Electronic Selectivity Tuning in Titanium(III)-Catalyzed Acetylene Cross-Dimerization Reactions

The reactivity of open-shell titanium­(III) complexes
in organometallic
catalysis is associated with many open questions, in particular regarding
the electronic structure of catalytic intermediates and transition
states. The unpaired electron density in principle allows for radical-type
reactivity, while at the same time empty d orbitals allow more traditional
cis-coordination insertion pathways. In this paper we investigated
the (Cp*)2TiIII-catalyzed cross-dimerization
of aliphatic and aromatic acetylenes, focusing on the reactivity of
two different aliphatic acetylenes with a series of different aromatic
acetylenes. The applied aliphatic acetylenes 1a (4-methylpent-1-yne)
and 1b (N,N-dimethyl-N-propargylamine) have the same size but different electron-accepting
abilities. The better π-accepting substrate 1b shows
a higher reactivity and selectivity than substrate 1a in the studied cross-dimerization reactions. Stronger π back-donation
from the titanium-localized SOMO to the substrate thus seems to be
a reasonable explanation for the improved selectivity of substrate 1b. DFT calculations indeed suggest a stronger binding of
substrate 1b as compared to that of 1a in
the selectivity-determining steps of the reaction, thus leading to
faster insertions and higher selectivities with substrate 1b.