Alkynyliodonium sulfonates

The first stable class of alkynyl(aryl)iodonium salts were the tosylates, 9, prepared by the interaction of HTIB (5), with terminal alkynes in refluxing chloroform [Eq. (1)] [11-13]. Unfortunately, this method suffers from lack of generality, separation problems from the concomitantly formed alkene salt, 8, and low product yields of 9. 

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. 

A more general, simpler procedure [20] takes advantage of the in situ formation of the p-oxo-bis-triflate (6 R=CF3) and its interaction with a sila- or tin-acetylene [Eq. (2)]. This methodology affords a wide variety of stable, alkynyl(phenyl)iodonium triflates 10 in good to excellent yields and is applicable to the synthesis of the parent [21] ethynyl(phenyl)iodonium triflate (10 R=H) from n-Bu3SnC=CH. 

YC=CSnBu3 + PhICN OSO2CF3 CHaClj. YC=CIPh OSO2CF3 + BujSnCN 

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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]. 

Alkynyliodonium ions, 1 and 2, are hypervalent iodine species in which one or two alkynyl ligands are bound to a positively charged iodine(III) atom. They are sensitive to nucleophiles, especially at the /1-carbon atom(s) of the alkynyl ligand(s), and for that reason, the isolation of stable alkynyliodonium salts generally requires the incorporation of nucleofugic anions. A list of known alkynyliodonium compounds (i.e. as of 4/1/94), containing 134 iodonium salts derived from 103 iodonium ions, and references (5-45) to their preparation and characterization are presented in Table 1. Among these compounds, alkynyl(phenyl)iodonium sulfonates and tetrafluoroborates are the most common, while alkynyl(alkyl)iodonium salts of any kind are unknown. 

When y-CH bonds are present in the R group of the alkynyliodonium ion, cyclopentenyl sulfones predominate. For example, the treatment of 5-phenyl-1-pentynyl(phenyl)-iodonium tetrafluoroborate with te/ra- -butylammonium benzenesulfinate in THF (i.e. homogeneous conditions) affords a moderate yield of l-phenylsulfonyl-3-phenylcy-clopentene and a low yield of the corresponding alkynyl sulfone (equation 51)32. With appropriately constructed alkynyliodonium ions, annulated cyclopentenyl sulfones are obtained (equations 52 and 53)32. 

The reactions of / -ketoethynyl- and ) -amidoethynyl(phenyl)iodonium triflates, 17 and 18, with sodium / -toluenesulfinate illustrate the synthetic potential of alkynyliodonium salts33. Although the direct attachment of a carbonyl group to the / -carbon atom of the triple bond in alkynyliodonium ions might be expected to facilitate alkynyl sulfone formation via the Ad-E mechanism, this mode of reactivity has not been observed. Instead, the MC pathway with carbenic insertion dominates and affords sulfones containing the... 

The propensity of the thiocyanate ion for alkynylation with alkynyliodonium ions has also been demonstrated with a series of bis(phenyliodonium)diyne triflates (equations 61 and 62)43. The efficient production of diynediyl dithiocyanates in these reactions may be contrasted with the favored formation of mono- and bis-cyclopentenyl sulfones from bisiodonium diyne salts and sodium/ -toluenesulfinate (see equation 57)86. 

The presence of catalytic amounts of cuprous triflate or silver(I) sulfonates exerts a remarkable influence on the activation energy and regiochemistry of alkynyl(phenyl)-iodonium tosylate and mesylate decompositions5,6. Such reactions proceed in acetonitrile at room temperature and afford moderate yields of alkynyl tosylates and mesylates (equations 82 and 83)5,6. It is noteworthy, however, that the treatment of alkynyliodonium triflates (R = n-Bu, r-Bu) with cuprous triflate in acetonitrile does not afford alkynyl triflates6. Silver(I) catalysis has similarly been applied to the conversion of bis(alkynyliodo-nium) tosylates to bisalkynyl tosylates (equation 84)43. As might be expected, mono-tosylate esters are also produced in these reactions. 

Several mechanisms for the catalytic action of Cu(I) and Ag(I) have been considered6. Among these, the metal-assisted addition-elimination sequence shown in equation 85 and illustrated with cuprous triflate was deemed most consistent with various control studies. A mechanism not discussed but equally plausible is the metal-assisted MC sequence depicted in equation 86. The greater separation of iodonium-sulfonate ion pairs in acetonitrile versus benzene should provide the tosylate (or mesylate) ions with sufficient mobility to add to the /7-carbon atom of the alkynyliodonium ion. 

Although mechanisms for the production of (/ -sulfonyloxyvinyl)iodonium salts from terminal alkynes via alkynyliodonium salts can be envisioned (e.g. equation 174), they are not consistent with similar transformations of internal alkynes. The generation of vinyl cations, or iodine-bridged counterparts78, and their capture with sulfonate ions to give... 

The reaction of alkoxide ions with alkynyliodonium salts is unproductive, leading to only decomposition products rather than the desired alkoxyacetylenes. Similarly, reaction of R3SiO does not lead to any siloxyalkynes. In contrast the softer sulfonate, carboxylate, and phosphate nucleophiles all readily react with alkynyliodonium salts leading to the corresponding alkynyl sulfonate, carboxylate and phosphate esters [4]. 

Interaction of alkynyliodonium compounds with arylsulfinate salts is particularly interesting. When the R group of the alkynyl moiety lacks a y-CH bond, alkynyl sulfones (89, 90) are formed in excellent isolated yields [70, 71] [Eqs. (45), (46)]. When y-CH bonds are available, the intermediate unsaturated carbene (Scheme 3-3) prefers insertion over rearrangement and hence cyclopentenyl sulfones, 91, predominate, although some alkynyl sulfone formation is also observed as illustrated in [Eq. (47)] [72]. 

Formalistically, reactions 63 and 64 are nucleophilic acetylenic substitutions (Sa -A) with the corresponding anions (sulfonate, phosphate or carboxylate) acting as nucleophiles and alkynyliodonium species 97-99 as the electrophilic substrates. However, the actual details of the mechanism are considerably more complex (equation 66). 

Single-crystal X-ray structural data are all consistent with the pseudo-trigonal bipyramidal, or T-shaped geometry, of alkynyliodonium species. In all known cases, the aryl group occupies an equatorial position, whereas the alkynyl moiety and the counter ion occupy apical positions. The alkynyl-iodine bond length is about 2.0 A and the I -O distances to the nearest sulfonate anion vary from 2.56 to 2.70 A. The C—I—O bond angles vary from 166° to 172° and the C—I—C bond angles are between 90° and 95°. 

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... 

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