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2 Homometallic Alkoxides

2 Homometallic Alkoxides

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450 Alkoxo and Aryloxo Derivatives of Metals<br />

2.2.2 Biphenoxides and Bis(phenol)methanes<br />

Both biphenol (from oxidative coupling of phenols) and bis(phenol)methane (formaldehyde/phenol<br />

condensation) as well as their substituted derivatives have been used<br />

as ligands (Scheme 6.4). 44,45 The binaphthol unit (typically generated by oxidative<br />

coupling of 2-naphthol) has also been extensively used to generate chiral aryloxides<br />

bound through both aryloxide oxygen atoms. 46,47 The low solubility and small steric<br />

size of parent binaphthol has led to the development of synthetic routes to substituted<br />

derivatives (Scheme 6.4). 48–50 The chiral ligand 6,6 0 -dimethyl-3,3 0 ,5,5 0 -tetra-tert-butyl-<br />

1,1 0 -biphenyl-2,2 0 -diol has been synthesized, resolved, and applied to Mo-catalysed<br />

metathesis reactions. 51,52 A variety of chelating, diaryloxo ligands have been applied<br />

to metal-catalysed olefin polymerization studies. Bridging groups include methylene, 53<br />

ethylene, 54 sulfur, 55 tellurium, 56 and sulfinyl units. 57 The metal coordination chemistry<br />

of a large number of other bis-phenolic reagents has also been investigated, e.g. those<br />

derived from salicylic acid and diammines (Scheme 6.5). 58 In some of these ligands<br />

the phenoxy groups are chemically non-equivalent.<br />

OH<br />

OH<br />

Bu t Bu t<br />

Bu t<br />

SiR3<br />

OH<br />

OH<br />

SiR3<br />

OH<br />

OH<br />

Bu t<br />

(i) 2NaH<br />

(ii) ClCH2OMe (i) 2BuLi<br />

(ii) H +<br />

2.2.3 Bis-salicylaldimines<br />

Me 3C<br />

Me<br />

Me<br />

OH OH<br />

OMOM<br />

OMOM<br />

Br<br />

OSiR3<br />

OSiR3<br />

Br<br />

Scheme 6.4<br />

(i) 2BuLi<br />

(ii) C2F4Br2 (i) 2NaH<br />

(ii) ClSiR3<br />

CMe3<br />

H +<br />

Br<br />

OMOM<br />

OMOM<br />

The bis-aryloxide ligands obtained by condensation of two equivalents of salicylaldehyde<br />

with diammines, exemplified by bis(salicylaldehyde)ethylenediimine (salen),<br />

constitute one of the most important classes of ligands in coordination chemistry. 59<br />

Br<br />

Br<br />

OH<br />

OH<br />

Br

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