2 Homometallic Alkoxides

Homometallic Alkoxides 145
The formation of esters by the Tischtchenko reaction was also demonstrated by
Nakai 987 who showed that magnesium dimethoxide or aluminium triethoxide were
effective catalysts.
4.15.3 Meerwein–Ponndorf–Verley Reaction
Another type of reaction involving a metal alkoxide and a carbonyl compound was
noticed as early as 1925 by Verley 988 and Meerwein and Schmidt, 989 who observed that
alkoxides of magnesium, calcium, and particularly aluminium could catalyse reduction
of aldehydes in the presence of excess ethyl alcohol as shown by Eq. (2.336):
RCHO C CH3CH2OH Al⊲OEt⊳3
↽ ⇀ RCH2OH C CH3CHO ⊲2.336⊳
Removal of the more volatile acetaldehyde from the reaction medium is easily achieved
with a stream of dry nitrogen or hydrogen to drive the reaction to the right.
In 1926, Ponndorf devised a method in which both aldehydes and ketones could
be reduced to alcohols by adding excess alcohol and aluminium triisopropoxide990 (Eqs 2.337–2.339).
R2CDO C Me2CHOH Al⊲OPri ⊳3
⇀
↽ R2CHOH C Me2CDO
(distil)
(2.337)
MeCOCH2CH2CH2Br C Me2CHOH Al⊲OPri⊳3 !MeCH⊲OH⊳CH2CH2CH2Br C Me2CDO
⊲2.338⊳
Me2CHOH C MeCHDCHCHO Al⊲OPri ⊳3
! MeCHDCHCH2OH C Me2CDO (2.339)
The process of reducing carbonyl compounds (aldehydes or ketones) to alcohols is
therefore known as the Meerwein–Ponndorf–Verley reaction. Although alkoxides of a
number of metal(loid)s such as sodium, magnesium, titanium, zirconium, iron, boron,
aluminium, tin, and antimony have been used for these reactions, those of aluminium
are by far preferred, since they tend to give the minimum degree of side reactions.
The use of aluminium isopropoxide over other alkoxides was also preferred by Young
et al. 991 as well as by Adkins and Cox. 71
Meerwein et al. 992 originally suggested a mechanism for the reaction (Eq. 2.340)
involving coordination of the carbonyl oxygen to the aluminium alkoxide which thereby
functioned as a catalyst in the redox reaction. 993
R 0 R 00 CO C Al⊲OCHR2⊳3
⇀
↽ R 0 R 00 CO ! Al⊲OCHR2⊳3
⇀
↽ ⊲R 0 R 00 CHO⊳⊲R2CHO⊳2Al OCR2
⇀
↽ ⊲R 0 R 00 CHO⊳⊲R2CHO⊳2Al C R2CO ⊲2.340⊳
As a result of mechanistic studies by a number of chemists such as Woodward et al., 994
Jackman et al., 995–997 McGowan, 998 Rekasheva and Miklykhin, 999 Williams et al., 1000
and Doering et al., 1001,1002 it appears that the reaction involves a six-membered cyclic
transition state 1003,1004 in which hydrogen transfer occurs from the ˇ-C–H bond of an
alkoxide to a coordinated ketone or aldehyde: