Iridium complexes stereochemistry

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

Interesting, optically acive complexes were obtained upon reaction of organosilicon hydrides. The reactions of R3SiH with transition metal complexes represent a key step in catalytic hydrosilylation reactions. Catalytic activation of silicon hydrides has been proposed to arise from an oxidative addition process to a transition metal centre. Such a process has been shown to be reversible via a reductive elimination step (equation 9). The stereochemistry of addition to an iridium complex was shown to occur in a cis fashion59 (equation 10). 

The influence which the other ligands have on the alkylation of d complexes is illustrated by the addition of methyl iodide to the tris(phos-phine)-rhodium complex (XL) (82a) but not to the similar complex RhCl(CO)(PPh3)2 in which a CO group has replaced a phosphine. However, the analogous iridium complex IrCl(CO)(PPh3)2 reacts with methyl iodide (see Section II,B) (2J, 41, 67), The rhodium adduct (XLI) is novel inasmuch as it contains two molecules of methyl iodide, the second apparently being bound through iodine (82a). The detailed stereochemistry... 

The PMe2Ph complexes have been studied in particular detail [163-165], since their 1H NMR spectra lend themselves to assigning the stereochemistry of the complexes. Figure 2.88 shows the relationships between a large number of these complexes, which are in general typical of iridium(III) phosphine complexes. 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Stereoselective hydrogenations. The stereochemistry of the hydrogenation of a double bond catalyzed by this Ir(I) complex is markedly controlled by the presence of a carboxamide group. The effect is attributed to coordination between the CONH group and iridium. Reductions of the same substrates with Pd/C show no stereoselection.2... 

S. E. Landau, K. E. Groh, A. J. Lough and R. H. Morris, Large effects of ion pairing and protonic-hydridic bonding on the stereochemistry and basicity of crown-azacrown, and cryptand-222-potassium salts of anionic tetrahydride complexes of iridium(III). Inorg. Chem. 41, 2995-07 (2002). 

Alkyl and aryl isothiocyantes, RCNS, parallel CS2 in their capability to insert into M—H bonds and yield products having the JV-alkyl- or JV-aryl-formamide chelate ligand RN=CH=S. The iridium(III) complexes [Ir(X)2(RN=CH=S)(PPh3)2] (207 X = Cl, Br R = Me, Et, Ph) can be prepared from trans-[Ir(H)(X)2(PPh3)2] and the appropriate alkyl or aryl isothiocyanate. Two isomeric forms of (207) were deemed plausible, with (207a) the preferred stereochemistry for the alkyl products and (207b) for the aryl products.453... 

The preparation of bis(bipyridyl) (702) and bis(phenanthroline) cobalt(III) complexes (576) has been the subject of a number of papers. Interest centers on the possibility of cis-trans isomerism in these complexes, since it would seem that the close approach of the 6,6 -(bipyridyl) and 2,9-(phenanthroline) protons in the trans complex would make this stereochemistry unattractive for central ions such as co-balt(III), rhodium(III), or iridium(III). Cis complexes are well established, but a violet compound claimed to be anionic cobalt(II) species (21, 573, 591). 

Two new preparative methods for iridium(III) complexes were recently reported. One involves the use of a mixture of hexaehloro-iridates(III) and (IV) as the source of iridium (287a) the other involves the addition of hypophosphorous acid (47a). Gillard and Heaton (287b) have now demonstrated that all compounds IrL2X2" are cis also, they present additional evidence to confirm the cis stereochemistry of the rhodium analogs, although it would appear that the compound they identify as cis-Rli(phen)2Br2 Br-2H>0 is probably [phenH] [Rh(phen)-Br4] (cf. 529). 

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