2 Homometallic Alkoxides

Homometallic Alkoxides 143
On the basis of the above observations, Villani and Nord 980 concluded that the
catalytic activity of these metal alkoxides in the condensation of aldehydes depends
upon the nature of the metal alkoxides. Thus, when a strongly basic alkoxide like
sodium ethoxide is used, the aldol type of condensation occurs whereas with an acidic
alkoxide such as aluminium ethoxide, the simple esters are formed. However, weakly
basic alkoxides [Mg[Al⊲OEt⊳4]2, Ca[Al⊲OEt⊳4]2, orNa[Al⊲OEt⊳4], which are intermediate
between sodium ethoxide and aluminium ethoxide as regards their acidic or basic
character, cause the formation of glycol esters. The bifunctional activity of the complex
catalyst Mg[Al⊲OEt⊳4]2 or Ca[Al⊲OEt⊳4]2, may be due to the intermediate basicity
of the complex which is formed from strongly basic Mg⊲OR⊳2 and acidic Al⊲OR⊳3.
However, it has also been observed that aldehydes other than those containing an
˛-CH2 group do not form a glycol ester with mildly basic alkoxides; the condensation
of ˛-ethylbutyraldehyde results in the formation of a glycol ester only in the presence
of strongly basic sodium ethoxide.
The mechanism of the Tischtchenko reaction was discussed by Lin and Day 981
who proposed that in the initial stages, the aluminium atom of aluminium alkoxide
coordinates with the carbonyl group of the aldehyde (Eq. 2.329),
R
H
C O + Al(OR) 3
R
H
C O Al(OR) 3
+
(2.329)
Consequently a positive charge is induced on the carbonyl carbon atom which facilitates
the reaction to proceed further (Eq. 2.330):
R
H
C + O
O
+
H
CR
+ −
−
Al(OR) 3
R
H
C
O
H
C R
O Al(OR) 3
RCOOCH 2R + Al(OR) 3
(2.330)
by oxidation process (loss of hydride ion) coupled with simultaneous acceptance of
hydride ion (reduction) by another aldehyde molecule finally yielding the organic ester.
Mixed Tischtchenko reactions were also studied by Nord. 982–984 In these reactions he
isolated benzyl isovalerate and isoamyl benzoate from benzaldehyde and isovaleraldehyde
as well as benzyl acetate, ethyl acetate, and benzyl benzoate from benzaldehyde
and acetaldehyde. Orloff 985 also reported the synthesis of benzyl isobutyrate and
isobutyl benzoate from benzaldehyde and isobutyraldehyde. However, the mechanism
of mixed Tischtchenko reactions is rather complex. For example, in the reaction of
benzaldehyde and n-butyraldehyde, either of the carbonyl groups may coordinate with
the aluminium alkoxide. In fact the carbonium ion formed in butyraldehyde is more
active than that in benzaldehyde and further reaction may proceed with the carbonium
ion of the former type. The overall mechanism may be explained by the following
equations (2.331)–(2.334):
C3H7
H
C
O + Al(OR)3
C3H7
H
C
+ −
O Al(OR)3
(2.331)