Chemistry Reference
In-Depth Information
As already discussed for the cobaltacycle formation step, the enthalpic balance of the
CO insertion step should be made more favorable by coordinative saturation of the active
cobalt atom in the initially formed species with carbon monoxide or with some other Lewis
base present in the reaction medium.
2.2.4 Reductive Elimination
As with cobaltacyles, acylcobaltacycles have never been detected as intermediates in the
PKR. It is assumed that, as soon as they are formed, they will evolve by reductive elimination
into weakly bonded cyclopentenone dicobalt complexes which will readily decompose to
afford the cyclopentenone product and unstable cobalt residues (Scheme 2.14).
CO
R
L
OC
Co
O
OC
Co
OC
H
CO
CO
OC
L
CO
Co
Co
R
OC
R
[or]
- Co
2
(CO)
5
L
O
O
O
R
OC
L
Co
CO
OC
Co
OC
H
Scheme 2.14
Reductive elimination and release of the cyclopentenone.
While this step is irrelevant to the course of stoichiometric PKR, it becomes of critical
importance for the success of catalytic versions of the reaction. In this type of process (see
below), the survival of the catalytic cycle depends on the successful transfer of the dicobalt
carbonyl fragment from the cyclopentenone complexes to new alkyne and alkene (or enyne)
molecules entering the reaction cycle.
2.3 Catalytic Pauson-Khand Reaction
Sustainability concerns have in recent times fostered a shift in interest from stoichiometric
to cobalt-catalytic versions of the PKR.
20
Different classes of additives were reported to
improve catalytic PKR,
21
but no mechanistic study was reported until quite recently. Even
the effect of the pressure of CO used in the process on reaction rate remained unclear.
Thus, while initial reports on catalytic PKR involved the use of high temperatures and CO
pressures,
22
catalytic PKR at atmospheric CO pressure was reported later.
23
The only thor-
ough kinetic study on the PKR reported so far is from Verdaguer, Riera, and co-workers,
24
who studied by in situ FTIR the reaction of trimethylsilylacetylene with norbornadiene
in the presence of a catalytic amount of Co
2
(CO)
8
, and applied Blackmond's reaction
progress kinetic analysis
25
to the study of the mechanism of the reaction (Scheme 2.15).
