[10025-97-5] · Cl4Ir · Iridium(IV) Chloride · (MW 334.02)
(catalyst precursor for the stereoselective reduction of cyclohexanones1-3)
Physical Data: decomposes on heating.
Solubility: sol cold H2O, alcohol.
Form Supplied in: dark brown, hygroscopic, amorphous mass. Pure iridium tetrachloride does not seem to have been made. Products sold as such contain water (about 10%) and may also contain some chloroiridic acid [H2IrCl6]. The manufacturer should state the iridium content. The ammonium salt [(NH4)2IrCl6], which can be obtained in a pure state, can be used instead.
Iridium tetrachloride is a catalyst precursor for the reduction of unhindered (conformationally biased or rigid) cyclohexanones to cyclohexanols with the hydroxy group largely axial (often >95%). The reactions are carried out in 90% aqueous 2-propanol and require the presence of phosphorous acid, H3PO3, or its dimethyl or trimethyl ester. The phosphorus-containing compound acts as the main reducing agent in the system and 2-propanol as a very minor one. Dimethyl phosphite, (MeO)2P(O)H, is the most convenient to use as phosphorous acid is hygroscopic and trimethyl phosphite is both very toxic and has an unpleasant odor.
Experimentally the reaction is very simple. There is no need to exclude atmospheric oxygen or moisture. The reactants are merely heated at reflux until reaction is complete. The phosphorus compound should be present in at least 100% excess with respect to the ketone. Some examples of the reduction of simple cyclohexanones are given in eqs 1-4.2
Selectride reagents, developed by Brown,4,5 give equal or even greater selectivity in the reduction of cyclohexanones to axial alcohols, but require anhydrous reaction conditions, an inert atmosphere, and often low temperatures.
Hindered ketones such as 2,2,6-trimethylcyclohexanone are not reduced. This property has been used for the regio- and stereoselective reduction of some steroidal di- and triketones in which the hindered carbonyl group(s) are not reduced (eqs 5 and 6).6
Similar transformations can be carried out with K-Selectride (Potassium Tri-s-butylborohydride), but seem difficult to take to completion.7
The identity of the catalyst in these reductions is not known, but is presumably an iridium-phosphite complex. Some such complexes have been made in an attempt to shed light on the nature of the catalyst.8,9
Thomas R. B. Mitchell
Queen's University, Belfast, UK