Dicarbonyl rhodium, reduction

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

Numerous applications of this chemistry to the synthesis of leukotrienes have been reported, as illustrated for the preparation of 12-hydroxyeicosatetraenoic acid (189 Scheme 41).154 Reaction of the carbe-noid precursor (190) with furan in the presence of rhodium(II) acetate generated a furanocyclopropane, which on standing reverted to predominantly the (Z, )-isomer (191). Reduction of the dicarbonyl compound (191) gave the diol (192), which was then selectively converted to the bromide (193). Subsequently, (193) was coupled with the alkynide (194), followed by Lindlar reduction and deprotection to produce (189). The overall procedure is quite general and has been applied to a range of related compounds (195-199)135-138 and useful synthetic fragments (200 and 201).153... 

Hence the rhodium III solvated by lattice oxide ions and presumably extra framework oxide ions or hydroxo ligands (depending on the dehydration state) could be carboxylated reductively to rhodium I dicarbonyl according to one of the following reaction scheme depending on the hydration state... 

As the VCO absorption bands due to the dicarbonyl decreased an IR band at 2340 cm due to CO. developed gradually together with a set of absorptions around 2100-2000 cm- due to new linear carbonyls and absorptions around 1800 cm- presumably due to bridged carbonyls (14). CO- appearance was interpreted as an indication of the further reduction of the monovalent rhodium either by CO or via the water gas shift reaction producing H- which is reported to occur on monoculear monovalent carbonyls (19, 20). As rhodium I was reduced to the zerovalent state, the observed VCO bands were ascribed to Rh (C0)j2 compound, in view of the excellent agreement between the observed frequencies and those reported for Rh (C0)j- in CI C or nujol or when adsorbed on fully dehydrated zeolite. 

The following step could be the reductive elimination of the acetyl halide to react with methanol. The growth of IR bands in the 1710-1685 cm- domain might be interpreted as due to CH COI accumulation and possibly further reaction with substrates present in the medium. Nevertheless readdition of CO restored the monovalent rhodium dicarbonyl thus indicative that somehow CH COI was eliminated. 

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