2 Homometallic Alkoxides

464 Alkoxo and Aryloxo Derivatives of Metals
Ph
where R = CH2SiMe3; ArOH = HO this refers to 6.51 on the previous page.
Ph
Kinetic studies have shown a primary kinetic isotope effect upon deuteration of the
phenolic proton. 7 In some cases ligand exchange reactions restrict the stoichiometry
that can be obtained. One of the most thoroughly studied reactions of this type is
the attempted synthesis of the mono-aryloxide derivative by reaction of trimethylaluminum
[Al2Me6] with 2,6-di-tert-butyl-4-methylphenol (Eq. 6.52) 137 which leads to
the bis(aryloxide) owing to the two disproportionation reactions shown in Eq. 6.53. 138
Me 3C
where ArOH = HO CH3.
Me3C
[Al2Me6] C 2ArOH 2MeH
! 2[⊲ArO⊳AlMe2] ⊲6.52⊳
3[⊲ArO⊳AlMe2] ⇀
↽ [Al2Me5⊲OAr⊳] C [⊲ArO⊳2AlMe]
[Al2Me5⊲OAr⊳] ⇀
↽
Me3C
where ArOH = HO
.
CH 3
1
2 [Al2Me6] C [⊲ArO⊳AlMe2] ⊲6.53⊳
Me3C Changing the aluminium alkyl substrate to [AlBu3] allows isolation of the monoaryloxide
derivative. 89,139
3.6.2 By Oxygenation of Metal–Aryl Compounds
Under certain circumstances it is possible to insert oxygen into metal–aryl bonds
leading to the corresponding aryloxide. This reactivity is a subset of the much
larger study of the oxidation of organometallic species in general. 140,141 Mechanistic
studies show that for transition metal-aryls, an intermediate peroxo (possibly 2 -
bound) can precede formation of the metal aryloxide function. 142 The oxygenation
of cyclopalladated compounds has been achieved, leading to chelated aryloxides of
palladium. 143 Similarly the treatment of nickel metallacycles with N2O can lead to
essentially cyclometallated aryloxides. 144 The addition of electron-deficient olefins
to nickel benzyne complexes can lead to metallacycles which are then oxygenated
to chelating aryloxides. 145 Treatment of the complex [PtMe⊲PPh2C6F5⊳2⊲THF⊳] with
LiOH was found to produce the phenoxide [PtMe⊲PPh2C6F5⊳2⊲PPh2C6F4O⊳]. 146 The
reaction is believed to involve attack of coordinated hydroxide at the electrophilic
ortho carbon of a C6F5 group. Recently the insertion of O2 into a chromium–phenyl
bond has been investigated. 147 The migration of aryl groups to metal–oxo functions
can lead to metal aryloxides. 148,149 More aspects of this reactivity are discussed in
Section 6.2.8.