Iridium hydride, stereochemistry

FIGURE 10.1 The NMR spectra of some iridium hydrides (hydride region). Each stereochemistry gives a characteristic coupling pattern. 

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. 

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). 

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). 

Equation 9.84 depicts the reaction of aniline, norbomene, and [Ir(PEt3)2(CjH )jCl] to form an iridium-aminoalkyl complex. This process is thought to occur by oxidative addition of the aniline to form an amido hydride complex, followed by insertion of the strained alkene into the iridium-nitrogen bond. Products from oxidative addition of aniline to the same iridium species were shown to form in the absence of the olefin. The syn stereochemistry of the aminoalkyl product indicated that a migratory insertion pathway was followed. 

Trichlorosilane reversibly adds the Si—H bond across iridium(I) forming a stable iridium(III) hydride (XXXII) of uncertain stereochemistry (19). Such a complex has been proposed as an intermediate stage in the hydro-silation of olefins catalyzed by more additions of this type will be examined in the future. 

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