Thiols from diazonium salts

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

A preparation of quinoline-3-thiol from the diazonium salt, according to prior literature, and using potassium ethyl xanthate, abnormally threw down a solid during extraction of the product into ether. This solid, filtered and dried, exploded on prodding with a metal spatula. The solid had an nmr spectrum consistent with the expected product, an 5-aryl-O-ethyl xanthate. Although the author appears aware of only the arenediazosulfide hazard, this must have been the 5 -arenediazoxanthate. A misprint renders quinoline as quinine. 

For Pd-catalyzed cross-coupling reactions the organopalladium complex is generated from an organic electrophile RX and a Pd(0) complex in the presence of a carbon nucleophile. Not only organic halides but also sulfonium salts [38], iodonium salts [39], diazonium salts [40], or thiol esters (to yield acylpalladium complexes) [41] can be used as electrophiles. With allylic electrophiles (allyl halides, esters, or carbonates, or strained allylic ethers and related compounds) Pd-i73-jt-allyl complexes are formed these react as soft, electrophilic allylating reagents. 

Sulfur can be introduced into a diazonium salt by the SN1 reaction shown in Figure 5.51. In order to prevent the reagent from effecting a double (rather than a mono-) arylation at the sulfur atom, potassium xanthogenate instead of sodium sulfide is used as the sulfur nucleophile. The resulting S-aryl xanthogenate C is hydrolyzed. In this way diarylsulfide-free aryl thiol B is obtained. 

The thiol (SH) group is introduced by reaction with potassium ethyl xanthate followed by acid hydrolysis. The phenylsulfanyl (phenylthio, SPh) group results from reaction with benzenethiolate ion. Sodium disulfide, Na2S2, yields diaryl disulfides. The arsonic acid group is introduced using Bart s reaction, in which a diazonium salt is reacted with sodium arsenite in the presence of a Cu(II) salt (Scheme 8.23). 

Esters can be obtained by reacting phosphinodithioic acids with alcohols (9.449), or by reacting alkali metal salts with alkyl halides (9.450) or benzene diazonium chloride (9.451). Phosphinodithioate esters can also be obtained from halophosphoranes and thiols (9.452) or disulphides and secondary phosphines (9.453). 

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