Alkynyl thioethers

The inverse isomerization mode, providing allenyl thioethers from alkynyl thioethers, is also known [309, 310] for example, 124 is first deprotonated with n-butyllithium and then protonated with ammonium chloride [311] (Scheme 1.55). 

Thiols react directly with non-activated alkynes [15] and with 1-alkynyl thioethers [16] to yield alkenyl thioethers in good yield (>76%), whereas thiocyanate anions only add to non-activated alkynes under acidic phase-transfer catalytic conditions on the addition of mercury(II) thiocyanate. Terminal alkynes are converted into vinyl thiocyanates, but disubstituted alkynes also form vinyl isothiocyanates [17]. Major by-products are the ketones formed by solvolysis of the alkynes. 

Scheme 3.22. Stannylcupration of alkynyl ethers [87a] and alkynyl thioethers (TBDMS = t-butyidimethylsilyl) [87bj.

In situ preparation of the vinylidene pentacarbonylchromium complexes was also reported by Fischer et al. [7]. Treatment ofthe acylchromate complexes 18, prepared by acylation of pentacarbonylchromium dianion, with trifiuoroacetic anhydride in the presence of DBU afforded very labile pentacarbonylchromium vinylidene complexes 19, which were, without isolation, reacted with several electron-rich alkynes such as ynamines, alkynyl ethers, and alkynyl thioethers to give the corresponding cyclobutenylidene chromium complexes in reasonable yield [8] (Scheme 5.5). 

Substituted 5-chloro-l,2,3-thiadiazoles react with simple hetero nucleophiles by displacement of the chlorine, but reaction with aryl- and alkyllithiums gives alkynyl-thioethers via attack at sulfur and then ring cleavage with loss of nitrogen. A similar ring cleavage occurs, but by a different mechanism, when the... 

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. 

An interesting approach to allenyl thioethers has been reported by Huang and Xiong, which is based on a novel three-component reaction. Treatment of 1-alkynyl-phosphine oxides 310 with lithium alkylthiolates 311 in the presence of an aldehyde 312 provides the expected highly substituted allenyl thioethers 313 in good yield (Scheme 8.84) [173]. Unfortunately, this procedure could not be extended to arylthio-lates. 

Bromination of the enol ether product with two equivalents of bromine followed by dehydrobromination afforded the Z-bromoenol ether (Eq. 79) which could be converted to the zinc reagent and cross-coupled with aryl halides [242]. Dehydrobromination in the presence of thiophenol followed by bromination/dehydrobromination affords an enol thioether [243]. Oxidation to the sulfone, followed by exposure to triethylamine in ether, resulted in dehydrobromination to the unstable alkynyl sulfone which could be trapped with dienes in situ. Alternatively, dehydrobromination of the sulfide in the presence of allylic alcohols results in the formation of allyl vinyl ethers which undergo Claisen rearrangements [244]. Further oxidation followed by sulfoxide elimination results in highly unsaturated trifluoromethyl ketonic products (Eq. 80). 

Besides processes (1) and (2), the reader should be aware that nucleophilic attacks on alkynes are treated in other chapters of this book, dealing with rearrangements, cyclizations, polyacetylenes, cyclic acetylenes and perhaps others. A number of publications overlap with ours in different ways and at different levels -. They treat individual alkynes or families " , e.g. acetylene, diacetylenes , acetylene dicarboxylic esters haloacetylenes , alkynyl ethers and thioethers > ynamines , fluoro-alkynes ethynyl ketpnes , nitroalkynes , etc. synthetic targets, e.g. pyrazoles , if-l,2,3-triazoles , isothiazoles , indolizines S etc. reagents, e.g. nitrones , lithium aluminium hydride , heterocyclic A -oxides - , azomethine ylids - , tertiary phosphorus compounds , miscellaneous dipolar nucleophiles - , etc. The reader will appreciate that all of these constitute alternate entries into our subject. 

Palladium(0)-catalysed coupling of an orf/to-halophenolic ether (thioether) with a terminal alkyne (or with an alkynylboronic ester ) and ring closure promoted with an electrophile - iodine has been most often used - is an excellent method to make both benzothiophenes °° ° and benzofurans. ortfto-AIkynyl-phenols can be comparably closed with palladium catalysis in the presence of copper(II) halides to give the corresponding 3-halo-benzofurans, ° and ortfto-alkynyl pyridin-2- and -3-yl acetates likewise ring close with iodine, generating furopyridines. ... 

SYNTHESIS OF THIOETHERS, SULFONES, AND RELATED COMPOUNDS Example 5.5 Preparation of an Alkynyl Alkyl Sulfide [103],... 

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