2 Homometallic Alkoxides

Homometallic Alkoxides 5
to completion by the presence of bases such as ammonia, pyridine, or alkali metal
alkoxides.
Another generally applicable method, particularly in the case of electronegative
elements, is the esterification of their oxyacids or oxides (acid anhydrides) with alcohols
(Section 2.6), and removing the water produced in the reaction continuously.
In addition to the above, alcoholysis or transesterification reactions of metal alkoxides
themselves have been widely used for obtaining the targeted homo- and heteroleptic
alkoxide derivatives of the same metal. Since the 1960s, the replacement reactions
of metal dialkylamides with alcohols has provided a highly convenient and versatile
route (Section 2.9) for the synthesis of homoleptic alkoxides of a number of metals,
particularly in their lower valency states.
The metal–hydrogen and metal–carbon bond cleavage reactions have also been
exploited in some instances (Section 2.10.2).
The following pages present a brief summary of the general methods used for the
synthesis of metal and metalloid alkoxides applicable to specific systems. Tables 2.1
and 2.2 in Section 2.1 (pp. 6–14) list some illustrative compounds along with their
preparative routes and characterization techniques.
2.1 Reactions of Metals with Alcohols (Method A)
The facility of the direct reaction of a metal with an alcohol depends on both the
electropositive nature of the metal and the ramification of the alcohol concerned.
In view of the very feeble acidic character of nonfluorinated alcohols [even
weaker than that of water: pK a values (in parentheses) of some alcohols
are CH3OH(15.8), CH3CH2OH(15.9), ⊲CH3⊳2CHOH(17.1), ⊲CH3⊳3COH(19.2),
CF3CH2OH(12.8), CH3⊲CF3⊳2COH(9.6), ⊲CF3⊳2CHOH(9.3), ⊲CF3⊳3COH(5.4)], this
route is more facile with lower aliphatic and fluorinated alcohols.
2.1.1 s-Block Metals
2.1.1.1 Group 1 metals (Li, Na, K, Rb, Cs)
The more electropositive alkali metals react vigorously with alcohols by replacement
of the hydroxylic hydrogen (Eq. 2.2):
M C ⊲1 C y⊳ROH ! 1
n [MOR.yROH]n C 1
2 H2 " ⊲2.2⊳
M D Li,Na,K,Rb,Cs;RDMe, Et, Pr i , Bu t ; 3,6,26,27
y D 0.
M D Li; R D Bu t , CMe2Ph; 28
y D 0.
M D K, Rb, Cs; R D Bu t ; 29
y D 1.
M D K, Rb; R D Bu t ; 29
y D 0.
The alkali metals react spontaneously with sterically compact aliphatic alcohols
(MeOH, EtOH, etc.) and the speed of the reaction increases with atomic number of the
metal, Li < Na < K < Rb < Cs, corresponding to a decrease in ionization potential of
the alkali metals. The ramification of the alkyl group is also important, as shown by the