Alkenes, production

FIGURE 5 7 The first formed carbocation from 3 3 dimethyl 2 butanol is secondary and rearranges to a more stable tertiary carbocation by a methyl migration The major portion of the alkene products is formed by way of the tertiary carbocation... 

Step (2) Ethanol acts as a base to remove a proton from the carbocation to give the alkene products (Deprotonation step)... 

Wnte structural formulas for all the alkene products that could reasonably be formed from each of the following compounds under the indicated reaction conditions Where more than one alkene is produced specify the one that is the major product... 

Identify the alkene product in each of the following Wittig ... 

Entry Imine Alkene Product Additive Yield (%) cis/trans ee (%) (cis)... 

One problem with elimination reactions is that mixtures of products are often formed. For example, treatment of 2-bromo-2-methylbutane with KOH in ethanol yields a mixture of two alkene products. What are their likely structures ... 

How many alkene products, including E,Z isomers, might be obtained by dehydration of 3-methy)-3-hexanol with aqueous sulfuric acid ... 

Compound A, C10H 8O, undergoes reaction with dilute H2S04 at 50 °C to yield a mixture of two alkenes, QpH ir>. The major alkene product, B, gives only cyclopentanone after ozone treatment followed by reduction with zinc in acetic acid. Identify A and B, and write the reactions. 

Complete reduction to the alkane occurs when palladium on carbon (Pd/C) is used as catalyst, but hydrogenation can be stopped at the alkene if the less active Lindlar catalyst is used. The Lindlar catalyst is a finely divided palladium metal that has been precipitated onto a calcium carbonate support and then deactivated by treatment with lead acetate and quinoline, an aromatic amine. The hydrogenation occurs with syn stereochemistry (Section 7.5), giving a cis alkene product. 

Trans stereochemistry of the alkene product is established during the second reduction step when the less hindered trans vinylic anion is formed from the vinylic radical. Vinylic radicals undergo rapid cis-trans equilibration, but vinylic anions equilibrate much less rapidly. Thus, the more stable trans vinylic anion is formed rather than the less stable cis anion and is then protonated without equilibration. 

According to Zaitsev s rule, formulated in 1875 by the Russian chemist Alexander Zaitsev, base-induced elimination reactions generally (although not always) give the more stable alkene product—that is, the alkene with more alkyl substituents on the double-bond carbons. In the following two cases, for example, the more highly substituted alkene product predominates. 

In the elimination of HX from an alkyl halide, the more highly substituted alkene product predominates. 

Treatment of an alkyl halide with a strong base such as KOH yields an. alkene. To find the products in a specific case, locate the hydrogen atoms on each carbon next to the leaving group. Then generate the potential alkene products by removing HX in as many ways as possible. The major product will be the one that has the most highly substituted double bond—in tints case, 1-methylcyclohexene. 

What s so special about periplanar geometry Because the sp3 a orbitals in the reactant C-H and C-X bonds must overlap and become p it orbitals in the alkene product, there must also be some overlap in the transition state. This can occur most easily if all the orbitals are in the same plane to begin with—that is, if they re periplanar (Figure 11.181. 

The elimination of HC1 from the isomeric menthyl and neomenthyl chlorides shown in Figure 11.20 gives a good illustration of this trans-diaxial requirement. Neomenthyl chloride undergoes elimination of HC1 on reaction with ethoxide ion 200 times as fast as menthyl chloride. Furthermore, neomenthyl chloride yields 3-menthene as the major alkene product, whereas menthyl chloride yields 2-nienthene. 

The difference in reactivity between the isomeric menthyl chlorides is due to the difference in their conformations. Neomenthyl chloride has the conformation shown in Figure 11.20a, with the methyl ancl isopropyl groups equatorial and the chlorine axial—a perfect geometry for L2 elimination. Loss of the hydrogen atom at C4 occurs easily to yield the more substituted alkene product, 3-menthene, as predicted by Zaitsev s rule. 

G Loss of a neighboring H+ in a fast step yields the neutral alkene product. The electron pair from the C-H bond goes to form the alkene r bond. 

Predict the major alkene product of the following El reaction ... 

The teal value of the Wittig reaction is that it yields a pure alkene of defined structure. The C=C bond in the product is always exactly where the OO group was in the reactant, and no alkene isomers (except E,Z isomers) are formed. For example, Wittig reaction of cyclohexanone with methylenetriphenyl-phosphorane yields only the single alkene product methylenecyclohexane. By contrast, addition of methylmagnesium bromide to cyclohexanone, followed by dehydration with POCI3, yields a roughly 9 1 mixture of two alkenes. 

Name the following amine, including R,S stereochemistry, and draw the product of its reaction with excess iodomethane followed by heating with Ag20 (Hofmann elimination). Is the stereochemistry of the alkene product Z or E Explain. 

In 1970, it was disclosed that it is possible to achieve the conversion of dimethylformamide cyclic acetals, prepared in one step from vicinal diols, into alkenes through thermolysis in the presence of acetic anhydride." In the context of 31, this two-step process performs admirably and furnishes the desired trans alkene 33 in an overall yield of 40 % from 29. In the event, when diol 31 is heated in the presence of V, V-dimethylforrnamide dimethyl acetal, cyclic dimethylformamide acetal 32 forms. When this substance is heated further in the presence of acetic anhydride, an elimination reaction takes place to give trans olefin 33. Although the mechanism for the elimination step was not established, it was demonstrated in the original report that acetic acid, yV, V-dimethylacetamide, and carbon dioxide are produced in addition to the alkene product."... 

For lists of reagents (with references) that condense with aldehydes and ketones to give alkene products, see Ref. 58, pp. 167,180. For those that give the alcohol product, see Ref. 58, pp. 575, 868, 875, 878, 901, 910. 

Methoxybenzaldehyde and methyl diethoxyphosphonoacetate were reacted via the Wittig-Horner-Emmons route to give the corresponding alkene product [85] (see a more detailed description in [42]). 

Figure 7.29. (Top) Molecular representations based on X-ray structural data of the diazo compound 88N2 and the alkene product 89Z (the migrating hydrogen is shown in black in both reactant and product). (Bottom) Schematic reaction path showing the minimal structural changes in the transition from the diazo compound to the product, via the probable transition structure 88TS.

Aryl alcohol oxidase from the ligninolytic fungus Pleurotus eryngii had a strong preference for benzylic and allylic alcohols, showing activity on phenyl-substituted benzyl, cinnamyl, naphthyl and 2,4-hexadien-l-ol [103,104]. Another aryl alcohol oxidase, vanillyl alcohol oxidase (VAO) from the ascomycete Penicillium simplicissimum catalyzed the oxidation of vanillyl alcohol and the demethylation of 4-(methoxymethyl)phenol to vanillin and 4-hydro-xybenzaldehyde. In addition, VAO also catalyzed deamination of vanillyl amine to vanillin, and hydroxylation and dehydrogenation of 4-alkylphenols. For the oxidation of 4-alkylphenol, the ratio between the alcohol and alkene product depended on the length and bulkiness of the alkyl side-chain [105,106]. 4-Ethylphenol and 4-propylphenol, were mainly converted to (R)-l-(4 -hydroxyphenyl) alcohols, whereas medium-chain 4-alkylphenols such as 4-butylphenol were converted to l-(4 -hydroxyphenyl)alkenes. 

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