Stoichiometric Organic Reactions

7 Stoichiometric Organic Reactions and Acid-Catalyzed Reactions in Ionic Liquids 

Stoichiometric - or, more simply, non-catalytic - reactions are an important and rapidly expanding area of research in ionic liquids. This section deals with reactions that consume the ionic liquid (or molten salt) or use the ionic liquid as a solvent. 

This chapter describes several examples of organic reactions in SCCO2, SCH2O, and other fluids (e.g. ethane, ethene, xenon, trifluoromethane), for which a few detailed descriptions of experimental procedures are included. 

The first experiments which were carried out in the author s laboratory on organometallic phase-transfer catalysis were concerned with the reduction of nitrobenzenes (4) to anilines (5) by triiron dodecacarbonyl. Such a conversion was reported to occur in benzene containing methanol at reflux for 10-17 h, with the hydridoundecacarbonyltriferrate anion as the likely key intermediate (16). It was our expectation that the trinuclear iron hydride should be generated by phase-transfer catalysis and if so, effect reduction of nitro compounds (4) under exceedingly mild conditions. Indeed this was the case, as illustrated by the results shown in Table I (17). Not only is the reaction complete in 2 h or less using sodium hydroxide as the aqueous phase, benzene as the organic phase, and benzyltrieth-ylammonium chloride as the phase-transfer catalyst, but it occurs at room temperature and requires less metal carbonyl than when the reaction was 

Crown ether catalysis of the reduction is also useful (Table I), with potassium hydroxide as the base and 18-crown-6 as the catalyst (18). 

Yields of Anilines (5) Obtained from 4 and Fe3(CO)i2 in the Presence or Absence of a Phase-Transfer Catalyst 

What is especially intriguing is the reverse behavior exhibited by the use of ruthenium carbonyl as the metal carbonyl. This reaction, which is catalytic in both Ru3(CO)2 and quaternary ammonium halide (which accelerates the rate of formation of the hydride intermediate), occurs in much higher yield under a carbon monoxide than a nitrogen atmosphere (22). The reaction conditions used for the Ru3(CO)12-catalyzed reaction are much milder than those reported using the water gas shift reaction [100°C, 500 psi] (25). 

The simpler iron carbonyls, Fe(CO)5 and Fe2(CO)9, can also deoxygenate nitrobenzenes to anilines in an aqueous base-organic solvent system. With these carbonyls, however, a phase-transfer catalyst is not only unnecessary, its presence results in reduced product yields Furthermore, the nitro compound must be present to induce attack of hydroxide ion on Fe(CO)s t0 give HFe(CO)4 [and possibly HFe2(CO)8 ] (77). Such a phenomenon was also noted by Pettit and co-workers (23) in their excellent work on the use of water gas shift reaction conditions for the Fe(CO)5-cat-alyzed deoxygenation of nitrobenzenes. 

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As in stoichiometric organic reactions, the application of nonvolatile ionic liquids can contribute to the reduction of atmospheric pollution. This is of special relevance for non-continuous reactions, in which complete recovery of a volatile organic solvent is usually difficult to integrate into the process. 

Ikushima, Y., and Arai, M. 1999. Stoichiometric organic reactions. Pp. 259-279 in Chemical Synthesis Using Supercritical Fluids (P.G. Jessop and W. Leitner, eds.). Wiley-VCH, New York. 

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