2 Homometallic Alkoxides

482 Alkoxo and Aryloxo Derivatives of Metals
stopped completely (Eq. 6.65). 11
Bu t
Ph
Ph
Bu t
O
O
Ph
Ta
Bu t Ph
Bu t
CHSiMe 3
CH 2SiMe 3
∆
NO REACTION
(6.65)
It is also possible for di-aryloxide ligands to undergo cyclometallation. Chisholm
et al. have shown that addition of 2,20-methylene-bis(6-tert-butyl-4-methylphenol) (di-HOAr) to [W2(NMe2⊳6] leads to a product [W2⊲ 2-H)( 2-NMe2)(di-OAr)(di-OAr-
H)(NMe2)(HNMe2)] in which the central methylene group of one di-aryloxide ligand
had been metallated. 232 The reaction involves the addition of the C–H bond to the initial
W 6C
2 core to yield a W 8C
2 (WDW) species. Mechanistic studies showed that in fact
an equilibrium existed between the metallated compound and [W2(di-OAr)2(NMe2)2].
Addition of pyridine or PMe3 to this latter compound yielded the corresponding
metallated adducts. Remarkably it was shown that the formation of the adduct [W2(di-
OAr)2(NMe2)2(PMe3)] preceded CH bond activation; i.e. the coordination of the donor
ligands “turns on” the W 6C
2 core. 233
A variety of aryloxide and alkoxide ligands undergo CH bond activation by Sn(IV)
metal centres. 234 The reaction appears to be restricted to arene CH bonds and can lead
to five- and six-membered stanacycles. Mechanistic studies using substituted orthoarene
rings in 2,4,6-triarylphenoxides imply an electrophilic substitution pathway. 235
Mixed alkyl, aryloxides do not undergo cyclometallation but chloro and amido ligands
can act as leaving groups to generate HCl and amine respectively. In the case of
dimethylamido derivatives of tin the dimethylamine generated by CH bond activation
sometimes remains in the coordination sphere of the metal. 234
5.2 Insertion Chemistry of Metal Aryloxide Bonds
Transition metal aryloxides undergo a number of insertion reactions with small
molecules. Some of this reactivity is of fundamental importance, and in a number
of cases important mechanistic studies have been carried out.
5.2.1 Insertion of Carbon Monoxide (Carbonylation)
The insertion of carbon monoxide into metal aryloxide bonds appears to be restricted
to later transition metal complexes. The initial products of these reactions are aryloxycarbonyls,
which may be stable or undergo further reaction. Three examples of this
type of reaction are shown in Eqs (6.66)–(6.68). 236–239