Alkynylations with alkynyl iodonium salts

Another useful reagent for the preparation of alkynyl lodonium Inflates is [cyano(trifluoromethylsulfonyloxy)(phenyl)]iodine [/i7, 138, 139, 140] prepared from iodosobenzene, trimethylsilyl tnflate, and trimethylsilyl cyanide (equation 71). This reagent reacts with various stannylacetylenes under very mild conditions to form the corresponding alkynyl iodonium salts in high yields [139] (equation 72)... 

The unique reactivity pattern of alkynyl iodonium salts discussed in Sections II,A.2 and II,D,la can also serve as two-carbon conjunctive reagents in the synthesis of pyrroles, dihydropyrroles, and indoles. Feldman et al. found that combination of alkyl or aralkyl tosylamide anions 101 with phenyl(propynyl)iodonium triflate (102) furnishes the corresponding dihydropyrroles 103 (95JOC7722) (Scheme 28). 

Alkynyl(aryl)iodonium salts where aryl was the o-carboxyphenyl group were obtained from silylalkynes and o-iodosobenzoic acid [139]. It is noted that the first alkynyl iodonium salts were obtained in low yield from terminal alkynes. This approach was perfected recently, so that a range of substrates upon reaction with p-MeC6H4IO in aqueous HBF4, with catalysis by HgO, afforded alkynyl (tolyl)iodonium salts [140]. 

A number of 1-hydroxybenziodoxoles and related heterocycles, more efficiently through their triflates, upon reaction with RC = CSiMe3 afforded either alkynyl iodonium salts or the corresponding cyclic iodanes, depending on the conditions. Some of these compounds reacted with NaN3 and were converted into /J-azidoalkenyl derivatives as illustrated in Scheme 55 [127,167,168]. 

The same salt from acetylene afforded similarly adducts with furan and 1,3-diphenyl i sobenzofuran. A number of alkynyl iodonium salts underwent also [2 + 3] cycloaddition with dipolarophiles such as a-diazocarbonyl compounds, nitrile oxides, etc., allowing the preparation of iodonium salts with an alkenyl or a heterocyclic moiety [7],... 

Enynes were prepared in good yield from alkynyl iodonium salts and alkenylcopper reagents, stereospecifically. This approach was suitable for the synthesis of conjugated enynes, using a trisubstituted alkene with complete retention of its geometry [47], 1,3-Diynes were similarly obtained by coupling alkynyl iodonium... 

Appropriately substitued alkynyl iodonium salts afforded with nucleophiles cyclopentene derivatives. This annulation can be either [5 + 0], when all carbon atoms come from the alkyl chain of the alkynyl moiety, or a [2 + 3] process, in which three carbon atoms come from the nucleophile. Competition between [5 + 0] and [2 + 3] annulation may occur in some cases. 

An alternative approach for alkynyl carboxylates involved reaction between [bis(acyloxy)iodo]benzenes and lithium acetylides [59]. Alkynyl iodonium salts afforded with sodium carboxylates in the presence of water 1-acyloxyketones heating in an excess of acetic acid gave similarly a-acetoxy ketones [60], Alkynyl tosylates and mesylates were obtained from the thermal decomposition of isolable alkynyl iodonium sulphonates. 

Thiocyanation was very effective with several alkynyl iodonium salts, in a simple process avoiding the previously used cyanogen chloride or mercury compounds best yields of alkynyl thiocyanates were obtained using potassium thiocyanate in DMF [66],... 

Another superior method using alkynyl iodonium salts involved the preparation of S-alkynyl O, O-dialkylphosphorodithioates high yields were obtained under phase transfer catalysis [67]. Similarly, with potassium p-toluenethiosulphonate were obtained esters of the general formula p-TolS02SC=CR [68]. 

Other pathways, with trimethylsilyl azide and the appropriate iodonium salts, involved the formation of 3-alkyl-1-azido-cyclopentenes (Table 9.3.) or Z-fi-azidoalkenyl iodonium salts [69], Several types of alkynyl iodonium salts afforded with lithium diphenylamide push-pull ynamines overcoming some inherent limitations of other methods. 

Triphenylphosphine with alkynyl iodonium salts in sunlight gave quantitatively the corresponding phosphonium salts, some of which could not be obtained by other methods [73]. Trialkyl phosphites were also reactive towards alkynyl iodonium salts several diaikyl alkynylphosphonates were prepared in good yield, e.g. [74] ... 

In a related area, interest in the isolation of alkynyl iodonium salts has led to the synthesis of analogous isoelectronic xenon(II) derivatives. These have been obtained by reaction of XeF2 with BulC=CLi or RC=CSiMe3 in BF3 OEt2 (181) ... 

Acetylenic ethers and esters represent an important class of functionalized acetylene derivatives of the hypothetical alkynols, RC=COH. The chemistry of acetylenic ethers has been well developed since their first preparation about 100 years ago Several exhaustive reviews covering the literature on acetylenic ethers and their analogs up to 1985 have been published in the last 30 years. In contrast to acetylenic ethers, esters of alkynols were unknown until the mid-1980s when the first preparation of alkynyl tosylates was reported. In the following years a wide variety of alkynyl carboxylate, phosphate and sulfonate esters has been prepared from alkynyl iodonium salts. The chemistry of these novel derivatives of alkynols has been summarized in a recent review. In the last 10 years considerable interest and research activity has arisen toward alkynols themselves and such derivatives as alkynolate salts and silyl ynol ethers. The present chapter will cover the chemistry of acetylenic ethers and esters as well as related derivatives of ynols with emphasis on new developments in this subject during the last 5-10 years. 

The reaction of sulfinates with alkynyl iodonium salts was successful, as these substrates are less easy to oxidize. Nevertheless, Waser and Chen demonstrated that EBX reagents can also be useful to synthesize alkynyl sulfones, as they allow a new one-pot procedure starting directly from Grignard reagents (Scheme 36) [161]. In this protocol, DABSO (DABCO-S02) is added after formation of the Grignard reagent. Addition of DME and TIPS-EBX 52 gives aryl alkynyl sulfones in 46-85% yield. Eor base sensitive substrates, it was also possible to start from aryl iodides and use a palladium catalyst. 

Kang, S.K. Lee, H.W. Jang, S.B. Ho, P.S. (1996) Palladium-catalyzed cross-coupling reactions of aryl-iodonium, alkenyl-iodonium and alkynyl-iodonium salts and iodanes with terminal alkynes in aqueous-medium, Chem. Commun., 835-6. 

As mentioned in the previous section, the treatment of secondary phosphine oxides with alkynyl iodonium salts is a reliable method for the preparation of phosphorus-carbon(sp) bonds. This chemistry can be extended to secondary phosphites for the preparation of alkynylphosphonates (Schane 4.367) [541]. The reactions were carried out under mild conditions in reaction vessels that were open to the atmosphere. 

Two approaches have been applied for the synthesis of imidazo[2,l + thia/,ole ring systems. Reaction of 2-mercapto-benzimidazole with perfluoro-2-methylpent-2-ene in the presence of triethylamine gave compound 419 (Equation 192), and cyclocondensation of 2-imidazolidinethione with the alkynyl(phenyl)iodonium salt 420 afforded product 421 (Equation 193) <2001RCB1446, 2003JCM715>. 

Carbonylative cross-coupling of various iodonium salts bearing transferable aryl, heteroaryl, alkenyl, alkynyl residues with phenylboronic acid takes place under mild conditions giving the respective ketones in high yields (Equation (16)). The yields of competing cross-coupling do not exceed 8%. 

There are two approaches for the preparation of alkenyl(phenyl)iodonium salts reaction of an activated alkene with an iodine(III) species and addition to a triple bond, either of simple alkynes or of alkynyl(phenyl)iodonium salts. 

In some instances the iodonium salt is not isolated but may react in situ with nucleophiles, for example upon hydrozirconation of alkynyl(phenyl)iodonium salts with Cp2Zr(H)Cl (Scheme 42) these salts were used for the stereoselective synthesis of some halogenated alkenes [128]. 

The palladium/copper-cocatalyzed coupling of the readily available trisubsti-tuted alkenyl(phenyl)iodonium triflates 77 with alkynyl- and alkenylstannanes proceeds under exceedingly mild conditions with retention of geometry of the alkenyl ligand of the iodonium salt (Scheme 35) [60]. 

Alkynyl(phenyl)iodonium salts can be used for the preparation of substituted alkynes by the reaction with carbon nucleophiles. The parent ethynyliodonium tetrafluoroborate 124 reacts with various enolates of /J-dicarbonyl compounds 123 to give the respective alkynylated products 125 in a high yield (Scheme 51) [109]. The anion of nitrocyclohexane can also be ethynylated under these conditions. A similar alkynylation of 2-methyl-1,3-cyclopentanedione by ethynyliodonium salt 124 was applied in the key step of the synthesis of chiral methylene lactones [110]. 

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

Alkynyl(phenyl)iodonium salts can be effectively used in palladium(O) or copper(I) catalyzed coupling or carbonylative coupling reactions with various organozirconium complexes (Scheme 57) [117-119]. 

Recent progress on the use of hypervalent iodine reagents for the construction of carbon-het-eroatom (N, O, P, S, Se, Te, X) bonds is reviewed. Reactions of aryl-A3-iodanes with organic substrates are considered first and are loosely organized by functional group, separate sections being devoted to carbon-azide and carbon-fluorine bond formation. Arylations and alkenyla-tions of nucleophilic species with diaryliodonium and alkenyl(aryl)iodonium salts, and a variety of transformations of alkynyl(aryl)iodonium salts with heteroatom nucleophiles are then detailed. Finally, the use of sulfonyliminoiodanes as aziridination and amidation reagents, and reactions of iodonium enolates formally derived from monoketones are summarized. 

These highly reactive yet stable species are strong electrophiles of tetraphilic character, since nucleophiles may attack three different carbon atoms (a,/ ,a ) and iodine. In most reactions the first step is a Michael addition at fi-C with formation of an alkenyl zwitterionic intermediate (ylide) which normally eliminates iodoben-zene, generating an alkylidene carbene then, a 1,2-shift of the nucleophile ensues. The final result is its combination with the alkynyl moiety which behaves formally as an alkynyl cation. The initial adduct may react with an electrophile, notably a proton, in which case alkenyl iodonium salts are obtained also, cyclopentenes may be formed by intramolecular C-H 1,5-insertion from the alkylidenecarbenes ... 

Using this method, several mesogen diacetylenes were obtained [49]. Palladium-catalysed coupling of an allylic cyclic carbonate with 1-pentynyl phenyliodonium tetrafluoroborate to give an enyne was highly successful [50]. Alkynyl iodonium triflates and lithium salts of diethyl 2-[(diphenylmethylene)amino]malonate were used for the preparation of alkynyl-a-amino acid derivatives, e.g. [51] ... 

The iodonium triflate (460 mg, 1 mmol) was added to a stirred slurry of anhydrous sodium p-toluene sulphinate (180 mg, 1.01 mmol) in dichloromethane (15 ml) at 20°C under nitrogen. After 15 min water (10 ml) was added and the phases were separated the aqueous layer was extracted with additional dichloromethane (2 x 5 ml), and the combined organic extracts were dried. The filtered solution was treated with hexanes (30 ml) and concentrated. The solid residue was purified by radial chromatography (silica gel, 200-400 mesh, dichloromethane-hexanes) to afford 3-tosyl-bicyclo[3.2.0]-3-heptene-2-one (197 mg, 75%), m.p. 164-165°C. The method is general for the preparation of sulphones with a cyclopentenone moiety other alkenyl iodonium salts gave alkynyl sulphones with sulphinates (Section 9.4.4). 

Stable esters coming formally from alkynols and carboxylic, sulphonic or dialkyl phosphoric acids were prepared for the first time via alkynyliodonium salts with the corresponding anion. Several alkynyl carboxylates were obtained by anion exchange of alkynyl iodonium tosylates the initially formed salts, PhI+C=CR RCO2, were converted spontaneously into the esters on elution through a chromatographic column packed with an anion-exchange resin (Pol+ArCOj) ... 

Equally good yields were also obtained from alkynyl iodonium triflates without phase transfer catalysis [65]. With arenesulphinic acids in methanol the reaction stopped at the stage of Z-/J-sulphonylalkenyl iodonium salt [41]. 

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