Alkynes catalytic hydration

Catalytic hydration and alcoholation of unsaturated compounds such as alkenes or alkynes would be a high value-adding step in the synthesis of compounds of complicated structure as well as in the large-scale production of industrially useful simple compounds. The activation of the O-H bond of water, alcohol, or carboxylic acid by transihon metals is relevant to a variety of such catalytic processes. 

K. Tani and Y. Kataoka, begin their discussion with an overview about the synthesis and isolation of such species. Many of them contain Ru, Os, Rh, Ir, Pd, or Pt and complexes with these metals appear also to be the most active catalysts. Their stoichiometric reactions, as well as the progress made in catalytic hydrations, hydroal-coxylations, and hydrocarboxylations of triple bond systems, i.e. nitriles and alkynes, is reviewed. However, as in catalytic hydroaminations the holy grail", the addition of O-H bonds across non-activated C=C double bonds under mild conditions has not been achieved yet. 

Table 1 Selected results on catalytic hydration of terminal alkynes RCCH.a...

Nolan and coworkers applied cationic gold(l) complexes of N-heterocyclic carbenes at elevated temperatures in the catalytic hydration of terminal and internal alkynes at very low catalyst loadings of 1,000 ppm (0.1%) down to 10 ppm (0.001 %). The thermal stability of the gold(l) carbene complex appears to be critical to achieve those results [123] (Scheme 21b). Iron Iron(III) chloride catalyzes... 

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. 

The catalytic hydration of alkynes with a variety of ruthenium catalysts has been recently reviewed [117]. The addition of water to terminal alkynes catalyzed by... 

Cyclic acetals were produced from terminal alkynes and ot,0) diols with cationic iridium(lll) complex [Ir(CH3)(OTf)(CO)(H20)(PPh3)2](OTf), exclusively in the absence of water at room temperature. Whereas direct catalytic hydration of the alkynes was not possible, the cyclic acetals were hydrolyzed separately to methyl... 

The catalytic hydration of alkynes in water was developed with water soluble c -(TPPTS)2PtCl2 and (DPPETS)PtCl2 complexes [TPPTS = tris(sodium w-benzenesulfonate)phosphine, DPPETS = ( /M-Na03SC6H4 2PCH2CH2P wt-CgH4S03Na 2)] [129, 130]. These complexes catalyze the hydration of water soluble 3-pentyn-l-ol and 4-pentyn-l-ol to 5-hydroxy-2-pentanone through a mechanism involving a 5-endo-dig and 5-exo-dig cyclization step, respectively, as shown in Scheme 71 for 3-pentyn-l-ol. 

Hintermann L, Labonne A (2007) Catalytic hydration of alkynes and its application in synthesis. Synthesis 1121-1150... 

Hiscox W, Jennings PW (1990) Catalytic hydration of alkynes with Zeise s dimer. Organometallics 9 1997-1999... 

Hartman JW, Hiscox WC, Jennings PW (1993) Catalytic hydration of alkynes with platinum (II) complexes. J Org Chem 58 7613-7614... 

Lucey DW, Atwood JD (2002) Insight into the selective room-temperature Platinum(II) catalytic hydration of alkynes in water. Organometallics 21 2481-2490... 

Scheme 5 Au(I)-acidic catalytic hydration of alkynes in aqueous methanol...

On the basis of these results and Damiano s report [28], Darcel et al. described an iron-catalyzed hydration of terminal alkynes using catalytic amounts of iron(III) chloride (10 mol%). The reaction selectively leads to the corresponding methyl ketone derivatives (Scheme 11) [29]. Iron(II) species such as FeCl2 or Fe(OAc)2 were not able to promote the reaction, the starting phenylacetylene remained unchanged after several days at 75°C. 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

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). 

The intramolecular addition of the O-H bond to alkynes catalyzed by palladium complexes has been developed by K. Utimoto et al. (Eq. 6.59) [104]. An alkynyl alcohol can be converted to a cyclic alkenyl ether in the presence of a catalytic amount of [PdCl2(PhCN)2 or [PdCl2(MeCN)2] in ether or THE at room temperature. When the reaction was carried out in MeCN-H20 under reflux in the presence of a catalytic amount of PdCl2, hydration of the acetylenic alcohol occurred and the ketoalcohol was obtained in good yield instead. 

Enzyme-like Acceleration in Catalytic Anti-Markovnikov Hydration of Alkynes to... 

The analogous hydroxylation of alkynes to produce ketones is enhanced by the co-catalytic effect of Aliquat and platinum(IV) chloride-carbon monoxide [3] it is assumed that HPtCI(CO) is the active hydration species. C-S and C-Br bonds are cleaved under the reaction conditions. 

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. 

Recently, a new class of supramolecular CpRu-containing catalysts for hydration of alkynes has emerged. These catalysts are based on the supramolecular self-assembly of monodentate ligands through hydrogen bonding association, as shown in Scheme 10.9 [41-43]. The remarkable activity of catalytic systems such as 15-17... 

The generally accepted pathway for the hydration of alkynes are the generation and subsequent tautomerization of an intermediate enol. The use of fairly concentrated acids, usually H2S04, is necessary to achieve suitable reaction rates. Addition of catalytic amounts of metal salts, however, greatly accelerates product formation. In most cases mercury(II) salts are used. Mercury-impregnated Nafion-H [with 25% of the protons exchanged for Hg(II)] is a very convenient reagent for hydration 35... 

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... 

Mercuric sulfate catalyzed hydration of cyclopropylacetylenes in aqueous sulfuric acid, like other monosubstituted alkynes, gave mainly the corresponding methyl ketone accompanied by small amounts of ring-opened prouducts (equation 168)236. Similar results were obtained using HgO in trichloroacetic acid, with catalytic amounts of BF3-Et20 and methanol. 

Except for very reactive alkynes, acid-catalyzed hydrations are usually sluggish. This slow hydration can be overcome by the addition of catalytic amounts of mercury(II) salts. Such hydrations are generally mild and will tolerate the presence of other functional groups. Specific examples of mercury-catalyzed hydrations are discussed in the next section. 

The hydration of alkynes is also accomplished by use of catalytic amounts of palladium and gold salts.305 The mildness of this reaction is demonstrated by the preparation of 5-oxo-prostaglandin derivatives (equation 202). In this connection, it should be noted that attempted use of other metal salts to catalyze C—-C triple bond hydrations has met with little success.306... 

As mentioned earlier (Section 45.9.6.2) aqueous solutions of RuC13 xH20 catalyse the hydration of alkynes. Detailed mechanistic studies indicate that the catalytic activity is dependent upon the combined concentrations of [RuCl4(OH2)2] and [RuC13(OH2)]2., 613... 

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