Transition metal alkoxide complexes ligands

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... 

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. 

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 ... 

R.R. Schrock, M.I.T. Have you or anyone else prepared platinum(IV) metallacyclobutane complexes with alkoxide ligands in place of chlorides One might expect the alkoxide complexes to behave considerably differently than the chloro complexes, perhaps like early transition metal complexes. 

While the notion that the alkoxides derived from aliphatic alcohols are poor nucleophiles toward 7r-allylmetal complexes has prevailed over the years, much progress made in the recent past has rendered the transition metal-catalyzed allylic alkylation a powerful method for the O-allylation of aliphatic alcohols. In particular, owing to the facility of five- and six-membered ring formation, this process has found extensive utility in the synthesis of tetrahydrofurans (THFs) (Equation (29))150-156 and tetrahydropyrans (THPs).157-159 Of note was the simultaneous formation of two THP rings with high diastereoselectivity via a Pd-catalyzed double allylic etherification using 35 in a bidirectional synthetic approach to halichondrin B (Equation (30)).157 The related ligand 36 was used in the enantioselective cyclization of a Baylis-Hillman adduct with a primary alcohol (Equation (31)).159... 

There would appear to be two distinct modes of reactivity of early transition metal alkyls with O2. When the metal is not in its highest oxidation state, an O2 complex of variable stability may form, and its subsequent reactivity may or may not involve the metal-carbon bond. The formation of remarkable stable 0x0 alkyls is an example of this pathway. In contrast, d°-alkyls react with O2 by a radical chain mechanism that invariable leads to formation of alkoxide complexes labile alkylperoxo ligands are clearly imphcated as intermediates in these reactions. 

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