Alkynyliodonium tosylates

Alkynyl(phenyl)iodonium salts can be efficiently coupled with various organo-copper reagents. Direct coupling of alkynyliodonium tosylates 132 with vinyl-... 

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]. 

Likewise, alkynyliodonium tosylates can be coupled with dialkyl- and diphenyl cuprates 136 to afford the respective alkyl- and phenyl-substituted alkynes 137 (Scheme 56) [114]. An interesting example of this reaction involves the coupling of (trimethylsilyl)ethynyl iodonium triflate with cubyl cuprate generated in situ from iodocubane 138 [116]. 

The palladium catalyzed alkoxycarbonylation of alkynyliodonium tosylates 132 in methanol or ethanol in the presence of trialkylamine proceeds under mild conditions to give carboxylates 144 in good yield (Scheme 58) [120]. 

Various 2-substituted benzofurans 165 are obtained by the interaction of iodo-nium salts 164 with sodium phenoxide in methanol (Scheme 63) [126, 127]. This reaction proceeds via the intramolecular alkylidene carbene insertion into the ortho-CH bond of the phenoxy ring. Furopyridine derivatives 167 can be prepared similarly by the intramolecular aromatic C-H insertion of the alkylidenecarbenes generated by the reaction of alkynyliodonium tosylates 166 with potassium salts of 4- or 3-hydroxypyridines [128]. 

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... 

Dimethyl-l-butynyl(phenyl)iodonium tosylate has been employed with bis-(diphenylphosphino)methane to give the (terr-butyl)phospholium tosylate shown in equation 7892. The initial formation of an alkynylphosphonium ion with a free phosphino group has been proposed. Intramolecular cyclization of this intermediate followed by a 1,3-prototropic shift would lead to the observed product. Evidence for the probability of such an intermediate is provided by the fact that the alkynyliodonium tosylate, like the... 

Lithium dialkynylcuprates behave similarly with alkynyliodonium tosylates and lead to conjugated diynes (equation 133)109. Unsymmetrical diynes can be prepared with moderate selectivity by this method, although they are accompanied by symmetrical diynes derived from the alkynyliodonium component. The treatment of lithium diphenyl- and dialkylcuprates with alkynyliodonium tosylates has also been investigated and affords alkyl- and phenyl-substituted alkynes (equation 134)109. 

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... 

Dialkyl 1-alkynylphosphonates (22) were formed in good to high yields by treatment of 1-alkynyliodonium tosylates (23) with trialkyl phosphites the reaction is exothermic with trimethyl phosphite but requires heating with triethyl or triisopropyl phosphite. The addition of trimethyl or triethyl phosphite to 3-alkylene-2-oxindoles (24) has led to new product types for a, -unsaturated carbonyl compounds, i.e. a stable tri-alkoxyphosphonium zwitterion (25), and a C-alkylated phosphonate (26). ... 

Improvements [18] and modifications [19] in this procedure have provided product yields of the alkynyliodonium tosylates of 60-90% as well as broader applicability to a greater variety of P-alkyl groups R. These modified procedures are also applicable to the formation of methanesulfonates, 3803 , as well as p-N02C6H4S03 salts. 

Reaction of vinylcopper reagents, 62, with alkynyliodonium tosylates results in conjugated enyne 63, [53] [Eq. (24)]. The reaction is stereospecific with retention of olefin geometry. By appropriate order of addition, either of the two possible isomeric trisubstituted olefin isomers, 63, can be obtained in good isolated yields and excellent (>99%) stereoselectivity. Likewise, conjugated diynes, 65, are obtained [54] in the reaction of dialkynylcuprates, 64, with alkynyliodonium tosylates [Eq. (25)]. This method may be used for the preparation of unsym-metrical diynes. The mechanism of these coupling reactions is not understood at present. 

Finally, alkynyliodonium tosylates are subject to alkoxycarbonylation and formation of alkynoic esters, 66, via Pd-catalyzed CO insertion under very mild conditions [55] [Eq. (26)]. 

Reaction of alkynyliodonium sulfonates, 9, in dry acetonitrile in the presence of catalytic amounts of AgOTs or CuOTf leads to the formation of alkynyl sulfonates, 70, in reasonable yields [Eq. (30)] [18], In a similar manner bis(alkynyliodonium) tosylates, 36, give modest yields of bisalkynyl tosylates, 71, accompanied by some monotosylates, 72 [Eq. (31)] [41]. 

Trialkyl phosphites undergo reaction with alkynyliodonium tosylates, resulting in dialkyl alkynylphosphonates, 102, via an Arbuzov-type process [78] [Eq. (55)]. 

Preparation A common synthetic approach to alkynyliodonium salts involves the reaction of an electrophilic X -iodane with a terminal alkyne or its silylated, stannylated, or lithiated derivative. In the early 1980s, Koser and coworkers found that [hydroxy(tosyloxy)iodo]benzene 75 reacts with terminal alkynes 344 upon gentle heating in chloroform or dichloromethane to form alkynyliodonium tosylates 345 in moderate to low yield (Scheme 2.98) [199,471,476]. 

Polymer-supported alkynyliodonium tosylates 34 can be prepared by treatment of reagent 4 with terminal alkynes in the presence of p-toluenesulfonic acid (Scheme 5.20) [57]. Polymers 34 are effective alkynylating reagents toward sodium sulfinates and benzotriazole [57]. 

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]. 

The alkynylation of sulfur nucleophiles works well with alkynyliodonium tosylates and triflates as long as the sulfur atom is not too electron-rich, else oxidation reactions dominate. For example, alkynyl thiocyanates [38, 39, 75], thiotosylates [76], and phosphorodithioates [77] can be accessed in good yields (Scheme 8, A). The alkynylation of thioamides is also possible, but in this case the product obtained is imstable and spontaneously cyclizes to give a thiazole (Scheme 8, B) [78, 79]. The alkynylation of sulfinates with alkynyliodOTiium triflates or tosylates gives an efficient access towards alkynyl sulfones (Scheme 8, C) [80, 81]. If C-H bonds are easily accessible, carbene C-H insertion products can... 

Copper-catalyzed carbonylative coupling of ( )-a-(ethylsdanyl)-vinyl zircono-cene chloride derivatives with alkynyliodonium tosylates has been reported to be a mild method for the preparation of vinyl alkynyl ketones in good yields (Scheme 12.30) [39]. 

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