Metal aryloxides synthesis

Lewis acids of chiral metal aryloxides prepared from metal reagents and optically active binaphthol derivatives have played a significant role in asymmetric synthesis and have been extensively studied.23 However, in Diels-Alder reactions, the asymmetric induction with chiral metal aryloxides is, in most cases, controlled by steric interaction between a dienophile and a chiral ligand. This kind of interaction is sometimes insufficient to provide a high level of enantioselectivity. 

In this section, we will highlight the development in the use of metal alkox-ides for the synthesis of new and interesting organometallic compounds, many of these are either inaccessible or difficult to synthesize by common synthetic procedures. We will not discuss (a) the chemistry of organometallic compounds containing alkoxides as supporting ligands, for which excellent reviews by Chisholm and co-workers (154, 513, 514) are available and (b) intramolecular cyclometalation (i.e., C—H bond activation) reactions of metal aryloxides due to the availability of an excellent account of this topic in a review article by Rothwell (515). Furthermore, a brief mention of the use of a related metal derivative (i.e., metal aryloxide) will be made merely for comparison. 

It may not be out of place to note that metal aryloxides ate finding continuous growing importance as a synthon for the synthesis of new and interesting metal alkyls, which are often difficult to synthesize by other routes. 

In the 1978 book very little space was devoted to metal aryloxides because this area had received scant attention, but the intervening years have seen a resurgence of activity involving the synthesis and characterization of many novel compounds and... 

Although the development of metal atom vapour technology over the past three decades has shown tremendous utility for the synthesis of a wide range of organometallic compounds (many of which were inaccessible by conventional techniques), the use of this technique for the synthesis of metal alkoxides and related derivatives does not appear to have been fully exploited. In 1990, Lappert et al. demonstrated the utility of this technique for the synthesis of M—O—C bonded compounds by the isolation of alkaline earth metal aryloxides. 

A much more widely used synthetic method entails the metathetical exchange reaction between alkali metal aryloxides and the metal halide. This procedure has been applied to the synthesis of lanthanide, actinide, and d-block metal aryloxides as well as derivatives of the main group metals (Eqs 6.23, 6.24, ° 6.25, " 6.26, 6.27, 6.28, 6.29, and 6.30 ° ). 

The homoleptic metal dialkylamides are an important class of compounds in inorganic chemistry. They are typically synthesized by treatment of the corresponding halide with lithium or sodium dialkylamide. Although involving an extra synthetic step, there are numerous examples where metal dialkylamide intermediates are useful in the synthesis of metal aryloxide compounds. The reaction normally involves the simple addition of the parent phenol to the metal dialkylamide in a nonprotic, typically hydrocarbon, solvent (Eqs 6.36, 6.37, and 6.38 ). 

The elimination of H2 by addition of phenolic reagents to metal hydrides is an excellent method for the synthesis of alkali metal aryloxide compounds (Eqs 6.54, ° 6.55, and 6.56 ). 

The method can also be applied to the synthesis of transition metal aryloxides, with mixed hydrido, aryloxides sometimes being observed and isolated (Eqs 6.51 and... 

Aryloxide (phenolate) ligands provide rigid and versatile ancillary ligand sets in order to study the multifunctional reactivity of alkylaluminum and alkylmagnesium reagents toward Ln-OR moieties. Several types of symmetrically substituted phenolate ligands — summarized in Chart 4 — were employed for the synthesis of a variety of heterobimetallic lanthanide(III) and lanthanide(II) metal complexes. Alkylation reactions revealed the preferred... 

The hydrogenation of aromatics has been a topic of interest since Sabatier s first synthesis of cyclohexane from benzene with metallic nickel. The role of Nb and Ta aryloxides as catalysts for this reaction was mentioned earlier. Another system that has been studied in detail comprises allyl and hydride complexes of cobalt, e.g., (i73-C3H5)Co[P(OMe)3]3. Like the Nb and Ta compounds cobalt gives cyclohexane with cis stereoselectivity. The active species is probably the hydride, generated from (allyl)CoL3 on hydrogenolysis, which reacts with arenes in a stepwise manner. 

Metal alkoxides and aryloxides, of the general formnla M(OR)x, can be considered as derived from the alcohol ROH by replacement of the hydroxylic hydrogen atom by the metal. These complexes have been known for many years Ti(OEt4) was prepared in 1924 by Bischoff and Adkins/ and the synthesis was attempted as early as 1875 by Demargay although pure product was apparently not obtained. Potential technological applications have resulted in increased activity in this area in recent years (see Sol-Gel Synthesis of Solids)... 

Although metal alkoxides were successfully used as a synthon for the synthesis (see Scheme 9) of more interesting homo- and heteroleptic metal alkyls, they have not attained the same importance as their sterically hindered aryloxide analogues. This finding might be due to the general solubility of both the products [i.e., desired metal alkyls and alkali metal (generally lithium) alkoxides] in hydrocarbon solvents. This limitation has made a cleaner separation of the products more difficult. 

The syntheses, physical properties, and molecular structures of alkoxides and aryloxides have been discussed in CCC (1987).161 The alkoxides of scandium and yttrium were reviewed in CCC (1987).1 There have been more recent developments in this area and the impetus for this chemistry has been the developments in materials research. Metal alkoxides and /3-diketonates can be used as precursors for oxide and nonoxide thin films.162 The stable M—O bond and the volatility of the metal alkoxides are important features of this area of chemistry. This has lead to more research in this area particularly in synthesis, NMR, and X-ray crystallography. 

The synthesis and characterization of a family of mono-Gp dichloro complexes with disubstituted aryloxo ligands has been reported, and their molecular structures provide some means of quantifying the number of electrons donated to the metal center by an aryloxide ligand. These complexes can be reduced by Grignard reagents or LiBu11 in the presence of enynes. The formation of metallacyclic derivatives (Scheme 348) was observed for the Cp but not for the Cp complexes, as deduced by NMR spectroscopy. The complexes have been investigated as catalysts... 

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