Carbonyl compounds alkynes hydration

The addition of acetylides to oxiranes yields 3-alkyn-l-ols (F. Sondheimer, 1950 M.A. Adams, 1979 R.M. Carlson, 1974, 1975 K. Mori, 1976). The acetylene dianion and two a -synthons can also be used. 1,4-Diols with a carbon triple bond in between are formed from two carbonyl compounds (V. Jager, 1977, see p. 52). The triple bond can be either converted to a CIS- or frans-configurated double bond (M.A. Adams, 1979) or be hydrated to give a ketone (see pp. 52, 57, 131). 

You have had earlier experience with enols m their role as intermediates m the hydration of alkynes (Section 9 12) The mechanism of enolization of aldehydes and ketones is precisely the reverse of the mechanism by which an enol is converted to a carbonyl compound... 

There are two degrees of unsaturation since the compound C H lacks four H s from being an alkane. The addition of 2 mol of H, excludes a cyclic compound. It may be either a diene or an alkyne, and the latter functional group is established by hydration to a carbonyl compound. The skeleton must be... 

Both acid and metal catalysis are usually required to accomplish hydration of alkynes to yield carbonyl compounds.34 The addition is usually regioselective, allowing for conversion of terminal alkynes to ketones. Hydration of acetylene to produce acetaldehyde used to be an industrially significant process but was replaced by the Wacker synthesis. 

Several preparative methods exist for the synthesis of 3(2//)-dihydrofuranones. 2,5-Disubstituted or 2,2,5,5-tetrasubstituted 3(2i/)-dihydrofuranones are usually prepared by reaction of sodium or lithium acetylide with a ketone to yield an alkynic alcohol which is then treated with a carbonyl compound in the presence of base to afford alkynic diols. Mercury catalyzed hydration of the resultant diols in the presence of acid affords the furanones in good yields (76JMC709). 

Hydration of alkynes yields carbonyl compounds and the ketone used in pyrylium syntheses has been successfully replaced by an alkyne (65CB334). Phenylacetylene, for example, reacts with 3-chloro-l-phenylprop-2-en-l-one to yield the 2,6-diphenylpyrylium salt. 

Hydration reactions of alkynes and nitriles were studied over various zeolites in liquid phase, with ethanol as solvent. Alkyne hydration led to expected carbonyl compounds whereas the formation of the amide and of the corresponding ester was observed during nitrile hydration. 

The hydration reaction of alkynes leading to carbonyl compounds is generally carried out in dilute acidic conditions with mercuric 1on salts (often the sulfate) as catalysts (ref. 5). Only very reactive alkynes (phenylacety-lene and derivatives) can be hydrated in strong acidic conditions (HgSO ) without mercury salts (ref. 6). Mercury exchanged or impregnated sulfonic resins have also been used in such reactions (ref. 7). Nevertheless, the loss of the catalyst during the reaction and environmental problems due to the use of mercury make this reaction method not as convenient as it should be for the preparation of carbonyl compounds. 

Moreover, contrary to alkyne hydration where no adsorption of the carbonyl compound was detected, the problem is complicated here by the saturation of the strong acidic sites by the formed amide, the concentration of which shows a rapid stabilization against time (Fig.3). Consequently the reaction selectivity greatly depends on the ester percentage. The behaviour of the amide itself over the studied zeolites confirms this observation the conversion of the amide into ester goes faster on the HY2 g zeolite than on the Hg and on the HMg zeolites. This later point, together with the comprehension of the different mechanisms in relation with the zeolite properties, will be discussed in a further paper. 

In a series of papers [30-35], Blum and co-workers reported the hydrogenation of alkenes, alkynes, and arenes in the presence of a hydrated ion-pair [0ct3NMe] [RhCl4(H20)n] . The quaternary ammonium tetrachlororhodate also catalyzes HTR of alkynes, alkenes and a,j6-unsaturated carbonyl compounds with polymethylhydrosiloxane [36, 37]. Recently, heterogenized and therefore recyclable quaternary ammonium halometallates, namely glass-encapsulated catalysts [38] and polymer-bound tetrachlororhodate [39], have been described. Both types of the insoluble catalysts effectively promote various processes including the hydrogenation of alkenes and the HTR of a,/9-unsaturated carbonyl compounds [40]. 

This reaction requires that the carboxyl group be altered to the carbonyl group of a ketone with the carbon skeleton remaining constant. The alkyne produced in part (f), n-Ci0H2iCECH, may be used to synthesize the ketone desired. Addition of water in the presence of H2SOi, and HgSOit results in hydration of the alkyne to a carbonyl compound. Hence,... 

A. special note should he made concerning hydriition reactions of tilkynes. Remember that hydration of alkencs leads to alcohols. Alkynes can be hyilraied. too. Markovnikov addition is achieved with an aqueous acidic Hg(ll) catalyst. Anti-Markovnikov addition occurs vi i a modified hydroboration-o.xidation sequence. Both initially give vinylic alcohols (or cnols) as products, but these are kinetieally and thermodytiamically unstable and i.sonieri/e to carbonyl compounds in a reaction called lautomerism. 

The latter is thermodynamically favorable because of the very strong carbon-oxygen double bond that is formed. It is kinetically rapid because the enol O—H bond is acidic and readily deprotonated, allowing the proton eventually to find its way to the nearby carbon. More details concerning the process will be upcoming when carbonyl compounds are discussed. Note that the hydration of alkynes is a new synthesis of aldehydes and ketones. 

Alkynes undergo hydroboration to give alkenylboranes, which can be oxidized to give carbonyl compounds with hydrogen peroxide. The net result of the two-step sequence is hydration, which gives aldehydes from terminal alkynes. 

Attraction between this acidic hydrogen and an oxygen of the hydrate (see 38) leads to proton transfer to oxygen and loss of water from the hydrate to form a new species (39), which is called an enol. An enol is formally defined as a molecule with an OH group attached directly to a C=C unit. As first observed in Chapter 10 (Section 10.4.5), where enols are formed by oxymercu-ration of alkynes, enols are very unstable and exist in an equilibrium with the corresponding carbonyl compound. Experiments show that this equilibrium between the enol form and the carbonyl form lies toward the carbonyl. 

Draw the structural formula of the enol formed in each alkyne hydration reaction then draw the structural formula of the carbonyl compound with which each enol is in equilibrium. 

Bell, R. P. "The Reversible Hydration of Carbonyl Compounds." Adv. Phys. Org. Chem., 4,1-29 (1966). Electrophilic Addition of Water to Alkenes and Alkynes... 

Like alkylboranes, dnylboranes can be converted into alcohols by treatment with basic peroxide (Fig. 10.75).The products are ends in this case, and they are in equilibrium with the corresponding carbonyl compounds. Here s an important point The regiochemistry of this reaction is the opposite of the regiochemistry of the hydration reaction of alkynes. This situation is exactly the same as the one that exists with the alkenes Hydration of an alkene gives overall Markovnikov addition, whereas the hydroboration/oxidation sequence gives the anti-Markovnikov product. 

Further, Lapworth s suggestion that enols are intermediates in these reactions has been established to apply to numerous other reactions of carbonyl compounds as well. Enols are intermediates in the hydration of alkynes (see Section 9.12) and the decarboxylation of P-keto acids and malonic acid derivatives (see Section 18.16). They are also intermediates in a number of biochemical processes including glucose metabolism and fatty acid biosynthesis. 

In a process analogous to the hydration of alkenes, water can be added to alkynes in a Markovnikov sense to give alcohols—in this case enols, in which the hydroxy group is attached to a double-bond carbon. As mentioned in Section 12-16, enols spontaneously rearrange to the isomeric carbonyl compounds. This process, called tautomerism, interconverts two isomers by simultaneous proton and double-bond shifts. The enol is said to tautomerize to the carbonyl compound, and the two species are called tautomers (tauto, Greek, the same mews, Greek, part). We shall look at tautomerism more closely in Chapter 18 when we investigate the behavior of carbonyl compounds. Hydration followed by tautomerism converts alkynes into ketones. The reaction is catalyzed by Hg(II) ions. 

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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