Iodocarbonyl compounds

Bis(5> m-collidine)iodine(I) tetrafluoroborate in DMSO has been found to be a convenient reagent for the conversion of alkanes to a-iodocarbonyl compounds. When dihydrofuran (305a) and dihydropyran (305b) are the substrates, this reaction affords the corresponding a-iodolactones 306 (Scheme 77). This method converts certain glycals such as 307 to their corresponding a-iodo-a,/3-unsaturated lactones 309, presumably because of elimination of a molecule of acetic acid from the initially formed lactone 308 (86S727) (Scheme 78). 

Reduction of 3-iodocarbonyl compounds (equation 2, n = 1) with a low-valent metal, - such as metallic zinc, generates homoenolates of esters, nitriles and ketones, and represents a convenient new entry to homoenolates. This reductive method also allows the preparation of higher homologs (n > 2). 

Reduction of 3-iodocarbonyl compounds with a zinc/copper couple in polar solvents (e.g. DMF, DMA) > generates homoenolates of esters, nitriles and ketones - (Scheme 24). These species are not well defined, but they appear to be very similar to those obtained by the silyloxycyclopropane route. 

It had been assumed for some time that the radicals derived from a-iodocarbonyl compounds would be electrophilic in character. More recently, however, rate studies... 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

The conditions employed for iridium-catalyzed carbonylation (ca. 180-190 °C, 20-40 bar) are comparable to those of the rhodium-based process. A variety of iridium compounds (e.g., I1CI3, IrU, H2I1CI6, Ir4(CO)i2) can be used as catalyst precursors, as conversion into the active iodocarbonyl species occurs rapidly under process conditions. In a working catalytic system, the principal solvent component is acetic acid, so the methanol feedstock is substantially converted into its acetate ester (Equation (2)). Methyl acetate is then activated by reaction with the iodide co-catalyst (Equation (3)). Catalytic carbonylation of methyl iodide formally gives acetyl iodide (Equation (4)) prior to rapid hydrolysis to the product acetic acid (Equation (5)). However, it is difficult to establish the true intermediacy of acetyl... 

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