Aryloxides metal derivatives

Alkoxides and aryloxides demonstrate similar chemistry to that of hydroxides in that it is possible to prepare mixed metal or double metal derivatives similar to hydroxo salts such as Na2[Sn(OH)6]. The formation of mixed metal alkoxides, e.g. Na2[Zr(OEt)6] and Ln[Al(OPri)4]3, is typically a result of the electron deficiency (Lewis acidity) of the metal centers in units of the type M(OR)x or M(OAr)JC (x < 5). This then leads either to oligomerization via alkoxide bridges or, in the presence of other alkoxides, to the formation of mixed metal compounds. 

The coordination of an alkoxide or aryloxide can lead to a mixed metal derivative.149 In some cases, ligand exchange can take place to generate a neutral adduct (equation 55).IS4... 

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. 

The addition of CS2 to the thallium aryloxides [Tl(OAr)] (Ar = 4-methyl, 4-bulyl, 4-rerr-butyl, 3,5-dimethylphenyl) has been shown to be a good synthetic route to the corresponding [Tl(S2COAr)] salts, which can be used to generate transition metal derivatives. ... 

It is also common that mixed metal derivatives of the lanthanides and group 1 metals are the products of synthesis. This can occur for small aryloxides leading to clusters, e.g. [La2Na3(/r4-OAr)3(/z2-OAr)6(dioxane)5] (OAr = OQH4Me-4), as well as in the generation of more discrete species with bulkier aryloxides, e.g. [(thf)Li(/i2-OAr)2La(OAr)2(THF)] (OAr = OC6H3Pr -2,6). In some situations the alkali metal interacts with the aryloxide jr-nucleus, e.g. [Cs( 7r-Ar-0)2La(OAr)2] (OAr = OC6H3P1I-... 

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]. 

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. 

Lanthanide aryloxides have proved to be excellent precursors to homoleptic lanthanide alkyls (B, Eq. 13) [140], The reaction can be conducted in non-polar solvents because of the good solubility of the starting compounds. The formation of insoluble alkali metal aryloxides is the driving force (kinetic control). Complexes derived from aliphatic alcohols [141] and acetylacetonates [131] are... 

Heterogeneous diene polymerization catalysts based on modified and unmodified silica-supported lanthanide complexes are known as efficient gas-phase polymerization catalysts for a variety of support materials and activation procedures (see Sect. 9). Metal siloxide complexes M(()SiR3 )x are routinely employed as molecular model systems of such silica-immobilized/ grafted metal centers [196-199]. Structurally authenticated alkylated rare-earth metal siloxide derivatives are scarce, which is surprising given that structural data on a considerable number of alkylated lanthanide alkoxide and aryloxide complexes with a variety of substitution patterns is meanwhile available. 

Chiral metal alkoxides M(OR)4 have been developed as asymmetric variants of ordinary Lewis acids, such as A1C13 and ZrCU, and are used as catalysts for selective carbon-carbon bond formation. Thus, starting from bidentate l,l -bi-2-naphthol derivatives (BINOL) and SnCU, a series of chiral tin(iv) aryloxides 221 (Figure 7) was prepared and successfully applied to the enantioselective Diels-Alder reaction <2006TL873>. Similar silocanes obtained from menthone- or camphor-derived 2,2 -biphenols have been obtained and their configuration was analyzed by NOE differential spectroscopy (NOEDS) <1997JOC7156>. 

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)... 

The compounds containing more sterically demanding alkoxide ligands have exhibited an extraordinary variety of structures (see Figs. 38-46, and Table III), with the preparation of not only the new complexes, but of new types of derivatives being a commonplace event during the last decade. Many of the types of compounds discovered contain forms of bonding previously unknown in metal alkoxide (1, 6, 7, 34) and aryloxide (293, 294) chemistry. 

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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