group was adjacent to the carbonyl (see Table III). In this case 99% off olefinic compound was formed stereospecifically (46). 

The zeolite is rigid and ordered, and lacks conformational adaptability, in contrast to an enzyme, which can coil, uncoil, and twist around. Yet the zeolite can incorporate transition metal functions—these are of prime importance in enzyme catalysis—and it can effect redox reactions reactions over zeolites can be inhibited by competitive adsorption of reactants, products, solvents, or poisons—a phenomenon observed in biological and some other inorganic heterogeneous catalytic systems Rideal kinetics have been identified in some zeolite-catalyzed alkylations, a pattern which has its parallels in the enzyme field a few cases of stereospecificity (such as orfho-alkylation effects, unusual olefin isomer ratios), where a transition state not otherwise attainable intervenes, may exist. What better group of catalysts than zeolites might there have been to activate the evolutionary process in the dark, fermenting Pre-Cambrian seas some 1,000,000,000 years ago ... 

With proton transfer (k ) rate determining, the primary deuterium isotope effect is satisfied as well as the sensitivity of the reaction rates to (C-O) bond rupture. The latter is reflected in the (kjk2) ratio. It was noted that rearrangement in the carbonium ion of the ion pair (e.g., loss of optical activity at the a-carbon center in the reactant ester, Atj > fcj) would substantiate the ion pair mechanism. Results of this kind of experiment in the stereospecific m-elimination that is observed in ester pyrolyses is not compatible with the ion-pair mechanism. For example, the acetate of the erythro-isomsT of 2-deutero-l,2-dipenylethanol gave /runs-stilbene with predominant retention of deuterium (95.6 1.7%). In contrast, the threo-isomer gave Iranj-stilbene with predominant loss of deuterium (26.4+0.6% deuterium) . 

We have already analyzed the following reaction, where a small percentage of stereoselectivity is found. If the enantiomer of the reactant gives the same products, with the ratio 55 to 45, is the reaction stereospecific Explain. 

The Diels-Alder reaction is stereospecific. The diene and dienophile approach suprafacially with respect to both reactants to form the new a bonds, while the original geometry is minimally shifted (Section 5.3.1). Because of this, groups that are cis in the dienophile remain cis in the cyclohexene. The stereochemical relationships in the diene are maintained as well, which limits the number of possible regio- and stereoisomers. The endo/exo ratio can he controlled, often by the use of a catalyst. An example is shown in Equation 8.28 [39]. CWoroaluminate A-l-butylpyridinium chloride (AlCljiBPC) is an ionic liquid which has been used as a substitute for water in these reactions. 

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