Chiral compounds iridium catalysts

Figure 4.7-4 Influence of perfluoroalkyl substituents (compounds a or b) and of anions X on the enantiomeric excess in the hydrogenation of imine 8a using chiral iridium catalysts 10-12 [12].

Chiral amines have been conveniently prepared also by asymmetric reductive amination of ketones using iridium catalysts and intriguing results with up to 96% ee have been obtained by Zhang and co-workers employing a catalytic system based on Ir./-binaphane in the presence of Ti(OPr )4 and iodine (Scheme 61). Water-soluble aquo complexes [Cp lr(H20)3](0Tf)2 494, [CpP Ir(H20)2](0Tf)2 504, and [Cp Ir(bpy)(H20)](0Tf)2 505 have been used to catalyze the reductive amination of hydrosoluble aldehydes and ketones as well as the dehalogenation of alkyl halides. The activity is markedly pH dependent and inactivation of the catalyst takes place reversibly on increasing the solution basicity due to Ir(H20), deprotonation and formation of mono- or dinuclear hydroxo complexes which are catalytically inactive. The structure of one of these compounds, [Cp Ir(bpy)(OH)]OTf 506, which reversibly forms from 494 around pH 6.6, is presented in Figure 42. 

Shibata and Tsuchikama subsequently developed the enantio- and diastereoselec-tive [2 + 2 + 2] cycloaddition reactions of tetraynes with monoynes. These reactions proceeded in the presence of the same chiral iridium catalyst to give helically chiral quinquearyl compounds, possessing four consecutive axial chiralities, with perfect... 

The synthesis of axially chiral compounds via the transition-metal-catalyzed enan-tioselective [2 - - 2 - - 2] cycloaddition is described in this chapter. Cobalt-, iridium-, and rhodium-based catalysts are currently available for this purpose. The characteristic of these three catalysts may be summarized as follows. 

The catalytic asymmetric hydroboration of 1,1-disubstituted alkenes R R C=CH2 with pinacolborane has been attained (<92% ee) by using complexes of iridium with chiral phospine-oxazolidines as catalysts (2.5 mol%). The same class of compounds has been shown to facilitate hydroboration (<98% ee) catalysed by chiral complexes of CuCl with a bidentate A)-heterocyclic carbene (NHC) containing SOg" as an additional ligating group. ... 

Asymmetric Transfer Hydrogenation of Ketones. The first reports on asymmetric transfer hydrogenation (ATH) reactions catalyzed by chiral metallic compounds were published at the end of the seventies. Prochiral ketones were reduced using alcohols as the hydrogen source, and Ru (274,275) or Ir (276) complexes were used as catalysts. Since then, many chiral catalytic systems for homogeneous ATH of ketones, imines, and olefins have been developed (37,38,256,257,277-289). The catalytic systems are usually based on ruthenium, rhodium, or iridium, and the ATH of aryl ketones is by far the most studied. Because of the reversibility of this reaction, at high conversions, a gradual erosion of the ee of the product has been frequently reported. An azeotropic 5 2 mixture of formic acid/triethylamine can be used to overcome this limitation. 

Another application of this reaction is in a synthesis of the alkaloid xenovenine 9.150 (Scheme 9.46), one of many isolated from tropical frogs, which acquire the compound from their diet. While asymmetric allylation provided the original stereogenic centre, Suzuki coupling, the Wittig reaction and reductive amination were each employed for key bond-forming reactions. Additional syntheses of xenovenine can be found in Section 6.2. The readily available allylic carbonate 9.152 was reacted with the diacylammo-nia derivative 9.153 in the presence of an iridium complex and the chiral pre-catalyst (Scheme 9.47). The... 

Metolachlor A process for making the herbicide Dual Magnum. Developed by Ciba-Geigy from 1982. The catalyst is a chiral iridium ferrocenyl compound (the Josephson ligand). In 2008, more than 10,000 tons of metolachlor per year were being made by this process. 

A method for the asymmetric hydrogenation of seven-membered cyclic imines of benzodiazepinones and benzodiazepines has recently been published. The chiral cyclic amines generated from these reactions make up the cores of many natural products and clinical drugs. Iridium-bisphosphine catalyst systems were investigated and found to give promising enantios-electivities which could be improved upon addition of morpholine tri-fluoroacetate. The optimum conditions, applied to model compound 50, are shown in Scheme 14.19, giving 51 in excellent enantioselectivity and complete conversion. 

---