The Mechanism of the Pauson-Khand Reaction: Hypothesis, Experimental Facts, and Theoretical Investigations - The Pauson-Khand Reaction - page 36

From both acylated species Ga and Gb , reductive elimination gives way to the same
coordinated cyclopentenone product H which can be released by decomposition of the
complex (stoichiometric systems) or, under the adequate conditions, substituted by a new
alkyne unit to regenerate complex A (catalytic systems).
This high level theoretical study has had a very strong impact on further investiga-
tion dealing with different aspects of the PKR from a mechanistic perspective. The key
aspects of its importance are: i) the theoretical analysis provided further support to the
Magnus mechanism (accepted as the working paradigm for research on this topic), while
complementing it with an effective rational of previous experimental observations and a
theoretical frame for designing new experiments; ii) the determination of the particular
structures of the intermediates and transition states involved in this mechanism (Figure 2.4)
allowed careful study of their most relevant features in both geometrical and electronic
aspects.
Regarding the geometrical aspects, it is worth noting that the essential backbone of
the complex was maintained throughout the process, with the Co-Co bond being retained
(2.326-2.695 A) and the Co 2 atom firmly attached to the acetylene moiety (Co 2 -C 1 and
Co 2 -C 2 bond lengths of 1.952-2.257 A). The acetylene unit in A had a C-C distance
of 1.336 A and a C-C-H angle of 142.9 ◦ , this clearly indicating strong backbonding
from the Co atoms. On the other hand, the olefin in C presented a nearly planar ge-
ometry and long Co-C distances, this showing a weak coordination with no significant
backbonding.
For the electronic aspects, natural population calculations were used to determine the
charge distribution in each step. Comparison of the evolution of the charge on both Co
atoms showed that Co 2 not only acted as a spectator, structural moiety but as a reservoir of
electrons for Co 1 . On the other hand, the acetylene moiety was negatively charged in the
first stages of the process (as a result of the above-mentioned strong back donation from
the Co atoms) while the ethylene moiety was essentially neutral, this situation changing at
the insertion stage.
These fundamental considerations provide the frame for further research on more par-
ticular issues as the regio- and stereoselectivity of the PKR, not only from computational
but also experimental approaches.
2.4.2 Regioselectivity. Early Steps of the Pauson-Khand Reaction
The above-mentioned analysis of the electronic characteristics of the intermediates and
transition states served as a basis for a preliminary theoretical rational of the experimentally
observed regioselectivity. 16a Particularly, the lack of polarization at the alkyne, or the alkene
observed in the transition state for the insertion step, ( TS1 , see above) suggested that the
regioselectivity had to be based on steric effects.
Analysis of this TS1 structure for the model reaction with acetylene and ethylene showed
that, while for the alkyne the C 1 position was clearly less hindered than C 2 , for the olefin
there was no apparent difference between both carbon atoms, both C 3 and C 4 occupying
relatively unhindered positions. This was consistent with the experimentally observed lack
of selectivity regarding the alkene and high selectivity regarding the alkyne counterpart in
the reaction between terminal alkenes and alkynes (Scheme 2.18).