2 Homometallic Alkoxides

Metal Aryloxides 509
co-workers have demonstrated that group 4 metal binaphthoxide derivatives can be
used to generate isotactic polymers and oligomers from ˛-olefins. 45
The development of “Schrock type” metathesis catalysts containing chiral binaphtholate
and biphenolate ligands to carry out asymmetric metathesis reactions has been
achieved. 50–52,266
Some recent developments include the synthesis of poly-aryloxides based upon the
binol nucleus. Examples include bis(binaphthol)methane 267 and binaphthols containing
phenolic substituents in the 3,3 0 -positions. 268
6.2 Normal Metal Aryloxides
6.2.1 Group 1 Metal Aryloxides
The aryloxide derivatives of the group 1 metals and particularly lithium and sodium are
important starting points for the synthesis of other metal aryloxides. They are generated
by addition of the phenol to either the alkyls (lithium, routinely BunLi), hydrides
(lithium and sodium), or the metal (lithium, sodium, and potassium). As with other
electropositive metals, the chemistry of the group 1 elements with 2,4,6-trinitrophenol,
metal picrates, is typically carried out with aqueous or other solutions of the corresponding
cations. The structural chemistry of group 1 metal compounds continues to
attract considerable attention. 269 This interest stems not only from the diverse structures
adopted by these compounds but also by the recognition that the degree of aggregation
of group 1 metal reagents can strongly influence their reactivity. 270 In the case
of the aryloxide derivatives of these metals the degree of oligomerization is sensitive
to the steric nature of the phenoxide nucleus as well as the presence of Lewis bases.
The degree of association in the solid state has been determined by X-ray diffraction
methods for a variety of group 1 metal aryloxides (Tables 6.10–6.13). Also the structures
of simple phenoxides [MOPh] (M D Li, Na, K, Rb, and Cs) have been examined
by powder diffraction using ab initio structure solutions. 271 Many of these studies have
been stimulated by the industrially important Kolbe–Schmitt synthesis, which involves
the solid-state carboxylation of a group 1 metal phenoxide. 272 Potassium, rubidium,
and caesium phenoxide were shown to be isostructural by powder diffraction with
two distinct metal environments within infinite chains. Besides a distorted octahedral
environment, three-coordinate alkali metals were present with weak phenyl ring
interactions. 271 The powder structures of [C6H5OK.xC6H5OH] (x D 2, 3) have also
been investigated. 273 In both compounds there are polymeric chains with potassium
surrounded by five oxygen atoms and one -bound phenyl ring.
The lithium derivative of the bulky ligand 2,6-diphenyl-3,5-di-tert-butylphenoxide
forms a cyclic trimer, [Li( 2-OAr)]3 with alternating long/short Li–O distances and
two-coordinate Li atoms. 274 A number of polymeric structures of unsolvated sodium
phenoxides have been determined (Table 6.11), e.g. [NaOC6H5]n 275 and [Na( 3-O- 6- C6H4Me-4)]n. 276 The parent phenoxide has chains of Na2O2 rings stacked together
using Na–O andNa– -arene interactions. In the 4-methyl derivative there are also
-interactions between the phenoxide nucleus and sodium. Recently the structure of
[Cs(OC6H3Pri 2-2,6)] has been determined to consist of infinite chains held together by
Cs–O andCs– -arene interactions. 277