Ruthenium imine/amine complexes

Examination of a series of imines of differing electronic properties showed that a change in the rate-determining step of this stoichiometric C=N hydrogenation occurs as the imine becomes more electron-rich. Hydrogenation of N-iso-propyl-(4-methyl)benzilidene amine led to an amine complex of ruthenium. In addition, the C=N hydrogenation was accompanied by isomerization of the imine to a ketimine (Eq. (47)). 

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

Primary amines at a primary carbon can be dehydrogenated to nitriles. The reaction has been carried out with a variety of reagents, among others, IF5,"9 lead tetraacetate, 20 nickel peroxide,121 NaOCl in micelles,122 S g-NiSO, 2-1 and CuCl-02-pyridine.124 Several methods have been reported for the dehydrogenation of secondary amines to imines.125 Among them126 are treatment with(l) iodosylbenzene PhIO alone or in the presence of a ruthenium complex, 27 (2) Me2SO and oxalyl chloride, 2" and (3) f-BuOOH and a rhenium catalyst. 29... 

Like ruthenium, amines coordinated to osmium in higher oxidation states such as Os(IV) are readily deprotonated, as in [Os(en) (NHCH2CH2NH2)]2+ [111614-75-6]. This complex is subject to oxidative dehydrogenation to form an imine complex (105). An unusual Os(IV) hydride, [OsH2(en)J2+ [57345-94-5] has been isolated and characterized. The complexes of aromatic heterocyclic amines such as pyridine, bipyridine, phenanthroline, and terpyridine are similar to those of ruthenium. Examples include [Os(bipy )3 ]2+ [23648-06-8], [Os(bipy)2acac] [47691-08-7],... 

Oxidation of amines to imines.1 In the presence of this ruthenium complex secondary amines are oxidized to imines in >70% yield. This reaction is particularly useful for preparation of 1-azadienes. 

We have already seen that imines may be formed by the oxidative dehydrogenation of co-ordinated amines and that this is a commonly observed process, particularly in macrocyclic systems. Likely mechanisms for these dehydrogenations were suggested in Chapter 5, which emphasised the role of the variable oxidation state metal ions in the process. These reactions are quite general and many examples involving iron or ruthenium complexes have been studied in detail. 

Treatment of [RuC1(NH3)5]2+ with Ag(02CCF3), followed by zinc amalgam reduction and addition of amine yields [Ru(L)(NH3)5]2+ (L = cyclohexylamine, benzylamine, methylamine).192 Oxidation of these complexes with Br2 produces the corresponding ruthenium(TII) species [Ru(L)(NH3)5]3+.192 Subsequent oxidation of the amine ligand can readily occur to give imine and nitrile products, explaining the relatively few complexes of this type that have been isolated (see Section 45.4.2). 

Another useful reduction process is asymmetric transfer hydrogenation (ATH) where the hydrogen is transferred from the solvent, often isopropanol, to the ketone or imine function to produce the enantiopure alcohol or amine. For example, Baratta et alP made ruthenium complexes containing the (/ ,S)-Xyliphos ligand to reduce a simple ketone to (5)-l-(3-trifluoromethylphenyl)ethanol, used in the synthesis of the fungicide (5)-MA20565 (Scheme 3). 

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