The Mechanism of the Pauson-Khand Reaction: Hypothesis, Experimental Facts, and Theoretical Investigations - The Pauson-Khand Reaction - page 28

CO CO CO

CO

CO CO CO CO

CO

R Co R

R Co Co

HO Co CO Co CO

Co CO

H H

Ph
Ph

CO

CO P

Ph 2 P

O

n

Figure 2.2 Studies on the isolation of the intermediates bearing the coordinated alkene.

2.2.2 Cobaltacycle Formation

Cobaltacycle formation is the most important step in the mechanistic pathway of the
PKR because it is in this step where the regiochemistry and the stereochemistry of
the final cyclopentenone adduct are established. The available experimental data in-
dicate that, in the absence of geometrical restrictions, the alkene-substituted complex
Co 2 (CO) 5 (alkyne)(alkene) readily evolves into a coordinatively unsaturated cobaltacycle.
The stereoelectronic requirement for this step to proceed is the achievement of a synperi-
planar arrangement between the carbon-cobalt bond and the carbon-carbon double bond
participating in the metathesis-like process (Scheme 2.10).

R R

CO

OC CO OC

CO

Co

OC Co Co CO OC Co

OC OC

R

R

Scheme 2.10 The alkene insertion step.

As we have already mentioned, the olefin coordination process is likewise kinetically
controlled. Thus, for olefins where the double bond depicts diastereotopic faces, coordi-
nation involving the less hindered face will be favored. Even if this step is irreversible,
the alkene moiety of a Co 2 (CO) 5 (alkyne)(alkene) complex can experience two types of
low-energy process before cobaltacycle formation: i) Rotation of the alkene ligand with
respect to the complex, and ii) fluxional processes of the Co(CO) 2 (alkene) moiety leading
to interconversion between pseudoaxial and pseudoequatorial positions. 19
Rotation of the alkene ligand (Scheme 2.11) is a highly relevant process in connection
with the regioselectivity of the PKR (whenever non-symmetrically substituted alkenes are
involved in the process) or with its stereoselectivity (when non-symmetrically substituted
alkynes and alkenes depicting diastereotopic faces are simultaneously involved in the
process).
Fluxional processes at a single Co(CO) 2 (alkene) moiety have, in turn, no direct influence
on the regio- or the stereochemistry of the PKR product.
Although considerations based on the conformational analysis of the
Co 2 (CO) 5 (alkyne)(alkene) complexes are normally used to predict or to rationalize