Other Ruthenium-Catalyzed Transformations

M Determined by HPLC analysis using DAICEL CHIRACEL AD-H (hexane/2-propanol 9 1). M Absolute configuration was determined by comparison of the sign of specific optical rotation with the reported one. idl Reaction was carried out at room temperature. t l Reaction was carried out at 0 °C. 

Transition Metal-Catalyzed Transformations other than Pd, Rh, and Ru 

Besides the already described Pd-, Rh- and Ru-catalyzed transformations, many other transition metals have also been used in domino processes, albeit to a lesser extent. Co- and also Ni-catalyzed transformations constitute the largest group in this section, though other examples include Cu, W, Mo, Fe, Ti, Cr, Au, Pt, Zr, and Lanthanide-catalyzed reactions. 

Co-catalyzed transformations are concerned mainly with the [2+2+2] cycloadditions of three alkyne groups to give arenes. Another important reaction is the [2+2+1] cycloaddition of alkynes, alkenes and CO to give cyclopentenones, which is the well-known as Pauson-Khand reaction [272]. 

The Pauson-Khand reaction is the Co-induced formation of cyclopentenones from ene-ynes and CO. One impressive example of a domino Pauson-Khand process is the synthesis of fenestrane 6/4-15, as reported by Keese and colleagues [278]. The transformation is initiated by a double Grignard reaction of 4-pentynoic acid 6/4-12, followed by protection of the formed tertiary hydroxyl group to give 6/4-13. The Co-induced polycyclization of 6/4-13 led directly to the fenestrane 6/4-15 

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Associated to copper(II) pre-catalysts, bis(oxazolines) also allowed the asymmetric Diels-Alder and hetero Diels-Alder transformations to be achieved in nearly quantitative yield and high diastereo- and enantioselectivities. Optically active sulfoximines, with their nitrogen-coordinating site located at close proximity to the stereogenic sulfur atom, have also proven their efficiency as copper ligands for these asymmetric cycloadditions. Other precursors for this Lewis acid-catalyzed transformation have been described (e.g., zinc salts, ruthenium derivatives, or rare earth complexes) which, when associated to bis(oxazolines), pyridine-oxazolines or pyridine-bis(oxazolines), led to efficient catalysts. 

Other radical-based transformations are ruthenium-catalyzed oxidative dimerizations of phenols [263] and reductive dimerizations [264], The isomerization of chiral c/s-epoxides to tram-epoxides catalyzed by 2-10 mol% TpRu(py)2Cl proceeds at 100 °C in 95-98% yields with inversion of configuration [265], A radical or SN2 mechanism was discussed for this process. 

Previous reviews have dealt with metal-catalyzed [93] and stoichiometric [94] oxidation of amines in a broad sense. This section will be limited to the selective oxidation of tertiary amines to N-oxides. Amine N-oxides are synthetically useful compounds [95, 96] and are frequently used as stoichiometric oxidants in osmium-[97-99] manganese- [100] and ruthenium-catalyzed [101,102] oxidations, as well as in other organic transformations [103-105]. Aliphatic tert-amine N-oxides are usefid surfactants [96] and are essential components in hair conditioners, shampoos, toothpaste, cosmetics, and so on [106]. Chiral N-oxides have been used in asymmetric catalysis involving metal-free catalytic transformations [107] as well as metal-catalyzed reactions where the N-oxide serves as a ligand [107, 108]. Chiral tertiary amine N-oxides were recently used as reagents in asymmetric epoxidation of a,(3-unsaturated ketones [109]. 

The ruthenium-iodide-catalyzed carbonylation of alcohols is greatly complicated by the facility with which the same system catalyzes the competitive water-gas shift and homologation reactions. The resulting inability to totally isolate the reaction(s) of interest necessitates that conclusions are based on observations which are less direct than in other systems discussed in this article. Further work, aimed at determining the nature of the proposed transformations, perhaps through the use of model compounds, would appear to be required to unravel the finer mechanistic details of the system. 

The diols (97) from asymmetric dil droxylation are easily converted to cyclic sii e esters (98) and thence to cyclic sulfate esters (99).This two-step process, reaction of the diol (97) with thionyl chloride followed by ruthenium tetroxide catalyzed oxidation, can be done in one pot if desired and transforms the relatively unreactive diol into an epoxide mimic, ue. the 1,2-cyclic sulfate (99), which is an excellent electrophile. A survey of reactions shows that cyclic sulfates can be opened by hydride, azide, fluoride, thiocyanide, carboxylate and nitrate ions. Benzylmagnesium chloride and thie anion of dimethyl malonate can also be used to open the cyclic sulfates. Opening by a nucleophile leads to formation of an intermediate 3-sidfate aiuon (100) which is easily hydrolyzed to a -hydroxy compound (101). Conditions for cat ytic acid hydrolysis have been developed that allow for selective removal of the sulfate ester in the presence of other acid sensitive groups such as acetals, ketals and silyl ethers. 

More recently, catalytic hydrogenations of alkenes by other catalysts in water have been explored. For example, water-soluble ruthenium complex RuCl2(TPPTS)3 has been used for the catalytic hydrogenation of unsaturated alkenes (and benzene). Hydrogenation of nonactivated alkenes catalyzed by water-soluble ruthenium carbonyl clusters was reported in a biphasic system. The tri-nuclear clusters undergo transformation during reaction but can be reused repeatedly without loss of activity. The organometallic aqua complex [Cp Ir (H20)3] " ... 

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