Transition Metal-Alkoxo Complexes

Free alkoxide and aryloxide anions are Bronsted bases with pK values of the corresponding alcohols ranging from 5 to 20 in water. The basicity is highly dependent on the electronic properties of the alkyl or aryl moieties. For example, the pK value of hexafluoro-tert-butanol, (CF3)jMeCOH, is 9.6, which is considerably lower than the pK value of tert-butanol (19.2), but roughly the same as that of phenol (9.9). Such differences in electronic, as well as steric, environments often leads to the different structures and reactivity patterns for compounds containing similar ancillary ligands, but different alkoxides or aryloxides. 

A common structural feature of transition metal hydroxide, alkoxide, and aryloxide ligands is their tendency to bridge two or more metals. The extent of such aggregation is reduced when the complexes contain more sterically hindered alkoxides, such as OBu, OCBu y bulky trialkylsiloxides, and 2,6-di-lerl-butylphenoxide.  

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As noted again later in the section on late-transition-metal-alkoxo complexes, the role of hard/soft and dir-pir interactions in controlling the properties of these compounds has been debated. In some cases, the "ir-repulsion has been cited as a factor that leads to the instability of these complexes, but in other cases arguments have been made that the reactivity of these complexes can be explained without invoking "ir-donation. In other cases, ar-donation from an amido group into an unoccupied orbital of a 16-electron late transition metal complex has been proposed to account for the structures of such species. ... 

Early-transition-metal-alkoxo complexes are also stable toward other typical organome-tallic processes, such as reductive elimination. Hydridometal-alkoxo, alkylmetal-alkoxo, and arylmetal-alkoxo complexes of the early metals are known, but none of them undergoes reductive elimination to form a carbon-oxygen bond. Early-metal-alkoxo complexes also do not tend to undergo migratory insertion processes. In fact, this lack of insertion was exploited by Jordan to illustrate coordination of alkenes to d metal centers. " ... 

In parallel with the catalytic reactivity of early-transition-metal-alkoxo complexes, the catalytic reactivity of lanthanide-alkoxo and -phenoxo complexes has been developed. The homogenous hydrogenation of a variety of arene substrates can be catalyzed by niobium... 

Oxidative addition of the O-H bond to transition metal complexes gives hydrido(hy-droxo), hydrido(alkoxo) or hydrido(carboxylato) complexes (Eq. 6.1), but web-characterized complexes obtained as primary products from the reaction of the compound, XO-H (XO-H = water, alcohol, and carboxylic acid) with late transition metals are quite rare [1]. Furthermore, the crystal stractures of very few complexes of this type have been reported. In this section we will survey late transition metal complexes resulting from activation of water, alcohol, and carboxylic acid. 

Before discussing the preparation of late transition metal complexes resulting from the activation of O-H bonds by late transition metal complexes, we wbl describe metathesis methods for the preparation of hydrido(hydroxo), hydrido(alkoxo), and hydrido(carboxylato) complexes. Though many methods of preparation of transition metal hydroxides, alkoxides, etc. by a metathesis reaction have been reported [1], only a limited number of examples of the preparation of hydrido(hydroxo), hydri-do(alkoxo) complexes etc. by metathesis are available. 

Hydrido(alkoxo) complexes of late transition metals are postulated as intermediates in the transition metal-catalyzed hydrogenation of ketones (Eq. 6.17), the hydrogenation of CO to MeOH, hydrogen transfer reactions and alcohol homologation. However, the successful isolation of such complexes from the catalytic systems was very rare [32-37]. 

Thus, the selectivities, deactivation mechanisms, and potential transformations of alkoxo and amido intermediates in such reactions are not well understood. It is even rare for transition metal amido and alkoxo complexes to be clearly identified as intermediates in catalytic chemistry. The hydrogenation of imines and ketones presumably involves such intermediates [68], but they have not been clearly detected in these reactions [69]. The catalytic reduction of CO on surfaces may involve alkox-ides, but well-characterized homogeneous analogs are unusual [58]. 

Transition metal catalyzed allylation by using allylic carbonates is one of the most versatile allylation methods. The Pd-catalyzed reaction is believed to proceed by the following mechanism. Eirst of all, oxidative addition of the C-0 bond to Pd(0) gives (r/ -allyl)(carbonato)palladium(II) complex, which undergoes decarboxylation to give the alkoxo complex [70]. The resulting (t] -allyl)(alkoxo)palladium(II) complex immediately reacts with nucleophiles (HNu)... 

Bond cleavage reactions of 0-H, S-H and N-H bonds by late transition metal complexes are also known to provide alkoxo-, aryloxo-, thiolato- and amidometal complexes. 

Many transition metal complexes contain anionic oxygen donor ligands, and many of these complexes display structures and reactivity that resemble that of more conventional organometallic compounds containing metal-carbon bonds. In some cases the alkoxo ligand is the site of reaction, and in other cases the alkoxide is an ancillary ligand. This section will focus on three main classes of compounds alkoxides (including aryloxides and the parent hydroxides), carboxylates, and p-diketonates. 

More specifically, calculations have suggested that the ruthenium-alkyl complex in Equation 6.53 reacts with arene to exchange covalent ligands by a process closely related to a a-bond metathesis mechanism. Computational studies of the reactions of a simple iridium-alkyl and alkoxo complex with alkanes to generate new metal-alkyl complexes have also suggested that a mechanism is followed that involves many of the characteristics of a classic a-bond metathesis transition state. However, calculations of the mechanisms of these two processes imply that the transition state contains some degree of M-H bonding. 

A few final comments should be made on the insertions of substrates containing C-C multiple bonds into the bonds between a transition metal and an electronegative heteroatom. First, insertions of olefins into related thiolate and phosphide complexes are as rare as insertions into alkoxo and amido complexes. Reactions of acrylonitrile into the metal-phosphorus bonds of palladium- and platinum-phosphido complexes to give products from formal insertions have been observed, and one example is showm in Equation 9.90. However, these reactions are more likely to occur by direct attack of the phosphorus on the electrophilic carbon of acrylonitrile than by migratory insertion. Second, the insertions of alkynes into metal-oxygen or metal-nitrogen covalent bonds are rare, even though the C-C ir-bond in an alkyne is weaker than the ir-bond in an alkene. 

The early transition metals are dissolved via a complex mechanism involving oxidation of alkoxide ligands with formation of extremely reactive alkoxo-radicals that in turn attack the metal, forming soluble alkoxide complexes already at the anode ... 

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