The Mechanism of the Pauson-Khand Reaction: Hypothesis, Experimental Facts, and Theoretical Investigations - The Pauson-Khand Reaction - page 38

With alkynes bearing two sterically similar but electronically different substituents, how-
ever, polarization on the alkyne turned out to exert a strong influence on the regiochemical
outcome of the reaction (Scheme 2.20).

O O

H H

R'

Co 2 (CO) 8 Co 2 (CO) 8

+

R R'

R

H H

R' R

R = H, R' = CH 3
R = H, R' = CO 2 CH 3

R = CH 3 , R' = CO 2 CH 3

Scheme 2.20 Regioselectivity in the PK reaction with differently polarized alkynes.

This issue was addressed by a theoretical study on the basis of a trans effect claimed to
affect the preferred olefin coordination position and, in consequence, the position for olefin
coordination and insertion 27 (Table 2.1).
In this work the authors argued that, with polarized alkynes such as methyl 2-butynoate
(Table 2.1, R CH 3 ,R

CO 2 CH 3 ), the carbonyl ligand in eq cis position with respect
to the negatively charged C 2 carbon atom was more labile than the others, and this causes
the reaction to proceed through olefin coordination and insertion from this point. This
study correctly reproduced the experimental observations for alkynes bearing sterically
similar but electronically different substituents, although it had the flaw of obviating the
possibility of a Curtin-Hammett situation with the coordination of the olefin not being the
regioselectivity-determining step (see Section 2.2).
A similar approach, based on the polarization of the acetylene unit in the
Co 2 (CO) 6 (alkyne) complex was attempted for the case of S -alkyl-substituted alkynes,

= =

Tab l e 2 . 1 Atomic charges (e − ) based on natural population analysis of the acetylenic
carbons in complexes A 1-3 and first carbonyl dissociation energies (kcal

mol − 1 ) for the
formation of the different isomeric dicobalt pentacarbonyl alkyne complexes B .

·

OC
OC

CO OC

OC

CO OC

OC

CO OC CO

CO CO CO

Co Co Co Co

- CO

R CO R' R R' R CO R' R CO R'

Beq cis Beq tr

A Bax

E A → B

Starting complex R R' q(C R ) ( R' ) q Bax Beq cis Beq tr

A 1 H 3 − 0.31 − 0.18 − 0.13 −

a

−

b

24.2

A 2 H 2 CH 3 − 0.36 − 0.33 − 0.03 37.1 26.4 26.7

A 3 CH 3 CO 2 CH 3 − 0.14 − 0.34 + 0.20 37.9 24.5 −

c

a Optimized structure exhibited a small imaginary frequency and reoptimization yielded B 1 eq tr . b Optimization yielded
B 1 eq tr . c Optimization yielded B 3 eq cis .