Alkenes formation from alcohols

Rnally, it will be remembered that (a) the strong organic acids such as ben-zenesulfonic acid and related aromatic sulfonic acids can be prepared by the sulfo-nation of the aromatic ring (Equation 8.75) through electrophilic aromatic substitution and that (b) add chlorides of these acids (Equation 8.76) on reaction with alcohols will produce aryl sulfonate (e.g., toluenesulfonate) esters (Equation 8.77) used earlier (Chapter 8) to effect alkene formation from alcohols. 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

Ion-exchange resins can also be used for dehydration [8]. The superacidic Na-fion-H catalyzes selective alkene formation from 1-propanol, 2-propanol, and tert-butyl alcohol at 433 K [45]. Ion-exchange resins have found industrial application in the production of isobutylene from ieri-butyl alcohol [46]. 

As introduced in Section 2.2.4, monobenzylic alcohols are inherently poor substrates for reduction using NaBH TFA. However, notable exceptions have been discovered and this tactic has a synthetic niche as illustrated with reduction products 101-105. Diastereo-selective introduction of deuterium was reported in the studies involving tetrahydroisoquinoline 102.89 Note that alkene formation from the carbocation intermediates... 

The above simple process cannot be applied to the preparation of the homo-logues a higher temperature is requir (di-n-amyl ether, for example, boils at 169°) and, under these conditions, alkene formation predominates, leading ultimately to carbonisation and the production of sulphur dioxide. If, however, the water is largely removed by means of a special device (see Fig. Ill, 57,1) as soon as it is formed, good 300 of ethers may be obtained from primary alcohols, for example ... 

We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid catalyzed dehydration (Section 5 9) or from alkyl halides by dehydrohalogenation (Section 5 14) Because our designated starting material is tert butyl alcohol we can combine its dehydration with bromohydrm formation to give the correct sequence of steps... 

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

The high selectivity of the catalyst in forming ( )-alkenes can be used in interesting ways (eq. 1). For example, in acetone-iie solution, within 15 min at room temperature allyl alcohol is converted to nearly pure enol (E)-26. Under these mild conditions, the product slowly isomerizes to the more stable aldehyde tautomer. We know of one other report of rapid enol formation from allyl alcohol, using a Rh... 

Under certain conditions, the trifluoroacetic acid catalyzed reduction of ketones can result in reductive esterification to form the trifluoroacetate of the alcohol. These reactions are usually accompanied by the formation of side products, which can include the alcohol, alkenes resulting from dehydration, ethers, and methylene compounds from over-reduction.68,70,207,208,313,386 These mixtures may be converted into alcohol products if hydrolysis is employed as part of the reaction workup. An example is the reduction of cyclohexanone to cyclohexanol in 74% yield when treated with a two-fold excess of both trifluoroacetic acid and triethylsilane for 24 hours at 55° and followed by hydrolytic workup (Eq. 205).203... 

In this chapter we will discuss some aspects of the carbonylation catalysis with the use of palladium catalysts. We will focus on the formation of polyketones consisting of alternating molecules of alkenes and carbon monoxide on the one hand, and esters that may form under the same conditions with the use of similar catalysts from alkenes, CO, and alcohols, on the other hand. As the potential production of polyketone and methyl propanoate obtained from ethene/CO have received a lot of industrial attention we will concentrate on these two products (for a recent monograph on this chemistry see reference [1]). The elementary reactions involved are the same formation of an initiating species, insertion reactions of CO and ethene, and a termination reaction. Multiple alternating (1 1) insertions will lead to polymers or oligomers whereas a stoichiometry of 1 1 1 for CO, ethene, and alcohol leads to an ester. 

As already indicated, a number of values of pA)i2o have been measured experimentally, including those just described. Many more can be evaluated from a combination of free energies of formation of alcohols and the relevant alkenes in aqueous solution [AGf(aq)].47 This is illustrated in Scheme 11 in which 1.364 corresponds to 2.303RT at 25°C in kcal mol 1 and AG( (aq) is abbreviated as AG°. ... 

One important difference between aqueous solution and alkyl cations com-plexed by a single water molecule is that bulk water is far more basic than an isolated molecule [82]. For this reason alkene formation is thermodynamically more favourable in aquae, providing a more favourable all-over route for acid-catalysed elimination from protonated alkyl compounds. Gas-phase studies of water clustered protonated alcohols provides the link between the isolated alkyl cation and the water solution [83-85]. 

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