Metal aryloxides bond activation

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. 

For this type of transformation, the insolubility of one of the products (i.e., lithium aryloxide) in pentane appears to be the driving force. Another distinct feature of metal aryloxides is their susceptibility to undergo intramolecular C—H bond activation (i.e., cyclometalation) reactions to afford new organometallic systems supported by aryloxide ligands (515). 

A variety of low valent aryloxide derivatives of the early transition metals undergo intramolecular CH bond activation. Attempts to isolate the d -species [M(OAr)3] or [M(OAr)2Cl] (OAr = 2,6-di-rcrt-butylphenoxide or 2,6-di-phenylphenoxide M = Nb, Ta) by reduction of the corresponding d -chloride leads instead to bis-cyclometallated compounds (Eq. 6.60). ... 

The thermal stability of the dialkyls [(ArO)2MR2] is a function of all three variables metal, aryloxide, and alkyl. With 2,6-di-terf-butylphenoxide the bis-benzyls for all three metals undergo elimination of toluene and formation of cyclometallated products generated by activation of iert-butyl CH bonds (see Section 5.1). 

A variant of direct metallation, cocondensation, has been utilized to prepare the first <7-bonded organocalcium complex, (SiMe3)2CH 2Ca(l,4dioxane)2, in addition to a transient heteroleptic calcium hydride, CeHsCaH by C-H activation (equation 3). The heteroleptic hydride is highly reactive and decomposes upon warming the reaction vessel above —78 °C. The product has been identified by its reaction with alcohols, affording quantitatively a series of calcium aryloxides. 

The aryloxide ligand is able to undergo cyclometallation at various metal centres via a number of mechanistic pathways. The reactivity can involve activation of the ortho-CH bond of the phenoxy nucleus itself, as well as aliphatic, benzylic, or aromatic CH bonds of attached substituents. The products of these reactions are typically stable four-, five- or six-membered oxa-metallacycles. ... 

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