1 research outputs found
Mechanism of the Ti<sup>III</sup>-Catalyzed Acyloin-Type Umpolung: A Catalyst-Controlled Radical Reaction
The titaniumÂ(III)-catalyzed
cross-coupling between ketones and
nitriles provides an efficient stereoselective synthesis of α-hydroxyketones.
A detailed mechanistic investigation of this reaction is presented,
which involves a combination of several methods such as EPR, ESI-MS,
X-ray, in situ IR kinetics, and DFT calculations. Our findings reveal
that C–C bond formation is turnover-limiting and occurs by
a catalyst-controlled radical combination involving two titaniumÂ(III)
species. The resting state is identified as a cationic titanocene-nitrile
complex and the beneficial effect of added Et<sub>3</sub>N·HCl
on yield and enantioselectivity is elucidated: chloride coordination
initiates the radical coupling. The results are fundamental for the
understanding of titaniumÂ(III)-catalysis and of relevance for other
metal-catalyzed radical reactions. Our conclusions might apply to
a number of reductive coupling reactions for which conventional mechanisms
were proposed before