2 Homometallic Alkoxides

148 Alkoxo and Aryloxo Derivatives of Metals
point of acetone (Eq. 2.347):
Al(OBut OH O
O OH
Et
Et
)3
+ + (2.347)
O
76%
Since the reaction conditions are nonacidic, this method can be valuable for substances
that would not tolerate acidic conditions or the presence of transition metal ions. 1010
OH
O
O
Al(OBu t )3
90% O
OH
OH
+ + (2.348)
The lanthanide (especially samarium) alkoxides serve as highly effective catalysts
1011–1013 for Oppenauer-type oxidation of alcohols to aldehydes and ketones
(Eq. 2.349):
OH
acetone (excess)
ButOSmI2 (cat.)
65 °C, 24 h
95%
O
OH
(2.349)
The main advantage of Oppenauer oxidation (Eqs 2.346–2.349) is its high selectivity.
For example, the reaction takes place under very mild conditions and is highly
specific for aldehydes and ketones, so that CDC bonds (including those conjugated with
CDO bonds) and many other polyfunctional molecules containing sensitive groups that
are destroyed by the conditions of many other oxidations and reductions may be present
without themselves being reduced.
In conclusion, Oppenauer oxidation may also be regarded as an elimination of
hydride (H ž ž ) ion. The reverse reaction is therefore hydride addition to carbonyl, as in
the Meerwein–Ponndorf–Verley reduction (Section 4.15.3):
Oppenauer oxidation
Meerwein−Ponndorf−Verley reduction
H
C
O
Al(OR)2
C
O
C O
H
C O
Al(OR) 2