2 Homometallic Alkoxides

Homometallic Alkoxides 147
On further refluxing, the insoluble product Ti⊲OC6H4CH2O⊳⊲OC6H4CHO⊳⊲OPr i ⊳ in
benzene, again 0.5 mole of acetone was liberated and the soluble product thus obtained
corresponded to Ti2⊲OPr i ⊳⊲OC6H4CH2O⊳⊲OC6H4CHO⊳.
Similar results were obtained by the reactions of zirconium alkoxide with salicylaldehyde
1008 except that the rate of these reactions was faster and, unlike the titanium
derivative, the product of the 1:2 molar reaction of Zr⊲OPr i ⊳4.Pr i OH with HOC6H4CHO
(salicylaldehyde) did not precipitate; instead the product Zr2(OPr i )(OC6H4CH2O)3
(OC6H4CHO) was directly obtained.
4.15.4 Oppenauer Oxidation Reaction
The reverse of the Meerwein–Ponndorf–Verley reduction (Section 4.15.3), occurs
when a ketone (as a hydride acceptor) in the presence of base is used as the oxidizing
agent. It is reduced to a secondary alcohol (Scheme 2.7), and the reaction is known as
Oppenauer Oxidation. 1009
It involves heating the alcohol to be oxidized with an aluminium alkoxide in the
presence of a carbonyl compounds, which acts as the hydrogen acceptor. The reaction
is an equilibrium process and proceeds through a cyclic transition state.
R2CHOH C Al⊲OCHMe2⊳3
! R2CHOAl⊲OCHMe2⊳2 C Me2CHOH
O
R2C Al(OCHMe2)2
H
C
O
Me2
Al(OCHMe2)3 + R2C
Scheme 2.7
The ketones most commonly used are acetone and butanone. Aluminium tertbutoxide
is usually employed as the base catalyst (Eq. 2.346).
Me
Me Me
CH
80%
Me
CH
Me
Me
Al(OBu t ) 3
O
CHCH(OH)Me + Me2C
CHCOMe + Me2CHOH
O
(2.346)
Cyclohexanone, benzoquinone, and fluorenone have also been used as strong hydrogen
acceptors in order to allow reactions to be carried out at temperatures above the boiling