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O
H
Me
Co 2 (CO) 8 cat.
+
TMS
Me
Si
, CO
Me
H
Exo (major)
Scheme 2.15 Catalytic PKR between trimethylsilylacetylene and norbornadiene.
From the experimental data obtained in this study, the rate equation 2.1 for the cat-
alytic intermolecular PKR between trimethylsilylacetylene and norbornadiene (NBD) was
deduced.
k [Co 2 (CO) 8 ] 1 . 3 [NBD] 0 . 3 1 . 2 [CO] 1 . 9
v =
(2.1)
The reaction turned out to be zero order with respect to alkyne, 1.3 order with respect
to the catalyst, between 0.3 and 1.2 order with respect to NBD, and
1.9 order with
respect to CO. On the basis of this rate equation, the authors suggested that a complex
reaction mechanism was in operation in the catalytic intermolecular PKR of norbornadiene.
However, the positive dependence on NBD and the negative dependence on CO, together
with the observation that the corresponding dicobalt alkyne complex I (Scheme 2.16),
clearly identified by IR spectroscopy, was acting as a major resting state, meant they
could infer that the CO-alkene ligand exchange process was an energetically demanding
equilibrium preceding the rate-limiting step. In this manner, the following step in the
sequence, consisting of alkene insertion to give cobaltacycle III , would be the rate-limiting
step in the catalytic process. In this scenario, a higher concentration of NBD and lower
concentration of CO are beneficial for the kinetics of the reaction, since they favor formation
of the key intermediate II .
A very important conclusion is that these experimental data are consistent with the
overall catalytic cycle shown in Scheme 2.16, which is in agreement with the original
Magnus proposal and with the theoretical knowledge accumulated on the reaction (see next
section).
2.4 Theoretical Studies
2.4.1 General Approach to the Mechanism
Despite the great interest in the Pauson-Khand reaction, only in the last decade, 30 years after
its discovery, has significant progress been made in computational approaches to elucidate
and understand its mechanism. Only limited attempts to explain the diastereoselectivity ob-
served when using chiral auxiliaries, by semi-empirical and molecular mechanics methods,
were published in the late 1990s 19c,26 (see below).
It was not until 2001 that the first comprehensive study on the full reaction mechanism
was published by Nakamura and Yamanaka. 16a In this study, performed at the DFT level
with simple substrates, the authors confirmed the feasibility of the Magnus mechanism,
while shedding some light on the particular issues affecting the reaction outcome in different
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