Ruthenium complexes imines

Zhang et al. [49] prepared a chiral ruthenium complex coordinated by a pyridine-bis(imine) ligand (structure 43 in Scheme 21). 

A tentative mechanism includes ruthenium-induced isomerization of the initial allylic alcohol via (hydrido) (7T-allyl)ruthenium complex 167 to the corresponding Ru-bound enol 168. This in. ( ////-generated nucleophile complex can then add to aldehydes or imines under formation of the desired products. 

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... 

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. 

The mechanism will vary in precise detail according to the metal. In the case of ruthenium complexes, it is quite common to observe a conproportionation and the formation of a ruthenium(iv) intermediate. In other cases, the unavailability of the metal oxidation states precludes reaction. For example, cobalt(m) complexes of cyclam cannot be oxidised to imine species because although a cobalt(ii)/cobalt(m) couple is possible, the cobalt(n) oxidation state is not accessible under oxidative conditions. In the case of metal ions which can undergo two oxidation state changes, alternative mechanisms which do not involve radical species have been suggested. 

Asymmetric hydrogen transfer shows promise for use at industrial scale because ruthenium complexes that contain chiral vicinal diamino 164 or amino alcohol 165 ligands allow the reductions of substrates such as aryl ketones and imines to be achieved under mild conditions.13 207... 

Metal complexes enable one to employ molecules that are thermally unreactive toward cycloadditions by taking advantage of their ability to be activated through complexation. Most of the molecules activated by transition-metal complexes involve C-C unsaturated bonds such as alkynes, alkenes, 1,3-dienes, allenes, and cyclopropanes. In contrast, carbonyl functionalities such as aldehydes, ketones, esters, and imines seldom participate in transition-metal-catalyzed carbonylative cycloaddition reactions. Recently, such a transformation was reported via the use of ruthenium complexes. 

Murai et al. [28] found that the reaction of a,/J-unsaturated imines with CO results in a [4+1] cycloaddition to give unsaturated y-lactams (Eq. 12). For the reaction of imines which contain a /1-hydrogen, the initially produced /J,y-un-saturated y-lactams are isomerized to the stabler a,/J-unsaturated isomers. This success can be attributed to the facile coordination of the sp2 nitrogen of the substrates to a ruthenium center that assembles the substrates to the ruthenium complex. 

One place to look for good alcohol racemization catalysts is in the pool of catalysts that are used for hydrogen transfer reduction of ketones. One class of complexes that are excellent catalysts for the asymmetric transfer hydrogenation comprises the ruthenium complexes of mono sulfonamides of chiral diamines developed by Noyori and coworkers [20, 21]. These catalysts have been used for the asymmetric transfer hydrogenation of ketones [20] and imines [21] (Fig. 9.9). 

In order to facilitate recycling of the multiple TsDPEN-functionalized dendrimer catalysts, the same group recently reported the synthesis of a novel form of hybrid dendrimer ligands by coupling polyether dendrons with peripherally TsDPEN-functionahzed Newkome-type poly(ether-amide) dendrimer (Figure 4.28) [90]. The solubility of these hybrid dendrimers was found to be affected by the generation of the polyether dendron. The ruthenium complexes produced were applied in the asymmetric transfer hydrogenation of ketones, enones, imines and activated... 

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). 

The Mukaiyama aldol reaction involves the addition reaction of a TMS-enol ether to an aldehyde. Loh et al. have investigated the reaction of l-methoxy-2-methyl-l-trimethylsiloxypropene with aliphatic and aromatic aldehydes in chloride, [BF4] and [PFg] ionic liquids. The yields varied considerably and it was found that the chloride ionic liquids gave the best yields (50-74%) [230]. Ruthenium complexes have been used in the addition of aUyl alcohols to aldehydes and imines in [BMIM][Pp6] [231] (and later in a very similar paper [232]). The addition of a cootalyst such as indium(iii) acetate was found to dramatically improve the yields in some cases and it was foimd that the ionic Uquid/catalyst combination could be recycled. Examples of these reactions are shown in Scheme 5.2-100. 

Rhodium and ruthenium complexes with chiral diphosphine ligands (L) are mainly active in hydrogenation of functionalized alkenes or ketones. In the hydrogenation of C=N bonds these complexes were less effective. And the first attempts to hydrogenate the imine precursor of the important herbicide Metolachlor (trade name DUAL) was not very successful (Scheme 7.6.). 

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