Bifunctional Ligand-Assisted Catalytic Ketone α‑Alkenylation with Internal Alkynes: Controlled Synthesis of Enones and Mechanistic Studies

Here,
we describe a detailed study of the rhodium­(I)-catalyzed,
bifunctional ligand-assisted ketone α-C–H alkenylation
using internal alkynes. Through controlling the reaction conditions,
conjugated enamines, α,β- or β,γ-unsaturated
ketones, can be selectively accessed. Both aromatic and aliphatic
alkynes can be employed as coupling partners. The reaction conditions
also tolerate a broad range of functional groups, including carboxylic
esters, malonates, secondary amides, thioethers, and free alcohols.
In addition, excellent E-selectivity was observed
for the tetra-substituted alkene when forming the α,β-unsaturated
ketone products. The mechanism of this transformation was explored
through control experiments, kinetic monitoring, synthesizing the
rhodium–hydride intermediates and their reactions with alkynes,
deuterium-labeling experiments, and identification of the resting
states of the catalyst.