Alkene selective hydration

Fig. 3.26. cis-Selective hydration of a chiral, racemic, trisubstituted alkene with induced diastereoselectivity A corresponds to the extent of the substrate control of diastereoselectivity. 

The mechanistic complexity of hydroboration-oxidation stands m contrast to the simplicity with which these reactions are carried out experimentally Both the hydrobo ration and oxidation steps are extremely rapid reactions and are performed at room tern perature with conventional laboratory equipment Ease of operation along with the fact that hydroboration-oxidation leads to syn hydration of alkenes and occurs with a regio selectivity opposite to Markovmkov s rule makes this procedure one of great value to the synthetic chemist... 

The hetero-Diels-Alder reaction between a,p-unsaturatcd ketoesters and nucleophilic alkenes has been described in two concurrent and independent reports (220, 222). As with acylphosphonates, these proved to be excellent substrates for catalyst 269c. The reaction proceeds efficiently in THF at low temperatures providing the cycloadduct in >99% ee at -78°C. Indeed, the impressive selectivity exhibited under these conditions allows the reaction to be conducted at a convenient temperature of 0°C, using the hydrated catalyst 266c in the presence of molecular sieves, Eq. 181. Observed diastereoselectivities... 

Together with glutathione conjugation, hydration is a major pathway in the inactivation and detoxification of arene oxides. Exceptions to this rule will be treated when discussing polycyclic aromatic hydrocarbons. Arene oxides are good substrates for microsomal EH, as evidenced in Table 10.1, where hydration of selected arene oxides, alkene oxides, and cy-cloalkene oxides by purified rat liver epoxide hydrolase is compared. The hy- ... 

Solid superacidic Nafion-H was also found to be effective in the hydration of acyclic alkenes.16 Isopropyl alcohol was produced with 97% selectivity in hydrating propylene17 at 150°C, whereas isobutylene yielded tert-butyl alcohol18 with 84% selectivity at 96°C. 

Recently, there have been many new catalysts developed for the hydration of alkenes, although most are more suited for industrial purposes. These catalysts include zeolites such as pentasil,283 mordenite286 and ferrierite,183 as well as those containing heavy metals,287 heteropolyacids288 and sulfonic acid exchange resins.289 It is reported that some of these new catalysts perform as high as 99.6% conversions and 99.4% selectivity, as illustrated in the hydration of propene into 2-propanol (equation 192).290... 

Unfortunately, these reactions tend to afford mixtures. Similar mixtures are observed when using 2-cy-clohexen-l-ol,315 3-cyclohexen-l-ol,31s,32s or the corresponding ethers or esters.326-327 The regio- and stereo-selectivity of the hydration of bicyclic alkenes has also been carefully examined (equation 2 1 3).328-330... 

The hydroxy group must be located on one of the doubly bonded carbons of the original alkene, so first draw all of the alkenes that meet this criterion. Examine the alkenes to determine whether it is possible to selectively add the OH group to the desired carbon. Remember that we can add the OH with either Markovnikov orientation (acid-catalyzed hydration or oxymercuration-reduction) or anti-Markovnikov orientation (hydroboration-oxidation), but we will have difficulty selecting between two carbons that are similarly substituted. 

For commercial processes, formed supports are more useful. Compared with other supports, fumed oxide supports showed new catalytic effects [41]. Some intensively investigated applications for these supports are abstracted in the following. SiC>2 pellets have been successfully introduced in a new generation of precious metal supports in vinylacetate monomer production [42]. This resulted in better selcctivities and an up to 50% higher space-time yield compared with supports based on natural alumo-silicates. In alkene hydration fumed silica pellets serve as a support for phosphoric acid. In this case, an increased catalyst lifetime and a higher space-time yield were observed [43]. Pyrogenic TiC>2 powder can be used as a starting material for the manufacture of monolithic catalysts [44] for the selective reduction of NOv with ammonia. 

Heteropolyacids are much more active than mineral acids for several types of homogeneous reactions in both organic solvents and aqueous solution [4, 8]. The enhancement is generally greater in organic solvents. For the hydration of isobutene in a concentrated aqueous HPA solution (above 1.5 mol dm-3), the reaction rate is about 10 times greater than for mineral acids [21]. This rate enhancement is attributed to the combination of stronger acidity, stabilization of protonated intermediates, and increased solubility of alkenes [21]. In this case, the selectivity is also much improved with HPA catalysts. 

As was already mentioned, the standard procedure for acid catalyzed alkene hydration exhibits a rather low selectivity. On the other hand, the use of a hydroxymercuration-reduction sequence leads to the exclusive formation of Markovnikov s alcohols. A nearly exclusive anti-Markovnikov s hydration is achieved via a hydroboration-oxidation reaction (see Section 2.4). The result in both these cases is the net addition of H2O, but the basic differences in the reaction mechanisms unambiguously determine a reversed regioselectivity pattern. 

Stereospecific hydration of olefins is another reaction involving a coordinated hydroxide. This reaction with maleate monoester (Figure 6.12, top) involves essentially alkene hydration by the coordinated nucleophile, hydroxide. The reaction is stereospecific in terms of the site of reaction, as it selects five-membered ring formation exclusively over six-membered... 

Mechanistic hypotheses play an important role in developing new catalytic and selective heterofunctionalizations of alkenes. Two basic reaction cycles for metal-catalyzed hydroalkoxylation (and hydration, for R = H) of alkenes can be postulated (Scheme 2). One pathway leads to Markovnikov products via activation of the nucleophile, oxy-metallation, and protonolysis (hydro-de-metallation) (Scheme 2a). Alternatively to the inner sphere syn-oxymetallation depicted in Scheme 2a, external anti-attack of the nucleophUe to coordinated olefin is plausible. The oxidation state of the metal remains constant in this cycle. The alternative hydrometallation pathway (Scheme 2b) proceeds via oxidative addition of the H-OR bond, hydrometallation of the olefin, and reductive elimination to the anti-Markovnikov addition product [3,4]. 

Catalytic conversions of allenes are sometimes considered models for catalytic reactions of alkenes, even though allene reactivity is more closely comparable to that of alkynes rather than alkenes. The catalytic hydration of allenes was achieved by means of a cationic gold(I) complex with a carbene steering ligand, (IPr)AuCl/ AgOTf (5 mol%), in dioxane (rt, 4—9 h) in fair yield [180]. Attack of water is selective for the terminal carbons, whereas regioselectivity in nonsymmetric substrates is controlled by steric, electronic, and solvation factors. 

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