The Mechanism of the Pauson-Khand Reaction: Hypothesis, Experimental Facts, and Theoretical Investigations - The Pauson-Khand Reaction - page 44

intramolecular PKR with BINAP as a ligand. 32 Modeling of the possible transition states for
the alkene insertion step resulted in accurate reproduction of the experimental observations,
thus allowing rationalization of how the chiral induction takes place (Figure 2.12).
In a similar approach, combining QM/MM ONIOM with full DFT calculations, the use
of chiral N -oxides as promoters for the enantioselective PKR was studied. 33 In this case,
the asymmetric induction was obtained from the selective oxidation of one CO ligand by
the chiral, enantiopure N -oxide (Figure 2.13, top). After this initial selection, the barrier
for racemization of the pentacarbonyl complex was demonstrated to be significantly higher
than the barrier for alkene insertion, this being responsible for the transfer of chirality to
the final product (Figure 2.13, bottom).

HO

OC CO

OC Co Co CO

CO

Acetone DME

HO

R S R S

OC CO

H

eq

OC
OC

Co Co CO

CO

B R

eq

3.3

2.6

ax

HO 1.7

ax

H

1.2

ax

A

ax

0.7

OC CO

0.2

eq

0 0

eq

OC
OC

Co Co CO

O

N

H

B S

O

OC
OC

CO

Co Co

CO

H

OH

12.0
10.8

HO

HO

OC CO

OC CO

OC
OC

Co Co CO

OC Co Co CO

CO

HO

HO

B R B R

2.4 2.4

H OC CO

OC CO H

OC
OC

Co Co CO

OC Co Co CO

CO

0 0

1.2 1.2

C R C R

H

H

Figure 2.13 Top: BNO-promoted selective loss of CO from Co 2 (CO) 6 (alkyne) complex and
energies (in kcal mol − 1 ) of the four lower transition states. Bottom: Energetic barrier for the

interconversion between enantiomers of the alkene complexes C R/S .

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