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reason, it has evolved over the years into a powerful and well-established procedure for
the assembly of five-membered rings. In a parallel manner, PK-type reactions involving
transition metals other than cobalt have also importantly developed.
2
O
1) Co
2
(CO)
8
R
2
R
1
H
R
1
R
2
H
2)
R
3
R
3
H
Scheme 2.1
A prototypic Pauson-Khand reaction.
From the perspective of reaction mechanisms, the Pauson-Khand reaction (PKR) is a
complex process (three fragments are combined and three covalent bonds are built in a
single step), taking place essentially as a black box. Thus, in stoichiometric reactions the
starting materials are normally the dicobalt hexacarbonyl complex of the reacting alkyne
and the counterpart olefin, and cyclopentenone formation takes place without detection of
any intermediate reaction.
A mechanistic sequence for the reaction was proposed by Magnus in 1985.
3
It was based
on general knowledge of organometallic chemistry, with the single purpose of facilitating
the rationalization of the observed diastereoselectivity of a particular set of examples of
the reaction (Scheme 2.2). This proposal, however, had the virtue of accommodating the
known experimental facts of the reaction and has gained universal acceptation as the
Magnus
mechanism
of the PKR.
R
R
TBSO
TBSO
Co
TBSO
Co
Co
Co
Co
Co
R
CO
H
H
Co
Co
R
R
TBSO
TBSO
TBSO
Co
R
Co
O
O
O
H
H
H
Scheme 2.2
The Magnus' mechanistic proposal.
In fact, considerations based on the Magnus mechanism have led to the rationalization
of regiochemical and stereochemical outputs of the PKR, and theoretical calculations
(see below) have shown the energetic viability of the proposed pathway. A more general
formulation of this mechanistic proposal for the PKR can be found in Scheme 2.3.
Thus, four main events are involved in the pathway connecting the starting alkyne
dicobalt hexacarbonyl complex with the product cyclopentenone: i) Ligand substitution,
where the reacting alkene becomes associated with the initial complex, ii) cobaltacycle
formation, of outmost importance since the regiochemistry and the stereochemistry of the
