Phosphines acetic anhydride process

Other companies (e.g. Hoechst, now Celanese) have developed a slightly different process in which the water content is low in order to save CO feedstock [1], In the absence of water it turned out that the catalyst precipitates. Also, the regeneration of ihodium(III) is much slower. The formation of the trivalent rhodium species is also slower because the HI content is much lower when the water concentration is low. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilisation of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts (Li, ammonium, phosphonium, etc). Later, we will see that this is also important in the acetic anhydride process. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives [1]. 

To a much smaller extent non-enzymic processes have also been used to catalyse the stereoselective acylation of alcohols. For example, a simple tripeptide has been used, in conjunction with acetic anhydride, to convert rram-2-acctylaminocyclohexanol into the (K),(R)-Qster and recovered (S),(S)-alcohol[17]. In another, related, example a chiral amine, in the presence of molecular sieve and the appropriate acylating agent, has been used as a catalyst in the conversion of cyclohexane-1(S), 2(/ )-diol into 2(S)-benzoyloxy-cyclohexan-1 f / j-ol1 IS]. Such alternative methods have not been extensively explored, though reports by Fu, Miller, Vedejs and co-workers on enantioselective esterifications, for example of 1-phenylethanol and other substrates using /. vo-propyl anhydride and a chiral phosphine catalyst will undoubtedly attract more attention to this area1191. 

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]. 

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