From hydration of alkynes

D Ketones and Aldehydes from Hydration of Alkynes (Section 9-9F)... 

Mercury(II) oxide and acetic acid effect the cyclization of l,4-diaryloxybut-2-ynes to 4-aryloxymethylchromenes. The transformation was attributed to cyclization of the butanone which resulted from hydration of the alkyne (72JHC489). However, it has since been shown that similar butanones do not cyclize to chromenes under the cyclization conditions (78JOC3856). Instead, a mechanism is proposed which involves a charge-induced Claisen rearrangement which is triggered by 7r-complex formation between the metal ion... 

Hydration of alkynes (via oxymercuration) gives good yields of single compounds only with symmetrical or terminal alkynes. Show what the products would be from hydration of each compound. 

As demonstrated below, a Lewis acid-mediated reaction was utilized in the synthesis of dihydro[b furan-based chromen-2-one derivatives from l-cyclopropyl-2-arylethanones and allenic esters <070L4017>. The TiCh-catalyzed anti-Markovnikov hydration of alkynes, followed by a copper-catalyzed O-arylation was applied to the synthesis of 2-substituted benzo[6]furan <07JOC6149>. In addition, benzo[6]furan-based heterocycles could be made from chloromethylcoumarins <07SL1951>, substituted cyclopropanes <07AGE1726>, as well as benzyne and styrene oxide <07SL1308>. On the other hand, DBU-mediated dehydroiodination of 2-iodomethyl-2,3-dihydrobenzo[6]furans was also useful in the synthesis of 2-methylbenzo[Z>]furans <07TL6628>. 

This reaction was first reported by Fittig and Schrohe in 1875 and subsequently extended by Kutscheroff in 1881. It is an acid-catalyzed hydration of alkynes into ketones. In this reaction, dilute sulfuric acid and mercuric salt are used as catalysts, and mercuric chloride can form a complex with acetylene in aqueous solution. This reaction has been used to prepare ketones from higher alkynes, such as propyne, and vinylacetylene as well as in commercial production of acetaldehyde from acetylene. ... 

The elements of HjO can be added to the carbon-carbon triple bond of an alkyne by the same two reactions used for the hydration of alkenes, namely hydroboration-oxidation and acid-catalyzed hydration. Even though the reagents are similar, the products from hydration of alkenes and allgmes are quite different. 

From a historical perspective it is interesting to note that the first NHC-Au(I) complex was isolated in 1973, more than 20 years before the findings of Arduengo et al, by Minghetti and Bonati [6]. Nevertheless, from this point on it took more than 25 years before Teles et al described, only in a footnote, the first use of an NHC-Au(I) complex as a catalyst for the hydration of an alkyne [3]. This was soon followed by the work of Herrman and coworkers who studied the NHC-Au(I) mediated hydration of alkynes in more detail [7]. In 2005, Nolan and coworkers prepared and discussed the structure of a large array of different NHC-Au(I) chlorides [8]. In this study, some of the NHC-Au(I) catalyst precursors most applied nowadays were prepared for the first time. 

From what you have learned about enols and the hydration of alkynes, predict what product is formed by the acid-catalyzed hydration of CH3CH2CH2C = COCH3. Draw a stepwise mechanism that illustrates how it is formed. 

A related study was reported in 1992 by Blum and Alper [20]. The authors showed that the ion pairs generated from RhCls and a quaternary anuncmium salt (Aliquat 336 ) promoted the hydration of alkynes (Scheme 7). 

The ketones resulting from the hydration of alkynes are useful chemical intermediates. For these reasons, attempts to develop nonmercury alkyne hydration were pursued. The system Nafion(Pd ) resin catalyst was the first reported to be active for the hydration of alkynes, including a-hydroxy alkynes, giving in high selectivity a-hydroxy-ketones [78]. At the same time, Cacchi and coworkers described the Pd(ll)-catalyzed hydration of enynes as key-step involved in a sequential process starting from vinyl triflates and affording y-hydroxy-a, p-enones [79]. 

A three-compruient addition for the hydration of alkynes was accomplished to form efficiently 3-oxabicyclo[3.1.0]hexanes from 2-(arylmethylene)cyclopropyl-carbinols, terminal arynes, and alcohols (Scheme 12) [126]. 

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. 

The degradation of alkynes has been the subject of sporadic interest during many years, and the pathway has been clearly delineated. It is quite distinct from those used for alkanes and alkenes, and is a reflection of the enhanced nucleophilic character of the alkyne C C bond. The initial step is hydration of the triple bond followed by ketonization of the initially formed enol. This reaction operates during the degradation of acetylene itself (de Bont and Peck 1980), acetylene carboxylic acids (Yamada and Jakoby 1959), and more complex alkynes (Figure 7.18) (Van den Tweel and de Bont 1985). It is also appropriate to note that the degradation of acetylene by anaerobic bacteria proceeds by the same pathway (Schink 1985b). 

The hydration of C-C triple bonds represents one of the most atom economical and environmentally friendly oxidation reactions [37], Recently, Nolan and co-workers reported the cationic [Au(lPr)][SbF ] system, which was generated in situ from [AuCl(lPr)] and AgSbF. The catalyst system showed remarkable activity in the hydration of a large range of alkynes, at An loadings as low as 10 ppm (typically 50-100 ppm), under acid-free conditions (Table 10.6) [38],... 

---