Alkynyliodonium tosylates synthesis

A similar coupling of alkynyliodonium tosylates 132 with dialkynylcuprates 134 leads to conjugated diynes 135 (Scheme 55) [114,115]. This method can be used for the preparation of unsymmetrical diynes in moderate yield. Recently, this coupling was employed in the synthesis of various liquid-crystalline diaryl-diacetylenes [115]. 

Alkynyliodonium tosylates have been similarly employed with the potassium salts of 3- and 4-hydroxypyridines for the synthesis of furopyridines (Scheme 58) [160]. As expected, the 3-hydroxypyridines afford mixtures of regioisomer-ic furopyridines, corresponding to carbenic insertions into the non-equivalent ortho-CH bonds of the pyridine ring. 

The synthesis of the first alkynyliodonium tosylates was achieved by the treatment of terminal alkynes with [hydroxy(tosyloxy)iodo]benzene (HTIB) (equation 8)8,10,11. Such reactions are generally conducted with an excess of alkyne in chloroform at reflux, although they can be carried out at room temperature, and dichloromethane can be employed as solvent. This procedure is, however, restricted to terminal alkynes in which R is either an aryl group or a bulky alkyl group. With linear alkyl groups (i.e. R = n-Pr, n-Bu, fl-C5Hn), phenyl(/ -tosyloxyvinyl)iodonium tosylates are obtained instead (equation 9)8. In some cases (R = /-Pr, /-Bu), mixtures of alkynyl- and (/ -tosyloxyvinyl)iodonium tosylates are produced8. ter -Butylacetylene appears to be the optimum substrate for this approach and has been employed with a series of [hydroxy(tosyloxy)iodo]arenes for the synthesis of various aryl(ter/-butylethynyl)iodonium tosylates (equation 10)9. 

The incorporation of propyne and 1-hexyne into alkynyliodonium salts with HTIB and its mesyloxy analog has been accomplished with the aid of a silica bead desiccant (equation ll)5,6, but the yields of the products are low. A better method for the synthesis of alkynyliodonium tosylates in which R is a linear alkyl group entails the treatment of (trimethylsilyl)alkyne/iodosylbenzene mixtures in chloroform with boron trifluoride etherate. When aqueous sodium tosylate is added to the resulting solutions, alkynyliodonium tosylates are produced and can be isolated from the organic phase (equation 12)7. The... 

While vinylsilanes and -stannanes have been used primarily for the synthesis of vinyliodonium salts with one or two / -alkyl substituents in the vinyl moiety, the treatment of alkynes with oxyiodanes permits the introduction of oxygen functionality at jft-carbon. The conversion of terminal alkynes with [hydroxy(tosyloxy) iodo]benzene (HTIB) to alkynyliodonium tosylates (equation 8) and/or (j5-tosyloxyvinyl)iodonium tosylates [TsOC (R)=CHiPh, "OTs R = n-Pr, n-Bu, n-C5Hn, i-Pr, i-Bu] (equation 9), depending on the size of R, has already been discussed8,11. In at least three cases, E Z mixtures were... 

The alkynylation of phosphorus nucleophiles has been less investigated (Scheme 7). Ochiai and co-workers first demonstrated in 1987 that the alkynylation of triphenyl-phosphine was possible with alkynyliodonium tetrafluoroborate salts under light irradiation (Scheme 7, A) [69]. The reaction most probably involves radical intermediates. In 1992, Stang and Critell showed that light irradiation was not needed if alkynyliodonium triflates were used [70]. Later, this methodology could be extended to other triaryl- or alkyl phosphines [71, 72]. In 1990, Koser and Lodaya also reported the synthesis of alkynylphosphonates by the Arbusov reaction of alkynyliodonium tosylates with trialkyl phosphites (Scheme 7, B) [73]. Alternatively, the same compotmds can be obtained by the reaction of alkynyliodonium tosylates with sodium phosphonate salts [74]. 

Recent examples of the rearrangement or alkynylation pathway include conversions of arylethynyl- and er -butylethynyl(phenyl)iodonium tosylates 24 and 25 to alkynylphosphonates, -selenides, and -tellurides with the appropriate anion salts in DMF (Scheme 50) [145-147], and a similar synthesis of push-pull selenides and tellurides from alkynyliodonium triflates containing electron-withdrawing groups in the alkynyl moiety [148]. 

Reactions of alkynyliodonium salts with multidentate nucleophiles can be employed for the synthesis of heterocyclic compounds. Recent examples include preparations of thiazoles, selenazoles, and 2-mercaptothiazoles by the treatment of alkynyliodonium mesylates or tosylates with thioamides, selenoamides, and ammonium dithiocarbamate (Scheme 62) [169-171]. A novel hetero-Claisen rearrangement of tricovalent iodine(III) intermediates was proposed to account for the 2,4-disubstitution pattern of the thiazoles [169]. 

Various flve-membered heterocycles can be prepared by inter- or intramolecular addition/cyclizations of appropriate nucleophiles with alkynyliodonium salts via alkylidene carbene intermediates [856, 978, 979]. The intermolecular variant of this cyclization has been employed in the synthesis of 3-substituted-5,6-dihydroimidazo[2,l-( ]thiazoles [997], 2-substituted imidazo[l,2-a]pyrimidines [998] and 2-substituted-imidazo[l,2-fl]pyridines [999]. In a representative example, 2-substituted imidazo[l,2-fl]pyridines 744 were synthesized in good yield by cyclocondensation of 2-aminopyridine (742) with alkynyl(phenyl)iodonium tosylates 743 under mild conditions (Scheme 3.293) [999]. The mechanism of this cyclization involves... 

Up to now, no efficient alkynylation of oxygen nucleophiles with EBX reagents has been reported. Also, in the case of nitrogen nucleophiles, alkynyUodonium salts remain the reagents of choice. Nevertheless, Cossy and co-workers reported in 2013 that the alkynylation of sulfonamides was possible with TMS-EBX 36 [157], Interestingly, no alkynylation was observed in the case of carbamates, although these substrates are readily alkynylated with alkynyliodonium salts. Selective alkynylation of the tosyl amide in the presence of a carbamate was possible. This selectivity was exploited for the synthesis of tetrahydropyrazine heterocycles. 

---