Alkynyl iodonium sulfonates

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. 

The present procedure, similar to that of Fujita for the preparation of alkynyl(phenyl)iodonlum tetrafluoroborates, RC5CIPh BF4, is simpler, much more general and in most cases gives significantly better yields. Table I gives yields of alkynyl(phenyl)iodonium sulfonates prepared by this procedure. 

Alkynyl(phenyl)iodonium sulfonates are stable, microcrystalline substances that can be stored and used indefiniteiy with little or no decomposition. They have been employed in the formation of aryl (2-furyl)iodonium tosylates, alkynyl sulfonate, carboxylate and phosphate esters, tricoordinate vinyliodinane species,fO and alkylidenecarbene-iodonium ylides. <... 

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. 

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. 

TABLE 4. Synthesis of alkynyl sulfones from alkynyl(phenyl)iodonium salts... 

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 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. 

Replacement of an unsaturated phenyliodo group. Formation of unsaturated sulfones by a tandem Michael addition-elimination is a highly efficient process that allows the synthesis of (Z)-l,2-bis(benzenesulfonyl)alkenes from (Z)-jS-(benzenesul-fonyl)alkenyliodonium salts. In S-ketoethynyl(phenyl)iodonium salts the electron-withdrawing power of the ketone group is weaker, the Michael addition is followed by carbene formation. Cyclopentenones are formed. A seemingly direct substitution of alkynyl(phenyl)iodonium salts gives alkynyl sulfones efficiently. ... 

The first successful preparation of acetylenic esters involved the use of alkynyl (phenyl)iodonium salts 97-99, which are now readily available by a variety of methods. The first reported members of acetylenic esters, alkynyl sulfonates 94, have been prepared by treatment of alkynyl(phenyl)iodonium tosylates or mesylates (97) with catalytic CuOTf or AgOTf in dry acetonitrile (equation 63) . ... 

Ochiai and coworkers have used these tandem Michael-carbene-insertion process in a nice synthesis of cyclopentene derivatives as illustrated by 69 and 70 using alkynyl(phenyl)iodonium salts (equation 31). Similarly, PhS02H addition to alkynyl(phenyl)iodonium salts in methanol results in the formation of e.g., 71 that, upon treatment with Et3N, gives both rearranged sulfone 72 and the cyclopentene 73 via the unsaturated carbene, as illustrated by the example in equation 32. 

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