Alkynyliodonium tetrafluoroborates

Interaction of sila-alkynes with excess iodosobenzene 4 in the presence of excess triethylox-onium tetrafluoroborate in dichloromethane at room temperature leads [27] to the formation of alkynyliodonium tetrafluoroboiates 12 [Eq. (4)]. A variation of this procedure employs Et20 BF3 followed by treatment with aqueous NaBp4 [Eq. (5)] [27, 28]. To date only alkyl-and aryl-substituted homologs, along with the silyl ethynyliodonium tetrafluoroborates, 12, have been reported via these procedures. 

Interestingly, treatment of Me3SiC = CH under these conditions does not yield the parent iodonium tetrafluoroborate (12, R=H). The parent system can be obtained by reaction of the silyl system with 48 % HF [Eq. (6)] [28], Direct conversion of a terminal alkyne to 12, via treatment with the p-oxo-bis-BF4 13, has also been reported [29] [Eq. (7)]. However, the generality and scope of this interesting and simple reaction was not reported. 

Similarly, treatment of t-BuCsCH with a 1 1 mixture of iodosobenzene and CF3SO3H (PhIO —TfOH) is reported to give the t-butylethynyl(phenyl)iodonium triflate [30]. Benzilic oxidation and formation of the acylalkyne 15, rather than alkynyliodonium salt, was observed in the reaction of 14 with PhIO and BF3 OEt2 in dioxane [Eq. (8)] [27]. 

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The production of alkynyliodonium tetrafluoroborates from alkynylsilanes with PhIO BF3 may be rationalized in a similar way (equation 7)31. 

Two methods for the direct conversion of terminal alkynes to alkynyl(phenyl)iodo-nium tetrafluoroborates have been reported, but their generality has not been documented in the literature. The ter -butylethynyl salt has been prepared by the generation of lithium tert-butylethynyl(trifluoro)borate and its coupling with iodosylbenzene by the standard method (equation 29)80. The treatment of 1-pentyne and phenylacetylene with diphenyl-ju-oxodiiodine(III) bistetrafluoroborate in dichloromethane likewise affords the corresponding alkynyliodonium tetrafluoroborates (equations 30)29. 

Despite the synthetic possibilities suggested by this early study, the chemistry of the alkynyliodonium salts lay dormant until the mid-1980s. In 1986, Ochiai and his coworkers published an important communication which shaped much of the later thinking on the reactions of alkynyliodonium ions with nucleophiles28. When / -dicarbonyl enolates are treated with alkynyliodonium tetrafluoroborates containing a long (> three carbons) alkyl chain, derivatives of cyclopentene are produced. This is illustrated in equation 41 for the... 

The contrasting behavior of alkynyliodonium tetrafluoroborates and alkynyliodonium triflates with triphenylphosphine (i.e. photochemical vs thermal activation) documented in the foregoing studies seems unlikely to be due to the different anions. Perhaps the thermal alkynylations are simply too slow to be observed at-78 °C in coordinating solvents such as THF, the conditions employed in the photochemical study. 

So far, this method has been employed for the synthesis of (1) (Z)-(jS-sulfonylvinyl)-iodonium tetrafluoroborates from alkynyliodonium tetrafluoroborates with sulfinic acids in methanol (equation 46)32,84, (2) (Z )-(/ -azidovinyl)iodonium tetrafluoroborates from alkynyliodonium tetrafluoroborates with Me3SiN3, H20 (i.e. HN3) in dichloromethane (equation 110)102 and (3) (Z )-/ -bromo- and (jS-chlorovinyl)iodonium halides from alkynyliodonium tetrafluoroborates with lithium halides in acetic acid or with HX in methanol (equation 111 and 112)103. 

The seminal paper which led to the revival of alkynyliodonium chemistry was published by Ochiai et al in 1986. The reaction of p-diketone enolates with alkynyliodonium tetrafluoroborates containing an alkyl chain led to the formation of cyclopentene derivatives such as (81). 1 ... 

Reaction of P-functionalized alkynyliodonium triflates, 11, with LiNPh2 results in various push-pull ynamines, 67, in 43-66% isolated yields [56] [Eq. (27)]. Treatment of alkynyliodonium tetrafluoroborates with Me3SiN3 in wet CH2CI2 results in the stereoselective formation of (Zy-P-azidovinyl iodonium salts 68 [Eq. (28)] in 50-91 % isolated yields [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]. 

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 alkyl chain in such MCI reactions need not be restricted to the alkynyliodonium component. For example, the treatment of sodium 2-n-hexyl- 1,3-indandionate with 1-propynyl(phenyl)iodonium tetrafluoroborate in THF affords the spiro-nonene system shown in equation 4228. 

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. 

When / -dicarbonyl enolates are allowed to react with alkynyliodonium salts, typically in ter/-butyl alcohol or THF, alkynyl- and/or cyclopentenyl- -dicarbonyl compounds are obtained. The product compositions are largely regulated by the migratory aptitude of R in the alkynyl moiety and the availability of alkyl side chains for the MC-insertion (MCI) pathway (equation 45). These divergent modes of reactivity are nicely illustrated by the reactions of the 2-phenyl-1,3-indandionate ion with ethynylfphenyl)- and 4-methyl-1-hexynyl(phenyl)iodonium tetrafluoroborates (equation 1 15)27 2. 

The treatment of alkynyl(phenyl)iodonium tetrafluoroborates in dichloromethane with aqueous sodium periodate affords the corresponding alkynyliodonium periodates24. However, except for the ter t-buty analog, which has been characterized by X-ray analysis (Table 3), these compounds are unstable to autooxidation and readily decompose to carboxylic acids (equation 150). 

The treatment of the thermodynamic enolate of 2-methylcyclohexanone with ethynyl(phenyl)iodonium tetrafluoroborate in tetrahydrofuran affords 2-ethynyl-2-methylcyclohexanone (equation 267). This is the only example known to this author of the alkynylation of an unactivated monocarbonyl compound with an alkynyliodonium salt. However, the earlier conclusion that simple enolates do not undergo alkynylation with alkynyliodonium salts (section II.D.7 and II.G) needs to be revvised. 

The onium transfer reaction between alkynylphenyliodonium tetrafluoroborates and triphenylarsine afforded high yields of 1-alkynyltriphenylarsonium tetrafluoroborates.However, this reaction appeared to be mechanistically at variance with the generally admitted patterns of reactivity of alkynyliodonium salts. Reaction of phenyl(phenylethynyl-2- C)iodonium tetrafluoroborate (88) (99% enriched) led to the arsonium salts (89) with more than 95% of enrichment on the p>carbon atom. Although the Michael carbene pathway was not totally excluded, the ligand coupling pathway seemed therefore more important. 

Single-crystal X-ray structural data have been reported for the following alkynyliodonium compounds the parent ethynyl(phenyl)iodonium triflate (368) [495], phenylethynyl(phenyl)iodonium tosylate (369) [477], cyanoethynyl(phenyl)iodonium triflate (370) [458], propynyl(phenyl)iodonium periodate (371) [502], trimethylsilylethynyl(phenyl)iodonium triflate (372) [503], 3,3,3-trifluoropropynyl(phenyl)iodonium triflate (373) [504], bis(alkynyl)iodonium triflate 374 [505] and complexes of ethynyl(phenyl)iodonium [506] and l-decynyl(phenyl)iodonium [507] tetrafluoroborates with 18-crown-6 (Figure 2.13). 

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