Chemistry Reference
In-Depth Information
8D.2.4.
2
) Binding Modes of the Lewis
Acid/Carbonyl Complexation
σ
- and
π
- (
η
1
and
η
There are two binding modes in the Lewis acid and carbonyl complexation either by
carbonyl lone pair electrons to form a σ - ( η
1
) bond or by carbonyl π system to complex
in an η
2
form (Fig. 8D.2) [9]. Main group elements, early transition metals, and lan-
thanides (d
0
) generally complex to a carbonyl compound in σ - ( η
1
) binding mode with
varying angles (B- O = C: 112 ∼ 119, Al - O = C: 136 + 4, Sn - O = C: 127 + 10, Ti - O = C:
125+12). π - ( η
2
) Bonding is observed in the
d
-electron late transition metal, which is
favorable for back donation to an electron-poor carbonyl compound, for example,
π - bonding Cp
2
Zr(II)-benzophenone complex [10], in sharp contrast to σ - bonding
Cp *
2
Ti(IV)Cl - DMF complex [11] (Fig. 8D.3 ). Signifi cantly, one enantioface of the car-
bonyl compound can be shielded in π-bonding so that a nucleophile can be directed
toward the other enantioface.
8D.2.5. In - Plane versus Out - of - Plane Complexation
The modes of the Lewis acid/carbonyl complexation can thus be defi ned by the bond
length (γ), bond angle (θ), and dihedral angle (ϕ) [9], in the continuum from in-plane
(ϕ = 0) σ - complexation mode to π - complexation mode via out - of - plane complexation
(ϕ ≠ 0) (Fig. 8D.4 ). Signifi cant example of out-of-plane complexation is seen in the X-ray
M
O
O
M
R
RH
H
1
)
2
)
(
σ
η
π
(
η
Figure 8D.2.
Ph
Cp
Cp*
Cp
Cl
Zr
Ti
O
Ph
Ph
O
Cp*
167.0°
O
Me
Zr
N
Cp
Ph
Me
H
Cp
Figure 8D.3.
M
r
ϕ
θ
H
O
R
Figure 8D.4.
