Ruthenium alkoxides

Limitations of the reaction due to the substitution pattern of the allylic alcohols were overcome by the use of tetrapropylammonium perruthenate (TRAP) as a catalyst and monosubstituted, disubstituted and trisubstituted allyl alcohols were converted into the corresponding saturated aldehydes and ketones [5]. Intermediacy of the ruthenium alkoxide in this reaction was evidenced from the complete lack of reactivity of the trimethylsilyl ether derived from the allylic alcohol. 

RuCls-catalyzed alcohol oxidation of carveol (4, Table 4) similarly showed a higher rate with TBHP than with PHP, suggesting that the rate-limiting step in ruthenium catalyzed oxidation of alcohols may involve reaction of a ruthenium alkoxide with RO2H, resulting in formation of the carbonyl compound with simultaneous reoxidation of the ruthenium (Scheme 4). 

The reaction can proceed through the formation of metal hydride species. Moreover, it was found that in case of ruthenium catalysts, the reaction rate can be significantly improved by addition of a base to the reaction mixture [20], The base promotes the formation of ruthenium alkoxide that further undergoes a -elimination to give, in sequence, a mono- and a dihydride complex (Scheme 18.8) [21], the latter being assumed as an active form of the catalyst [22]. 

Hasanayn, P. Morris, R. H. Symmetry aspects of H2 splitting by five-coordinate d6 ruthenium amides, and calculations on acetophenone hydrogenation, ruthenium alkoxide formation, and subsequent hydrogenolysis in a model trans-Ru(]4)2(diamine)(diphosphine) system. Inorg. Chem. 2012,51,10808-10818. 

In the proposed mechanism, the dearomatised pincer ligand deprotonates the alcohol, aromatising the pyridine ring, with no change in oxidation state, to form the ruthenium alkoxide (Figure 12.3). The hemi-labile amine arm dissociates from the Ru metal and allows for the requisite a s-coordination of the alcohol relative to the metal to be adopted, facilitating p-hydride elimination and generating the Ru-bound aldehyde. 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

This aldol condensation is assumed to proceed via nucleophilic addition of a ruthenium enolate intermediate to the corresponding carbonyl compound, followed by protonation of the resultant alkoxide with the G-H acidic starting nitrile, hence regenerating the catalyst and releasing the aldol adduct, which can easily dehydrate to afford the desired a,/3-unsaturated nitriles 157 in almost quantitative yields. Another example of this reaction type was reported by Lin and co-workers,352 whereas an application to solid-phase synthesis with polymer-supported nitriles has been published only recently.353... 

Scheme 3.7 Generation of the active hydride catalyst by hydrogen transfer from formic acid or iso-propanol via /5-hydride elimination from formate or alkoxide intermediates. The square represents a vacant site on ruthenium.

A different mechanism is operative with the 16-electron complex RuCl2(PPh3)3 (24) (Scheme 20.11). Here, the dichloride complex (25) is rapidly converted into a dihydride species (26) by substitution of both chloride ligands with alkoxides and subsequent eliminations similar to the conversion of 18 to 20 described above [46, 47]. Subsequently, the ruthenium dihydride species 26... 

Ruthenium(III) catalyses the oxidative decarboxylation of butanoic and 2-methylpropanoic acid in aqueous sulfuric acid. ° Studies of alkaline earth (Ba, Sr) metal alkoxides in amide ethanolysis and of alkali metal alkoxide clusters as highly effective transesterification catalysts were covered earlier. Kinetic studies of the ethanolysis of 5-nitroquinol-8-yl benzoate (228) in the presence of lithium, sodium, or potassium ethoxide revealed that the highest catalytic activity is observed with Na+.iio... 

When aldehydes, with or without a hydrogen, are treated with aluminum ethoxide, one molecule is oxidized and another reduced, as in 9-69, but here they are found as the ester. The process is called the Tishchenko reaction. Crossed Tishchenko reactions are also possible. With more strongly basic alkoxides, such as magnesium or sodium alkoxides, aldehydes with an a hydrogen give the aldol reaction. Like 9-69, this reaction has a mechanism that involves hydride transfer.751 The Tishchenko reaction can also be catalyzed752 by ruthenium complexes.753 by boric acid,754 and, for aromatic aldehydes, by disodium tetracarbonylferrate Na2Fe(CO)4,755 OS I, 104. 

This section is actually devoted to the description of rhenium alkoxides, as the technetium ones are rarely studied. The latter are reviewed in a short appendix at the end. The chemistry ofrhenium in lower oxidation states is much alike that of ruthenium and therefore even for rhenium, the lower oxidation state complexes with jt-acceptor ligands are described in this chapter. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

An interesting variant is the in situ preparation of transition metal alkoxides from the corresponding halogenides and subsequent reaction with an azolium salt to form the NHC transition metal complex [69]. This works particularly well with rhodium, iridium and ruthenium where [(ii -cod)MCl]j (M = Rh, Ir) and [Cp RuCl]2 are readily available [57,58,71]. 

Allylic alcohols are isomerized via direct interaction of the ruthenium atom with alcohol. /3-Elimination of ruthenium hydride from metal alkoxide yields a ruthe-nium-enone species C which undergoes insertion of the olefinic moiety into the Ru-H to form an oxyallylic intermediate D. As a result, the hydrogen atom shifts from the a- to y-position of the allylalcohol. Protonolysis of the oxyallylic species leads to a saturated carbonyl compound and cationic unsaturated species, [CpRu(PPh3)2] A. 

Finally, the last few years have seen the first examples of the use of molecular-imprinted, polymer-supported catalysts for achieving product selectivity. The imprinted cavities are tailored in such a way that the course of a chemical reaction is directed towards one of the possible products. In the previous section it has already been shown that molecularly imprinted polymers used as microreactors are able to impart to a given reaction a different regio- and stereo-selectivity with respect to the same reaction in solution. Attempts towards an imprinted enantio-selective catalyst were reported by Gamez and co-workers who employed as template monomer an optically active, polymerisable ruthenium complex bearing in its coordination sphere an enantiomerically pure alkoxide [121]. After polymerisation, the alkoxide was split off and the resulting polymer-supported catalyst was used for enantio-selective hydride transfer reductions. The obtained selectivity was higher than for a polymer prepared without the optically active alkoxide but lower than for the same ruthenium complex in solution. 

A combination of isopropanol and an alkali hydroxide or alkoxide together with Ru or Ir catalyst and a chiral ligand constitutes the reduction system. The ligands include proline, chiral cfr-l-amino-2-indanols, and the following 74, ° 75, 76, 77. f5)-Propargylic alcohols (>97% ee) are produced when the ketones are treated with the ruthenium complex 2 in isopropanol (without added base). ... 

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