Ethers, alkynyl, hydration

Acid-Catalyaed Hydration of Alkynyl Ethers and Thioethers. 207... 

Acid-catalyzed hydration of alkynyl ethers and thioethers... 

The earliest evidence for vinyl cations as intermediates comes from thorough studies by Jacobs and co-workers and Drenth and co-workers of the acid catalyzed hydration of alkynyl ethers (3-7) and thioethers (8-10). In particular, the hydration of the following compounds has been investigated ... 

Clearly, a large body of diverse evidence indicates that the acid-catalyzed hydration of alkynyl ethers and thioethers proceeds via a rate-determining protonation through a vinyl cation. However, these vinyl cations are unique in that they have a resonance form where the positive charge resides on the... 

Once again, a large amount of diverse evidence indicates the intermediacy of a vinyl cation in electrophilic additions to arylacetylenes. As in the case of the hydration of alkynyl ethers and thioethers, the vinyl cation formed is especially stable because of resonance interaction and charge delocalization with the adjacent rr center of the aromatic system. 

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. 

To a suspension of iodosylbenzene (1.1 g, 5 mmol) and the appropriate 1-trimethylsilyl-l-alkyne (5 mmol, prepared from 1-alkyne and trimethylchlorosilane in nearly quantitative yield, or obtained commercially) in chloroform (10 ml) was slowly added boron trifluoride etherate (710 mg, 5 mmol) at 0°C. The mixture was stirred at room temperature for 3-4 h, then recooled at 0°C and a solution of p-toluenesulphonic acid hydrate (3.8 g, 20 mmol) in water (20 ml) was added the resulting mixture was stirred vigorously for a few minutes. The organic phase was separated and the aqueous phase was washed with additional chloroform. The combined organic phase was washed with water, dried and concentrated. The residual oil solidified upon addition of ether. The solid was filtered, washed with ether and air-dried to give alkynyl phenyliodonium tosylates (62-89%). 

It is remarkable that the mechanistic parameters, rates included, for the acid-catalyzed hydration of compounds (2) and (3), are in general, similar to those observed in the reaction of the corresponding alkenes. This has been illustrated in detail by Richey and Richey (1970) and will be discussed in section IIIA2a. The hydration of triple bonds may be faster than that of double bonds, as shown by the main product obtained from the alkynyl alkenyl ether under 9 of Table 1. 

An intramolecular version of alkyne hydration was reported in 2006 by Belting and Krause [127] providing an efficient route to tetrahydrofuranyl ethers 32. This transformation consists in a tandem cycloisomerization-hydroalkoxylation of homopropargylic alcohols 31 in the presence of an alcohol in a dual catalyst system (a gold precatalyst and a Bronsted acid) under mild conditions (Scheme 13). The reaction proceeds satisfactorily with terminal and internal alkynes, with bis-homopropargylic alcohols and alkynyl phenols to provide cyclic acetal skeletons that occur in a variety of natural products. Substituted furanones can be obtained by gold(III)-catalyzed activation of alkynes by heterocyclization and subsequent 1,2-alkyl shift [128]. 

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