Thioethers metal/ammonia

Reduction of sulfoxides to thioethers.1 Use of hydrogen halides for this reduction was first reported in 1909 and is still a viable method. This reduction has assumed importance since chiral sulfinyl groups are valuable in asymmetric syntheses and are eliminated in two steps reduction to the ether followed by catalytic hydrogenation or metal/ammonia reduction. The first step can now be carried out with several reagents, as shown by this comprehensive review (349 references). 

As far as propargyl thioethers are concerned, the substrates in this section follow all the principles discussed for propargyl ethers and propargylamines in the two preceding sections. For alkyl propargyl thioethers typical bases used are sodium amide in liquid ammonia, alcoholate or alkali metal hydroxide [178, 186-189, 191, 287-291], and again some derivatives of carbohydrates have been used successfully [292, 293], If an ester group is also present in the molecule, the reaction can be accompanied by a hydrolysis to the carboxylate [294]. 

If one or more of the hydrogen atoms of a non-metal hydride are replaced formally with another group, R—e.g., alkyl residues—then derived compounds of the type R-XHn-i, R-XHn-2-R, etc., are obtained. In this way, alcohols (R-OH) and ethers (R-O-R) are derived from water (H2O) primary amines (R-NH2), secondary amines (R-NH-R) and tertiary amines (R-N-R R") amines are obtained from ammonia (NH3) and thiols (R-SH) and thioethers (R-S-R ) arise from hydrogen sulfide (H2S). Polar groups such as -OH and -NH2 are found as substituents in many organic compounds. As such groups are much more reactive than the hydrocarbon structures to which they are attached, they are referred to as functional groups. 

Solutions of these metals in liquid ammonia effect (i) the reduction of a range of functional groups such as carbonyl and acetylenic and also conjugated and aromatic systems, and (ii) cleavage of benzyl and allyl ethers and thioethers. These reactions are usually carried out by the general procedure of adding the metal to a solution of the substrate in liquid ammonia to which dry methanol or ethanol or t-butanol has been added to provide a ready proton source (alcohols are more acidic than ammonia).34... 

Although a-metallated allenic thioethers are readily formed from the corresponding thioethers and strong bases, it is in general more practical to generate these intermediates from the isomeric acetylenic sulfides. This can be done either with butyllithium (or BuLi TMEDA) [9] in THF or with alkali amides in liquid ammonia [44] ... 

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