Alkynyliodonium carboxylates

A more general and efficient approach to alkynyl carboxylates, also thought to involve alkynyliodonium carboxylate intermediates, entails the treatment of bis(acyloxyiodo)-arenes with alkynyllithium reagents (equation 88)81. These reactions are best conducted in the presence of 2-nitroso-2-methylpropane in order to suppress oxidative coupling of the lithium acetylides by the acyloxyiodanes. 

Alkynyl carboxylate esters, 75, are obtained in the reaction of lithium acetylides with bis(acyloxy)iodobenzene, 73 [Eq. (32)] [59], These reactions are likely to proceed through the intermediacy of the respective alkynyliodonium carboxylates (74), although no such salts have been isolated to date as they spontaneously decompose, via loss of iodobenzene, to the alkynyl carboxylates, 75. Only benzoate esters (75 R = CgHj) are sufficiently stable to isolate and store pure for longer periods. Simple alkylcarboxylates such as acetates are not stable although the hindered pivaloate ester (65 R = t-Bu, R = t-Bu) has been isolated in low yield [59]. Among the reasons for the instability of these esters is their sensitivity to moisture they both readily add water and undergo subsequent hydrolyses [60]. Because of the sensitivity to moisture, the isolated yield [41] of bisalkynyl benzoates, 76, from the bisalkynyliodonium triflates, 35, is only 6-15% [Eq. (33)]. 

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]. 

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) ... 

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). 

Reaction of trimethylsilylalkynes with 16 in dichloromethane in the presence of BF3 OEt2 at room temperature followed by heating in methanol at 60 °C results in the stable heterocyclic alkynyliodonium salts 17 [Eq. (9)] [31]. These species represent intramolecular iodonium salts where the counterion is a carboxylate. Unlike the acyclic alkynyl(phenyl)iodonium carbox-ylates 18, that are unstable to isolation and decompose to the corresponding alkynyl benzoates 19 [Eq. (10)] [32], the cyclic analogs 17 are readily isolable. 

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]. 

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). 

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