2 Homometallic Alkoxides

Homometallic Alkoxides 35
(b) When the reaction is carried out with a higher boiling alcohol, the refluxing
temperature can be lowered in the presence of benzene and the side reactions are
thus minimized.
(c) The technique has the added advantage that it can be used with different stoichiometric
ratios of the reactants. Thus the mixed alkoxides of many elements have
been prepared.
(d) A simple oxidimetric method developed for estimation of ethyl alcohol or
isopropyl alcohol present in the azeotrope makes it possible to follow the progress
of the reaction quantitatively.
2.7.5 Solubility Factor
The solubility factor has been found to be of some use for the synthesis of
insoluble alkoxides. For example, when titanium ethoxide or isopropoxide is treated
with methanol, an instantaneous reaction occurs with the separation of insoluble
methoxide. 270 The fractionation process is not necessary in cases where such insoluble
derivatives are obtained.
The insoluble methoxides are generally not preferred as starting materials for alcoholysis
reactions, because they requires a longer refluxing period for completion of the
reaction. Bradley et al. 274 and Mehrotra, 305 however, made the interesting observation
that zirconium methoxide, after prolonged refluxing with excess tert-butyl alcohol 274
or tert-amyl alcohol 305 in the presence of benzene, finally yielded monomethoxide
tri-tert-alkoxide only:
Zr(OMe) 4 C 3R t OH
(excess)
! Zr(OMe)(OR t ⊳3 C 3MeOH " ⊲2.88⊳
where R t D tert-butyl or tert-amyl.
The final product was found to be a stable dimeric species [⊲R t O⊳3Zr⊲ -
OMe⊳2Zr⊲OR t ⊳3], which did not undergo further alcoholysis for steric reasons.
In the light of the above factors, the alcoholysis reactions of a few metal alkoxides
may be briefly summarized. In the alcoholysis reactions of boron alkoxides
with primary alcohols, Mehrotra and Srivastava 292 observed that reactions proceed to
completion conveniently, but with tertiary alcohols, (tert-butyl alcohol) mono-alkoxy
di-tert-butoxide was the final product. The non-replaceability of the last alkoxy group
with tert-butyl alcohol was ascribed to steric factors. It was observed that the reaction
was fast in the beginning but slowed down at the later stages after the formation of
the mixed alkoxide. Presumably steric hindrance of the mixed alkoxide B(OR)(OBu t )2
prevented the close approach of another molecule of tert-butyl alcohol.
The alcoholysis reactions of tin(IV) alkoxides are comparatively faster than those of
the silicon and germanium analogues and proceed to completion without any catalyst.
Bradley 26 thus prepared a number of primary, secondary and tertiary alkoxides by the
alcoholysis reactions of tin tetraisopropoxide isopropanolate with various alcohols in
the presence of benzene (Eq. 2.89):
Sn(OPr i ⊳4.Pr i OH C 4ROH ! Sn(OR) 4 C 5Pr i OH " ⊲2.89⊳
However, in the alcoholysis reaction of tin tetraisopropoxide isopropanolate
with tert-heptyl alcohol in refluxing toluene or benzene, Gupta 311 could isolate