Transition metal alkoxide complexes

The use of supported metal complexes in transesterification reactions of TGs is not new. An earlier patent claimed that supported metals in a hydroxylated solid could effectively catalyze transesterification. The catalyst preparation used an inert hydrocarbon solvent to attach transition metal alkoxide species to the support surface. The reaction, however, was carried out in the presence of water. The author claimed that water was essential in preparing materials with good catalytic activity. Among the metals employed, titanium catalysts showed the best activity. However, it was not clear from the preparation method if reproducibility could be easily achieved, an important requirement if such catalysts were to be commercially exploited. 

The hydrolysis of early transition metal alkoxides is an extremely facile reaction that makes the manipulation of these materials non-routine. The typical insolubility and high molecular complexity... 

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]. 

Once the hydroxy functionalised imidazolium salt is formed, it can be deprotonised and reacted with various metal complexes to form (transition) metal carbene complexes. The hydroxy group ensures that the ligand can be coordinated even to metals that are normally reluctant to form stable carbene complexes. A good example is the deprotonation of a hydroxyethyl functionalised imidazolium salt with potassium hydride [36]. The potassium cation coordinates to the oxygen atom of the alkoxide sidechain and forms cubes as structural elements (see Figure 4.6). The carbene end then coordinates to the respective... 

Moore et al. [274] introduced a sulfonato functionality into a NHC wingtip group to make the resulting transition metal carbene complexes water soluble. Introduction of the functional group was achieved in a modification of the epoxide method previously used by Arnold et al. [33] and Glas et al. [12] for the synthesis of alkoxide functionalised carbenes (see Section 4.1). Reaction of an N-substituted imidazole with... 

Komiya, S., Taneichi, S., Yamamoto, A., Yamamoto, T. Transition metal alkoxides. Preparation and properties of bis(aryloxy)iron(ll) and bis(alkoxy)iron(ll) complexes having 2,2 -bipyridine ligands. Bull. Chem. Soc. Jpn. 1980, 53, 673-679. 

More recently (1994), discrete early transition metal (Ti, V, Nb, orTa) al-koxide complexes containing homochiral trialkanolamine ligands (392) were prepared and their usefulness as highly enantioselective catalysts was demonstrated. It is noteworthy that earlier work on the reactions of tetradentate triethanolamine with transition metal alkoxides was reported to yield insoluble products (6). 

A convenient and widely used mettiod for the preparation of late transition metal hydride complexes involves the generation of coordinated alkoxide from base and an alcohol, followed by elimination of an aldehyde or ketone. In one case, the alkoxide intermediate has been observed, while use of the labeled alcohol in Equation 3.120 has confirmed that the a-hydrogen becomes the hydride ligand. This reaction has even been used to produce a Co(III) hydride from a Co(III) aquo complex (Equation 3.121). Only primary and secondary alcohols can be used. 

Like late-transition-metal-amido complexes, pir-dir interactions between the electron pair on oxygen and the filled d-orbitals on the metal can affect the thermod)mamic stability and the reactivity of aUcoxo complexes. Naturally, this effect in metal-aUcoxo complexes is less pronounced than in metal-amido complexes because of tihe lower basicity of an alkox-ide. At the same time, the presence of two electron pairs on oxygen causes this effect in aUcoxo complexes to depend less on geometry than in amido complexes. Sudh Tr-interactions have been studied in detail by Caulton, and have been used to rationalize the geometries, nucleophilicity, and basicity of late metal alkoxides and amides (Figure 4.16). ... 

Many of the typical reactions that form transition metal-heteroatom bonds can be used to prepare late transition metal alkoxides. - For example, metathetical exchange between late metal halides and alkali metal salts of the corresponding alcohols often forms late metal alkoxides. Complexes that are more reactive than metal halides, such as metal acetates or triflates, are sometimes used when exchanges with halides are slow or reversible (Equation 4.66). Late metal fluorides can also be used for these exchanges, as shown in Equation 4.67. ... 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

Another factor that influences reaction kinetics is the extent of oligomerization molecular complexity) of the metal alkoxides. The molecular complexity depends on the nature of the metal atom. Within a particular group, it increases with the atomic size of the metal (see Table 3), thus explaining the tendency of divalent transition metal alkoxides (Cu, Fe, Ni, Co, Mn) to polymerize rendering them insoluble [129]. Molecular complexity... 

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