ADDITION OF WATER TO ALKYNES

By adding of water to alkynes other than acetylene, ketones are produced. The addition reaction occurs in the presence of H2S04 and HgS04 catalysts. 

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Recently, on the basis of the Markovnikov addition of water to alkynes, Trost et al. developed a three-component addition reaction of terminal alkynes, water, and methyl vinyl ketone, affording 1,5-diketones in DMF/water in the presence of ruthenium and indium catalysts (Eq. 4.38). 

The addition of water to alkynes at high temperatures has already been discussed in Section 4.3.3.1 (see Scheme 4.15) [273]. 

In fact, a mechanism for this reaction can be drawn that does not involve Pd at all, but let s assume that Pd is required for it to proceed. Cl- must be nucleophilic. It can add to Cl of the alkyne if the alkyne is activated by coordination to Pd(II). (Compare Hg-catalyzed addition of water to alkynes.) Addition of Cl- to an alkyne-Pd(II) complex gives a o-bound Pd(II) complex. Coordination and insertion of acrolein into the C2-Pd bond gives a new a-bound Pd(II) complex. In the Heck reaction, this complex would undergo P-hydride elimination, but in this case the Pd enolate simply is protonated to give the enol of the saturated aldehyde. 

Addition of water to alkynes preparation of aldehydes and ketones... 

Consideration of the oxidation level reveals diat while one carbon is reduced (the one to which hydrogen adds), die other is oxidized (die one to which the oxygen adds). There is no net change in oxidation level of the alkene functional group. Likewise die reverse processes of these addition reactions, namely, elimination of HX from alkyl halides and dehydration of alcohols to give alkenes, are not redox processes. Additions of water to alkynes is analogous. In this case, however, the product is a ketone, the oxidation level of the ketone is seen to be the same as the alkyne, and so no net change in oxidation level has occurred. 

The addition of water to alkynes is also aided by the presence of mercury (II) salts. The reaction is usually conducted in water, with the presence of a strong acid, such as sulfuric acid, and a mercury salt, such as HgS04 oi HgO. In this case the mercury is spontaneously replaced by hydrogen under the reaction conditions, so a second step is not necessary. The addition occurs with a Markovnikov orientation stereochemistry is not an issue. 

Section 11.6 discussed the acid-catalyzed addition of water to alkynes. The initial product of this reaction, called an enol, has a hydroxy group attached to one of the carbons of a CC double bond. The enol is unstable and rapidly converts to its tautomer, a carbonyl compound. The carbonyl and enol tautomers of acetone are shown in the following equation ... 

The C-C bond formation can also be obtained via a first-step addition of a heteroatom to alkynes. Thus, the reaction of the three components terminal alkyne, water and enone led to 1,5-diketone with atom economy, using the system CpRuCl(COD)/NH4PF6 and In(0S02CF3)3 as a cocatalyst [58,59] (Eq. 43). The mechanism is postulated to proceed by the ruthenium-catalyzed nucleophilic addition of water to alkynes to generate a ruthenium enolate intermediate allowing further insertion of enone and formation of 1,5-diketones after protonation. 

As described in the section Reactions Involving Addition of Water to Alkynes, the reaction of terminal alkynes, water, and a-vinyl ketones afforded 1,5-diketones in DMF-H20 (Eq. 12). Under similar conditions, in the presence of halide, ruthenium-catalyzed three-component coupling of alkyne, an enone, and halide ion formed vinyl halide (Eq. 17) [35]. 

Mercuric sulfate, HgS04 Acts as a catalyst for the addition of water to alkynes in the presence of aqueous sulfuric acid, yielding ketones (Section 8.4). 

Interestingly, two enzymes have been described that catalyze the addition of water to alkynes, resulting in the formation of alkenols acetylene carboxylate hydratase from Pseudomonas (E.C. 4.2.1.71), which converts propynoic acid to 3-hydroxyprope-noate1201. The latter tautomerizes to malonic semialdehyde. Acetylene dicarboxylate hydratase (E. C. 4.2.1.72) converts acetylene dicarboxylic acid to 2-hydroxyethylenedi-carboxylic acid, which spontaneously decarboxylates to pyruvate[21]. 

The mixture of mercury oxide, boron trifluoride-ether, and trifluoroacetic acid is a particularly effective catalyst for hydration of alkynes 84 mixtures of mercury oxide and boron trifluoride88 and of mercury oxide with sulfuric acid86 have also been used. Addition to acid-sensitive acetylenes is carried out under neutral conditions by means of mercuriacetamide or mercuri-/7-toluenesulfonamide.87 Acid exchange resins loaded with mercury ions can also be used for addition of water to alkynes.88... 

Addition of water to alkynes requires not only an acid catalyst but mercuric ion as well. The mercuric ion forms a complex with the triple bond and activates it for addition. Although the reaction is similar to that of alkenes, the initial product—a vinyl alcohol or enol—rearranges to a carbonyl compound. 

FIGURE 14.2 Carbonyl compounds synthesized by addition of water to alkynes (SiajBH is disiamyl-borane, lUPAC name bis (l,2-dimethylpropyl)borane. 9-BBN is 9-borabicyclo[3.3.1]nonane). 

The addition of water to alkynes (hydration) is one of the fundamental methods for generating carbonyl compounds from unsatmated hydrocarbon precursors... 

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