Chemistry Reference
In-Depth Information
CO
CO
OC
OC
CO
CO
H
OC
H
Co
H
CO
Co
- CO + H
2
CCH
2
+ CO - H
2
CCH
2
Co
+ CO
H
Co
Co
Co
H
CO
H
CO
CO
OC
Cobaltacycle
formation
OC
CO
OC
CO
Ligand substitution
CO
CO
CO
OC
O
H
Co
O
- Co
2
(CO)
6
+ CO
H
H
CO insertion
Co
Reductive elimination
CO
OC
H
CO
Scheme 2.3
General formulation of the Magnus mechanism of PKR.
final adduct become determined at this point, iii) CO insertion, and iv) reductive elimination
and decomplexation of a weakly bonded cyclopentenone-Co
2
(CO)
6
complex.
We will discuss in the next section the available knowledge of these steps in stoichio-
metrical PKR.
2.2
Stoichiometric Pauson-Khand Reaction
2.2.1 The Ligand Substitution Steps
It can be reasonably assumed that the ligand substitution process in stoichiometric PKR
takes place through a dissociative mechanism whose first step leads to a coordinatively
unsaturated dicobalt pentacarbonyl species (Scheme 2.4). Thus, operation under a CO
overpressure leads to a lowering in reaction rate (through prevention of CO dissociation),
while solvents with Lewis base character
4
(or Lewis base additives)
5
accelerate the reaction
likewise through stabilization of the coordinatively unsaturated intermediate and subsequent
labilization of CO ligands in the Lewis base-substituted complex.
6
CO pressure inhibits reaction
Lewis bases favor reaction
CO
CO
CO
CO
CO
CO
CO
L
R
Co
R
R
+CO
Co
Co
-CO, +L
Co
CO
-CO
Co
CO
Co
CO
H
H
H
CO
CO
CO
CO
CO
CO
High energy
pentacarbonyl complex
Lewis base stabilized
pentacarbonyl complex
Scheme 2.4
Mechanisms for CO dissociation.
An important aspect of the CO dissociation from the initial hexacarbonyl complex refers
to the enthalpy characteristics of this step. For PKR performed in the absence of nucleophilic
additives, under purely thermal conditions, this is with high probability the rate-determining
step of the reaction. In the search for milder reaction conditions and increased selectivity
