Hydration of Alkynes to Aldehydes and Ketones

Other organometallics also react with ketones and imines. For example, the alkyne anions that we prepared in Chapter 8 (remember that the p/[Pg.644]

Terminal alkyne anions are popular reagents for the acyl anion synthons (RCHjCO"). If this nucleophile is added to aldehydes or ketones, the triple bond remains. This can be con verted to an alkynemercury(II) complex with mercuric salts and is hydrated with water or acids to form ketones (M.M.T. Khan, 1974). The more substituted carbon atom of the al-kynes is converted preferentially into a carbonyl group. Highly substituted a-hydroxyketones are available by this method (J.A. Katzenellenbogen, 1973). Acetylene itself can react with two molecules of an aldehyde or a ketone (V. jager, 1977). Hydration then leads to 1,4-dihydroxy-2-butanones. The 1,4-diols tend to condense to tetrahydrofuran derivatives in the presence of acids. 

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... 

In 1998, Wakatsuki et al. reported the first anti-Markonikov hydration of 1-alkynes to aldehydes by an Ru(II)/phosphine catalyst. Heating 1-alkynes in the presence of a catalytic amount of [RuCljlCgHs) (phosphine)] phosphine = PPh2(QF5) or P(3-C6H4S03Na)3 in 2-propanol at 60-100°C leads to predominantly anti-Markovnikov addition of water and yields aldehydes with only a small amount of methyl ketones (Eq. 6.47) [95]. They proposed the attack of water on an intermediate ruthenium vinylidene complex. The C-C bond cleavage or decarbonylation is expected to occur as a side reaction together with the main reaction leading to aldehyde formation. Indeed, olefins with one carbon atom less were always detected in the reaction mixtures (Scheme 6-21). 

By the example of 34 different alkynes, it was convincingly demonstrated that the product of the treatment of [PtCLJ with CO at 40-110 °C is a very powerful alkyne hydration catalyst some of the reactions are shown on Scheme 9.7 [25], The best medium for this transformation is THF containing 5 % H2O. The reaction can also be performed in a water-organic solvent two-phase system (e.g. with 1,2-dichloroethane), however in this case addition of a tetralkylammonium salt, such as Aliquat 336, is required to facilitate mass transfer between the phases. After the reaction with CO, the major part of platinum is present as H2[ Pt3(CO)6 n], but the catalytic effect was assigned to a putative mononuclear Pt-hydride, [PtHCl(CO)2], presumably formed from the cluster and some HCl (supplied by the reduction of [PtCU]). The hydration of terminal acetylenes follows Markovnikov s mle leading exclusively to aldehyde-free ketones. 

A highly regioselective, efficient, and clean anti-Markovnikov hydration of terminal acetylenes has been realized through the use of catalytic amounts of Ru complexes.561 Typically, [CpRu(dppm)Cl] catalyzes the reaction at 100°C to give aldehydes in high yields (81-94%). Triflic acid or trifluoromethanesulfonimide effectively catalyzes the hydration of alkynes without a metal catalyst to afford Markovnikov products (ketones).562... 

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. 

Keto-enol tautomerism was discussed in Chapter 10 (Section 10.6) in connection with the hydration reaction of alkynes, including oxymercuration or hydroboration. The carbonyl form is more stable, and for most aldehydes and ketones only a tiny amount of enol is present. Acetone (2), for example, exists primarily as the ketone, and experiments show only 1.5 x 10 % enol. This experiment titrated acetone with diatomic bromine and measured the extent of reaction, which converted 2 to l-bromo-2-propanone (a-bromoacetone, 4). The second product in this reaction is HBr. By this experiment, the enol content of acetone—and presumably of other simple ketones—is remarkably small. 

The hydroboration/oxidation sequence is complementary to the direct, mercury(ll)-catalyzed hydration reaction of a terminal alkyne because different products result. Direct hydration with aqueous acid and mercury(IJ) sulfate leads to a methyl ketone, whereas hydroboration/oxidation of the same terminal alkyne leads to an aldehyde. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Alkynes, unlike alkenes, are not hydrated readily in aqueous acid unless a mercuric salt is present as a catalyst. Also, the products that are isolated are either aldehydes or ketones instead of alcohols. Even though the addition of one molecule of water to ethyne probably gives ethenol (vinyl alcohol) initially, this compound is unstable relative to its structural isomer (ethanal) and rapidly rearranges ... 

Anhydrous iron(III) halides catalyse coupling of alkynes and aldehydes.211 Simple terminal alkynes, R CH, react with aldehydes, R2CHO, to give ( ,Z)-1,5-dihalo-1,4-dienes (55). In contrast, non-terminal arylalkynes give ( ,)-o, /3-unsaturated ketones. The catalysts also promote standard Prins cyclization of homoallylic alcohols. Studies of intermediates and of alkyne hydration - together with calculations - all support FeX3 complex formation with alkyne as the activating step. 

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