Hydration, of terminal alkene

Figure 5. Correlation of rate of hydration of terminal alkenes with

Unfortunately, the reported [118] direct anti-Markovnikov hydration of terminal alkenes catalyzed by frans-PtHCl(PMe3)2 in the presence of aqueous NaOH and a phase-transfer catalyst at 60-100°C has proved to be irreproducible [119, 120]. 

Jeaiseai CM, Trogler WC (1986) Catalytic hydration of terminal alkenes to primary alcohols. Science 233 1069-1071... 

Other cations (Cu2+, Pd2+, Ru3+, Ni2+, Rh3+) incorporated into Nafion-H have been found to promote hydration.36 Other metals that catalyze hydration of alkynes include gold(III),37 ruthenium(in),38 and platinum(II) (Zeise s salt39 40 and halides40), p-Methoxybenzenetellurinic acid is very effective in the hydration of terminal alkynes 41 Similar to the hydration of alkenes, photochemical acid-catalyzed hydration of alkynes is possible ... 

The mercuric ion-catalyzed hydration of alkynes probably proceeds in a similar manner to the oxymercuration of alkenes (see Section 5.1). Electrophilic addition of Hg to the triple bond leads to a vinylic cation, which is trapped by water to give an vinylic organomercury intermediate. Unlike the alkene oxymercuration, which requires reductive removal of the mercury by NaBH4, the vinylic mercury intermediate is cleaved under the acidic reaction conditions to give the enol, which tautomerizes to the ketone. Hydration of terminal alkynes follows the Mai kovnikov rule to furnish methyl ketones. ° ... 

In order to obtain the enol as the product of the addition reaction, only one equivalent of BH3 can be allowed to add to the alkyne. In other words, the reaction must stop at the alkene stage. In the case of internal alkynes, the substituents on the boron-substituted alkene prevent the second addition from occurring. However, there is less steric hindrance in a terminal alkyne, so it is harder to stop the addition reaction at the alkene stage. A special reagent called disiamylborane has been developed for use with terminal alkynes ( siamyl stands for secondary iso amyl amyl is a common name for a five-carbon fragment). The bulky alkyl groups of disiamylborane prevent a second addition to the boron-substituted alkene. So borane can be used to hydrate internal alkynes, but disiamylborane is preferred for the hydration of terminal alkynes. 

Methylbut-2-enyl)phenol underwent hydration with aqueous phosphoric acid to give the 3-hydroxy analogue which by dehydration regenerated the alkene together with some of terminal alkene isomer. 

Anti-Markovnikov addition of H2O to olefins is of enormous importance in view of the production of linear alcohols directly from alkenes. It is a general phenomenon, however, that reaction of water with olefins and alkynes, as in the previous example, gives products of Markovnikov addition. The first anti-Markovnikov hydration of terminal alkynes (Scheme 42) with transition metal catalysis was reported in 1998 (227). A series of aliphatic and aromatic alkynes... 

Ketones can be prepared via oxidation of secondary alcohols, ozonolysis of alkenes, acid-catalyzed hydration of terminal alkynes, or Friedel-Crafts acylation. 

Strategy What is an immediate precursor of a primary alcohol " Perhaps a terminal alkene, which could be hydrated with non-Markovnikov regiochemistiy by reaction with borane Followed by oxidation with H2O2-... 

The rate of hydration is dependent on the substitution and hence the nucleophilicity of the C—C double bond.280 In general, trisubstituted alkenes are hydrated faster than terminal alkenes. Thus, the reaction is most commonly applied to the preparation of tertiary alcohols (equation 191).281... 

This combination of reagents h s been used to oxidize terminal vinyl groups to methyl ketones and is known as the Wacker oxidation. The nucleophile is simply water, which attacks the activated alkene at the more substituted end in an oxypalladation step. (3-Hydride elimination from the resulting a-alkyl palladium complex releases the enol, which is rapidly converted into the more stable keto form. Overall, the reaction is a hydration of a terminal alkene that can tolerate a range of functional groups. 

Isayama has reported an elegant procedure for the hydration of alkenes with molecular oxygen and triethylsilane catalyzed by a cobalt(II) complex followed by a reductive treatment with Na2S203 [41]. The reaction is efficient with terminal alkenes and a,/9-unsaturated esters. The radical nature of this reaction is ques-... 

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. 

Oxidation of organoboranes to alcohols is usually effected with alkaline hydrogen peroxide. The reaction is of wide applicability and many functional groups are unaffected by the reaction conditions, so that a variety of substituted alkenes can be converted into alcohols by this procedure. Several examples have been given above. A valuable feature of the reaction is that it results in the overall addition of water to the double (or triple) bond, with a regioselectivity opposite to that from acid-catalysed hydration. This follows from the fact that, in the hydroboration step, the boron atom adds to the less-substituted carbon atom of the multiple bond. Terminal alkynes, for example, give aldehydes in contrast to the methyl ketones obtained by mercury-assisted hydration. 

As a center of high electron density, the triple bond is readily attacked by electrophiles. This section describes the resnlts of three such processes addition of hydrogen halides, reaction with halogens, and hydration. The hydration is catalyzed by mercury(II) ions. As is the case in electrophilic additions to unsymmetrical alkenes (Section 12-3), the Markovnikov rule is followed in transformations of terminal alkynes The electrophile adds to the terminal (less snbstituted) carbon atom. 

Another example of several homogeneous catalysts combined together is a multica-talytic cascade reaction combining Pd, acid, and Ru catalysis (Fig. 8.18), where by coupling palladium-catalyzed oxidation, acid-catalyzed hydrolysis, and ruthenium-catalyzed reduction, the anti-Markovnikov olefin hydration can be achieved, resulting in primary alcohols from aryl-substituted terminal alkenes. 

Thus far, we have learned two ways to make aldehydes (i) a/ift-Maikovnikov hydration of a terminal alkyne or (ii) ozonolysis of an alkene, either of which is potentially reasonable here. However, in order to produce botii of these compounds from a single synthetic protocol, a key recognition is that they can be produced fiom ozonolysis of the following (hsubstituted alkene. 

Platinum(II) complexes have been described as catalysts for the hydration of both terminal alkenes in an anti-Markovnikov fashion and for symmetric maleic acid derivatives. 

Usually, the hydroxy cyclization is much faster than the direct nucleophilic addition of water to the alkyne to form the corresponding methyl ketone. However, hydration of the alkyne takes place with 1,6-enynes in which the alkene bears electron withdrawing substituents that reduce its reactivity in the electrophilic addition [227]. Thus, enyne 88a reacts with MeOH in the presence of gold(I)-catalyst to give 89 in excellent yield, whereas 88b with a p-nitrophenyl group at the terminal carbon of the alkene gives ketone 90 (Scheme 48). Formation of ketone 90 is the result of addition of water contained in the solvent, since addition of 4 A molecular sieves inhibits the hydration reaction [50, 51]. 

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