The Mechanism of the Pauson-Khand Reaction: Hypothesis, Experimental Facts, and Theoretical Investigations - The Pauson-Khand Reaction - page 39

TS1eq tr
15.2

TS1ax
13.8

CO OC

OC
OC

CO OC

OC

CO

TSeq tr ,ax
5.7

CO CO

Co Co Co

TS tr⁄⁄,
4.7

H CH 3 H CO CH 3 H CO CH 3

Ceq tr⁄⁄
3.4

Cax Ceq cis Ceq tr

Cax
2.8

Ceq tr
0.0

Figure 2.6 Computational analysis of the interconversion between the different olefin-
coordinated complexes C for the PKR between propyne and ethylene.

although with no success, this making apparent that a more complete theoretical analysis
was necessary for obtaining a good model for the regioselectivity with polarized alkynes. 28
In a more detailed study, focused on the olefin insertion step, 29 the possible modes
of olefin coordination in a Co 2 (CO) 5 (propyne)(ethylene) complex were analyzed. After
demonstrating that the perpendicular conformation of the olefin is preferred, the barriers
for olefin rotation and for the Co(CO) 2 (alkene) unit pseudorotation were determined. The
low barriers found demonstrated that the initial coordination position is not the key issue
for determining the regioselectivity of the reaction (Figure 2.6).

OC
OC

CO

TS1eq cis

CO

CO

Co

OC CO

2.185/2.177

18.8

TS1ax cis

Co

18.6

1.513

H CH 3

OC

2.816 Cax H

3.237 CH 3

TS1eq tr

D cis

16.6

OC
OC

CO TS1ax tr

15.2

2.246/2.237
2.965

CO

Co

OC CO

Co

H CO CH 3

1.516

CO

Ceq cis

H CH 3

D tr

4.2

Cax

OC CO

CO

2.265/2.251
2.935

2.0 Co Ceq cis

Ceq tr

0.5

D cis

1.4

H CO CH 3

0

D tr

Ceq tr

OC
OC

CO

OC
OC

CO

OC CO OC

OC Co

CO

CO

CO

CO Co

Co

2.010 Co

1.995

1.934

1.937

H CH 3

H CH 3

H CO CH 3 H CO CH 3

1.992 1.979

1.953 1.960

TS1eq tr TS1eq cis TS1ax tr TS1ax cis

Figure 2.7 Analysis of the transition states for the olefin insertion step ( TS1 ) as a function of
the different isomers of intermediate C , in the PKR of propyne and ethylene.