2 Homometallic Alkoxides

40 Alkoxo and Aryloxo Derivatives of Metals
to Nb(OR)5. The probable mode of this reaction can be represented by Eqs (2.105)
and (2.106):
Nb(NEt2⊳4 C 4ROH ! Nb(OR) 4 C 4Et2NH " ⊲2.105⊳
Nb(OR) 4 C ROH ! Nb(OR) 5 C 1
2H2 " ⊲2.106⊳
Bradley and co-workers 332–334 have nicely demonstrated the importance of metal
dialkylamides as starting materials for preparation of the alkoxides of a wide range of
metals.
Reactions of M(NR2)4 (M D V, Nb, R D Et; Cr, R D Et or Pr i ;andMo,RD Me)
with 1-adamantyl alcohol have been investigated by Wilkinson and co-workers 169 and
isolated mononuclear alkoxides of these metals in the tetravalent state.
The reaction represented by Eq. (2.101) is, however, sometimes accompanied by a
change in the oxidation state of the metal. For example, Cr(NEt2)4 reacts with primary
and secondary alcohols 335 according to Eq. (2.107). Only tertiary alcohols and the
sterically demanding 3,3-dimethyl-2-butanol, 336 which are not prone to this type of
oxidation, are known 331 to give chromium(IV) alkoxides.
2Cr(NEt 2⊳4 C 7R 0 R 00 CHOH ! Cr(OCHR 0 R 00 ⊳3 C R 0 R 00 CO C 8Et2NH " ⊲2.107⊳
Similar to the alcoholysis reaction of tetrameric aluminium isopropoxide 293 with tertbutyl
alcohol, the alcoholysis reaction of dimeric aluminium tris(dimethyl amide) 337,338
with tert-butyl alcohol is also slow owing to steric factors. However, the amide route
affords finally the tris product [Al⊲OBu t ⊳3], (Eq. 2.108) instead of the mixed product of
the type Al2⊲OBu t ⊳5⊲OPr i ⊳ which was finally obtained in the reaction of [Al⊲OPr i ⊳3]4
with an excess of Bu t OH.
Al2⊲NMe2⊳6 C 5Bu t OH !
5Me2NH
Al2⊲OBu t ⊳4⊲ -OBu t ⊳⊲ -NMe2⊳
Bu t OH ⊲excess⊳
! Al2⊲OBu t ⊳6 C Me2NH ⊲2.108⊳
The reactions of Bi(NMe2)3 with alcohols afford soluble and volatile alkoxides of
bismuth: 339
Bi(NMe 2⊳3 C 3ROH ! Bi(OR) 3 C 3Me2NH " ⊲2.109⊳
where R D Pr i (less soluble and non-volatile), CH2CH2OMe, CH2CH2NMe2,
CHMeCH2NMe2, CMe2Et.
Although the utility of metal dialkylamides for the synthesis of metal alkoxides
(including tert-butoxides) had been established for many years, the progress in this
direction was rather slow up to the 1980s. Since then the alcoholysis reactions of metal
bis(trimethylsilyl) amides involving sterically hindered mono- and multi-dentate (with
more recent emphasis on specially designed donor-functionalized) alcohols to reduce
the tendency of molecular aggregation and to increase the solubility and volatility of
the resulting mono- or di-nuclear alkoxide derivatives of even more electropositive and
larger size metals, such as heavier alkaline earths and lanthanide elements, have played
a significant role in the development of exciting homometallic alkoxide systems 340,341
as shown by Eqs (2.110–2.112):