Metal aryloxides cyclometallated

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). 

The metals thorium and uranium (as with the chemistry in general of these elements) dominate actinide aryloxide chemistry. Synthetic strategies normally focus on the halides reacting with group 1 metal aryloxides or reaction of metal amides with phenols. An important piece of early work was the demonstrated interconversion of eight-coordinate [UMe4(dmpe)2] and [M(OPh)4(dmpe)2] (M = Th, U Table 6.20) by addition of phenol or MeLi to each substrate respectively. The reaction of the cyclometallated... 

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). 

The formation of rings that contain a thioether linkage does not appear to be catalyzed efficiently by Ru, even when terminal olefins are present. On the other hand, molybdenum appears to work relatively well, as shown in Eqs. 30 [207] and 31 [208]. Under some conditions polymerization (ADMET) to give poly-thioethers is a possible alternative [26]. Aryloxide tungsten catalysts have also been employed successfully to prepare thioether derivatives [107,166,169]. Apparently the mismatch between a hard earlier metal center and a soft sulfur donor is what allows thioethers to be tolerated by molybdenum and tungsten. Similar arguments could be used to explain why cyclometalated aryloxycarbene complexes of tungsten have been successfully employed to prepare a variety of cyclic olefins such as the phosphine shown in Eq. 32 [107,193]. 

Late metal alkoxides can also imdergo decomposition by pathways other than 3 hydrogen ehmination. For example, several rhodium- aryloxide complexes, (PPli3)jRh(OAr), undergo decomposition by cyclometallation to ehminate free alcohols and form metalla-cycles (Equation 4.81). ... 

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. ... 

The addition of 4-methylphenol to the ruthenium species [Ru(>j -Me2P-CH2)(Me) Me3)4] produces the cyclometallated aryloxide [Ru(0- / -C6H4)(PMe3)4]. Reaction with CO and CO2 was found to lead to insertion into the metal-aryl bond to produce five- and six-membered metallacycles respectively. ... 

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). ... 

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