Mechanism of Cobalt-Catalyzed Heterodimerization of Acrylates and 1,3-Dienes. A Potential Role of Cationic Cobalt(I) Intermediates

Coupling
reactions of feedstock alkenes are promising, but few
of these reactions are practiced industrially. Even though recent
advances in the synthetic methodology have led to excellent regio-
and enantioselectivies in the dimerization reactions between 1,3-dienes
and acrylates, the efficiency as measured by the turnover numbers
(TONs) in the catalyst has remained modest. Through a combination
of reaction progress kinetic analysis (RPKA) of a prototypical dimerization
reaction and characterization of isolated low-valent cobalt catalyst
precursors involved, several important details of the mechanism of
this reaction have emerged. (i) The prototypical reaction has an induction
period that requires at least 2 h of stir time to generate the competent
catalyst. (ii) Reduction of a Co­(II) complex to a Co­(I) complex and
subsequent generation of a cationic [Co(I)]+ species are responsible for this delay. (iii) Through RPKA using
in situ infrared spectroscopy, same excess experiments reveal inhibition
by the product toward the end of the reaction, and no catalyst deactivation
is observed as long as the diene is present in the medium. The low
TON observed is most likely the result of the inherent instability
of the putative cationic Co­(I) species that catalyzes the reaction.
(iv) Different excess experiments suggest that the reaction is first
order in the diene and zero order in the acrylate. (v) Catalyst loading
experiments show that the catalyst is first order. The orders in the
various regents were further confirmed by variable time normalization
analysis. (vi) A mechanism based on oxidative dimerization [via Co­(I)/Co­(III)-cycle]
is proposed. Based on the results of this study, it is possible to
increase the TON by a factor of 10 by conducting the reaction at an
increased concentration of the starting materials, especially the
diene, which seems to stabilize the catalytic species.