2 Homometallic Alkoxides

146 Alkoxo and Aryloxo Derivatives of Metals
R2CHO OCHR2
O
Al
O
C C
R
R
H
R′′
R′
R2CHO OCHR2
R
R
•
O
C
Al
H
O
C
R′
R′′
As a consequence of the coordination of the carbonyl oxygen with aluminium
isopropoxide ⊲R D Me⊳, a partial positive charge will be developed on the carbon
atom; this could be the rate-determining step in this reaction mechanism. This view
was supported experimentally by McGowan 998 who varied the electron density on the
carbonyl carbon atom by substituting an electronegative group X in the para-substituted
acetophenones XC6H4COCH3. However, this did not constitute a conclusive proof of
the mechanism. Furthermore, the postulated mechanism has the expected stereochemical
consequences, and this aspect has been demonstrated by Jackman et al. 995–997 and
Doering et al. 1001–1002 However, kinetic measurements 996,1005,1005a are complicated by
the oligomeric nature of the aluminium alkoxides.
It is noteworthy that in some cases reduction with metal alkoxides, including aluminium
isopropoxide, involves free-radical intermediates (single electron transfer (SET)
mechanism). 1006
The potentialities of the Meerwein–Ponndorf–Verley reaction as an alternative
method for preparation of metal alkoxides have also been studied (Eqs 2.341–
2.345): 146,1007,1008
Ti⊲OPr i ⊳4 C 4Cl3CCHO ! Ti⊲OCH2CCl3⊳4 C 4Me2CDO 146
Zr⊲OPr i ⊳4 C 4Cl3CCHO ! Zr⊲OCH2CCl3⊳4.2Me2CDO C 2Me2CDO 146
⊲2.341⊳
⊲2.342⊳
2-Hydroxybenzaldehyde (salicylaldehyde) reacts with isopropoxides of titanium
1007,1008 and zirconium 1008 with displacement of isopropoxo group(s) as isopropyl
alcohol to give mixed derivatives (Eqs 2.343–2.345), which on heating undergo reduction
of the aldehyde function with elimination of acetone.
Ti⊲OR⊳4 C xHOC6H4CHO benzene
!
room temp ⊲RO⊳4 x Ti⊲OC6H4CHO⊳x C xROH ⊲2.343⊳
where R D Et, Pr i ,Bu t ; 1007 x D 1, 2.
benzene
⊲RO⊳4 x Ti⊲OC6H4CHO⊳x ! ⊲RO⊳4 2x Ti(OC6H4CH2O⊳x C xR
reflux
0 CHO ⊲2.344⊳
where R D Et, Prn ; 1008 R0 D Me, Et; x D 1, 2.
The reaction of Ti⊲OPri⊳4 with salicylaldehyde in 1:2 molar ratio initially liberated
only one mole of acetone and an insoluble product (Eq. 2.345):
Ti⊲OPr i ⊳4 C 2HOC6H4CHO
toluene
! Ti⊲OC6H4CH2O⊳⊲OC6H4CHO⊳⊲OPr
reflux
i ⊳ C ⊲CH3⊳2CDO ⊲2.345⊳